How Will Delayed Planting Influence Crop Diseases in 2019?

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

Darcy Telenko, Extension Field Crops Pathologist, Purdue University

Figure 1. The Disease Triangle Concept

We keep getting this question, because as we write this, it is storming yet again in many locations in the Midwest. Rain, rain, and more rain has pushed back timely planting everywhere. Concern is starting to mount about not only yield loss simply from delayed planting, but what increased risk of yield loss due to disease there might be in 2019. As we consider this issue, we will use tar spot of corn and white mold of soybean as just two examples of where this could be an issue.

The Plant Disease Triangle. Remember that the plant disease triangle is the foundation for understanding how plant diseases develop and how to manage them. In order for a plant disease to occur you must have a virulent pathogen, a susceptible host plant, and favorable weather conditions to coincide at the same time. If any one of these three components is missing (or we implement a management strategy that removes or reduces one component) then a plant disease will not occur. When it comes to the host component, it not only matters that the host is generally susceptible but is also at a susceptible growth stage. Consider white mold of soybeans for a minute. All stages of soybean are susceptible to infection by the white mold fungus, but most infections occur through open flowers. Thus, the disease triangle is met when you have (1)white mold fungal spores flying around at the same time that (2)soybean flowers are open (susceptible stage), during, (3) cool and wet weather (favorable environmental condition)completing the triangle (Figure 1). The point here is that if we continue in a cool wet pattern, and delayed planting continues, we may quickly find ourselves with crops at susceptible growth stages when the weather is very conducive to disease.

Figure 2. White Mold in a Soybean Field

Delayed Planting and White Mold of Soybean. In 2017, we had an epic epidemic of white mold on soybean across the upper Midwest (Figure 2). One of the main reasons that the epidemic was so bad is that it was generally cool for a large portion of the season. This resulted in soybeans that moved very slowly from one growth stage to the next. When it came to flowering, soybeans bloomed for an extended period of time. This left them in a susceptible growth stage for about twice as long as normal. These cool conditions also coincided with wet weather that was favorable for the pathogen. In 2018, planting occurred reasonably on-time and we accumulated heat units quickly. Bloom started early in the season and was about half as long as it was in 2017. This meant that soybeans “escaped” infection in large portions of the upper Midwest. Fast-forward to 2019. If this cool rainy cycle persists, and planting is delayed, then soybeans may bloom later and over an extended period of time during wet/humid weather conditions. Keeping an eye on weather before and during the soybean bloom period along with consulting the Sporecaster smartphone app  can help you make the educated decision to spray fungicide or not.

Figure 3. Tar Spot Signs and Symptoms on Corn Leaves

Delayed Planting and Tar Spot of Corn. In 2018 Tar spot of corn (Figure 3) created quite a stir. The epidemic was widespread and caused some significant yield losses in areas that it occurred. The tar spot fungus is residue-borne. There is also decent evidence that it can survive over-winter on corn residue (Figure 4). Our laboratories have been investigating tar spot fungal survival on corn residue collected after snow-melt in Wisconsin and Indiana. Regardless of whether there was fall tillage performed or not, survival of tar spot fungal spores (ascospores) on the residue collected ranged between 15 and 40%, with an average around 20%. These are VERY preliminary findings (and the numbers might change once we finish counting and analyzing data), but the point is that there is viable tar spot fungal inoculum present in Midwest corn fields. Therefore, one component of the triangle is met! As for the other two components, corn is being planted later than normal and conditions are cool and wet. Again, if this cycle of cool and wet holds, conditions will be favorable for the fungus. Delayed planting of corn will also push corn into conducive growth stages for the fungus to infect and cause heavy yield losses (although, we have seen infection at all growth stages as long as there was green tissue available). One of the reasons that the 2018 tar spot epidemic was so significant, was that many areas of the upper Midwest had cool and excessively wet conditions around the V6 growth stage and again near or after the VT growth stages. When foliar diseases of corn start at early growth stages (V6 or V8) the risk for yield loss can be much higher than if they start after R2 or brown silk. Keep an eye on the weather between the V6 and R2 growth stages and consult with your local extension personnel to decide if a fungicide might be warranted for corn to prevent tar spot, or other foliar diseases.

Figure 4. Signs of the Tar Spot Fungus on Corn Residue

Scouting and Watching Weather Reports Might Pay in 2019. Once corn and soybeans are planted, take the time to scout and pay attention to the weather. While thorough scouting can take time, it may be worth it in 2019. Catching a plant disease early can be the difference in being successful in managing it or not. Pay attention to the weather leading up to, and during, the critical crop growth stages. This can also help you make an educated decision about in-season application of fungicides. If it is cool and humid/rainy, and the crop is at a susceptible growth stage, then a fungicide application might be warranted. If it is hot and dry and the crop moves quickly through susceptible growth stages, then a fungicide might not be warranted. Study the disease triangle and use it to your advantage. The 2019 field season could be a year that this knowledge might be handy!

For in-season updates follow us on Twitter and Consult our websites at the links below:

Dr. Damon Smith

@badgercropdoc

https://badgercropdoc.com/

 

Dr. Darcy Telenko

@DTelenko

https://extension.purdue.edu/fieldcroppathology/

 

For More information about tar spot, white mold, and fungicide efficacy consult the following resources:

  1. Tar spot Fact sheet
  2. Short Tar Spot Video
  3. Tar Spot Webinar 
  4. White Mold Fact Sheet
  5. Short White Mold Video
  6. White Mold Webinar
  7. Corn Fungicide Efficacy Table
  8. Soybean Fungicide Efficacy Table

Poor Soybean Seed Quality and Preparing for the 2019 Field Season

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Damon Smith, Associate Professor and Extension Specialist, Department of Plant Pathology, University of Wisconsin-Madison

Shawn Conley, Professor and Extension Specialist, Department of Agronomy, University of Wisconsin-Madison

Figure 1. Severe Phomopsis seed decay. Photo Credit: Craig Grau, University of Wisconsin-Madison.

As we finish off 2018 and look ahead to the 2019 planting season, soybean farmers need to be prepared for some potential seed quality issues. The 2018 crop was plagued by several problems, but one of the most substantial was a large amount of white, chalky, or black, and damaged seed (Figure 1). This damaged seed is impacting germination rates of soybean seed slated for the 2019 crop.

What caused this issue?

Most of this damage is a result of infection and colonization by a group of fungal species called Diaporthe. This group is implicated in diseases such as stem canker, pod and stem blight (Figure 2), and Phomopsis seed decay (Figure 3). Excessive rains at the end of August and throughout September and October resulted in a large amount of pod infection by Diaporthe. These infections combined with delayed harvest allowed for extensive seed colonization by these fungi. This resulted in Phomopsis seed decay which has led to visually damaged seed and the germination issues we are now seeing. To learn more about this group of fungi and the diseases they cause, visit the Crop Protection Network (CPN) website on pod and stem blight and Phompsis seed decay by clicking here. You can also download a PDF version of the CPN fact sheet on the same subject by clicking here.

How Do I manage this Problem at Planting in 2019?

Figure 2. Pod and stem blight of soybean

Soybean seed producers should try to clean seed to achieve less than 20% damaged seed in a seed lot. Multiple cleaning steps might be needed to achieve this level. While testing germination now is recommended, remember that testing germination again next spring and potentially just prior to delivery will also help you to understand the germination rate and determine if other management strategies need to be employed such as fungicidal seed treatments.

Seed treatments can help improve the germination rate of seed damaged by Diaporthe. However, you will need more than metalaxyl or mefonoxam active ingredients in your seed treatment. Metalaxyl and mefonoxam are good against Phytophthora and Pythium, but not effective against other organisms, like Diaporthe. Seed treatments with Phomopsis on the label have an additional fungicide (either a DMI or SDHI). Page 157 of the publication A3646 – Pest Management in Wisconsin Field Crops has a table of some of the seed treatments with Phomopsis on the label. Also available is the seed treatment efficacy table from the Crop Protection Network (CPN). You can download that publication by clicking here.

Figure 3. Damaged soybean seed as a result of Diaporthe infection.

We also recommend that as a farmer, you double check the percent germination on every seed lot prior to planting and adjust your seeding rates accordingly. Here are our recommendations for soybean seeding rate based on yield potential and white mold risk: The Soybean Seeding Rate Conundrum.

If I’m a Seed Producer, What Should I Do to Prevent this Problem Next Year?

Foliar fungicide applications during the growing season could reduce the damage from Diaporthe. Some work has demonstrated that fungicide applications between the R3 to the R5 growth stages might be useful in reducing damage. This might help improve seed quality, but not necessarily improve yield. For a list of fungicide products with efficacy ratings for soybean, take a look at this additional publication from the CPN by clicking here.

2018 Wisconsin Field Crops Pathology Fungicide Tests Summary Now Available

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Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison

Brian Mueller, Assistant Field Researcher, Department of Plant Pathology, University of Wisconsin-Madison

Each year the Wisconsin Field Crops Pathology Program conducts a wide array of fungicide tests on alfalfa, corn, soybeans, and wheat. These tests help inform researchers, practitioners, and farmers about the efficacy of certain fungicide products on specific diseases. The 2018 Wisconsin Field Crops Fungicide Test Summary is now available. These tests are by no means an exhaustive evaluation of all products available, but can be used to understand the general performance of a particular fungicide in a particular environment. Keep in mind that the best data to make an informed decision, come from multiple years and environments. To find fungicide performance data from Wisconsin in other years, visit the Wisconsin Fungicide Test Summaries page. You can also consult publication A3646 – Pest Management in Wisconsin Field Crops to find information on products labeled for specific crops and efficacy ratings for particular products. Additional efficacy ratings for some fungicide products for corn foliar fungicides, soybean foliar and seed-applied fungicides, and wheat foliar fungicides can be found on the Crop Protection Network website.

Mention of specific products in these publications are for your convenience and do not represent an endorsement or criticism. Remember that this is by no means a complete test of all products available.  You are responsible for using pesticides according to the manufacturers current label. Some products listed in the reports referenced above may not actually have an approved Wisconsin pesticide label. Be sure to check with your local extension office or agricultural chemical supplier to be sure the product you would like to use has an approved label.  Follow all label instructions when using any pesticide. Remember the label is the law!

Badger Crop Doc with Ashley Davenport for an AgPro Podcast

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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

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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

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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. /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!

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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

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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

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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

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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.