Badger Crop Doc with Ashley Davenport for an AgPro Podcast

Dr. Damon Smith, or @badgercropdoc, is an extension specialist and associate professor at the University of Wisconsin, Madison. We’re talking about different agronomic issues farmers have faced this year, including a newer one: tar spot. You can listen to the Podcast here.

Bacterial Leaf Streak of Corn Confirmed for the First time in Wisconsin

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

Carol Groves, Associate Researcher, University of Wisconsin-Madison

Brian Hudelson, Plant Disease Diagnostician, University of Wisconsin-Madison

Sue Lueloff, Assistant Plant Disease Diagnostician, University of Wisconsin-Madison

Figure 1. Symptoms of bacterial leaf streak on corn.

The 2018 corn production season in Wisconsin has been challenging to say the least. We had what looked to be some of the best corn production we ever had, and then the diseases started to move in. We have observed numerous foliar disease issues and have spent a lot of time trying to understand the tar spot epidemic in Wisconsin and surrounding states.

To add insult to injury, we have now confirmed bacterial leaf streak (BLS) of corn. You may remember that we have been on the lookout for this disease over the past several seasons, but have not confirmed it officially in the state until now. A corn sample was received in our Plant Disease Diagnostic Clinic this season from Pierce County with symptoms consistent with those for BLS (Fig. 1). The sample was confirmed positive in our clinic through multiple tests, including bacterial streaming and PCR. Subsequently, the sample has been confirmed positive by multiple laboratories, including the CPHST-Beltsville Laboratory.

Bacterial leaf streak (BLS) of corn was reported for the first time on corn in the U.S. in 2016, but was likely present in Nebraska since 2014. The first report was in Nebraska with subsequent reports coming in from other states in the U.S. corn belt. Other states where the disease has been confirmed include Iowa, Illinois, Colorado, Kansas, Minnesota, Oklahoma, South Dakota, Texas, and now Wisconsin.

What causes bacterial leaf streak and what are the symptoms?

Bacterial leaf streak is caused by a bacterium named Xanthomonas vasicola pv. vasculorum. It causes wavy narrow leaf lesions with wavy edges that are often brown in color. Lesions can appear translucent and have halos when backlit. Symptoms on corn have been observed as early as V7, starting in the lower canopy and moving up the canopy if weather conditions are favorable (wet weather, with hot temperatures). Little is known about the disease cycle, but researcher believe it can overwinter on corn residue. The bacterium is presumed to be spread by irrigation, splashing rain, or wind-driven rain. No injury is needed for the bacterium to enter the plant. It is unknown if the bacterium can be spread on, or in, seed and if there are alternative weed hosts.

Does bacterial leaf streak cause yield loss?

Little is actually known about the disease on corn in the U.S. Most researchers believe that yield loss is minimal if the disease moves in late in the season. If the disease moves in earlier and causes extensive leaf blighting during grain fill, then yield losses could be more substantial. Little is known about the effect of BLS on grain quality.

How do I manage bacterial leaf streak of corn?

Some corn hybrids appear to have better resistance to BLS than others. Work with your seed dealer to find a hybrid that is rated as resistant and fits your environment. Hybrid resistance will be key to manage this disease. BLS is caused by a bacterium, thus, fungicides are not effective in controlling this disease. Withholding irrigation has also been shown to not be effective as the disease can occur in drylands and irrigated fields. Managing corn residue through rotation may be helpful. Tillage and burying residue might also be an option, but managing soil erosion should be placed as a higher priority.

Other Resources about bacterial leaf streak

How do I get a diagnosis if I suspect bacterial leaf streak?

If you suspect that you have BLS in your corn crop in Wisconsin, leaf samples of corn plants can be sent in a sealed plastic bag with NO added moisture to the University of Wisconsin Plant Disease Diagnostic Clinic (PDDC). Information about the clinic and how to send samples can be found by CLICKING HERE.

What to Expect from Stalk Rot and Mycotoxins in Severely Diseased and Damaged Corn

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

Corn is looking pretty rugged in many areas of the Wisconsin corn belt. Areas in southern, southwestern, and south-central Wisconsin have experienced major foliar disease epidemics including the new disease, tar spot. Areas in eastern, east-central, and south-central Wisconsin have also seen heavy flooding and storm damage in corn fields. We have seen fields severely diseased, experiencing stalk rot, lodged, flooded – you name it, it has been a challenging finish to a season that had much promise.

How is tar spot affecting stalk integrity?

Figure 1. Stalks lodged due to reduced stalk integrity.

For corn foliar diseases such as northern corn leaf blight (NCLB) and gray leaf spot (GLS), it is well known that high severity can lead to stalk integrity issues. As foliage is damaged, less photosynthetic capacity is available from the leaves to produce carbohydrates for the plant. To fill an ear of corn, carbohydrates are needed from somewhere. In corn where the foliage is significantly damaged, the stalks become a considerable source to fill out the ear (a sink for nutrients). This leaves the stalk tissues devoid of carbohydrates leading to cell death and subsequent colonization of the stalk by fungal pathogens who are taking the opportunity to feed on a weak stalks. Thus, it isn’t uncommon to see stalk rots like Gibberella stalk rot, Fusarium stalk rot or Anthracnose stalk rot at higher incidence where high foliar disease pressure was observed (Fig.1). Where you find stalk rots, you often find root rots caused by the same pathogens. Root rot and stalk rot often go hand-in-hand.

Other causes for loss in stalk integrity can include large ears (nutrient sinks) that the plant can’t fill out, without using some of the stalk resources. In 2018 we saw many fields where the crop was moving through growth stages quickly and setting what appeared to be good yields. However, weather conditions changed midseason, with wet weather and more cloud cover, combined with nitrogen issues in some fields. This led to large ears that needed to be filled out, with again, limited photosynthetic capacity. The stalks were scavenged for carbohydrate, leaving them, again, with limited integrity.

Figure 2. An entire field lodged due to significant stalk rot

Now throw in some tar spot. Yet, another foliar disease that can limit photosynthetic capacity of the corn plant. We have observed many fields with significant stalk integrity issues. Whether just tar spot, or tar spot combined with GLS, NLCB, and/or stalk scavenging just for carbohydrates – stalks are in bad shape in many areas of Wisconsin. This is resulting in significant lodging issues in many fields, especially those hit with bad storms over the last several weeks (Fig. 2). Harvesting fields with low stalk integrity early will be key to protect yield potential. Conduct a “pinch” test or “push” test to determine which field have lower stalk integrity. Simply pinch stalks or push stalks to a 30 degree angle. Those plant that are soft and easily pinch or don’t pop back up after pushing, have stalk integrity issues. If 30-50% or more of these plants are identified with stalk integrity problems, they should be harvested first, to prevent yield losses from lodging.

What about tar spot, lodged corn, and mycotoxins?

Mycotoxins have not been implicated in the organisms reported to cause tar spot in Latin America. However, that doesn’t mean that other organisms that cause mycotoxins might not be present on harvested grain or silage. As plants dry down they can no longer actively fight fungal infection. We have looked at many brown and drying leaf samples from corn plants with tar spot. We do find many other fungal organisms, including Fusarium-organisms, which can produce mycotoxins. So while tar spot itself may not lead to mycotoxins, opportunistic fungi that colonize secondarily may result in elevated mycotoxin levels.

In addition, corn that has lodged and is in contact with the wet and saturated ground is at risk of being colonized by organisms that produce mycotoxins. Many of the known mycotoxin-producing fungi are found in the soil and on residue on the surface of the soil. If lodged corn is in contact with the ground and there is good moisture, it is possible that the ear and plant are being colonized and mycotoxins are being produced. So while your combine might be able to pick a plant up and harvest the ear, beware that it might be heavily colonized with organisms that produce mycotoxins. If taking corn for silage, lodged plants run the risk of significant hygiene issues in the bunker, including mycotoxins issues.

Where else can mycotoxins come from?

Figure 3. Diplodia ear rot on an ear of corn.

Corn ears don’t have to touch the ground to be infected with ear-rot fungi, they can also be colonized by ear-rot fungi through the silks. Given the kind of crazy year we have had, ear rot might be a significant concern in fields that saw erratic weather this season. Ear rots caused by fungi in the groups Diplodia (Fig. 3), Fusarium, and Gibberella will be the most likely candidates to watch for as you begin harvest.  Fusarium and Giberrella are typically the most common fungi on corn ears in Wisconsin.  This group of fungi not only damage kernels on ears, but can also produce mycotoxins.  The toxins of main concern produced by these organisms are fumonisins and vomitoxin and can threaten livestock that are fed contaminated grain.  Thus grain buyers actively test for mycotoxins in corn grain, and feed managers monitor silage for mycotoxin levels to be sure they are not above certain action levels established by the U.S. Food and Drug Administration (FDA).

The FDA has established maximum allowable levels of fumonisins in corn and corn products for human consumption ranging from 2-4 parts per million (ppm).  For animal feed, maximum allowable fumonisin levels range from 5 ppm for horses to 100 ppm for poultry. Vomitoxin limits are 5 ppm for cattle and chickens and 1 ppm for human consumption.

For more information about ear rots and to download a helpful fact sheet produced by a consortium of U.S. corn pathologists, CLICK HERE.

How do I reduce mycotoxin risks at harvest?

Before harvest, farmers should check their fields to see if moldy corn is present. Sample at least 10-20 ears in five locations of your field. Pull the husks back on those ears and observe how much visible mold is present. If 30% or more of the ears show signs of Gibberella or Fusarium ear rot then testing of harvested grain is definitely advised. If several ears show 50-100% coverage of mold testing should also be done. Observe grain during harvest and occasionally inspect ears as you go. This will also help you determine if mycotoxin testing is needed.

If substantial portions of fields appear to be contaminated with mold, it does not mean that mycotoxins are present and vice versa. For example, Diplodia ear rot does not produce mycotoxins. However, if you are unsure, then appropriate grain samples should be collected and tested by a reputable lab.  Work with your corn agronomist or local UW Extension agent to ensure proper samples are collected and to identify a reputable lab.

For more information on mycotoxins and to download a fact sheet, CLICK HERE.

Helpful information on grain sampling and testing for mycotoxins can be found by CLICKING HERE.

For a list of laboratories that can test corn grain for mycotoxins, consult Table 2-16 in UW Extension publication A3646 – Pest Management in Wisconsin Field Crops.

How should I store corn from fields with ear rots and mold?

If you observe mold in certain areas of the field during harvest, consider harvesting and storing that corn separately, as it can contaminate loads; the fungi causing the moldy appearance can grow on good corn during storage.  Harvest corn in a timely manner, as letting corn stand late into fall promotes Fusarium and Gibberella ear rots.  Avoid kernel damage during harvest, as cracks in kernels can promote fungal growth.  Also, dry corn properly as grain moisture plays a large roll in whether corn ear rot fungi continue to grow and produce mycotoxins.  For short term storage over the winter, drying grain to 15% moisture and keeping grain cool (less than 55F) will slow fungal growth.  For longer term storage and storage in warmer months, grain should be dried to 13% moisture or less. Fast, high-heat drying is preferred over low-heat drying. Some fungi can continue to grow during slow, low-heat drying. Also, keep storage facilities clean.  Finally, mycotoxins are extremely stable compounds: freezing, drying, heating, etc. do not degrade mycotoxins that have already accumulated in grain. While drying helps to stop fungal growth, any mycotoxins that have already accumulated prior to drying will remain in that grain. The addition of acids and reducing pH can reduce fungal growth but will not affect mycotoxins that have already accumulated in harvested grain.

For more information on properly storing grain and to download a fact sheet on the subject, CLICK HERE.

References

Munkvold, G.P. and White, D.G. Compendium of Corn Diseases, 4th Edition. APS Press.

In addition, This article is a compilation of the following previously written resources:

Smith, D.L. 2016. Wisconsin Late-Season Corn Disease Update. http://fyi.uwex.edu/fieldcroppathology/2016/09/07/wisconsin-late-season-corn-disease-update/.

Smith, D.L. and Mitchell, P. D. 2016. Wet Wisconsin: Moldy Corn and Crop Insurance. http://ipcm.wisc.edu/blog/2016/09/wet-wisconsin-moldy-corn-and-crop-insurance/.

 

 

Holy Tar Spot, Batman!

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

Figure 1. Tar Spot on Corn From the Arlington Agriculture Experiment station in 2018.

The phone has been ringing off the hook over the past week. In fact, this morning I had to dump my voicemail as it wouldn’t take any more messages. Why is that? For those of you who have been traveling the southern Wisconsin and Northern Illinois countryside, you know why. Holy tar spot, Batman!

Tar spot (Fig. 1) is no longer a cosmetic leaf disease in Wisconsin and Illinois. We have seen epic levels this season, resulting in severe damage in some fields and early dry-down of corn. Tar spot is a relatively new disease in the U.S. and Wisconsin. It is caused by a fungus called Phyllachora maydis. Tar spot causes small tar-like spots on the surface of corn leaves. For great information about tar spot and what it looks like, consult this Purdue Extension fact sheet. Tar spot was first found in the U.S. in 2015. In 2016 and 2017, tar spot was identified in Green, Iowa, Grant, and Lafayette counties in Wisconsin. In 2018 confirmations have been made in these same general areas, but also has expanded to include reports from as far north as Columbia Co. over to the eastern side of the state, including Fond Du lac and other areas of the cron belt. Severity has ranged from simple cosmetic damage to complete death of entire fields. Unfortunately, it is hard to discern why the epidemic is so significant here in the upper Midwest in 2018. Some anecdotal thoughts include hybrid susceptibility, environmental impacts, and synergistic reactions between multiple organisms.

In Latin America Phyllachora maydis can be found in a complex with another fungus called Monographella maydis. In areas where the complex occurs significant yield loss has been described. However, in the U.S. Monographella maydis has not been found in complex with Phyllachora maydis. In addition, a third organism, Coniothyrium phyllachorae, has also been found to be associated with the complex. This organisms has also not been implicated in the epidemics in the Midwest. So, our epidemic of 2018 is a real head-scratcher.

What is known about tar spot?

Figure 2. Tar spot with “Fisheyes” on the upper surface of corn leaves.

Tar spot is favored by cool conditions (60-70 F) and high relative humidity (averages above 75%). It is generally accepted that when Phyllachora maydis occurs by itself, damage is cosmetic in nature and does not result in significant yield reductions. This has been most of the case since 2016 when tar spot was first identified in Wisconsin. In Latin America where Phyllachora maydis can form complexes with Monographella maydis and/or Coniothyrium phyllachorae, “fisheye” symptoms along with severe necrosis and early dry down can be observed. In Latin America, it has been reported that when in complex, damage from tar spot can result in yield loss as much as 30%. The main method to manage tar spot in Latin America is to use resistant hybrids. Little is known about effective fungicides, especially fungicides that might be available in the U.S.

So is tar spot behaving the same in the Midwest U.S. as in Latin America?

The simple answer is, I don’t know. We can easily find the presence of Phyllachora maydis on affected corn in the Midwest. However, we aren’t able to easily find the other organisms that have been implicated. However, we do find “fisheyes” and significant necrosis on corn leaves (Figs. 2 and 3). It could be that Phyllachora maydis is forming a complex with other organisms in the Midwest. Things don’t have to be the same as in Latin America. One simple observation is that in areas where gray leaf spot (GLS) got an early foothold on corn, we have seen more severe epidemics of tar spot. Is that the complex in Wisconsin? Not sure, but definitely a lot of damage in some fields by both organisms. This is why we are working hard right now to identify organisms implicated in causing this damage. We need to know what the complex might be here in Wisconsin and the Midwest.

What is the impact on yield and do I need to worry about mycotoxins?

Figure 3. Tar spot with “Fisheyes” on the lower surface of a corn leaf.

We are not sure what the impact on yield will be yet. This is a hard question to answer and will depend on when the epidemic started in a field and how severe symptoms will be. If I had to make an educated guess, on grain corn, damage will likely range from none, to low test weights on moderately affected fields, to some yield loss in fields that were hit early. On silage corn, it might be difficult to pack the bunker as dry-down is occurring rapidly and moisture might be too low to make quality silage. Mycotoxins have not been implicated in the organisms reported to cause tar spot in Latin America. However, that doesn’t mean that other organisms that cause mycotoxins might not be present on harvested grain or silage. While we don’t expect there to be an issue with mycotoxins, I would encourage you to continue to test grain and feed to be sure mycotoxins are not present.

What should I do with fields affected by tar spot?

Scout these fields. It will be important to make timely harvest decisions. For fields that will be taken for grain, consider harvesting these fields early. One observation we have made is that in fields affected by early epidemics and dry-down, stalks have been scavenged for nutrients to finish filling the ear. This is leaving stalk integrity in some of these fields as highly questionable, with various stalk rots moving in. Timely harvest will be key in these fields before excessive lodging occurs. For silage, other considerations, such as high-moisture corn, might be needed where corn is too dry to effectively pack the bunker. Also, be sure to continue to test for mycotoxins just to be sure there aren’t other organisms capitalizing on already stressed corn plants. DO NOT apply fungicides at this point. Fungicides will not be effective after the milk growth stage and most fungicides have a pre-harvest interval that has already passed for both silage and grain.

What is the Wisconsin Field Crops Pathology program doing to understand tar spot?

While frustrating for those of you who have to deal with this disease, we do need to take our time to try to figure out what the cause of the epidemic is. If we don’t know what to control, how do we develop a management plan? You wouldn’t just take antibiotics, if you didn’t know what caused your sore throat, would you?

We are working with Dr. Nathan Kleczewski at the University of Illinois to improve our understanding of this pathogen in the U.S. If you would like to confirm tar spot on corn, or provide samples for research purposes, you can send samples to the University of Wisconsin Plant Disease Diagnostic Clinic. If we can get a better handle on what is causing the the significant damage on corn, we can make informed decisions about how to manage it. Our program is also rating and taking notes in fungicide trials where we have tar spot. We aren’t sure we will find differences, but taking notes on timing of application and products will inform our recommendations. We also are looking for hybrid trials where we could take notes on resistant hybrids. Finally, we have initiated a late-season field trial to begin to understand the epidemiology of this disease. If we can understand when it moves in and how long it takes to see symptoms after infection, we might be able to better apply in-season management strategies. In summary, we are working on trying to find some answers, but research and acquiring relevant data will take a little time.

Other Resources

Invasive Species Compendium – Phyllochora maydis

USDA-ARS Fact Sheet – Tar Spot of Corn

University of Illinois Pest Bulletin

2018 Pest Management Update Meeting Series Announced

The schedule for the Wisconsin Pest Management Update meeting series has been set. Presentations will include agronomic pest management information for Wisconsin field and forage crops. Speakers include Mark Renz and Rodrigo Werle, weed scientists, Damon Smith, plant pathologist, and Bryan Jensen, entomologist.

The format will be the same as in 2017. Meetings will either be in the morning or afternoon On November 12-16, 2018. Simply choose a day/location to attend with each meeting running 3 hours. Note that several locations and contacts have changed since 2017 (marked with * in the meeting flier). Please read the informational flier carefully and make sure you contact the appropriate person at your desired location.

2018 Pest Management Update Highlights:

  • Integrated Pest Management Updates in corn, soybeans, alfalfa, and small grains: Update on new products and/or use of existing products as well as brief highlights of the 2018 pest situations in each crop.
  • Waterhemp management
  • Dicamba off-target research
  • Pollinator Training
  • Soybean cyst nematode training and management

Please make your reservation with the host contact at least one week prior to the scheduled meeting date.

Three hours of Certified Crop Advisor CEU credits in pest management are requested for each session.

To download a PDF of the flier, CLICK HERE.

Wisconsin Late-Season Soybean Disease Update

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

The calls have been coming in this past week on a couple of soybean diseases. In the southern third of the state most of the calls have centered on sudden death syndrome or SDS. To the north, most questions pertain to Sclerotinia stem rot or white mold. I’ll discuss SDS in some detail, plus provide a detailed description of brown stem rot (BSR) which also typically shows up this time of year. Finally I’ll provide a brief update on the white mold situation.

Sudden death syndrome (SDS)

Symptoms of sudden death syndrome on soybeans

The first noticeable symptoms of SDS are chlorotic (i.e., yellow) blotches that form between the veins of soybean leaflets. These blotches expand into large, irregular, chlorotic patches (also between the veins), and this chlorotic tissue later dies and turns brown. Soon thereafter entire leaflets will die and shrivel. In severe cases, leaflets will drop off leaving the petioles attached. Taproots and below-ground portions of the stems of plants suffering from SDS, when split open, will exhibit a slightly tan to light brown discoloration of the vascular (i.e., water- conducting) tissue. The pith will remain white or cream-colored. In plants with advanced foliar symptoms of SDS, small, light blue patches will form on taproots and stems below the soil line. These patches are spore masses of the fungus that causes the disease.

Foliar symptoms of SDS can be confused with those of brown stem rot. However, in the case of brown stem rot (BSR), the pith of affected soybean plants will be brown. In addition, roots and lower stems of plants suffering from BSR will not have light blue spore masses.

Once symptoms of SDS are evident, yield losses are inevitable. Yield losses can range from slight to 100%, depending on the soybean variety being grown, the plant growth stage at the time of infection and whether or not SCN is present in a field. If SDS occurs after reproductive stages R5 or R6, impact on yield is usually less compared to the development of SDS at flowering that can lead to substantial yield losses. When SCN is present, the combined damage from both diseases can be substantially more than the sum of the damage expected from the individual diseases.

SDS is caused by the soilborne fungus, Fusarium virguliforme (synonym: F. solani f. sp. glycines). F. virguliforme can overwinter freely in the soil, in crop residue, and in the cysts of SCN. The fungus infects soybean roots (by some reports as early as one week after crop emergence), and is generally restricted to roots as well as stems near the soil line. F. virguliforme does not invade leaves, flowers, pods or seeds, but does produce toxins in the roots that move to the leaves, causing SDS’s characteristic foliar symptoms.

SDS cannot be controlled once plants have been infected. Foliar fungicides have NO effect on the disease.Recently a new seed treatment has been identified that has efficacy against SDS. The active ingredient fluopyram can be found in the seed treatment iLeVo and is rated “very good” in multi-state trials. Other methods of control include using SDS-resistant varieties whenever possible in fields with a history of the disease; however, keep in mind that SDS-resistant varieties with maturity groups suitable for Wisconsin and other northern regions (groups I and II) can be limited. If SDS and SCN are both problems in the same field, planting an SCN-resistant soybean variety may also be beneficial in managing SDS. Do not delay planting soybeans to avoid symptoms of SDS.  In Wisconsin, it has been demonstrated that the benefits to yield when planting early outweigh the benefits of reduced SDS symptoms if planting is delayed. Improve soil drainage by using tillage practices that reduce compaction problems. Rotation, while useful in managing other soybean diseases, does not appear to significantly reduce the severity of SDS. Even after several years of continuous production of corn, F. virguliforme populations typically are not reduced substantially. Research from Iowa State University has shown that corn (especially corn kernels) can harbor the SDS pathogen.

For more information CLICK HERE to download a full color fact sheet on SDS. A short video on SDS can also be viewed by CLICKING HERE.

Brown stem rot (BSR)

Symptoms of BSR in soybean stems compared with a healthy soybean stem in the center.

Symptoms of BSR are usually not evident until late in the growing season and may be confused with signs of crop maturity or the effect of dry soils. The most characteristic symptom of BSR is the brown discoloration of the pith especially at and between nodes near the soil line. This symptom is best scouted for at full pod stage. Foliar symptoms, although not always present, typically occur after air temperatures have been at to below normal during growth stages R3-R4, and often first appear at stage R5, peaking at stage R7. Foliar symptoms include interveinal chlorosis and necrosis (i.e., yellowing and browning of tissue between leaf veins), followed by leaf wilting and curling. Yield loss as a result of BSR is generally greatest when foliar symptoms develop. The severity of BSR symptoms increases when soil moisture is near field capacity (i.e., when conditions are optimal for crop development).

Foliar symptoms of BSR can be confused with those of sudden death syndrome (see description below). However, in the case of sudden death syndrome (SDS), the pith of affected soybean plants will remain white or cream-colored. In addition, roots and lower stems of plants suffering from SDS (but not those suffering from BSR) often have light blue patches indicative of spore masses of the fungus that causes SDS.

BSR is caused by the soilborne fungus Cadophora gregata. There are two distinct types (or genotypes) of the fungus, denoted Type A and Type B. Type A is the more aggressive strain and causes more internal damage and plant defoliation than Type B. P. gregata Type A also is associated with higher yield loss. P. gregata survives in soybean residue, with survival time directly related to the length of time that it takes for soybean residue to decay. Thus, P. gregata survives longer when soybean residue is left on the soil surface (e.g., in no till settings) where the rate of residue decay is slow. P. gregata infects soybean roots early in the growing season. It then moves up into the stems, invading the vascular system (i.e., the water-conducting tissue) and interfering with the movement of water and nutrients.

Several factors can influence BSR severity. Research from the University of Wisconsin has shown that the incidence and severity of BSR is greatest in soils with low levels of phosphorus and potassium, and a soil pH below 6.3. In addition, C. gregata and soybean cyst nematode (Heterodera glycines) frequently occur in fields together, and there is evidence that BSR is more severe in the presence of this nematode.

BSR cannot be controlled once plants have been infected. Foliar fungicides and fungicide seed treatments have NO effect on the disease. Use crop rotations of two to three years away from soybean with a non-host crop (e.g., small grains, corn, or vegetable crops), as well as tillage methods that incorporate plant residue into the soil. Both of these techniques will help reduce pathogen populations by promoting decomposition of soybean residue. Also, make sure that soil fertility and pH are optimized for soybean production to avoid overly low phosphorus and potassium levels, as well as overly low soil pH. Finally, grow soybean varieties with resistance to BSR. Complete resistance to BSR is not available in commercial varieties. However several sources of partial resistance that provide moderate to excellent BSR control are available. Also, some, but not all, varieties of soybean cyst nematode (SCN) resistant soybeans also are resistant to BSR. Most soybean varieties with SCN resistance derived from PI 88788 express resistance to BSR. However, the same is not true of varieties with SCN resistance derived from Peking. Therefore growers should consult seed company representatives about BSR resistance when selecting a variety with SCN resistance derived from this source. You can download a full color fact sheet on BSR by clicking here. You can also CLICK HERE to view a short video on BSR.

White Mold

White, fluffy growth of the white mold fungus on a soybean stem

Symptoms of white mold are becoming pretty apparent in Wisconsin. White fluffy growth (mycelium) is readily evident. Incidence in the northern half of the state is higher. We have visited fields as far north as Wausau and Pulaski, Wisconsin and have observed incidence ranging from 0% to 30% of plants infected. Reports from areas in the northwest indicate white mold present, but not as high of incidence levels. As we move to the southern portion of Wisconsin, white mold can be found, but at reasonably low levels. Most of the soybean crop is at the R5 growth stage, with some earlier maturing fields approaching R6. Questions have arisen about spraying fungicide now to reduce the damage caused by white mold and preserve yield. The short answer is NO. The reason is that the primary means of infection by the white mold fungus is through soybean flowers. These infections happened weeks ago. Therefore, the optimal time to spray would be when flowers were out. A low level of plant-to-plant transmission can occur late in the season in soybeans. However, this rate is low enough, that spraying to prevent it does not produce favorable results.

How much soybean yield might I lose from white mold?

Research has demonstrated that for every 10% increase in the number of plants that are infected with white mold at the R7 growth stage, you can expect between 2 to 5 bushels of yield loss. Thus, the fields I mentioned earlier will likely range from little detectable yield loss (3% incidence) to as high as 10 bushels lost (20% incidence).

What should I do if I see white mold in my soybean field now?

Get out and survey your fields for white mold. It is a good idea to determine how much white mold you have in your fields, so you can make some educated harvest decisions. One way to move white mold from one field to the next is via combines. You could clean your combine between each field, but this can be time consuming. So by determining which fields have no white mold and which fields have the most white mold, you can develop a logical harvest order by beginning your harvest on fields with no white mold and working your way to the heavily infested fields. This will help reduce spread of the white mold fungus to fields that aren’t infested. You can also make some decisions on your rotation plan and future soybean variety choices based on these late season observations.

If you would like to learn more about white mold and management of this disease, CLICK HERE to download a fact sheet from the crop protection network. You can also watch a short video about white mold by CLICKING HERE.

Late Season Corn Foliar Disease Update and Hail-Damaged Corn

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

Scouting by my team and phone calls from extension personnel, consultants, and farmers have made it evident that there are several foliar diseases of corn showing up  in this first part of August. Gray leaf spot (GLS), northern corn leaf blight (NCLB), and tar spot have all been found in various locations over the last week or so. It is becoming very late in the season to try to control GLS or NCLB. Current data on tar spot indicate it likely doesn’t need to be controlled. Thus, there is likely not much to do at this point, but to document which fields have which diseases. This can help in fall scouting to make harvest decisions, as fields with higher levels of leaf disease may not have experienced any yield loss, but might have stalk integrity issues, which could lead to lodging. Determining which fields might be more prone to lodging can help establish harvest order to minimize any losses due to severely lodged plants. Below is more information about each foliar disease.

Gray leaf spot (GLS)

Figure 1. Gray leaf spot on a corn hybrid. Photo Courtesy of Craig Grau, University of Wisconsin-Madison

Gray leaf spot is cause by a fungus named Cercospora zeae-maydis. During times of very warm temperature and high humidity (greater than 90%), GLS can increase rapidly on susceptible hybrids. In fields with large amounts of corn residue (e.g. corn-on-corn rotation, minimal tillage, etc.) GLS may be more prominent due to higher levels of inoculum. Symptoms start as small narrow, blocky lesions that might be tan in the center and have a darker margin (Fig. 1). Lesion can increase in size and number and will typically move from lower leaves to upper leaves. Yield loss is most prominent when lesions reach the ear leaves either 2 weeks before tasseling or two weeks after tasseling. Currently, in Wisconsin, we have seen few fields where lesions have reached the ear leaves prior to brown silk. However, in a small number of fields planted to a susceptible hybrid, there has been rapid increase to the ear leaves prior to tassel. In those fields a fungicide application may result in adequate yield protection to cover the cost of fungicide application. See my previous article on how to make the decision to spray fungicide on corn.

Northern Corn Leaf Blight (NCLB)

Figure 2. Northern corn leaf blight on corn.

Northern corn leaf blight is caused by the fungus Setosphaeria turcica. The fungus is most active when wet weather coincides with temperatures between 65 F and 80 F. During these conditions, the fungus will readily make microscopic spores (called conidia) inside the symptomatic areas of leaves and those spores will get splashed onto more leaves. Therefore, the disease typically moves from the lower canopy, up the corn plant as the season progresses. When temperatures get above 80 F and it is dry, growth and spread of the fungus slows dramatically. This is why little NCLB was observed in July, but is showing up now. It is all about the temperature at which the fungus likes to grow. Lesions initiate as cigar-shaped lesions on lower leaves. When conditions are conducive lesions can expand and increase, moving rapidly up the plant (Fig. 2). Occasionally a gray-to-black fuzzy growth is evident in the center of lesions. This growth is sporulation of the fungus. Like GLS, yield loss is greatest when lesions reach the ear leaf either two weeks before or two weeks after tasseling. Again, consult my previous article on how to make the decision to spray fungicide on corn.

Tar Spot

Tar spot is a relatively new disease in the U.S. and Wisconsin. It is caused by a fungus called Phyllachora maydis. Tar spot causes small tar-like spots on the surface of corn leaves. For great information about tar spot and what it looks like, consult this Purdue Extension fact sheet. Tar spot was first found in the U.S. in 2015. In 2016 and 2017, tar spot was identified in Green, Iowa, Grant, and Lafayette counties in Wisconsin. In 2018 confirmations have been made in these same general areas. In Latin America Phyllachora maydis can be found in a complex with another fungus called Monographella maydis. In areas where the complex occurs significant yield loss has been described. However, in the U.S. Monographella maydis has not been found in complex with Phyllachora maydis. Furthermore, Phyllachora maydis is not known to cause yield loss on corn in the U.S. While it can be a striking disease, fungicide applications are not recommended for tar spot in the U.S. Much more work is needed to characterize this pathogen and understand the disease. We are working with Dr. Nathan Kleczewski at the University of Illinois to improve our understanding of this pathogen in the U.S. If you would like to confirm tar spot on corn, or provide samples for research purposes, you can send samples to the University of Wisconsin Plant Disease Diagnostic Clinic.

What about Spraying Fungicide After Hail Damage?

The best study on this subject was conducted by my colleagues at Iowa State University a couple years back. They found that for the most part application of fungicide after hail does not result in any benefits. Especially after the R2 growth stage. We also had an opportunity to look at a natural hail event in 2014 at Arlington. This happened around VT.  We were also unable to find a significant difference in treating with  a fungicide versus not treating after late season hail-damage. In addition, it isn’t likely that fungal infections will increase after hail. In fact in the Iowa State University study, they found a negative correlation between hail damage and fungal disease. Hail CAN increase Goss’s wilt risk. However, Goss’s wilt is caused by a bacterium. Thus, fungicide application does not work for this disease. For more information on Goss’s Wilt and how to manage it click here. In summary, given the current market prices and the fact that corn is generally through the silking period, fungicide application on hail-damaged corn is not needed.

Wisconsin White Mold Risk Maps – July 29, 2018

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

**These maps are for guidance only and should be used with other sources of information and professional advice when determining risk of white mold development. For field-specific predictions, we encourage you to use the Sporecaster smartphone application. These predictions will be most accurate for your specific location. Information about Sporecaster and how to download can be found by clicking here. Sporecaster takes into account crop phenology, in addition to weather parameters, to make field-specific recommendations. The maps below are based on weather only and you must determine if your crop is currently phenologically at risk for infection. For more information on white mold and how to manage it, see my previous post.**


Map Legends: No color = model is inactive and risk of apothecia in the field is not likely; Gray = apothecia might be present, but likelihood of apothecial presence is extremely low; Blue = low risk of apothecia; Yellow = medium risk of apothecia in the field; Red = high risk of apothecia in the field. Model predictions must be combined with soybean growth stage and canopy characteristics to aid in timing of fungicide sprays.


 

Figure 1. White Mold Risk Map for Non-irrigated Fields – July 29, 2018

Cooler, dry weather has changed the white mold risk map over the last several weeks for non-irrigated (dryland or rain-fed) soybean fields (Figure 1). Cool weather has generated some areas of high risk in the southwest, western, and north-central regions of the state. However, the dry conditions have dissipated much of the moderate and low risk areas that showed several weeks ago. Risk in general is much more spotty than it has been. The soybean crop is also moving through growth stages much more rapidly than in 2017. Thus, the risk window for infection by the white mold fungus will end soon. Remember that once the crop has finished flowering risk of new infections is low to non-existent. In addition, late applications (R4 and later growth stages) of fungicide will not be needed for white mold control.

 

Figure 2. White Mold Risk Map for irrigated fields planted on 15″ row-spacing – July 29, 2018

Risk remains high across much of the state for irrigated soybeans planted to 15″ rows (Figure 2). Risk is starting to dissipate in areas in the east-central region, due to dryer conditions. However, a fungicide application should be considered if irrigating and soybeans are flowering and planted to a 15″ row-spacing. Note that if you have irrigation in your field but are not actually irrigating, you should be using the non-irrigated model above to make an accurate prediction of white mold risk.

 

Figure 3. White Mold Risk Map for irrigated fields planted on 30″ row-spacing – July 29, 2018

Risk of white mold in irrigated fields planted to 30″ row-spacing has decreased a bit in some areas, since the last post (Figure 3). However, Much of the state is at risk if irrigating on 30″ spaced soybeans. In these areas, a fungicide application should be considered if soybeans are being irrigated, canopy is nearly closed, and flowers are present. Note that if you have irrigation in your field but are not actually irrigating, you should be using the non-irrigated model above to make an accurate prediction of white mold risk.


These models were developed at the University of Wisconsin-Madison in conjunction with Michigan State University and Iowa State University to identify at-risk regions which have been experiencing weather favorable for the development of white mold fungus apothecia. Weather information and maps are provided by the Soybean Integrated Pest Information Platform for Extension and Education (iPIPE), which is managed by ZedX, Inc. These models predict when apothecia will be present in the field using combinations of 30-day averages of maximum temperature, relative humidity, and maximum wind speed. Using virtually available weather data, predictions can be made in most soybean growing regions. Based on these predictions, a map is generated under three scenarios (non-irrigated soybeans, soybeans planted on 15″ row-spacing and irrigated, or soybeans planted on 30″ row-spacing and irrigated). The maps are colored to show the likelihood of apothecial presence within a region.  If the model is predicting high risk (red) in your area for your planting scenario, the soybeans are flowering, and the canopy is somewhat closed, then the white mold risk is high. If your fields are at-risk, we recommend to consult your local extension personnel or other research-based resources for the best in-season management options for your area.

Wisconsin White Mold Risk Maps – July 15, 2018

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

**These maps are for guidance only and should be used with other sources of information and professional advice when determining risk of white mold development. For field-specific predictions, we encourage you to use the Sporecaster smartphone application. These predictions will be most accurate for your specific location. Information about Sporecaster and how to download can be found by clicking here. Sporecaster takes into account crop phenology, in addition to weather parameters, to make field-specific recommendations. The maps below are based on weather only and you must determine if your crop is currently phenologically at risk for infection. For more information on white mold and how to manage it, see my previous post.**


Map Legends: No color = model is inactive and risk of apothecia in the field is not likely; Gray = apothecia might be present, but likelihood of apothecial presence is extremely low; Blue = low risk of apothecia; Yellow = medium risk of apothecia in the field; Red = high risk of apothecia in the field. Model predictions must be combined with soybean growth stage and canopy characteristics to aid in timing of fungicide sprays.


Figure 1. White Mold Risk Map for Non-irrigated Fields – July 15, 2018

Hot, dry weather continues to push risk for white mold down in non-irrigated (dryland or rain-fed) soybean fields (Figure 1). While risk is high in some locations of the state, there is much more yellow on the map compared to last week, indicating medium risk. Continued areas of high risk are present along Lake Michigan and in the central and southwest portions of Wisconsin. In these areas, a fungicide application should be considered if the soybean canopy is nearly closed and flowers are present.

Figure 2. White Mold Risk Map for irrigated fields planted on 15″ row-spacing – July 15, 2018

Risk remains high across the state for irrigated soybeans planted to 15″ rows (Figure 2). A fungicide application should be considered if irrigating and soybeans are flowering and planted to a 15″ row-spacing. Note that if you have irrigation in your field but are not actually irrigating, you should be using the non-irrigated model above to make an accurate prediction of white mold risk.

Figure 3. White Mold Risk Map for irrigated fields planted on 30″ row-spacing – July 15, 2018

Risk of white mold in irrigated fields planted to 30″ row-spacing has increased significantly since last week (Figure 3). Much of the state is at risk if irrigating on 30″ spaced soybeans. In these areas, a fungicide application should be considered if soybeans are being irrigated, canopy is nearly closed, and flowers are present.


These models were developed at the University of Wisconsin-Madison in conjunction with Michigan State University and Iowa State University to identify at-risk regions which have been experiencing weather favorable for the development of white mold fungus apothecia. Weather information and maps are provided by the Soybean Integrated Pest Information Platform for Extension and Education (iPIPE), which is managed by ZedX, Inc. This model predicts when apothecia will be present in the field using combinations of 30-day averages of maximum temperature, relative humidity, and maximum wind speed. Using virtually available weather data, predictions can be made in most soybean growing regions. Based on these predictions, a map is generated under three scenarios (non-irrigated soybeans, soybeans planted on 15″ row-spacing and irrigated, or soybeans planted on 30″ row-spacing and irrigated). The maps are colored to show the likelihood of apothecial presence within a region.  If the model is predicting high risk (red) in your area for your planting scenario, the soybeans are flowering, and the canopy is somewhat closed, then the white mold risk is high. If your fields are at-risk, we recommend to consult your local extension personnel or other research-based resources for the best in-season management options for your area

To Spray or Not to Spray Fungicide on Corn for Grain or Silage?

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison

Treating field corn, for grain, with fungicide has become a common practice in the Midwest. With so many fungicide programs and formulations, and the re-emergence of yield-limiting corn diseases, like northern corn leaf blight (NCLB) and gray leaf spot (GLS), foliar fungicide application has demonstrated an ability to reduce foliar disease severity and increase grain yield under some circumstances.

Figure 2: A computer simulation of 5% NCLB severity on a corn leaf.

Figure 1: A computer simulation of 5% NCLB severity on a corn leaf.

How do I know if disease is active at the time I want to spray?  While I hate talking about threshold levels for managing disease, it can be helpful in your decision making process to know what might be severe. While scouting look in the lower portion of the canopy. If some foliar disease symptoms are present in the lower canopy, make a visual estimation of how frequent (percentage of plants with lesions) the disease is in a particular area and how severe (how much leaf area is covered by lesions).  The lower leaves aren’t responsible for much yield accumulation in corn, but spores produced in NCLB and GLS lesions on these leaves can be splashed up to the ear leaves where disease can be very impactful. So by scouting the lower canopy and getting an idea of how much disease is present, you can “predict” what might happen later on the ear leaves to make an informed spray decision. The other consideration you should make while scouting is the resistance rating that the hybrid has for NCLB and/or GLS. If it is rated as resistant, then NCLB or GLS severity might not be predicted to get very severe, while in a susceptible hybrid, NCLB or GLS might be present on 50% or more of plants at high severity levels. Note however, that even if a hybrid is rated as resistant, it can still get some disease. Resistance isn’t immunity! If NCLB is present on at least half the plants and severity is at least 5-10% and weather is forecast to be rainy and cool, a fungicide application will likely be needed to manage the disease. So what does 5% leaf severity look like? Figure 1 is a computer generated image that shows 5% of the corn leaf with NCLB lesions. You can use this image to train your brain to visually estimate how severe the disease might be on a particular leaf. As for fungicide choice and timing, I consider that further below.

What fungicide should I spray and should I spray at all? My question is what are you trying to do? Control a disease or simply boost grain yield? Fungicide should be used as a tool to control a disease and preserve yield. There is no silver bullet fungicide out there for all corn diseases. However, there are many products which work well on a range of diseases. The Corn Fungicide Efficacy table lists products that have been rigorously evaluated in university research trials across the country. You can see there are several products listed that perform well on both NCLB and GLS. So obviously, if a disease is present and you are trying to control the disease, you might expect more return on your investment, compared to simply spraying fungicide and hoping that there might be a yield increase.

Paul et al. (2011) conducted research to investigate the return on investment (ROI) of using fungicide at low and elevated levels of disease. Data from 14 states between 2002 and 2009 were used in the analysis. They looked at 4 formulations of fungicide products across all of these trials. I won’t go into detail about all products, but will focus on one here, pyraclostrobin. This is the active ingredient in Headline® Fungicide. In all, 172 trials were evaluated in the analysis and Paul et al. found that on average there was a 4.08 bu/acre increase in corn grain yield when pyraclostrobin was used. So there does appear to be some increase in yield with the use of fungicide, but in our current market, will this average gain cover the fungicide application?

 

Figure 2. Break-even scenarios for corn when foliar fungicide was applied.

The suggested application rate for Headline® Fungicide is 6 to 12 fl oz/acre. My latest cost sheets indicate that at the 6 fl oz/acre rate, the cost of the product alone would be about $12/acre. Note that this does not include the custom applicator cost. This is a variable expense that would need to be added in to get an accurate ROI for your operation. Today we can estimate that we might sell corn grain somewhere between $3 and $4 per bushel. We can then use the cost of the fungicide product and the price of grain to figure out how many bushels of corn we need to make in the crop that would be treated with pyraclostrobin vs. not treating. Figure 2 is a table with various corn prices along the vertical axis and fungicide costs per acre along the horizontal axis. The cells indicate the bushels of corn per acre needed to break even when using a fungicide at the corresponding cost and corn grain sale price. Using the above scenario, we see that with corn priced between $3 and $4 per acre and a fungicide application cost of $between$10 and $15/acre, we would need to gain 2.5 – 5.0 bushels per acre when using Headline® Fungicide in the current corn market. Obviously these calculations are for just one product, but you can do the same for your farm and fungicide program and use the table to figure out what break-even yield gain you will need to cover your costs.

What are the odds of getting that 2.5 to 5 bushel per acre yield gain when using Headline® Fungicide? Paul et al. went further and calculated the probability of return at various corn prices and fungicide costs. They did separate analyses for foliar disease severity less than 5% and greater than 5%. In our current corn market with around $3/bu corn prices and a cost of Headline® Fungicide at $15/acre (fungicide plus custom application), Paul et al. found that at low foliar disease levels (<5% severity) the odds of a positive ROI using the fungicide would be around 50%. The odds of a positive ROI improve if disease severity is greater than 5%. In their calculations with higher levels of disease (>5% severity), the odds of a positive ROI would be between 60% and 70%. The morale of this story is that if you are going to use fungicides on corn, they should be targeted toward fields that will have, or are at risk, for disease!

So what about fungicide application timing? One of the best times to apply fungicide to maximize any benefits for grain corn is during tasseling (VT) and into the silking (R1-R2) timing. In multiple site-year studies across the corn belt of the U.S., application of fungicide on grain corn at VT resulted in over a 7 bu average yield gain. In Wisconsin, the average at the VT timing is about 5.5 bushels. However, this level of yield gain only materializes when a yield limiting foliar disease is active and moving up the canopy. You can check out results of the fungicide trials and the performance of various products over the last few years in Wisconsin by visiting my Fungicide Test Summaries page.

Finally, be aware that in some cases, application of fungicide in combination with nonionic surfactant (NIS) at growth stages between V8 and VT in hybrid field corn can result in a phenomenon known as arrested ear development. The damage is thought to be caused by the combination of NIS and fungicide and not by the fungicide alone. To learn more about this issue, you can CLICK HERE and download a fact sheet from Purdue Extension that covers the topic nicely. Considering that the best response out of a fungicide application seems to be between VT-R2, and the issues with fungicide plus NIS application between V8 and VT, I would suggest holding off for any fungicide applications until at least VT.

What about fungicide on corn for silage? This practice has been gaining increased interest over the last several years. Dr. Felipe Cardoso’s animal science laboratory at the University of Illinois has published several peer-reviewed papers describing the physiological changes in the corn plant treated with several fungicides, that result in improved feed quality. In those studies yield was often not directly impacted by the fungicide application, but fibrous changes in the corn plant improved feed conversion to milk production in cows fed silage corn treated with fungicide.

Figure 3. Ear rot of corn.

Another possible benefit to treating both silage corn and grain corn with fungicide is the potential reduction in mycotoxin accumulation. Mycotoxins are secondary metabolites produced by fungi. There are 400-500 different known mycotoxins. In corn in Wisconsin, we typically are most concerned with deoxynivalenol (DON or vomitoxin) and fumonisins. These mycotoxins are produced by fungi in the group Fusarium which can cause ear rot issues (Fig. 3) and also stalk rot issues in corn. Recent studies by colleagues in Ontario, Canada have demonstrated that the triazole containing fungicide Proline (active ingredient: prothioconazole) applied at R1 (silking), or shortly after the beginning of silking, can reduce DON levels in corn grain, compared to not treating. The Field Crops Pathology Laboratory at the University of Wisconsin-Madison set out to determine if Proline, and other fungicides and programs, had a similar effect on silage corn treated with fungicide.

Our trial was established at the Arlington Agricultural Research Station located in Arlington, WI in 2017. The brown midrib (BMR) corn hybrid ‘P0956AMX’ was chosen for this study. Corn was planted on 11 May and chopped on 13 Sep. Single applications of various fungicide products (Table 1) were applied at growth stages V6 (19 Jun), R1 (26 Jul), 5 days post R1 (31 Jul), and 10 days post R1 (4 Aug). Ear rot severity was assessed by visually rating five ears per plot on the day of harvest. Yield was determined by harvesting the center two rows of each plot using a small plot silage chopper with an onboard platform weigh system. Chopped sub-samples were collected from each plot and analyzed for deoxynivalenol (DON) content.

Consistent with other datasets, we found no significant difference in yield across treatments (Table 1). Likewise, little difference in quality could be found among all treatments. This could be due to the fact that we started with a high-quality BMR hybrid, thus not readily responding to physiological changes that correspond to increased feed quality when treated with fungicide.

All fungicide treatments resulted in a significant reduction in DON content compared to the non-treated control, except Delaro applied at V6 and Quilt Xcel applied at R1. Application of the experimental 1 fungicide applied at R1 resulted in the lowest DON content among treatments. Remaining treatments had comparable DON levels to experimental 1 except for Quilt Xcel at R1. These results were consistent with previous data from Canada indicating that there is a “sweet spot” of application timing (especially when using Proline), when the goal is to reduce DON. The window of application begins at R1 (silking) and ends around 10 days after the start of R1.

Additionally, our data have shown that the product Quilt Xcel does not reduce DON levels on par with some other products. In fact, in other trials, Quilt Xcel has resulted in higher levels of DON in grain corn. In grass crops like wheat, it has been shown that products containing the strobilurin fungicide class can increase DON levels over not treating. Therefore, these products are not recommended for application after flowering in wheat. This same phenomenon could be possible in corn. Thus, care should be taken when choosing products and programs specifically aimed at reducing DON levels in corn silage.  

Summary

As we approach the critical time to make decisions about in-season disease management on corn, it is important to consider all factors at play while trying to determine if a fungicide is right for your corn operation. Here is what you should consider:

1) Corn hybrid disease resistance score – Resistant hybrids may not have high levels of disease which impact yield.

2) Get out of the truck and SCOUT, SCOUT, SCOUT – Consider how much disease and the level of severity of disease present in the lower canopy prior to tassel.

3) Consider weather conditions prior to, and during, the VT-R2 growth stages – if it is cool and wet, disease may continue to increase in corn and a fungicide application might be necessary. If it turns out to be hot and dry, disease development will stop and a fungicide application would not be recommended.

4) Consider your costs to apply a fungicide and the price you can sell your corn grain – Will you gain enough out of the fungicide application to cover its cost?

5) Hold off with making your fungicide application in Wisconsin until corn has reached the VT-R2 growth stages – The best foliar disease control and highest likelihood of a positive ROI will occur when fungicide is applied at VT when high levels of disease are likely.

6) Dairy farmers should think about the overall goal of using fungicide on silage corn. If the goal is to simply alter the corn plant physiologically to improve feed quality, there are numerous products and application timings that have the potential to provide a benefit over not treating. However, if the goal is to target mycotoxins, specifically DON, certain fungicide products may need to be applied specifically during the short silking stage (R1-R2) of the corn plant, to reduce DON levels.

7) Be aware that every time you use a fungicide you are likely selecting for corn pathogen populations that will become resistant to a future fungicide application – Make sure your fungicide application is worth this long-term risk. To learn more about fungicide resistance, you can CLICK HERE to download a UW Extension fact sheet.

Other Resources

Wisconsin Field Crops Fungicide Information Page

Applying Fungicides to Corn Early in Iowa

References

Haerr, K.J., Lopes, N.M., Pereira, M.N., Fellows, G.M., and Cardoso, F.C. 2015. Corn Silage from corn treated with foliar fungicide and performance of Holstein cows. J. Dairy Sci. 98:8962-8972.

Kalebich, C.C., Weatherly, M.E., Robinson, K.N., Fellows, G.M., Murphy, M.R., and Cardoso, F.C. 2017. Foliar fungicide (pyraclostrobin) application effects on plant composition of a silage variety corn. Animal Feed Science and Technology. 225:38-53.

Paul, P. A., Madden, L. V., Bradley, C. A., Robertson, A. E., Munkvold, G. P., Shaner, G., Wise, K. A., Malvick, D. K., Allen, T. W., Grybauskas, A., Vincelli, P., and Esker, P. 2011. Meta-analysis of yield response of hybrid field corn to foliar fungicides in the U.S. Corn Belt. Phytopathology 101:1122-1132.

White, D.G., editor. 2010. Compendium of Corn Diseases. APS Press.