Wisconsin Corn and Soybean Disease Update and Forecast – July 21, 2022

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

Rain, and the return of more humid weather, has meant that risk of tar spot of corn and white mold of soybean has increased over the past week. Now is the time to think about your in-season management plan for both of these diseases. Let’s dig in a bit on what the risk looks like for each disease.

Tar Spot of Corn

This week we added Fond du Lac County to the tar spot map (Fig. 1). We also continue to see tar spot slowly increasing in plots and production fields on the Arlington research station. Looking back at our records from last season, we are tracking almost identically with what happened last year. I know folks think it is dry, but the tar spot fungus doesn’t care. It might move slow in these conditions, but the elevated humidity provides adequate leaf wetness for the disease to slowly progress. Should it start raining more regularly I expect the disease to pick up speed.

Corn is rapidly approaching (if not already at) the optimal window of opportunity (VT-R3) for spraying fungicide to control tar spot. Given the high risk for tar spot across much of the state (Fig. 2), now is the time to call in that fungicide application if you are planning on it. Given the possible constraints on locating a custom applicator, getting the order in earlier than later may ensure application of fungicide by the R3 corn growth stage. Get out and scout, scout, scout!

Figure 2. Tar Spot Risk for Wisconsin on July 21, 2022

White Mold of Soybean

White mold risk has increased from reasonably low last week, to mostly moderate across the state, this week (Fig. 3). Risk trends are also increasing, indicating that weather is continuing to become more favorable for white mold development. As we approach the R3 soybean growth stage, it will be important to make a decision on fungicide application, especially if you haven’t already applied a fungicide. If rain moves in over the next 7-10 days, expect risk to continue to increase. In irrigated fields we have been able to find apothecia (the mushroom-like structure that produces spores that infect soybean). This corroborates the increased risk we are seeing even in non-irrigated fields.

Figure 3. White mold risk in Wisconsin for July 21, 2022.

The Field Prophet Tool

For those who like all of their disease prediction tools in one place you might check out the Field Prophet version of the Tarspotter and Sporecaster apps. This tool consolidates all of our disease prediction tools into one convenient tool. The app also allows for true 7-day forecasting and will display 7-day trends to better inform your disease management decisions. Field Prophet, Inc is a startup company supported by UW-Madison and uses science-based information and the same models as Tarspotter and Sporecaster to deliver informative tools for agriculture clientele. You can also download and use Field Prophet for free for the next 6 months! You might find this tool as a handy alternative to Tarspotter and Sporecaster.

 

 

Wisconsin Corn and Soybean Disease Update and Forecast – July 14, 2022

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

Weather over the last week has been generally drier with milder temperatures in most of Wisconsin. Isolated storms have occurred and periods of leaf wetness have prevailed. So what does that mean for important plant diseases of corn and soybeans? Let’s break that down.

Tar Spot of Corn

As noted last week we found tar spot in Columbia county Wisconsin, which was the first report for 2022. This week we add Dane County to the list (Fig. 1). Tarspotter risk has remained high for much of the state over the last week due to milder temperatures and periods of leaf wetting events. It is important to note that conditions favorable for tar spot development are different than those for white mold in soybean. For white mold rain, more sustained wetting events, and cooler temperatures are required (see below). As of today (July 14, 2022) tar spot risk remains high or elevated for most of the state (Fig. 2). Over the next week the forecast is putting us a bit drier and hotter. Thus, the tar spot risk could continue to decline. However, remember that tar spot will continue to show up due to favorable weather 2 or more weeks back. The tar spot pathogen has a long incubation period (time from infection to tar spot appearance). Thus, you shouldn’t be surprised in finding tar spot at low levels over the next week. So should you spray fungicide now? If you can, wait until at least the VT (tasseling growth stage). The evidence is strong that the optimum window to spray fungicide to control tar spot is between the VT and R3 (milk) growth stages. Spraying before VT might leave corn plants vulnerable to a late-season tar spot increase. Thus, if you spray before VT, you might need to come back with a second application of fungicide closer to the R3 growth stage. For guidance on when/if to spray fungicide to manage tar spot, see my previous article.

Figure 2. Tar spot risk for Wisconsin on July 14, 2022.

White Mold of Soybean

White mold risk remains generally low and is dropping for most of the state of Wisconsin (Fig. 3). This is not surprising as temperatures have remained moderate with drier conditions. Based on the current risk and the 7-day forecast, fungicide applications can be held back. Folks should pay attention to the weather and Sporecaster risk as the crop moves into full bloom and early pod development. In recent years we have seen white mold risk increase during the late bloom time necessitating a fungicide application around the R3 growth stage. I would expect this same scenario to set up in 2022 in at least a portion of Wisconsin. Folks should monitor this situation carefully as we move ahead over the next 2 weeks. For more information on white mold and making the fungicide spray decision, see this previous article.

Figure 3. White mold risk for Wisconsin for July 14, 2022.

The Field Prophet Tool

For those who like all of their disease prediction tools in one place you might check out the Field Prophet version of the Tarspotter and Sporecaster apps. This tool consolidates all of our disease prediction tools into one convenient tool. The app also allows for true 7-day forecasting and will display 7-day trends to better inform your disease management decisions. Field Prophet, Inc is a startup company supported by UW-Madison and uses science-based information and the same models as Tarspotter and Sporecaster to deliver informative tools for agriculture clientele. You can also download and use Field Prophet for free for the next 6 months! You might find this tool as a handy alternative to Tarspotter and Sporecaster.

We Found Tar Spot of Corn in 2022, Now What?

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

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

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

It didn’t take long this year to find tar spot in the corn crop in the Midwest. Last week brought on the first county-wide reports of tar spot in Iowa and now we have found tar spot on corn in Columbia Co. Wisconsin as of July 6, 2022 (Fig 1). This is the earliest in the season, and the earliest growth stage of corn, that I have ever seen tar spot in Wisconsin. With that said, the severity is extremely low and does not necessitate spraying fungicide at the moment! So what should you do now?

Figure 1. Map of U.S. counties where tar spot has been confirmed in the 2022 season, as of July 7, 2022. Map source: https://corn.ipmpipe.org/tarspot/.

What should I do?

My advice is to get prepared and make sure you have the tools in place to deal with this problem. As I said last season, tar spot is here to stay and we need to simply be prepared and ready to fight the disease. The first line of defense is to know if you have had tar spot before. This will tell you if there is resident inoculum sources present that can initiate epidemics. If you have seen tar spot on your farm before, then assume the pathogen is present and in close proximity to corn (the host). Remember the disease triangle? The last component of the triangle is the weather. If there has been conducive weather then the triangle has been met and risk is high for finding tar spots. So how do you know if the weather is conducive? Well, there is an app for that!

Tarspotter and Field Prophet are both Smartphone applications that can help you determine if the weather has been conducive to put your corn crop at high risk of tar spot development. Figure 2 shows a map of Wisconsin from the Field Prophet version of the tool that is showing that weather has been highly conducive for the development of tar spots. The app DOES NOT tell you if the pathogen is present. We are working on this part of the triangle to improve our predictions, but you need to determine if the pathogen is present in your field. This tool just tells you if the weather has been conducive.

Figure 2. A Screen shot of a map developed in the Field Prophet app showing risk for tar spot development in Wisconsin as of July 7, 2022.

So what weather is conducive for tar spot development? Well, it is different that the weather needed to grow corn. Yes, precipitation is helpful, but more importantly, we need leaf wetness. Specifically leaf wetness at night. What gives leaf wetness this time of year other than rain? That would be high dew points and humidity. These variables are included in the models that run in Tarspotter and Field Prophet. We also include temperature (not as important as you think it would be) and precipitation. These variables are measured over the last 14 days and included in each daily run of the tool. We use the GPS on the smartphone to pull down cloud-based weather for a precise location. Thus, these results are site-specific. I also like to the use the Field Prophet version of the models as this version provides a 7-day trend line on how weather has been progressing and also allows for a true 7-day forecast. These additional tools can better help with the decision-making process.

My corn is at V8 or V10, should I spray Fungicide?

My short answer is no! The disease is just getting started. It is at low severity (Fig. 3) and is low in the canopy on leaves that are not going to contribute to yield. My advice is to use your prior knowledge of where tar spot occurred and the Tarspotter tool to help guide your scouting efforts. Get out into the fields and know what you are dealing with. Target field planted to known susceptible hybrids. Get yourself prepared and use those lower leaves to monitor severity and tar spot progress. Be ready to protect (put fungicide on) those leaves that contribute to yield (ear leaf and above), later on.

When should I spray fungicide? What should I use?

Figure 3. A single tar spot on a lower leaf of corn in Wisconsin on July 6, 2022.

Our recent work has shown that if you get the product right, you can generally control tar spot to the point to preserve your yield potential, with one well timed spray. So what is that timing of the single application? That would be between the VT (tasseling) and R3 (milk) growth stages. We determined this using a series of trials where we did single applications of fungicide for individual growth stages. Figure 4 shows the results of two trials from 2020 where the window of opportunity to reduce the severity of tar spot with a single application is between VT and R3. Yes, the two-spray program (V8+VT) did also control tar spot, but all the work was done by the VT application, not the V8-timed spray! Thus, if you chose to spray your 2022 crop at this point in the season, you stand a good chance of having to come back with a second application later in the season.

Also notice in figure 4 that Tarspotter was tested in 2020 and did not perform very well. This was an early iteration of the tool and we have since improved its performance. So please don’t judge the tool based on this figure. If you would like to see how Tarspotter performed in advising fungicide applications last season, check out this article based on the 2021 field season.

Now the questions is what fungicide should you choose? The short answer is that you have lots of options. You can learn more about fungicides and fungicide performance by CLICKING HERE. You can also check out the efficacy of various fungicides based on a collective of University research by viewing the “Fungicide Efficacy for Control of Corn Diseases” on the Crop Protection Network website. In addition, Dr. Darcy Telenko at Purdue University led an effort to publish data from a multi-state coordinated fungicide trial where we tested various fungicides during the 2021 epidemic.

Figure 4. Tar spot intensity after spraying fungicide once at each corn growth stage during the 2020 field season.

 

Figures 5 shows the tar spot severity, while figure 6 the corn yield, from those trials where a single application of each of the products was made at the VT growth stage. Clearly you have lots of options when it comes to products that can control tar spot. That is good news! Yes, some products do a bit better in preserving yield over others, but all fungicides tested in 2021 resulted in numerically higher yields than not-treating. Remember that 2021 was a banner year for tar spot. These results might not be as clear in a year where tar spot is not as intense.

Figure 5. Tar spot severity from multi-state trials where corn was treated with foliar fungicides at the VT corn growth stage, or not treated. Source: Telenko et al., 2022 – https://apsjournals.apsnet.org/doi/10.1094/PHP-02-22-0012-BR.

Figure 6. Corn yield from multi-state trials where corn was treated with foliar fungicides at the VT corn growth stage, or not treated. Source: Telenko et al., 2022 – https://apsjournals.apsnet.org/doi/10.1094/PHP-02-22-0012-BR.

The Conclusion

DON’T PANIC! This is just a call to be ready. Download the apps and know what the weather is doing. Use your prior knowledge and scouting in key locations to track tar spot. Get your management plan in place. Have your fungicide of choice available. Communicate with your custom applicator. Be ready to spray between the VT and R3 growth stages if you plan to use just one fungicide application and you are seeing tar spot increase. If you spray between the V8 and VT growth stages, be ready to monitor the smartphone apps and do more scouting as you might have to pull the trigger again later in the season. Get out and SCOUT, SCOUT, SCOUT!

Other Resources

New Research Update: Disease Development and Deoxynivalenol Accumulation in Silage Corn

Richard W. Webster, University of Wisconsin-Madison; Maxwell O. Chibuogwu, University of Wisconsin-Madison; Hannah Reed, University of Wisconsin-Madison; Brian Mueller, University of Wisconsin-Madison; Carol L. Groves, University of Wisconsin-Madison; Albert U. Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; Martin I. Chilvers, Michigan State University; Kiersten A. Wise, University of Kentucky; and Damon Smith, University of Wisconsin-Madison.

A new research update has just been published on the Crop Protection Network summarizing recent work conducted to understand how deoxynivalonol (DON or vomitoxin) accumulates in silage corn plants. We also took a look at managing this issue with fungicides. The summary of the work is below, or you can click here to read the entire research update.

Gibberella ear rot

Summary

  • Fusarium graminearum is a fungus that causes the two diseases, Gibberella ear rot and Gibberella stalk rot, which can lower yield and feed quality of silage corn.
  • This fungus produces a secondary metabolite called deoxynivalenol (DON; also known as vomitoxin) during development and colonization of the corn plant, which is toxic to both humans and livestock.
  • Our research found that infection, colonization, and production of DON by F. graminearum in ears and stalks of corn plants can differ, and suggests that the two diseases can occur independently of each other.
  • Foliar fungicides reduced foliar diseases in both years, but the effects of fungicide on DON concentrations across entire plants were inconsistent in 2019.
  • Scouting for Gibberella ear and stalk rot and testing for DON in silage corn is important even if visual ear symptoms are not present as DON may still be accumulating in the stalks.

Tar Spot Is Here To Stay…

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

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

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.

Insights on In-Season Corn Disease Management Decisions For the 2021 Season

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

Corn is approaching the V6-V10 range of growth stages in much of Wisconsin. With this, comes many questions about applying fungicide to control disease and preserve yield. What diseases are out there? What disease(s) should I focus on in-season? When should I spray? What should I spray? On top of these questions, we are also confronted with corn prices, which are notably optimistic this year, making folks think more about fungicide applications. So what should we consider for in-season disease management? Lets consider the diseases first, then the management decisions.

Figure 1. NCLB Lesions on a corn leaf

Northern Corn Leaf Blight (NCLB): The most diagnostic symptom of NCLB is the long, slender, cigar-shaped, gray-green to tan lesions that develop on leaves (Fig. 1).  Disease often begins on the lower leaves and works it way to the top leaves.  This disease is favored by cool, wet, rainy weather. Higher levels of disease might be expected in fields with a previous history of NCLB and/or fields that have been in continuous and no-till corn production. The pathogen over-winters in corn residue, therefore, the more residue on the soil surface the higher the risk for NCLB.  Management should focus on using resistant hybrids and residue management.  In-season management is available in the form of several fungicides that are labeled for NCLB. However, these fungicides should be applied at the early onset of the disease and only if the epidemic is expected to get worse.

While I hate talking about threshold levels for managing disease, it can be helpful in your decision making process to know what might be severe. While scouting look in the lower portion of the canopy. If some symptoms are present in the lower canopy, make a visual estimation of how frequent (percentage of plants with lesions) NCLB is in a particular area and how severe (how much leaf area is covered by NCLB lesions.  The lower leaves aren’t responsible for much yield accumulation in corn, but spores produced in NCLB lesions on these leaves can be splashed up to the ear leaves where disease can be very impactful. So by scouting the lower canopy and getting an idea of how much disease is present, you can “predict” what might happen later on the ear-leaves to make an informed spray decision.

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

The other consideration you should make while scouting is the resistance rating that the hybrid has for NCLB. If it is rated as resistant, then NCLB severity might not be predicted to get very severe, while in  a susceptible hybrid, NCLB might be present on 50% or more of plants at high severity levels. Note however, that even if a hybrid is rated as resistant, it can still get some disease. Resistance isn’t immunity! If NCLB is present on on at least half the plants and severity is at least 5-10% and weather is forecast to be rainy and cool, a fungicide application will likely be needed to manage the disease. So what does 5% leaf severity look like? Figure 2 is a computer generated image that shows 5% of the corn leaf with NCLB lesions. You can use this image to train your brain to visually estimate how severe the disease might be on a particular leaf. As for fungicide choice and timing, I consider that further below.

Figure 3. Gray Leaf Spot lesions on a corn leaf.

Gray Leaf Spot (GLS): Gray leaf spot typically starts as small blocky or jagged, light tan spots. These can expand to become long, narrow, rectangular lesions (Fig. 3) that may have yellow or orange halos around them. Gray leaf spot is typically worse when temperatures are warm and humidity is frequently above 90%. Thus, in Wisconsin, this disease is generally more frequent in the southern and southeastern portion of the state. Conditions that favor GLS often do not favor NCLB. The GLS pathogen and NCLB pathogen have different temperature requirements. Yield loss from GLS will be the greatest if lesions develop on the ear-leaves right before and right after tasseling. Like NCLB, hybrids rated as susceptible will generally suffer greater yield reductions due to gray leaf spot. Management of GLS should focus on choosing hybrids with excellent resistance and managing corn residue. Corn residue allows the pathogen to overwinter.

Like NCLB, fungicides can also be used to manage gray leaf spot. However, these should be applied as preventative applications. Thus, using the same rule of thumb to make a spray decision for GLS as for NCLB can help you make the decision to spray fungicide. As for fungicide choice and timing, I will also consider that further below.

Figure 4. Tar Spot Signs and Symptoms on Corn Leaves

Tar Spot: Tar spot is favored by cool conditions (60-70 F) and high relative humidity (averages above 75%). Over the last several seasons the tar spot pathogen, Phyllachora maydis, has been able to cause yield reductions in parts of the Midwest by itself. There seems to be no need for a second fungus, Monographella maydis to be present to cause “fisheye” symptoms along with severe necrosis and early dry down. In addition, work done in the Midwest shows that the tar spot fungus can overwinter on corn residue from the previous season. So like with NCLB and GLS fields with high levels of infested residue might be more prone to infection by the tar spot fungus.

Not a lot is known about hybrid resistance to tar spot. Losses as high as 50-60 bushels per acre have been recorded on some hybrids, while others in the same field were only marginally affected. Partial resistance might be present in Midwest on certain corn hybrids. However work is ongoing to understand which hybrids those might be. Fungicide applications have been shown to reduce tar spot levels. However, timing of application must coincide with disease onset and product choice is important. Mixed-mode-of action fungicides have been the most consistent in efficacy over the past several seasons (more on that below). More information on tar spot can be found by in the Tar Spot Electronic Book.

Figure 5. Eyespot symptoms on a corn leaf.

Eyespot: Eyespot typically first develops as very small pen-tipped sized lesions that appear water-soaked.  As the lesions mature they become larger (¼ inch in diameter) become tan in the center and have a yellow halo (Fig. 5).  Lesions can be numerous and spread from the lower leaves to upper leaves. In severe cases, lesions may grow together and can cause defoliation and/or yield reduction. Eyespot is also favored by cool, wet, and frequently rainy conditions.  No-till and continuous corn production systems can also increase the risk for eyespot, as the pathogen is borne on corn residue on the soil surface.  Management should focus on the use of resistant hybrids and residue management.  In-season management is available in the form of fungicides. Severity has to reach high levels (>50%) before this disease begins to impact yield. I often have eyespot present in my corn trials each year as we plant into continuous corn and use no-till. However, we typically do not see yield reductions from this disease even in non-sprayed plots. When scouting, note the disease and keep track of the severity. Again, fungicides should be applied early in the epidemic and may not be cost effective for this disease alone.

What Disease(s) Should I Focus on In-Season? Based on the information above, the greatest emphasis for Wisconsin should be placed on controlling NCLB, GLS, and tar spot (what I call the ‘Corn Foliar Disease Trifecta!). Most hybrids planted in Wisconsin will be resistant to eyespot.

What Should I Spray, and When Should I Spray for Corn Foliar Diseases In Wisconsin? Fungicide should be used to preserve yield and reduce disease level. There is no silver bullet fungicide out there for all corn diseases. However, there are many products which work well on a range of diseases. The Corn Fungicide Efficacy table lists products that have been rigorously evaluated in university research trials across the country. You can see there are several products listed that perform well on NCLB, GLS, and tar spot. So obviously, if a disease is present and you are trying to control the disease, you might expect more return on your investment, compared to simply spraying fungicide and hoping that there might be a yield increase.

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

Figure 6. Average yield preservation of QoI+DMI fungicide applications over not-treating across the U.S. corn belt at the V6, Vt, and V6+VT application timing.

Let’s Take a Closer Look at Corn Fungicide Return on Investment (ROI): In our current market, what are the odds that we might break even on a fungicide application in 2021? Today’s corn future price for September has a bushel of corn above $5.00. While most of the early work on fungicide use in corn has focused on Headline® Fungicide, much of the industry has transitioned to using multi-mode-of-action products. These would be products mostly containing strobilurin (QoI), triazole (DMI), and/or succinate dehydrogenase inhibitor (SDHI) fungicides in the same jug. Details about fungicides and fungicide mode of action can be found on the Fungicides for Field Crops Information Page. Products such as Headline AMP® or Quilt Xcel® would fall into the QoI + DMI category. These combination products have also been fairly consistent in response in my fungicide trials. You can find summaries of these trial results here. If we consider using Quit Xcel® at 10.5 fl oz or Headline AMP® at 10.0 fl oz, the list pricing of the products plus adding aerial application costs, result in a range of cost of $20 – $30 per acre to make the applications at VT. If we can sell corn for at least $5.00 per bushel then we would need to preserve 4 bu/acre to 6 bu/a in yield over not treating to break even! 

In a recent analysis of corn yield data where DMI+QOI products were applied at the tasseling period (VT) across the entire corn belt, the average yield preservation over not treating was 7.20 bu/a (Fig. 6). Thus, with the strong corn prices projected this fall, there could be some opportunity to come out ahead with a fungicide application, especially if forecasted rain events come to fruition and make disease issues increase. The projected probability of breaking even in the current economy with $5.00/bu corn is between 68% and 70% (Fig. 7). If we can sell our corn for a better price or make the applications cheaper, then the odds will continue to improve. We also know that in Wisconsin, the odds of breaking even improve further if NCLB or GLS are active and increasing during the tasseling period. Get out there and scout!

Figure 7. Probability of Breaking Even Based on Data from Across the U.S. (VT Application Timing)

So What About Fungicide Application Timing? We can investigate this question over the U.S. corn belt using the same dataset. Applications focused on an early (V6) timing, a VT-R2 timing, or a combination of V6 plus a VT-R2 application. Let’s again focus on the QoI+DMI products. Based on observations across the corn belt the V6 timing averaged almost 3 bu/a of preserved yield over not treating (Fig. 6). The VT application resulted in nearly 7.2 bu/a in preserved yield, while the two-pass program only offered a little over 8 bu/a. Clearly the higher average yield preservation occurs using a single application of fungicide at the VT-R2 timing. Wisconsin data has been consistent with this observation. Thus it is recommended that a single application of fungicide be used around the VT-R2 growth stages, when NCLB or GLS are active and increasing on or near the ear leaves.

What About Silage Corn and Ear Rot? When it comes to ear rot control and reducing the accumulation of mycotoxins in grain or silage corn, fungicide application should be made when white silks are out. Spores of fungicide that generally cause mycotoxin issues in the grain portion of corn will infect the plant through silks. Thus, apply fungicides during silking or within 5-10 days after silking starts, can be beneficial. Note that if the goal is to target mycotoxin production and reduce deoxynivalenol (DON) accumulation in the grain portion of the plant, products containing a DMI component should be used. Results where QoI + DMI products were used on silage corn can be found in our 2019 Fungicide Test Summary and again in our 2020 Fungicide Test Summary.

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

Summary

As we approach the critical time to make decisions about in-season disease management on corn, it is important to consider all factors at play while trying to determine if a fungicide is right for your corn operation in 2021. Here is what you should consider:

1) Corn hybrid disease resistance score for NCLB and GLS (and perhaps tar spot too, if known) – Resistant hybrids may not have high levels of disease which impact yield.

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

3) Consider weather conditions prior to, and during, the VT-R2 growth stages – if weather is conducive for NCLB, GLS, and or tar spot then disease may continue to increase in corn and a fungicide application might be necessary. If it turns out to be hot and dry, disease development will stop and a fungicide application would not be needed.

4) Consider your costs to apply a fungicide and the price you can sell your corn grain – Will you preserve enough yield out of the fungicide application to cover its cost? The 2021 season is looking decent to break even on that investment, but this is not the norm.

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

6) Be aware that every time you use a fungicide you are likely selecting for corn pathogen populations that will become resistant to a future fungicide application – Make sure your fungicide application is worth this long-term risk. See fact sheet A3878 below for more information.

Other Resources

Impact of foliar fungicide timing and fungicide class on corn yield response in the United States and Ontario, Canada

Fact Sheet: A3878 – Fungicide Resistance Management in Corn, Soybeans, and Wheat in Wisconsin

VIDEO: Researching Fungicide Applications for Grain-Corn, Effect on Yield & Cost Recovery

References

Groves, C.L., Kleczewski, N.M., Telenko, D.E.P., Chilvers, M.I., and Smith, D.L. 2020. Phyllachora maydisascospore release and germination from overwintered corn residue. Plant Health Progress. https://doi.org/10.1094/PHP-10-19-0077-RS.

Munkvold, G.P. and White, D.G., editors. 2016. Compendium of Corn Diseases, Fourth Edition. APS Press.

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

Wise, K., Mueller, D., Sisson, A., Smith, D., Bradley, and Robertson, A., editors. 2016. A Farmer’s Guide to Corn Diseases. APS Press.

Wise, K.A. and Smith, D.L., Freije, A., Mueller, D.S., Kandel, Y., Allen, T., Bradley, C.A., Byamukama, E., Chilvers, M., Faske, T., Friskop, A., Hollier, C., Jackson-Ziems, Kelly, H., Kemerait, B., Price, P., Robertson, A., and Tenuta, A. 2019. Meta-analysis of yield response of foliar fungicide-treated hybrid corn in the United States and Ontario, Canada. PLoS ONE 14(6): e0217510. https://doi.org/ 10.1371/journal.pone.0217510.

Wisconsin Corn Tar Spot Update – July 29, 2020

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

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