I Finally Got My Soybeans Planted and Now They Look Sick!

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

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

Phytophthora stem rot of a seedling soybean. Photo Credit: Craig Grau.

The 2019 growing season has been nothing but full of challenges for Wisconsin farmers and farmers throughout the Midwest. Weather and grain markets have not improved, combined with late-planting of all crops, including soybeans. Dryer weather recently has allowed many to catch up a bit on planting, but now the weather is turning wet again. With this wet weather right after planting, we start to get concerned about several seedling and early-season diseases that can show up, and the performance of seed treatments used to protect soybeans against the pathogens that cause these diseases.

What are the Pathogens of Primary Concern?

Soybeans are susceptible to several early diseases. A detailed list of those important in Wisconsin can be found HERE. You will notice in that list that there is an array of fungi and water-molds that can affect soybeans, compromising stands. More recently we have been very interested in tracking the water-molds. These organisms include Pythium and Phytophthora. Pythium can cause diseases such as Pythium seedling blight and root rot while Phytophthora can result in Phytophthora root and stem rot of soybean. When it comes to both of these diseases, several species within each pathogen genus can affect soybeans in Wisconsin. In fact, The Wisconsin Department of Agriculture, Trade and Consumer Protection Pest Survey Program and the Plant Industry Bureau Laboratory has tracked water-molds in soybean fields from since 2008. The latest results of this surveys can be found by CLICKING HERE. You will notice that there are actually two Phytophthora species and more than 5 Pythium species that can affect soybeans in Wisconsin. With the diversity of pathogens in the state and the wet spring we are having, it is no wonder that seedling issues are present in Wisconsin.

Will Seed Treatments Cure Poor Soybean Emergence?

The short answer is no. In the last 10 years we have seen a significant increase in the availability and use of seed treatments in soybeans. These seed treatments can be a simple single-mode-of-action fungicide or combined with multiple fungicides, insecticides, nematicides, and/or plant growth regulators. A detailed list of seed treatment products registered in Wisconsin for soybeans and other grain crops can be found on the What’s On Your Seed fact sheet. While we highly recommend the use of seed treatments to combat seed rots and seedling blights, it is important to realize that they are not perfect and can fail or under-perform for many reasons. Even if you used a seed treatment on your soybean seed in 2019, you may still notice emergence issues. There are many factors that play a role in the success of a seed treatment, including the correct choice of product against the right pathogens, weather, soil type, etc. For more information on the factors that can affect seed treatments check out the fact sheet posted HERE. If you are in a situation where you used a seed treatment and the stand is poor, check out this publication. This can give you some insight on what happened as you work through diagnosing the issue with your agronomist. There is not a one-size-fits-all seed treatment so it is important that if you have had issues with the performance of your seed treatment, you determine if a pathogen is involved and what species it might be. Knowing this information can help guide you in choosing the seed treatment most appropriate for controlling that particular pathogen in the future. If you need help diagnosing a potential seed decay or seedling disease, you can send a sample to the University of Wisconsin-Madison Plant Disease Diagnostic Clinic. Details on how to prepare and send a sample can be found on their website by clicking here.

Does Variety Resistance Help Improve Soybean Stands?

Yes! While resistance to Pythium in soybean isn’t well understood, there are resistant varieties deployed for managing Phytophthora. Both race-specific and field resistance (lower level of resistance to all races) are available in soybean varieties marketed in Wisconsin. There are often one or more race-specific Phytophthora resistance genes in commercial soybean varieties. The genes present in specific soybean varieties are listed each year in the University of Wisconsin Soybean Variety Test Results (UW-Extension publication A3654). Your seed dealer will also have this information.

Monitor the performance of the varieties you choose. When optimum disease conditions develop later in the growing season, scout those areas of the fields to look for stem rot development. If a large number of plants with Phytophthora stem rot are found, choose varieties with a different Rps gene and higher levels of partial resistance for next year. This pathogen does adapt to the Rps genes, but it is a slow process. Careful monitoring of plant performance is all that is needed. A listing of RPS genes and their relative effectiveness in Wisconsin can be found in the table below.

Race-specific Phytophthora resistance genes and their effectiveness in Wisconsin

Soybean genes Phytophthora races controlled Effectiveness in Wisconsin
Rps 1a 1, 2, 10, 11, 13-18, 24 limited effectiveness
Rps1b 1, 3-9, 13-15, 17, 18, 21, 22
Rps 1c 1-3, 6-11, 13, 15, 17, 21, 23, 24 effective in 75% of fields
Rps 1k 1-11, 13-15, 17, 18, 22, 24 effective in 99% of fields
Rps 3 1-5, 8, 9, 11, 13, 14, 16, 18, 23, 25
Rps 4 1-4, 10, 12, 16, 18-21, 25
Rps 6 1-4, 10, 12, 14-16, 18-21, 25
Rps 1k, 6 1-11, 12-22, 24, 25

Field resistance, also called field tolerance, is present at different levels in most soybean varieties marketed in Wisconsin. For example, even if a variety has a specific resistance gene that may not be effective, such as Rps 1a, against the races of Phytophthora sojae present in a field, the variety may perform better than other varieties with this gene because it has an adequate level of field resistance to Phytophthora. Field resistance can be overcome by high disease pressure especially in the seedling stage. A final note on resistance – in field where Phytophthora sansomeana is present, Rps genes may have little effect. No data currently exists on soybean variety performance against this fairly new pathogen of soybean in Wisconsin.

Wisconsin Winter Wheat Disease Update – June 4, 2019

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

Wheat heads are close to emergence on some earlier varieties of winter wheat in southern and south-central Wisconsin. Within the next week wheat heads will be emerging and anthesis (flowering) will be starting, with later varieties to follow. Now is the time to prepare for Fusarium head blight (FHB or scab) management. The Fusarium head blight Risk Model (http://www.wheatscab.psu.edu) is showing moderate to high levels of risk in the primary wheat growing region of the state over the next 72 hours (Figure 1). Pay close attention to the risk model and your local weather as we approach anthesis (flowering). I anticipate the risk to remain high as periods of rain and humidity persist. Fungicide products of choice to control FHB in Wisconsin include Caramba, Prosaro, and Miravis Ace. Multiple years of data in Wisconsin suggest that the best application window for any of these products begins at the start of anthesis until 5-7 days after the start of anthesis. Applying fungicide before anthesis or more than 7-10 days after anthesis will result in poor performance against vomitoxin accumulation. For information pertaining to recent fungicide studies on winter wheat in Wisconsin, CLICK HERE and scroll to page 12. Other reports can be found by CLICKING HERE.

Figure 1. Fusarium Head Blight Prediction Center 72-hour Outlook for Wisconsin.

Wisconsin Winter Wheat Disease Update – May 31, 2019

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

The University of Wisconsin-Madison Field Crops Pathology crew has spent the last several days scouting winter wheat variety trials and commercial wheat fields in south and south-central Wisconsin. Wheat at all locations observed had flag leaves fully emerged. Weather has been extremely wet and cool across the state. Despite the wet conditions, wheat was generally disease free in all locations visited.

In Sharon, WI wheat looked decent despite challenging winter and spring conditions. The stand was a bit uneven in terms of growth stage, but most varieties looked good. It won’t be a record yield year, but stands look better than anticipated at this location. Wheat at the Arlington location in central Wisconsin looked very good with strong stands combined with even growth stages across varieties. I anticipate yields to be decent. At the Fond du Lac location wheat was in okay shape, however several varieties did experience significant winterkill. Stands were like those in Sharon, with uneven growth stages within varieties.

Figure 1. Fusarium head blight prediction for May 31, 2019

While disease on wheat has been relatively non-existent in Wisconsin, weather has been extremely wet across the state. Considering these conditions, we are worried about the risk for Fusarium head blight (FHB) this year given the weather pattern we have been stuck in. Currently, the Fusarium head blight Risk Model (http://www.wheatscab.psu.edu) is showing mostly high levels of risk in the primary wheat growing region of the state (Figure 1). While no heads have emerged, heading will begin in the next 1-2 weeks. Pay close attention to the risk model and your local weather as we approach anthesis (flowering). I anticipate the risk to remain high as periods of rain and humidity persist. Fungicide products of choice to control FHB in Wisconsin include Caramba, Prosaro, and Miravis Ace. Multiple years of data in Wisconsin suggest that the best application window for any of these products begins at the start of anthesis until 5-7 days after the start of anthesis. Applying fungicide before anthesis or more than 7-10 days after anthesis will result in poor performance against vomitoxin accumulation. For information pertaining to recent fungicide studies on winter wheat in Wisconsin, CLICK HERE and scroll to page 12. Other reports can be found by CLICKING HERE. Flag leaves are out, get out and scout!

Wisconsin Winter Wheat Disease Update – May 21, 2019

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

Figure 1. Septoria leaf blotch on wheat leaves

Winter wheat in Wisconsin continues to move through growth stages at a fairly even pace. Winter wheat plots in our research program located at the Arlington Agricultural Research Station, Arlington, Wisconsin are just approaching the emerging flag leaf stage. Perhaps by the weekend or early next week, flag leaves will have emerged. Interestingly, this growth stage will likely occur almost at the same date as in 2018. Last season, we applied our Feekes 8 fungicide treatments on May 25th. So while it has been cool, and wheat appears to be moving through growth stages slowly, things aren’t too far off from 2018.

Weather remains very wet. Most wheat we have scouted this week appears to be clean of disease. One concern we have is the development of Septoria leaf blotch. In 2016 we had an early epidemic of this leaf disease, that impacted yield. The cool conditions are not particularly conducive for this disease, but the high humidity and wet conditions certainly are. Weather forecasts indicate warmer conditions over the next week, thus keep your eyes peeled for the development of the disease.

Figure 2. Pycnidia in a Septoria leaf blotch lesion.

Septoria leaf blotch can be identified by necrotic lesions that develop on leaves of winter wheat (Figure 1). Small fruiting structures (pycnidia) can often be identified inside the necrotic area of the lesions, with the naked eye or a good hand lens (Figure 2). Prolonged wet/humid conditions broken by a brief dry period, followed by more wet conditions, can favor infection. Temperatures between 60 and 77 F favor disease development. Septoria leaf blotch can be managed with varietal resistance (both race-specific and partial resistance) and also fungicides. For a list of effective fungicides for Septoria leaf blotch control, CLICK HERE to download a copy of the Small Grains Fungicide Efficacy Table. For more information pertaining to Septoria leaf blotch, and other related leaf blotch diseases, CLICK HERE to download a fact sheet.

Reports from the the mid south and plains states continue to indicate stripe rust is on the move. Continue to monitor and scout wheat as your crop moves into the flag leaf stage and to heading. If stripe rust moves in, a fungicide application may be warranted. As I indicated in my previous update, we did not have a stripe rust epidemic in 2018, in Wisconsin. Thus, there was no inoculum in the state to infect fall-sown wheat. Inoculum for an epidemic to initiate in 2019 will have to come from the southern U.S. The best way to make an educated decision to spray is to scout and catch the disease in its early stages. Continue to pay attention to extension reports as we track stripe rust from the southern U.S., northward.

How Will Delayed Planting Influence Crop Diseases in 2019?

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

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

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

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. /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/.