Tar Spot Is Here To Stay…

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

Roger Schmidt, Nutrient and Pest Management Program, University of Wisconsin-Madison

…let’s start working on how to manage it in Wisconsin (Some thoughts on how to do this in the “Other Resources” below). While we haven’t had major epidemics every year since 2016, tar spot has been found each year and caused problems in some corn fields since its discovery. Figure 1 show counties in the U.S. that tar spot has been found, to date. I’m sure there are others in Wisconsin, feel free to send me pictures or samples for confirmation.

 

Yes, 2018, and now 2021, have clearly seen significant tar spot epidemics that affect yield. What challenges will we be dealing with as we move into harvest in 2021? Here are my thoughts.

Silage Corn Harvest

I took a crack at this one in the video below.

Many are already chopping or have finished at this point. However, it is important to realize that tar spot can dry corn plants abnormally fast. Thus, packing the bunker and initiating fermentation can be a challenge. This can lead to secondary issues such as fungal growth in the bunker and/or increases in mycotoxins. Monitor things carefully and do the best job you can this year putting the crop in the bunker. This year isn’t the year to cut corners!

Grain Corn Harvest

Now is the time to get out there and see how much tar spot is in your grain corn crop and start making plans for order of harvest. If the crop is shutting down abnormally fast, then stalk integrity is going to start to become an issue. Prioritizing fields with high tar spot pressure for harvest first, will help to limit lodging issues and ease harvest headaches. Also, be sure to check for other issues as you scout, like ear rots, that can lead to mycotoxin issues. Get mycotoxin tests if you think you might have problems, before you put it in the bin. You don’t want to mix grain that is low in mycotoxins with grain that has high levels of mycotoxins. Incidentally, no mycotoxins have been implicated directly with tar spot.

More on the grain harvest situation can viewed in the video below.

Tar Spot Management Nuggets as Harvest Begins

The two big questions I have been getting lately about tar spot management are: 1) How is tar spot hybrid resistance looking? 2) How does fungicide work on tar spot?

First, hybrids…There aren’t any hybrids that we know of that are completely resistant. Yes, some are more partially resistant that others. Now is also a good time to look at hybrids. Those that show less disease and are still green are going to be better candidates to grow next year versus those that are showing lots of tar spot and have already dried down. Look at local hybrid trials, especially. Write down the numbers of hybrids that look good. Leave the others that look bad.

Now the fungicide question. Yes, there are some fungicides that work. Yes, even on susceptible hybrids they can help. However, like white mold in soybeans, fungicide efficacy is largely dependent on fungicide application timing. Miss the onset of the epidemics, or periods where the epidemic is rapidly increasing, and even a great fungicide will look terrible.

To help with the timing decision we have developed the Tarspotter smartphone app. We have been testing this app over the last couple of seasons. It is useful in that it can help you anticipate epidemics of tar spot to get fungicide applications on preventatively, when they will work the best. Figure 2 below shows the performance of a fungicide application applied using Tarspotter versus a single application at the VT growth stage versus the non-treated check. While the standard VT application did well, two-applications (one at the start of the epidemic at V10 and another at R2) were needed to hold tar spot off in 2021. As you can clearly see, Tarspotter can be used to make these complicated spray decisions and optimize fungicide performance.

Figure 2. Non-treated check on the left, fungicide applied at VT only in the center, and fungicide applied at V10 and R2 according to Tarspotter on the right.

As for fungicide products, we continue to see mixed-mode-of-action products work more consistently that single-mode-of-action products. This fall we should have some updated fungicide performance information that might shed more light on this subject. Stay tuned!

Other Resources

Tar Spot Web-book

Will a Second Fungicide Be Worth the Cost for Tat Spot Management?

Fact Sheet: Tar Spot

How Tar Spot of Corn Impacted Hybrid Yields During the 2018 Midwest Epidemic

Wisconsin Soybean and Corn Disease Update – August 2, 2021

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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

Camila Primieri Nicolli, Post-Doctoral Research Associate, Department of Plant Pathology, University of Wisconsin-Madison

It has been a while since I posted a corn and soybean disease update for Wisconsin in 2021, and that is because it has been reasonably quiet in the disease world up to this point. However, recent scouting and incoming reports indicate that this could change a bit as we move into August. Let’s take a look at what has happened so far this season and what to keep an eye out for over the next month or so.

The Soybean Situation

Figure 1. Phytophthora Root and stem rot of soybean

The Phytophthora Issue in Soybeans. During July we saw, and received reports, of some fields with Phytophthora root and stem rot of soybean (Fig. 1). I have received a lot of questions on why this happened. Are we seeing changes in the races of the Phytophthora organism in Wisconsin? My short answer is probably not. However, we have found a new species of Phytophthora in some fields that can affect soybean. The new species, Phytophthora sansomeana, can be found in mixed infections with P. sojae. Thus, even if we deployed the proper Rps resistances genes in our varieties, this “new” organisms might be causing some of the damage we observed this year.

We also are seeing fewer available soybean varieties with Rps 1K form of resistance. This form of resistance should be effective on about 99% of the fields in Wisconsin. Instead, we see varieties deployed that have the Rps 1c, Rps 1a, or no Rps gene indicated. Rps 1C is effective on about 75% of the acres in Wisconsin, just to give you some perspective. Thus, I don’t think this is necessarily an issue where we have seen race shifts in P. sojae, but a combination of issues where perhaps we aren’t deploying the correct resistance genes and we might have a new species of Phytophthora adding to the mix. You can learn more about managing Phytophthora root and stem rot of soybean in Wisconsin by clicking here. You will note that seed treatments can also be used to manage Phytophthora root and stem rot. You can click here to learn more about fungicide seed treatments and fungicide resistance in this group of organisms.

What’s Up with White Mold? For most of the state of Wisconsin, we are through the critical bloom time for infection by the white mold fungus. It is now really too late to make a fungicide application that is economically viable. However, scouting fields through August can help you determine what worked, what didn’t, and to figure out your harvest order. Remember, a great way to move the white mold fungus around is by contaminated combines at harvest. Start harvesting fields with no or low white mold incidence and work your way to those fields that look worse. Also consider cleaning combines between fields to limit movement of the fungal survival structures (a.k.a apothecia – the things that look like rat turds!) from one field to the next.

Look out for SDS. Now is also a good time to be scouting for sudden death syndrome (SDS) in soybeans. I’m not sure we will have a bunch of SDS this year in Wisconsin, but we will see pockets for sure. Knowing where you see it and what you did in 2021 can help with making variety and seed treatment decisions for 2022 and beyond. Remember that we do have decent partial resistance to SDS in many commercial varieties. Start here and choose the most resistant variety that fits your environment. Then consider layering a seed treatment (either Saltro or ILeVO) for improved management of SDS. You can learn more about seed treatment performance by studying the Fungicide Efficacy for Control of Soybean Seedling Diseases chart. You can also learn more about the performance of ILeVO in multi-state research trials by reading this report.

The Corn Situation

Corn in Wisconsin has been reasonably free of disease up to this point this season. However, we have noted a few foliar diseases beginning to pop up. We have observed gray leaf spot (GLS) becoming easy to find in most fields, while northern corn leaf blight (NCLB) is starting to show up in a handful of fields we have visited. We are also paying close attention to the tar spot and southern rust situations, I’ll expand on these below.

How bad is tar spot? The Tarspotter app has been running at moderate to high risk of tar spot increase over the last couple of weeks in Wisconsin. Our scouting has confirmed that tar spot is present in at least 5 counties so far in Wisconsin (Fig. 2). All but Grant County show tar spot to be easy to find, but it is present in the lower canopy at low severity. In Grant County, we had to hunt a long time to find 2 spots in a research field on a known susceptible. These observations align with Tarspotter as it indicated just moderate risk in the southwest quadrant of Wisconsin, with high risk from south central to the north. If you plan on spraying a fungicide to manage tar spot, we recommend that this be done soon, prior to the R3/R4 growth stage. The goal here is to protect the leaves from the ear leaf up from continued increase by the fungus. If you would like to learn more about tar spot check out the new web book published by the Crop Protection Network.

Figure 2. County-level confirmations of tar spot in the U.S. as of August 2, 2021.

Continue to scout for tar spot and let us know what you are finding. We are now accepting good pictures of tar spot to confirm its presence in counties where we have observed it in years past, in Wisconsin. In counites that tar spot has never been confirmed, we would like to get a physical sample to verify (Fig 2). Feel free to reach out to me if you do find tar spot or any other disease of corn or soybean for that matter.

Figure 3. County-level confirmations of southern rust on corn in the U.S. as of August 2, 2021.

Has southern rust hit Wisconsin yet? The short answer is NO, not that we can find. We have scouted and asked several folks in our network, and nobody has observed and lab-confirmed southern rust in Wisconsin. However, figure 3 show county-level confirmations of southern rust of corn in the U.S. Based on this map, I would not be surprised if southern rust is confirmed in the next week or so in Wisconsin. Like tar spot, fungicides can be applied up to the R3/R4 growth stage with some benefits. Spraying after R4 will not yield economic returns. To learn more about managing southern rust of corn, check out the electronic fact sheet from the Crop Protection Network.

Keep an eye on the soybean and corn disease situation and scout, scout, scout. Let us know what you are finding!

Wisconsin Soybean and Corn Disease Update – July 7, 2021

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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

Roger Schmidt, Nutrient and Pest Management Program, University of Wisconsin-Madison

Soybean White Mold Update

Figure 1. White mold risk for Wisconsin on July 7, 2021 from the Sporecaster smartphone app.

Figure 1 illustrates the calculated risk of white mold for select Wisconsin locations for non-irrigated soybeans, as determined by Sporecaster for July 7, 2021. This means that if soybeans are flowering and the area between rows is filled in more than 50%, risk is just moderate in most locations of the state, with the exception of the far northeast portions of the state. This moderate risk indicates that there may not be apothecia present in fields in these locations at this time, however, the situation needs to be monitored closely as we move from R1 to the R2 growth stage. With a cooler, wetter weather pattern over the next 5-7 days, I believe that the risk for white mold will increase. I’m expecting a later onset (closer to the R3 growth stage) of white mold for much of the state in 2021. Warmer weather up to this point has pushed the risk of white mold potentially later in the bloom period.

Current White Mold Management Recommendation

I wrote extensively about white mold management in my previous post. Take some time to read the management recommendations there. I think folks should be patient yet monitor the situation carefully over the next several weeks. Again, if calculated risk continues to rise, then a fungicide application may be warranted as we progress through the bloom period. Be sure to download the Sporecaster app to get tailored recommendations for your fields. You can also adjust the action thresholds in the app (my map above is set at the default 40% threshold) and run specific models for irrigated environments.

Corn Tar Spot Update

Figure 2. Tar spot risk for Wisconsin on July 7, 2021 based on the Tarspotter smartphone app.

Figure 2 shows the calculated risk from Tarspotter (our smartphone prediction tool for tar spot) for July 7, 2021, for various locations in Wisconsin. The action threshold for high risk is 40% using the updated Tarspotter model for 2021. As you can see, the present risk is high for much of the state. Cooler, wet conditions over the next week will keep risk moderate to high. We have scouted several locations in Wisconsin and have been unable to find tar spot at this time in the state. However, figure 3 shows that tar spot has been found in some surrounding states, at low levels.

Current Tar Spot Management Recommendation

Monitoring Tarspotter (be sure to download it to your smartphone) and scouting should be done at this time to determine the diseases present. Tracking this situation, not only for tar spot but other corn diseases, will also help you make an in-season fungicide spray decision as we approach the very important VT/R1 growth stage. For more on making the decision to spray fungicide on corn, see my previous post. Get out and scout, scout, scout!

Figure 3. Confirmed tar spot cases in the U.S. as of July 7, 2021.

Wisconsin Corn Tar Spot Update – July 29, 2020

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

Roger Schmidt, Nutrient and Pest Management Program, University of Wisconsin-Madison

No new county level first-detects of tar spot in Wisconsin have been added to the national tar spot map this week (Fig. 1). This week we are seeing just a a handful of new counties added in the Midwest corn belt. Note that all county level confirmations for 2020 are in areas where the disease has been previously reported.

Figure 1. Corn IpmPIPE tar spot confirmations for U.S. Counties as of July 29, 2020. Grey shading indicates past confirmation in the county. Orange shading indicates a positive confirmation for 2020. Yellow shading indicates a probable positive.

Figure 2. Tar Spot risk for various locations in Wisconsin for July 29, 2020.

Figure 2 shows the calculated risk from Tarspotter (our smartphone prediction tool for tar spot) for July 29, 2020, for various locations in Wisconsin. The action threshold for high risk is 75% using the updated Tarspotter model for 2020. As you can see, the present risk has continued to decline overall for the entire state with the exception of the far Northwest, where there is not a history of tar spot. Continued dryer and warm weather is less conducive for the development of tar spot, thus we see the risk continuing to decline. So while we can find tar spot in handful of fields, progress of the disease is slow. Remember, tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period, accompanied by extended periods of leaf wetness caused by dew, rain, or irrigation events. The newest Tarspotter tool captures all of these aspects and balances these in the calculations of risk in the map above.

The Recommendation

Tassels and silks have been out in the southern portion of the state. We are now in the tail-end of the window of opportunity for a fungicide application if you feel the risk for disease, including tar spot, is warranted. While tar spot is slow to develop, we have seen gray leaf spot (GLS) developing in the lower canopy and moving up. We continue to also scout for southern rust, but have had no confirmations of this disease so far in Wisconsin.

Do some scouting and check weather reports. If it is dry in your area and has been hot, then no disease may be present. You might be able to hold off on that fungicide application. If it has been humid and rainy and you have some disease present in the lower canopy, then a fungicide application might be warranted. Now is the critical time to pay attention to disease development and make a final fungicide spray decision. See my previous post for more information about making the decision to spray fungicide on corn.

More Tar Spot Information

  1. Tar Spot Fact sheet (Updated for 2020!)
  2. Short Tar Spot Video
  3. Tar Spot Webinar 
  4. Corn Fungicide Efficacy Table

Wisconsin Corn Tar Spot Update – July 23, 2020

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

Roger Schmidt, Nutrient and Pest Management Program, University of Wisconsin-Madison

No new county level first-detects of tar spot in Wisconsin have been added to the national tar spot map this week (Fig. 1). This week we are seeing more counties being added now in Iowa, Illinois, Indiana, and Michigan. Note that all county level confirmations for 2020 are in areas where the disease has been previously reported.

 

Figure 1. Corn IpmPIPE tar spot confirmations for U.S. Counties as of July 23, 2020. Grey shading indicates past confirmation in the county. Orange shading indicates a positive confirmation for 2020.

 

Figure 2. Tar Spot risk for various locations in Wisconsin for July 23, 2020.

Figure 2 shows the calculated risk from Tarspotter (our smartphone prediction tool for tar spot) for July 23, 2020, for various locations in Wisconsin. The action threshold for high risk is 75% using the updated Tarspotter model for 2020. As you can see, the present risk has declined overall for most of the state with exceptions for south-central and far Northwest Wisconsin. Dryer and warmer weather is less conducive for the development of tar spot, thus we see the risk slowly declining. Remember, tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period, accompanied by extended periods of leaf wetness caused by dew, rain, or irrigation events. The newest Tarspotter tool captures all of these aspects and balances these in the calculations of risk in the map above.

The Recommendation

Tassels and silks are out in the southern portion of the state. We are now in the window of opportunity for a fungicide application if you feel the risk for disease, including tar spot, is warranted. While tar spot is slow to develop, we have seen gray leaf spot (GLS) developing in the lower canopy and moving up. Do some scouting and check weather reports. If it is dry in your area and has been hot, then no disease may be present. You might be able to hold off on that fungicide application. If it has been humid and rainy and you have some disease present in the lower canopy, then a fungicide application might be warranted. Now is the critical time to pay attention to disease development and make a fungicide spray decision. See my previous post for more information about making the decision to spray fungicide on corn.

More Tar Spot Information

  1. Tar Spot Fact sheet (Updated for 2020!)
  2. Short Tar Spot Video
  3. Tar Spot Webinar 
  4. Corn Fungicide Efficacy Table

Wisconsin Corn Tar Spot Update – July 14, 2020

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

Roger Schmidt, Nutrient and Pest Management Program, University of Wisconsin-Madison

Last week brought our first detect of tar spot in Wisconsin for the 2020 field season. This week we are seeing more counties being added now in Iowa and Indiana, with some additional non-confirmed reports from southwest Wisconsin. Figure 1, show the location of the confirmed detections, which are all in the previously recorded range of the tar spot pathogen.

Figure 1. Corn IpmPIPE tar spot confirmations for U.S. Counties as of July 14, 2020. Grey shading indicates past confirmation in the county. Orange shading indicates a positive confirmation for 2020.

Figure 2. Tar Spot risk for various locations in Wisconsin for July 14, 2020.

Figure 2 shows the calculated risk from Tarspotter (our smartphone prediction tool for tar spot) for July 14, 2020, for various locations in Wisconsin. Figure 3 shows the risk for locations in southern and south-central Wisconsin. The action threshold for high risk is 75% using the updated Tarspotter model for 2020. As you can see, the present risk is elevated in much of the southern and southwestern portion of the state, the areas where reports are coming in. This is due to the fact that the weather continues to be relatively wet and humid for the past 30 days with decent rainfall. Cooler conditions this week are also contributing to the rising risk. Tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period, accompanied by extended periods of leaf wetness caused by dew, rain, or irrigation events. The newest Tarspotter tool captures all of these aspects and balances these in the calculations of risk in the map above.

The Recommendation

Figure 3. Tar spot risk for the southern and south-central zones of Wisconsin for July 14, 2020.

Tassels are starting to peek or are out in the southern portion of the state. We are now in the window of opportunity for a fungicide application if you feel the risk for disease, including tar spot, is warranted. Do some scouting and check weather reports. If it is dry in your area and has been hot, then no disease may be present. You might be able to hold off on that fungicide application. If it has been humid and rainy and you have some disease present in the lower canopy, then a fungicide application might be warranted. See my previous post for more information about making the decision to spray fungicide on corn.

More Tar Spot Information

  1. Tar Spot Fact sheet (Updated for 2020!)
  2. Short Tar Spot Video
  3. Tar Spot Webinar 
  4. Corn Fungicide Efficacy Table

Wisconsin Corn Tar Spot Update – July 9, 2020

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

Roger Schmidt, Nutrient and Pest Management Program, University of Wisconsin-Madison

Tar spot has been detected on corn for the first time during the 2020 season, this week in Michigan and Indiana. In both situations these were fields with a history of the disease. Incidence and severity is very low, suggesting that the recent warm, dryer weather may be keeping tar spot in check at these sites. Figure 1, show the location of these detections.

Figure 1. Corn IpmPIPE tar spot confirmations for U.S. Counties as of July 9, 2020. Grey shading indicates past confirmation in the county. Orange shading indicates a positive confirmation for 2020.

Figure 2. Tar Spot risk for various locations in Wisconsin for July 9, 2020.

Figure 2 shows the calculated risk from Tarspotter (our smartphone prediction tool for tar spot) for July 9, 2020, for various locations in Wisconsin. Figure 3 shows the risk for locations in southern and south-central Wisconsin. The action threshold for high risk is 75% using the updated Tarspotter model for 2020. As you can see, the present risk is elevated in much of the southern portion of the state, with areas of high risk in the western half of the state. This is due to the fact that the weather continues to be relatively wet and humid for the past 30 days with decent rainfall across this portion of the state, despite relatively warm temperatures. Tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period, accompanied by extended periods of leaf wetness caused by dew, rain, or irrigation events. The newest Tarspotter tool captures all of these aspects and balances these in the calculations of risk in the map above.

We have been scouting fields in the southern portion of the state where there has been a history of tar spot. We have been unable to find any symptoms of tar spot thus far. Fields generally look disease free, which is common this time of year in Wisconsin.

The Recommendation

Figure 3. Tar spot risk for the southern and south-central zones of Wisconsin for July 9, 2020.

Even if weather turns more favorable for tar spot, evaluate the likelihood that tar spot might develop early in your field. Remember, if you have no history of the disease, then the likelihood of local inoculum being present is low. Saving the fungicide application for later in the season might be a better option. If you have a history and you know you have a susceptible hybrid coupled with a no-till situation, then the risk is higher and you need to evaluate the economics of doing an application of fungicide in early or mid v-stages. Remember, if you do a V6-V8 application of fungicide, conditions could stay conducive later in the season for tar spot. Those early applications will “burn out” by the time the tasseling period rolls around. So if you do (or did) put a fungicide spray on at V6-V8, you might have to come back at VT-R2 with another application to protect plants during the reproductive phase, should favorable conditions for tar spot persist. Keep an eye on the weather and keep scouting!

More Tar Spot Information

  1. Tar Spot Fact sheet (Updated for 2020!)
  2. Short Tar Spot Video
  3. Tar Spot Webinar 
  4. Corn Fungicide Efficacy Table

Corn Disease and Nutritive Value Considerations for the 2019 Silage Harvest

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Damon L. Smith, Associate Professor and Extension Field Crops Pathology Specialist, University of Wisconsin-Madison

John Goeser, Adjunct Assistant Professor, Department of Dairy Science, University of Wisconsin-Madison and Animal Nutrition Director, Rock River Laboratory, Inc

The 2019 silage corn harvest is finally starting to ramp up in Wisconsin. With the excitement of finally getting into the field comes the need to be aware of the corn disease situation this season. As most of you will remember, the 2018 field season was an extreme challenge when it came to making quality corn silage in Wisconsin. Foliar diseases of corn, forced the plants to lose photosynthetic capability pre-maturely, resulting in cannibalization of stalks for carbohydrates to fill ears. Loss in stalk integrity meant extreme lodging, not to mention that is was a struggle to find optimum moisture in any field. Throw in frequent rains, and trying to chop on time to achieve quality fermentation was nearly impossible in 2018. The consequences of the challenging season are still being felt with poor quality, wild yeast issues, and higher than typical mycotoxin loads. So what does 2019 look like?

Foliar Disease of Silage Corn in 2019 

Compared to 2018, the foliar disease situation has been less significant in 2019. However, there are still some important diseases to consider as you prepare for harvest. Statewide, gray leaf spot did appear early again this season. However, unseasonably cool weather kept this disease relegated to the lower canopy. For most fields we have visited, gray leaf spot will likely be of little impact on yield and feed quality this year. 

Figure 1. Tar spot on a leaf of corn located in Arlington, WI on August 7, 2019. Photo Credit: Hannah Reed, University of Wisconsin-Madison.

The flipside of the cooler weather meant that tar spot (Fig. 1) has become an issue again this year. Tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period. Periods of extended leaf wetness further facilitate increase and spread. We have been right in the ideal growth zone for the pathogen that causes tar spot since the first part of August. Over the last month, tar spot has been found in many areas of the state (CLICK HERE to view the latest national map for tar spot confirmations), leading to the 4th straight field season where this disease has impacted silage corn. While the disease has moved in later this season, compared to 2018, it is moving quickly. Tar spot can kill leaves prematurely, or reduce photosynthetic capacity. 

 

Figure 2. Northern corn leaf blight on corn.

Northern corn leaf blight (NCLB; Fig. 2) can also be readily found in the upper canopy in some fields in 2019. This disease has historically been a more significant problem on silage hybrids, increasing when the weather is cool and the humidity high. Depending on the severity and interaction of both NCLB and tar spot, these diseases can influence whole plant moisture levels and also cause stalk-cannibalization, leading to increased risk for lodging. As you prepare to chop silage, scout fields to understand the severity of foliar disease levels along with whole plant moisture and kernel maturity. Fields with the highest levels of foliar disease should be closely monitored for whole plant moisture and prioritized for harvest first. Then work your way to those fields with less visible disease.       

Ear rots and mycotoxins of silage corn in 2019 

Figure 3. Gibberella ear rot on corn.

In 2018, corn production in Wisconsin was also plagued by high levels of Gibberella ear rot (Fig. 3) and high levels of deoxynivalenol (DON or vomitoxin) in finished grain and silage. Remember that vomitoxin is a secondary metabolite produced by the fungus that causes Gibberella ear rot. We believe that 2019 will be another year with high levels of Gibberella ear rot and vomitoxin levels. Weather has been wet, especially during silking on late-planted fields in 2019. This increases the risk of Gibberella ear rot. Furthermore, the fungus that causes Gibberella ear rot can cause Fusarium head blight (FHB or scab) in wheat. Vomitoxin can also accumulate in wheat grain resulting in unusable grain, or grain subjected to dockage at the elevator. The 2019 wheat season saw high levels of FHB in winter wheat, with subsequent reports of high levels of vomitoxin. Anecdotal reports of very high DON levels have been reported in wheat straw harvested in 2019. This situation further substantiates the possibility that corn might also be hit hard with Gibberella this year. When scouting fields, pull back some husks to see if there is visible ear rot. Note these fields where high levels of severity exist. Also, check fields for lodging and assess stalk integrity. The fungus that causes Gibberella ear rot can also cause Gibberella stalk rot. We also know that from some preliminary research, vomitoxin can accumulate in the stalk portions of the plant in addition to the ears. Fields with high levels of ear rot and/or stalk rot should be prioritized for harvest first. You might also consider keeping silage from these higher-severity fields separate from other fields you harvest. Also consider testing for nutritive quality and mycotoxin load as you chop silage, so you know how much vomitoxin is present and potentially from which fields. Information on testing grain and silage can be found by clicking here. An additional list of testing labs can be found in A3646-Pest Management in Wisconsin Field Crops in table 2-16. Remember that mycotoxins like vomitoxin are very stable. They cannot be removed by heating or freezing. When storing corn grain for long periods of time, we recommend drying grain down to 13%. This will help stop the continued growth of the fungus that can cause vomitoxin and reduce any subsequent accumulation of the mycotoxin. In silage corn production, harvesting at optimum moisture and packing the bunker and inducing fermentation and anaerobic conditions as quickly as possible will limit any further growth of the fungus and any additional accumulation of vomitoxin.

If you sprayed silage corn with fungicide in 2019, this might help reduce the levels of foliar disease, ear rot, and vomitoxin levels. However, it will not “cure” the situation nor is it anywhere near perfect. Research in 2018 demonstrated that fungicides could reduce disease levels, but in a year when weather conditions were conducive for ear rot and vomitoxin accumulation expectations needed to be lowered. In 2018 certain fungicide programs had the capability of reducing vomitoxin levels by 50% or more, but that still meant that a lot of the silage made was still considered unacceptable for feeding due to high vomitoxin levels. Remember that hybrid choice, in addition to treating with fungicide, can play an important role in how much vomitoxin is present and the nutritive value of the finished feed.

What are the impacts of poor silage quality and mycotoxin accumulation? 

Animal nutritionists have observed many impacts of mycotoxin and microbial growth challenges in animals, including dairy cattle. Performance and health issues can range from milk fat or milk protein percentage decreases, to decreased milk production and all the way on up to feed refusal, intestinal or gut hemorrhaging, and death. For this reason, nutritionists have devised guidelines for dietary limits of some mycotoxins to reduce harm to the animal. Dr. John Goeser has assembled the “Mycotoxin Guidelines and Dietary Limits” fact sheet to help producers better understand the potentially harmful toxin levels in the total diet (DM). You will see in that chart that for vomitoxin (DON), the suggested total mixed ration (TMR) concern limit is just 0.5 to 1.0 ppm for dairy cattle. The fact sheet also provides a helpful formula to understand the contribution of toxin in a particular component of feed, relative to the total diet.

Also recognize microbial growth (mold, yeast and negative bacteria) challenges will increase with wetter conditions. Both mycotoxin load and microbial contamination need to be checked if performance or health appear challenged for your herd. Start by checking the TMR and then work backward from there with your advisory team.

We are expecting a prolonged harvest this year due to unprecedented planting growing conditions earlier this season. As discussed previously, step up your crop scouting efforts to optimize harvest this year. Consider using the approach discussed in this recent Hoard’s Dairyman HD Intel newsletter to be proactive and stay in control this harvest.

The Take-Home

  1. Spend some time scouting fields for foliar, ear, and stalk disease. Prioritize harvest for fields with high disease severity.
  2. Be proactive. Consider testing corn silage for mycotoxins, specifically vomitoxin, a couple times as your farm begins harvest. If results come back greater than expected, consider increasing frequency so you know what you are dealing with before silage is ensiled.
  3. Consider keeping fields with high disease levels and/or high vomitoxin levels segregated from better feed. 
  4. Take time to target optimum harvest moisture and packing conditions to shorten time to anaerobic conditions and fermentation.
  5. Keep oxygen out of the silo. After the silo, bunker, pile, or bag have been sealed, continue watching for holes or leaks on a regular basis and repair damaged plastic or seams. 

Tar Spot Now Confirmed in Wisconsin in 2019

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

Hannah Reed, Graduate Research Assistant, Department of Plant Pathology, University of Wisconsin-Madison 

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

Figure 1. Corn IPM PIPE tar spot occurrence map as of August 7, 2019.

Tar spot has been found on corn in plots established to monitor for the disease in Arlington (Columbia Co.) and Lancaster (Grant Co.) WI (Fig. 1). In both cases the disease was present on hybrids known to be susceptible. At the Arlington location disease was found in just one small area of the field. Tar spot coverage was low to moderate on a few leaves (Fig. 2). Microscopy was used to observe ascospores from stromata, thus confirming the tar spot fungus (Fig. 3).

Figure 2. Tar spot on a leaf of corn located in Arlington, WI on August 7, 2019.

Tar spot was very hard to find in the Lancaster location. However, it was observed on several plants in one monitoring plot. In each case only 1-2 spots were observed.

What does this mean for you?

Figure 3. Asci and Ascospores of the tar spot fungus.

This means it is time to get back out and scout corn fields for tar spot. If you have had a history of tar spot and you know that you have a hybrid that is more susceptible and there is a large amount of infested residue, then you should monitor this situation closely. If tar spot is observed and you are irrigating or have had frequent rain, monitor this situation very closely. Tar spot seems to progress quickly in irrigated environments. Remember, that the window of opportunity to treat with a fungicide can pass rapidly as this disease can move quickly. Protecting this ear leaves before R3 can be important for preserving yield. There are many products that have demonstrated decent efficacy toward tar spot. You can find our 2018 fungicide test summaries by CLICKING HERE and scrolling down to pages 2-7. Work with your local extension personnel if you need help diagnosing the disease or need advice on spraying fungicides.

Wisconsin Mid-Season Corn Disease Update – August 2, 2019

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

The Tar Spot Situation

Figure 1. Tarspotter risk predictions for the state of Wisconsin on August 2, 2019.

Figure 1 shows the calculated risk from Tarspotter for August 2, 2019, for various locations in Wisconsin. As you can see, the present risk remains very low for most of the state. Continued warm and dry conditions have kept the risk low in Wisconsin. Tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period. We continue to scout fields in southern and southwestern WI and continue to find no tar spot in our travels. Tar spot has been observed now in multiple counties in Illinois, Indiana, and Michigan, along with one county in Iowa (Fig. 2). Continued dry weather is expected to keep this disease at non-existent or low levels in Wisconsin for the next couple of weeks.

Other Corn Diseases To Watch in Wisconsin

Figure 2. Corn IPM PIPE tar spot occurrence map as of August 2, 2019.

We continue to frequently find gray leaf spot (GLS) on corn. This disease is going to be problematic on some hybrids and in certain environments in Wisconsin. We are seeing GLS on ear leaves and severity is increasing. Remember, if you are going to spray fungicide, the idea is to spray preventatively before the disease reaches the ear leaves. Continue to scout fields and look in the lower canopy and watch movement of the pathogen and disease symptoms up the canopy. Optimal fungicide application timing if disease is progressing will be between VT and R3. See my previous article about making the fungicide spray decision.

Figure 3. Common rust on Corn. Photo Credit: Daren Mueller, Iowa State University, Bugwood.org

Southern rust has also been on our minds recently in Wisconsin. The disease is is caused by the fungus Puccinia polysora. Symptoms of southern rust are different from common rust (Fig. 3) in that they are typically smaller in size and are often a brighter orange color (Fig. 4). Pustules of southern rust also typically only develop on the upper surface and will be be more densely clustered. Favorable conditions for southern rust development are similar to those for common rust. high humidity and temperatures around 80F encourage disease development. However, very little free moisture is needed for infection to occur. Southern rust is typically a late-arriver in Wisconsin. When it does move in, it is usually in the southern and south-western portions of the state. Spores of this fungus have to be blown up from tropical regions or from symptomatic fields in the southern U.S. The fungus can not overwinter in Wisconsin. While southern rust epidemics can be rare events in Wisconsin, the disease can be serious when it occurs. In addition, when it occurs close to sinking, yield loss from the disease can be high. Thus, close monitoring of forecasts and scouting are needed to make timely in-seaosn management decision.

Figure 4. Southern rust on corn. Photo Credit: Emmanuel Byamukama, South Dakota State University, Bugwood.org

Currently the Corn Southern Rust iPIPE map is showing numerous confirmed cases of southern rust to our south, including an observations in a far northern Illinois county (Fig. 5). No confirmed cases have been identified in Wisconsin. However, close attention should be paid to this disease in 2019 as the confirmed cases this year have been earlier than in the past. This could mean that conditions are ripe for movement of southern rust inoculum into Wisconsin.

Management of Southern Rust

Figure 4. Corn IPM PIPE southern rust occurrence map for August 2, 2019.

Traditionally resistance was used to manage southern rust. However, in 2008 a resistance-breaking race of the southern rust fungus was confirmed in Georgia. Thus, most modern hybrids are considered susceptible to southern rust. Rotation and residue management have no effect on the occurrence of southern rust. The southern rust fungus has to have living corn tissue in order to survive and can not overwinter in Wisconsin. Fungicides are typically used to control southern rust in parts of the U.S. where this is a consistent problem. Efficacy ratings are also available for fungicides against southern rust on the Corn Fungicide Efficacy Table. Should southern rust make its way to Wisconsin prior to the “milk” (R3) growth stage in corn, it could cause yield reductions. Growers and consultants should scout carefully through the R3 growth stage and be sure to properly identify the type of rust observed. If you need assistance in identifying rust on corn, 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.

Other Useful Resources about Rusts on Corn

Purdue Extension Fact Sheet – Common and Southern Rusts of Corn

Video by Dr. Tamra Jackson-Ziems of the University of Nebraska – Identifying Rust Diseases of Corn