Southern Rust of Corn in Wisconsin

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

Figure 1. Southern rust on corn. Photo Credit: Emmanuel Byamukama, South Dakota State University, Bugwood.org

If you are like me, you have been paying attention to reports from the southern and other Midwest states indicating that southern rust of corn is making its way further north this year, a first in a couple of years. We have been watching and have indeed confirmed southern rust on field corn in Columbia County Wisconsin this week. You can follow current southern rust updates on the iPiPE site. Now that southern rust has been confirmed in Wisconsin, it is time to NOT PANIC. We just need to be diligent in scouting over the next several weeks so that we can make, first, silage harvest decisions, then grain harvest decisions. Grain yield reductions are greatest if southern rust moves in prior to the “milk” (R3) growth stage in corn. Much of the crop in Wisconsin is nearly past this, so the good news is that impacts on grain yield should not be significant. However, there could be impacts on silage corn harvest as southern rust, like other foliar diseases, can dry the crop prematurely. See my previous article on how silage corn might be impacted by foliar disease. I also would like folks to know which fields southern rust has been a problem this year for grain corn, and prioritize those fields for harvest first, as standability might be an issue in some of these fields.

Southern rust is caused by the fungus Puccinia polysora. Symptoms of southern rust are different from common rust in that they are typically smaller in size and are often a brighter reddish/orange color (Fig. 1). 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 include high humidity and temperatures around 80F. However, very little free moisture is needed for infection to occur. Southern rust is typically a rare occurrence in Wisconsin. When it does occur, it is usually in the southern and south-western portions of the state, with epidemics initiating late in the season. With that said, southern rust has made it to south-central Wisconsin this year. 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 prior to the R3 growth stage. Therefore close monitoring of forecasts and scouting are needed to make timely in-season management decisions.

Management of Southern Rust

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 available for fungicides against southern rust on the Corn Fungicide Efficacy Table. As I said previously, should southern rust make its way to Wisconsin prior to the “milk” (R3) growth stage in corn, it could cause yield reductions. A helpful matrix can be found here to help make the fungicide application decision, in order to limit impacts by the southern rust fungus. 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.

 

Figure 2. Common rust on Corn. Photo Credit: Daren Mueller, Iowa State University, Bugwood.org

Common rust is caused by the fungus Puccinia sorghi and is extremely common in Wisconsin, but often results in little yield loss. Most field corn hybrids planted in Wisconsin are very resistant to the disease. Symptoms can include chlorotic flecks that eventually rise and break through the epidermis to produce pustules of brick-red spores (Fig. 2). Typically these pustules are sparsely clustered on the leaf. They can also appear on other parts of the plant including the husks and stalks. Management for common rust primarily focuses on using resistant hybrids. Remember resistance is not immunity, so some pustule development can be observed even on the most resistant hybrids. Some inbred corn lines and specialty corn can be highly susceptible to common rust. Under these circumstances a fungicide may be necessary to control common rust. Most of the hybrids I have scouted this season have some pustules, however incidence and severity is relatively low. Therefore, a fungicide application to control common rust wasn’t needed to manage common rust for most of these hybrids in Wisconsin.

Other Useful Resources about Rusts on Corn

WisCONTEXT Article on Southern Rust

Ohio State University Article on How to Differentiate Common Rust from Southern Rust

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

References

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

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

Disease and Mycotoxin Considerations for Corn Silage Harvest In Wisconsin

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

We are quickly approaching that time of year where we will see silage choppers working the 2024 corn crop. This means it is time to understand overall crop health and how diseases might be affecting the crop so that you can make the best silage product you can.

Overall, the corn crop in Wisconsin looks good. Yes, I can find some sort of disease in any field I visit, but I would say for the most part, most fields that were planted relatively on time, and didn’t experience early-season flooding, have a decent crop. With that said, we need to monitor the crop closely as we approach harvest as foliar disease issues can sneak up quickly as cool, wetter weather moves in near the early fall harvest.

Foliar diseases such as southern rust, tar spot, gray leaf spot (GLS), and northern corn leaf blight (NCLB) can be problematic for silage production. Not only do these diseases lead to a reduction in overall silage quality, they can force the plant to scavenge carbohydrates in the stalk which can result in standability issues and lodging. These diseases can also influence whole plant moisture making harvesting at optimal moisture difficult. If silage is harvested at sub-optimal moisture, then packing the bunker properly can be a challenge which can lead to slow fermentation and continued growth of aerobic organisms like fungi. This can indirectly lead to an increase in mycotoxins and “mold” issues from these aerobic fungi.

Tar spot and silage harvest

Tar spot of corn has been an issue on silage corn in Wisconsin since 2016. In fact, the first finding of tar spot was on corn for silage that year. Since 2016 the largest and most consistent impacts happen on corn for silage vs. corn for grain. Yes, tar spot can result in significant grain losses, but tar spot can also affect the overall plant in other ways such as loss in dry-matter yield (Fig. 1) and whole plant moisture contents well below optimal, making bunker-packing a significant issue. In recent work in Wisconsin, we see significant dry-matter yield reductions when tar spot severity on the ear leaf reaches over 10% at the time of chopping. This can result in as much as 15% reduction in yield, with these impacts dramatically increasing at severity levels of 20% or more on the ear leaf (Fig. 1). If tar spot (or any foliar disease) is moving quickly in your silage crop, you might consider chopping a bit earlier to reduce dry-matter yield losses and to try to optimize whole plant moisture, prioritizing good bunker management. If moisture at chopping is not prioritized, then subsequent storage issues such as mold and mycotoxin concerns can arise indirectly, due to poor bunker management. This brings us to Gibberella ear and stalk rot in silage corn.

Figure 1. Impact of tar spot severity on the ear leaf at harvest time compared to dry-matter yield in whole-plant chopped silage corn.

Gibberella and silage harvest

Gibberella ear rot is caused by Fusarium graminearum (a.k.a. Gibberella zea). The same pathogen can cause Gibberella crown and stalk rot in corn. We have observed both diseases on silage corn in Wisconsin in recent years. Our environment here in the Great Lakes region makes a perfect place for this pathogen to cause these diseases. In addition to the damage that the pathogen can cause, the fungus can also produce various mycotoxins, most importantly deoxynivalenol (DON or Vomitoxin). Our laboratory has conducted quite a bit of research recently trying to understand where in the plant DON accumulates. In detached plant part experiments, we have noted that DON can accumulate in both the stalk and ear portions of the plant, AND that these two phases of accumulation are not linked to each other (Chibuogwu et al., 2024). The fungus can infect these parts separately at different times during the season and the subsequent accumulation of DON can happen differentially in the stalks vs. the ears. This is partially why you can go out to the field and scout for ear rot and not see a lot of infection (moldy ears), but still have high DON levels at chopping time. Some of that DON is likely accumulating in the stalks.

We have also been following the fate of DON in silage harvested and chopped from a brown midrib (BMR) hybrid and a dual-purpose hybrid that were grown in the field and treated with fungicides at white silk (R1). We chopped the plants in each plot and then used mini-silos (polyethylene bags vacuum-sealed using a commercial grade vacuum packer) to conduct a time-course experiment following DON levels in the mini-silos (Chibuogwu et al. 2025). In all cases we saw DON levels generally increasing in the first 30 days after chopping (Fig. 2). They then leveled off and became stable at 60, 90, and 120 days after chopping. Some of this increase could be due to oxygen still in the system during the first 30 days after chopping. DON-producing fungi are aerobic and continue to consume some of the minute levels of oxygen still in the system, thereby still producing DON. However, this likely only explains some of the DON levels we detected.

Figure 2. DON concentration of chopped field-grown silage corn over time of ensiling.

There are also “masked” or conjugated forms of DON that are detectable in routine DON analyses. One such conjugate is DON-3-glucoside (D3G). D3G can be produced by fungi or during a plant’s attempt to protect itself from the toxicity of DON. Either way, D3G is not detectable in routine test and must be tested for specifically. We investigated our samples further during the first 30 days of ensiling and found that the level of D3G at harvest, explained a significant level of the DON recorded in samples after 30 days of ensiling. This is to say that D3G present at harvest, is likely metabolized in the first 30 days of ensiling releasing DON and resulting in higher DON levels 30-days later (Fig 3). Again, this relationship only partially explains why DON increases in silage during the first 30 days of storage. The full explanation is likely due to both metabolization of D3G and continued fungal respiration leading to an increase in DON at feed out compared to when it was packed in the bunker.

Figure 3. Relationship of DON-3-Glucoside (log) at harvest and DON concentration after 30 days of ensiling.

The Take Home

So, what are we to do with all of this information? Well, knowledge is power. You need to balance foliar disease management with Gibberella/DON management when making high-quality silage in Wisconsin. Diseases like tar spot are the new normal. As you prepare to harvest, it is a good idea to get out in the field and see how bad the foliar disease is and how much ear rot you are seeing. You will want to prioritize harvest on fields showing more disease. In fields where there are high levels of tar spot, southern rust, or NCLB, monitor moisture carefully and try to chop to optimize moisture. Concentrate on good bunker hygiene and spend time packing the material as best you can, focusing on getting as much oxygen out of the system as you can. If a corn crop becomes too dry to make good silage, you might consider harvesting it for high-moisture grain to try to circumvent bigger issues that could arise at feed out, by making less than ideal silage. Finally, it is important to test for DON frequently and understand what you are dealing with. You want to start with the lowest levels of DON coming from the field that you can. DON will likely increase in the bunker no matter how well you pack it. Thus, starting with the lowest levels at harvest will help keep final levels of DON below critical thresholds. Moving forward, mycotoxin testing in corn should include not only DON but also for conjugates of DON that can be metabolized back to DON and increase the final DON concentration during ensiling.

Have a safe and productive silage harvest season!

Citations

Chibuogwu, M.O., Groves, C.L., Mueller, B., and Smith, D.L. 2024. Effects of fungicide application and corn hybrid class on the presence of Fusarium graminearum and the concentration of deoxynivalenol in ear and stalk parts of corn (Zea mays) used for silage. Plant Disease. https://doi.org/10.1094/PDIS-12-23-2662-RE.

Chibuogwu, M.O., Reed, H., Groves, C.L., Mueller, B., Barrett-Wilt, G., Webster, R.W., Goeser, J., and Smith, D.L. 2025. Influence of hybrid class and ensiling duration on deoxynivalenol accumulation and its derivative deoxynivalenol-3-glucoside while ensiling corn for silage. Plant Disease. https://doi.org/10.1094/PDIS-06-24-1166-RE.

Wisconsin Field Crop Disease Update – August 10, 2024

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

Tar Spot of Corn

Figure 1. Statewide tar spot risk as calculated by the Field Prophet tool for August 10, 2024.

You can find the most recent updates on tar spot confirmations across the U.S. here: https://corn.ipmpipe.org/tarspot/. The Field Prophet tool is also showing mostly moderate risk across the southern two-thirds of Wisconsin, with just the northern tier with high risk for tar spot development at this point in the season (Fig. 1). This means that the weather is less conducive for tar spot development and spread. We are also quickly approaching the end of the in-season management window for applying fungicides in corn for tar spot control. For the major foliar diseases of corn, including tar spot, the optimal window to apply fungicides and maximize return on investment, is between the VT/R1 and R3 growth stages. If your crop has reached the milk growth stage and you are still not seeing any disease and the Tarspotter risk isn’t high, you can hold off on the fungicide application. The likelihood of a yield-limiting tar spot epidemic is low in this situation. You can learn more about fungicide application for tar spot control here.

White Mold of Soybeans

Figure 2. Statewide white mold risk as calculated by the Field Prophet tool for August 10, 2024.

The risk for white mold according to Field Prophet is a bit higher compared to tar spot. Risk is generally high across the state with several pockets of low or medium risk. This means that weather has been conducive for the development of the mushroom-like structure (apothecia) of the white mold fungus that gives rise to spores that infect soybean. These risk estimates appear to be accurate as we continue to find apothecia in fields with a history of white mold (Figure 3). With that said, the soybean crop is quickly approaching the R4 growth stage or beyond, where the opportunity of infection by the white mold fungus does not exist. Thus, there is not a need to spray for white mold beyond the R4 growth stage. Similar to tar spot, the optimal timing of fungicide application ranges from the R1 growth stage to the R3 growth stage in soybeans. If you no longer see flowers on soybeans, then the risk for infection by the white mold fungus is low, despite conducive weather for the fungus.

Use Predictive Tools like a “Crystal Ball”

We continue to get questions about how the apps (Tarspotter, Sporecaster, and Field Prophet) should be used to make decisions about in-season disease management. We have designed these tools to be predictive. This means that the risk indexes you see are telling you that in the next week or two, you might see these diseases pop up. We developed these tools to act like a “crystal ball” so that you can look ahead and apply fungicides in a preventative manner. We know that fungicides work best when applied preventatively. In addition for diseases like tar spot, the fungus that causes this disease has a very long incubation period (time from infection to when you actually see tar spots). This time period can be as long as 30 days. So a high risk today, might indicate seeing tar spot show up even a month from now. Be sure to use these tools to look into the future and to prepare yourself, rather than being reactionary.

Figure 3. Apothecia of the white mold fungus at the base of a soybean stem.

Other Diseases to Watch

Figure 4: Northern corn leaf blight (NCLB) lesion on a corn leaf.

We are starting to find more and more northern corn leaf blight (NCLB; Figure 4) in some fields in Wisconsin. This makes sense as the weather recently has been cooler and wetter. Luckily most of the fungicides used for tar spot also have good efficacy against NCLB. Thus if you sprayed for tar spot, you have probably controlled NCLB too. For those with silage corn, especially brown mid-rib (BMR) hybrids, continue to scout your fields for NCLB. Some BMR hybrids are very susceptible to this disease. If NCLB begins to move quickly and you are approaching your harvest window, you might consider chopping a bit earlier in order to reduce damage by NCLB.

We also continue to watch the frogeye leaf spot situation in soybeans. Our beta-testing models for frogeye leaf spot have shown relatively low risk for this disease in Wisconsin this season. We also have not found any frogeye leaf spot to date. However, the risk is creeping up in southern Wisconsin. The window to treat for this disease ends around R5. Thus, I believe most of the soybeans in Wisconsin will escape frogeye leaf spot without the need to apply a fungicide this season. However, keep an eye on your fields and continue to scout to stay informed of the situation.

Mid-Season Corn and Soybean Disease Update and New Corn Fungicide ROI Calculator

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

Tar Spot and Corn

Figure 1. Statewide tar spot risk as calculated by the Field Prophet tool for July 22, 2024.

You can find the most recent updates on tar spot confirmations across the U.S. here: https://corn.ipmpipe.org/tarspot/. The Field Prophet tool is also showing mostly moderate to high risk across the state of Wisconsin (Fig. 1). This means that if your crop is between VT/R1 and R3 you should be actively scouting for tar spot and making the decision to apply a fungicide at this time. Our research has shown that one well-timed application of fungicide somewhere between VT/R1 – R3 will control tar spot enough for a yield response even in a heavy-pressure year. You can learn more about managing tar spot BY CLICKING HERE. If you think you found tar spot I would appreciate if you would let us know. We can enter the county level data into the Corn IPMPipe Map and contribute to the cause.

Gray Leaf Spot and Corn

We are also watching the gray leaf spot (GLS) situation closely. The southern and southwestern portions of the state have had enough heat and moisture that risk is likely high for this disease in those locations. For the rest of the state, risk isn’t as high, but likely moderate risk exists. Fortunately, the same fungicides that work well in controlling tar spot, also work well against GLS. Thus, another reason that a fungicide application may need to be considered at this point in the season.

Corn Fungicide ROI Calculator

With the tight margins in the corn market, and a high risk for disease due to wet weather, I have been getting a lot of questions about fungicide return on investment (ROI) this season. Fortunately, the scientists involved in the Crop Protection Network have been working on amassing fungicide performance and ROI data over the last few seasons and have launched a new interactive Fungicide ROI tool. The tool is research-based using data from across the entire U.S. corn production belt, including Wisconsin. During the studies included in this dataset, the diseases of interest were tar spot and also southern rust. Both are again diseases of interest this season. The tool can be used to run various scenarios that fit your farm and situation in the 2024 growing season. You can change the expected end-of-season disease severity levels, and the pricing of products included in the tool. Not all products labeled for corn can be found there, but most of the popular ones are included. You can also adjust the sale price for your crop, and expected farm yield, so that you can get real estimates of ROI and probability of breaking even for your situation. An example scenario can be found in figure 2 where I adjusted the tool to 180 bu/a yield and a corn sale price of $4 per bushel with expected high levels of tar spot or southern rust. You can see that several products result in a positive net benefit per acre using default product pricing. However, the tight margins this year, do result in lower breakeven probabilities. I would suggest running the tool for your situation. Be honest with yourself and put real numbers in the tool. This tool may be helpful for you if you are on the fence about spraying a fungicide this year. If you have a known resistant hybrid, your crop is already through to R3, and no disease has shown up yet, you might be able to help your financial situation by not spraying this season.

White Mold and Soybeans

Figure 3. Statewide white mold risk as calculated by the Field Prophet tool for July 22, 2024.

The risk for white mold according to Field Prophet is a bit spottier compared to tar spot. Mostly the northern and central portions of the state are at high risk with some pockets of moderate risk. The southeastern portion of Wisconsin is at low risk currently (Fig. 3). This is due to hotter temperatures several weeks back. Regardless of location, our models are telling us that the risk for white mold will continue to rise across the state over the next 7 to 10 days. If you are in a low-risk area and you are at R3 or beyond, you might not have much to worry about for this year when it comes to white mold. However, if you are in a moderate-risk zone, watch this situation carefully. If you are at R3 and the crop has good canopy, you might consider one late R3 application. If you are in a high-risk zone, the crop has canopied, and your soybean crop is in the bloom period, it is time to think about a fungicide application. These will be the areas I would expect to find white mold 1-3 weeks from now. If you would like to learn more about white mold management, check out my previous article HERE.

As always, get out and look at the crop. Scout, scout, scout!

It’s that time of year again: What to do about white mold of soybean?

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

Shawn Conley, Extension Soybean and Small Grains Agronomist, Department of Plant and Agroecosystem Sciences, University of Wisconsin-Madison

It is time for my annual reminder about white mold in soybeans, and its management. The 2023 season in Wisconsin was all about drought and heat. So far, the 2024 season has been on the opposite end of the spectrum. Don’t be complacent about managing white mold this year.

Figure 1. Apothecia, a small mushroom-like structure of the white mold fungus that give rise to spores, which infect soybean flowers.

Remember that the white mold fungus infects soybeans through open and senescing flowers, by spores that are born from small mushroom-like structures called apothecia (Fig. 1). Remember that if the bloom period gets extended due to cool weather, this can lead to an extended window for infection by the fungus. Often cool weather is a double whammy as it is good for the white mold fungus and slows down soybean crop development, thereby extending bloom. This could be at play this season, stay on top of your game!

Given the moderate temperatures and moisture we have been getting the risk for white mold is currently moderate to high for the central to northern tiers of the state, while the far southern tier is at generally low risk (Fig. 2).

Predicting White Mold 

The flowering growth stages are a critical time to manage white mold in-season. You can view a fact sheet and video on the subject. As you probably know, timing in-season fungicide sprays at the correct time during the soybean bloom period can be extremely difficult. To help solve this decision-making issue, models were developed at the University of Wisconsin-Madison in conjunction with Michigan State University and Iowa State University to identify at-risk regions which have been experiencing weather favorable for the development of white mold apothecia. These models predict when apothecia will be present in the field using combinations of 30-day averages of maximum temperature, relative humidity, and wind speed. Using virtually available weather data, predictions can be made in most soybean growing regions. To facilitate precise predictions and make the model user-friendly, we use the Sporecaster smartphone application for Android and iPhone and also the Field Prophet app for iPhones.

Figure 2. Map of white mold risk generated in the Field Prophet app for Wisconsin, July 8, 2024.

The purpose of the smartphone app is to assist farmers in making early season management decisions for white mold in soybean. The best time to spray fungicides for white mold is during flowering (R1 and R3 growth stages) when apothecia are present on the soil surface. If you have trouble growth-staging soybeans, here is a helpful guide on correctly identifying soybean growth stages.

University research has indicated that the appearance of apothecia can be predicted using weather data and a threshold of percent soybean canopy row closure in a field. Based on these predictions and crop phenology, site-specific risk values are generated for three scenarios (non-irrigated soybeans, soybeans planted on 15″ row-spacing and irrigated, or soybeans planted on 30″ row-spacing and irrigated). Though not specifically tested we would expect row-spacings of 22 inches or less to have a similar probability response to fungicide as the 15-inch row-spacing.

The Sclerotinia apothecial models that underlie the Sporecaster prediction tool have undergone significant validation in both small test plots and in commercial production fields. In 2017, efficacy trials were conducted at agricultural research stations in Iowa, Michigan, and Wisconsin to identify fungicide application programs and thresholds for model implementation. Additionally, apothecial scouting and disease monitoring were conducted in a total of 60 commercial farmer fields in Michigan, Nebraska, and Wisconsin between 2016 and 2017 to evaluate model accuracy across the growing region. Across all irrigated and non-irrigated locations predictions during the soybean flowering period (R1 to early R4 growth stages) were found to explain end-of-season disease observations with an accuracy of 81.8% using the established probability thresholds now programmed in the app. We have made additional improvements for 2024, to further refine accuracy. So if you have used Sporecaster before, you might want to check the version in the “Help and Info” button to be sure you have version 1.41.3 of Sporecaster.

We also know that for highly susceptible soybean varieties, the action threshold should be adjusted down. Research in the upper Mid-west demonstrated that for most soybean varieties the default action threshold depicted in the tool when you set up a field is accurate. However, some varieties are highly susceptible and the action threshold should be moved down from 40% to 20% for varieties that are known to be highly susceptible. this can improve accuracy of the tool and recommendation for fungicide application.

Not only can users run predictions of risk during the soybean bloom period for any field, but you can also set up visual maps to look at multiple sites simultaneously. An example for the state of Wisconsin can be found in figure 2, which represents risk for July 8, 2024, for non-irrigated soybeans. Currently, if soybeans are flowering, risk is moderate-to-high in much of Wisconsin for non-irrigated soybeans. I personally like the Field Prophet version of the prediction tool a bit better as it provides a 7-day historical trend of risk and also provides a 7-day true forecast. The historical trend lines can be viewed on the fly in your field list (Fig. 3). If the trend is moderate, but the slope of the line is increasing, then you might want to keep a close eye on the white mold risk. We find that the trends are much more informative for decision-making vs. just the daily instantaneous risk level.

Figure 3. 7-day historical white mold risk trend lines as depicted in the Field Prophet app on July 8, 2024.

What to Spray for White Mold?

If you have decided to spray soybeans for white mold, what are the best products to use? Over the last several years we have run numerous fungicide efficacy trials in Wisconsin and in conjunction with researchers in other states. In Wisconsin, we have observed that Endura applied as a single application at 8 oz at the R3 growth stage performs well. The window to spray runs from R1 to R3, but our recent data suggests waiting a bit into that window improves efficacy of the fungicide application vs. spraying at R1. If you do choose to spray at the R3 growth stage, be sure to focus on getting good canopy penetration with your fungicide spray. Soybean canopies at R3 can be dense and hauling more water, slowing your sprayer speed, and increasing spray pressure can all help improve spray penetration in those dense canopies. Other fungicide options also include Omega and Proline. You can view results of past fungicide evaluations for Wisconsin by CLICKING HERE. If you would like to run tailored estimations of return on investment for various fungicide programs, you can use another smartphone application called Sporebuster.

What is Sporebuster?

When a fungicide application is needed to control white mold in soybeans, Sporebuster can help determine a profitable program. You enter your expected soybean price, expected yield, and treatment cost. Sporebuster instantly compares ten different treatment plans at once to determine average net gain and breakeven probability of each. You can mark, save and share by email, the best plans for your farming operation.

The purpose of Sporebuster is to assist soybean farmers in making a fungicide program decision that is profitable for their operation. Sporebuster is meant to complement Sporecaster. Once Sporecaster recommends a fungicide application, Sporebuster can be used to determine a profitable program.

Information that drives Sporebuster is based on research from 2009-2016 from across the upper Midwestern US. Statistical models were developed based on these data that included white mold pressure and yield response from fungicide for 10 common fungicide programs. Due to recent changes in the markets, be sure to manually adjust the cost of the product you are interested in, so that the tool returns an accurate estimate of return on investment. My advice is to call local suppliers and see what products you can get and what the per acre cost will be to get the application done. Details about the research behind Sporebuster can be found by CLICKING HERE.

Helpful Smartphone Application Links

Sporecaster

  1. Click here to download the Android version of Sporecaster.
  2. Click here to download the iPhone version of Sporecaster.
  3. Click here to download the Field Prophet version of Sporecaster for the iPhone.

Sporebuster

  1. Click here to download the Android version of Sporebuster.
  2. Click here to download the iPhone version of Sporebuster.
  3. Here is a video on how to use Sporebuster and interpret the output.

Other Resources

  1. To watch an in-depth video on white mold management, CLICK HERE.
  2. To find more information on white mold, view a web book from the Crop Protection Network, CLICK HERE.

Scientific References

  1. Willbur, J.F., Fall, M.L., Blackwell, T., Bloomingdale, C.A., Byrne, A.M., Chapman, S.A., Holtz, D., Isard, S.A., Magarey, R.D., McCaghey, M., Mueller, B.D., Russo, J.M., Schlegel, J., Young, M., Chilvers, M.I., Mueller, D.S., and Smith, D.L. Weather-based models for assessing the risk of Sclerotinia sclerotiorum apothecial presence in soybean (Glycine max) fields. Plant Disease. https://doi.org/10.1094/PDIS-04-17-0504-RE
  2. Willbur, J.F., Fall, M.L., Byrne, A.M., Chapman, S.A., McCaghey, M.M., Mueller, B.D., Schmidt, R., Chilvers, M.I., Mueller, D.S., Kabbage, M., Giesler, L.J., Conley, S.P., and Smith, D.L. Validating Sclerotinia sclerotiorum apothecial models to predict Sclerotinia stem rot in soybean (Glycine max) fields. Plant Disease. https://doi.org/10.1094/PDIS-02-18-0245-RE.
  3. Fall, M., Willbur, J., Smith, D.L., Byrne, A., and Chilvers, M. 2018. Spatiotemporal distribution pattern of Sclerotinia sclerotiorum apothecia is modulated by canopy closure and soil temperature in an irrigated soybean field. Phytopathology. https://doi.org/10.1094/PDIS-11-17-1821-RE.
  4. Willbur, J.F., Mitchell, P.D., Fall, M.L., Byrne, A.M., Chapman, S.A., Floyd, C.M., Bradley, C.A., Ames, K.A., Chilvers, M.I., Kleczewski, N.M., Malvick, D.K., Mueller, B.D., Mueller, D.S., Kabbage, M., Conley, S.P., and Smith, D.L. 2019. Meta-analytic and economic approaches for evaluation of pesticide impact on Sclerotinia stem rot control and soybean yield in the North Central U.S. Phytopathology. https://doi.org/10.1094/PHYTO-08-18-0289-R.
  5. Webster, R.W., Mueller, B., Conley, S.P., and Smith, D.L. 2023. Integration of soybean (Glycine max) resistance levels to Sclerotinia stem rot into predictive Sclerotinia sclerotiorum apothecial models. Plant Disease. https://doi.org/10.1094/PDIS-12-22-2875-RE.

Whoomp! There it is! What to do about Tar Spot of Corn in 2024

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

Over the last several weeks we have seen confirmed positives for tar spot in parts of Iowa, Missouri, Kansas, Nebraska, Indiana, Illinois, Michigan, and now Wisconsin (Pepin Co.; Fig. 1). While it has been found in Wisconsin, the severity and incidence are extremely low and does not necessitate spraying fungicide at the moment! So, what should we do now?

My advice is to get prepared and make sure you have the tools in place to deal with this problem. As I said the last few seasons, 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 spot. 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. 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.

So what weather is conducive for tar spot development? Yes, precipitation is helpful, but more importantly, we need intermittent wet/dry cycles to give us intermittent 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 which is an influential variable too. These variables are measured over the last 14 days and 30 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. If you would like to learn more about the “nuts and bolts” that run behind the smartphone apps, you can find our research publication HERE.

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

Figure 2. Tar spot severity diagram indicating various levels of tar spot on corn leaves. Yield loss isn’t typically detectable in the field until severity reaches 10% or more on the ear leaf or leaves above this leaf.

My short answer is no! The disease is just getting started. If you find it in Wisconsin right now, it will be at low severity 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. Figure 2 shows various severity levels on a corn leaf. We don’t start to see yield loss until we reach about 10% severity on the ear leaves or above. Thus, you have time! Target fields 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 especially if the weather becomes increasingly conducive (think wet/dry cycles!) and/or your scouting indicates severity is increasing.

When should I spray fungicide? What should I use?

When making decisions on using a fungicide for tar spot management keep in mind that fungicide active ingredients are important. Products with multiple fungicide classes are preferred (QoI + DMI or QoI + DMI + SDHI). Products with multiple fungicide classes tend to provide better efficacy and delay the development of fungicide resistance. See the CPN Fungicide Efficacy Guide for specific products and their ratings for tar spot and other diseases.

Application timing is very important for tar spot management. It is best to use scouting and/or tar spot risk or profit tools like Tarspotter and Field Prophet to make informed decisions about when to apply fungicides for tar spot management. These apps use weather data to determine if the environmental conditions are favorable for tar spot to develop, and consequently optimize fungicide application timing.

In most years, a fungicide application will not be needed prior to the V10 growth stage. In most years, one well-timed (VT-R3 growth stages) fungicide will be sufficient to manage tar spot. Even in years where two applications appear to improve tar spot control, improved ROI is marginal over a single well-timed application.

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

Fusarium Head Blight in Wisconsin Winter Wheat: A Guide for Harvest Preparation

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

Shawn Conley, Extension Soybean and Small Grains Agronomist, Department of Plant and Agroecosystem Sciences, University of Wisconsin-Madison

This season, we’ve observed moderate to high levels of Fusarium head blight (FHB), also known as scab, in some Wisconsin winter wheat fields. The incidence and severity have varied based on location, the susceptibility of the wheat variety, and whether a fungicide was applied at or shortly after anthesis.

FHB has been more prevalent in the southern and south-central wheat-growing areas of the state. However, it can also be found in other areas, depending on the susceptibility of the wheat varieties grown. As you prepare for harvest, it’s crucial to scout your maturing wheat crop and estimate the potential damage from FHB.

FHB can cause direct yield loss, and the fungus that causes this disease can also produce deoxynivalenol (DON), also known as vomitoxin.

Identifying FHB

Figure 1. Fusarium head blight of winter wheat

In non-mature winter wheat or spring wheat fields, diseased spikelets on an infected grain head die and bleach prematurely, while healthy spikelets on the same head retain their normal green color (Fig. 1). Over time, premature bleaching of spikelets may progress throughout the entire grain head. If infections occur on the stem immediately below the head, the entire head may die. As symptoms progress, developing grains are colonized, causing them to shrink and wrinkle. Infected kernels often have a rough, sunken appearance and range in color from pink or soft gray to light brown. As wheat dries down, visual inspection of heads for scab will become more difficult.

Why FHB Identification is Important

FHB identification is important because it not only reduces yield but also decreases the quality and feeding value of grain. The FHB fungus may produce mycotoxins, including DON or vomitoxin, which can adversely affect livestock and human health when ingested.

The U.S. Food and Drug Administration has set maximum allowable levels of DON in feed for various animal systems. For beef and feedlot cattle and poultry, the limit is less than 10 ppm; for swine and all other animals, it’s less than 5 ppm. However, many professional animal nutritionists agree that in ruminating cattle, the level of DON in the total ration should be below 1 ppm.

Local grain elevators test for DON and discount loads of grain for unacceptable levels of the mycotoxin. Be sure to check with your local elevator about their thresholds for docking grain and discount schedule based on the level of DON detected BEFORE you bring a load for delivery.

Lodged Wheat and DON

We’ve noted many fields with high levels of lodging in Wisconsin this season. Research has shown that DON levels are significantly higher in lodged wheat compared to standing wheat. The longer wheat is lodged, the more DON accumulates. If you suspect that DON is a concern in your field and there is significant lodging, care should be taken during harvest to test grain for DON.

Preparing for Wheat Harvest

  1. Adjust combine settings to blow out lighter seeds and chaff. Research has shown that adjusting a combine’s fan speed from 1,375 to 1,475 rpms (100 rpms above standard configuration) and shutter opening to 90 mm (3.5 inches; 20 mm wider than the standard configuration) resulted in the lowest discounts at the elevator due to low test weight, percentage of damaged kernels, and level of the mycotoxin deoxynivalenol (DON; vomitoxin) present in the harvested grain. This strategy should be used only for fields expected to have high levels of scab, as harvested yield can also be reduced in field with normal kernels due to the higher air flow.
  2. Take special care when harvesting fields that are lodged. Remember that higher levels of DON are likely in fields with lodged wheat. Be sure to test grain so you know what the DON concentration is before taking the crop to the elevator.
  3. Know your elevator’s inspection and dockage procedure and discount schedule (each elevator can have a different procedure and discount rate).
  4. Scabby kernels do not necessarily mean high DON levels and vice versa. For example, in a 2014 fungicide evaluation, very low visible levels of FHB were observed for all treatments. However, when the finished grain was tested for DON, significant levels were identified for all treatments. Be sure to test and know what levels of DON are in your grain even if you didn’t see a high level of visible disease. Also, don’t assume that because a fungicide was used, there will be no DON.
  5. DON can be present in the straw, so there is concern regarding feeding or using scab-infected wheat straw. DO NOT use straw for bedding or feed from fields with high levels of scab. If in doubt, have the straw tested for DON levels.
  6. Do not save seed from a scab-infected field. Fusarium graminearum can be transmitted via seed. Infected seeds will have decreased growth and tillering capacity as well as increased risk for winterkill.
  7. Do not store grain from fields with high levels of scab. DON and other mycotoxins can continue to increase in stored grain.
  8. Harvest in a timely fashion to minimize elevator discounts and balance dockage due to FHB. Click here to read about some recent research on optimizing harvest timing in winter wheat.

For more information on Fusarium head blight research, click here.

References

  1. Bissonnette, K.M., Kolb, F.L., Ames, K.A., and Bradley, C.A. Effect of Fusarium head blight management practices on mycotoxin accumulation of wheat straw. Plant Dis. 102:1141-1147.
  2. Cowger, C., and Arellano, C. 2013. Fusarium graminearum infection and deoxynivalenol concentrations during development of wheat spikes. Phytopathology 103:460-471.
  3. Nakajima, T., Yoshida, M, and Tomimura, K. 2008. Effect of lodging on the level of mycotoxins in wheat, barley, and rice infected with Fusarium graminiearum species complex. J. Gen. Plant Pathol. 74:289-295.
  4. Salgado, J. D., Wallhead, M., Madden, L. V., and Paul, P. A. 2011. Grain harvesting strategies to minimize grain quality losses due to Fusarium head blight in wheat. Plant Dis. 95:1448-1457.

Wisconsin Winter Wheat Disease Update – May 24, 2024

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

Shawn Conley, Extension Soybean and Small Grains Agronomist, Department of Plant and Agroecosystem Sciences, University of Wisconsin-Madison

Figure 1. Stripe rust on a flag leaf of winter wheat.

Winter wheat in Wisconsin continues to move through growth stage ahead of average. This week I have seen many wheat heads emerged and anthers out. We have also confirmed stripe rust (Fig. 1) in Columbia and Dodge Counties. Now is the time to make a decision on a fungicide application in winter wheat.

In my previous post I mentioned the importance of balancing both stripe rust and Fusarium head blight (FHB) as we moved into the next several weeks. Now that anthers are emerging on wheat heads, the time is right to get a fungicide application out to control FHB. These fungicides will also control stripe rust. You can find an excellent list of fungicides that will control both FHB and stripe rust HERE.

Given the current stripe rust movement into the state (Fig. 2) and potential risk for FHB to increase (Fig. 3), I think a fungicide should be strongly considered to protect yield in winter wheat this season. I realize that the Fusarium risk tool is still showing much of Wisconsin in a low risk of FHB. However, with stripe rust moving in at this point and the rainy weather, choosing an FHB fungicide to apply now at anthesis is a wise decision in my opinion. The optimal timing of application of fungicide at this point in the season is the start of anthesis (50% of heads showing at least one anther) to 7 days after the start of anthesis. This will maximize control of FHB while also providing control of stripe rust and the mycotoxin deoxynivalenol (DON).

Figure 2. U.S. counties confirmed to have stripe rust on wheat – May 24, 2024.

 

Figure 3. Risk of Fusarium head blight based on the Fusarium risk tool – May 24, 2024.

As always, be sure to get out in the field and scout, scout, scout! Making an informed decision is key to success!

Wisconsin Winter Wheat Disease Update – May 16, 2024

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

Shawn Conley, Extension Soybean and Small Grains Agronomist, Department of Plant and Agroecosystem Sciences, University of Wisconsin-Madison

Winter wheat in Wisconsin continues to move through growth stages at record pace. We are about 10-14 days ahead on growth stages compared to this time in most years in Wisconsin. The warm spring and timely rain has pushed wheat very quickly.

As I mentioned last week, we continue to monitor the stripe rust situation. This week brings us a confirmed stripe rust positive in Tippecanoe Co. Indiana (Fig. 1). As I mentioned in my previous article, the likelihood is high that we will see stripe rust in Wisconsin this season. We have continued to scout for this disease and visited several variety and research location this week in southern Wisconsin. We have not found stripe rust yet. This doesn’t mean that it isn’t here. I still encourage you to scout and let us know if you find it or get it confirmed by our Plant Disease Diagnostic Clinic.

With the rapid growth stage changes happening, we are quickly approaching the time in the season that we need to be aware of risk and in-season management decisions for Fusarium head blight. Fusarium head blight (FHB) has typically been a more frequently occurring issue in Wisconsin. However, in recent years, our spring seasons have been exceptionally hot and dry leading to little disease. However, this season is different with moderate temperatures and adequate precipitation to make FHB an issue. Not only is the disease yield limiting, but the fungus that causes FHB can also produce the mycotoxin called deoxynivalenol (DON or vomitoxin). DON contamination above 2 ppm in finished grain can often lead to discounts at the elevator or outright rejection. Thus, this disease is worth managing.

Fortunately, we have some excellent in-season management options for FHB. Be sure you know the relative susceptibility of the varieties you have planted. We have excellent data showing significant reductions of FHB where we use a resistant variety and then layer a fungicide application on top. Varietal resistance works!

When it comes to fungicides for FHB, there are really just five products to choose from that are rated as “Good” on the “Fungicide Efficacy for Control of Wheat Diseases” table. Timing is everything when using a fungicide for FHB management. Be sure to time applications at the start of anthesis or within 5-7 days after the start. This is the ideal window of opportunity to control FHB and reduce DON levels in the finished grain. Spraying earlier than anthesis or later than about a week after the start of anthesis will result in lost efficacy, or no control of FHB. If you need help with growth staging, be sure to check out the “Visual Guide to Winter Wheat Development and Growth Staging.” Also, the fungicides rated “G” for FHB in the fungicide efficacy table are effective against stripe rust, so if that disease happens to move in now or later, a single application of fungicide at the anthesis timing should take care of both problems.

There is a disease prediction tool for FHB of wheat. You can find that tool at http://www.wheatscab.psu.edu. This tool should be monitored frequently as your crop approaches anthesis and soon after. It can help you determine if your crop is at risk, based on the weather conditions. Risk as of May 16, 2024 for FHB-suscpetible winter wheat varieties is currently estimated to be low (Fig. 2). However, given the 7-day forecast of rain and warm temperatures, I would suspect this to change to be more favorable for FHB risk in the coming 7-10 days. If you haven’t applied a fungicide yet this season, I would urge you to consider one, well-timed application targeting FHB this season.

The ‘Take Home’ for wheat management over the next several weeks.

  1. Plan to apply an FHB fungicide application – especially on susceptible varieties
  2. Shoot for Anthesis or up to 5-days after the start of anthesis for the fungicides rated “G” for FHB in this table.
  3. All of the available fungicides rated for FHB also are effective against stripe rust. Thus, one fungicide can manage both problems!
  4. You can go slightly earlier (Feekes 10.5; Efficacy slightly reduced compared to typical timing) up to 5-days after the start of anthesis for Miravis Ace to manage FHB.
  5. Watch the “Scab Alerts” – it isn’t perfect, but can help you make a decision (http://www.wheatscab.psu.edu).

Stripe Rust in Wisconsin in 2024?

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

Shawn Conley, Extension Soybean and Small Grains Agronomist, Department of Plant and Agroecosystem Sciences, University of Wisconsin-Madison

The last several winter  wheat seasons in Wisconsin have been very quiet when it comes to disease. Hot and dry weather has meant that while the inoculum for some pathogens might be in Wisconsin, we haven’t seen any epidemics of disease that needed active in-season management. That could change in 2024 as we watch the stripe rust situation in the south and mid-south.

The current stripe rust Ag Pest Monitor shows numerous counties in Louisiana, Arkansas, and Kansas positive for the disease (Fig. 1). Most recently, Cumberland County Illinois was also found to be positive. This latest positive is the earliest we have seen stripe rust move up this far in the “rust pathway” in a few years. Given that the winter wheat crop in Wisconsin is not yet to flag leaf growth stages, we need to watch the progression of this disease carefully. We are about to come into a critical time of the season, that if we have active stripe rust, we will need to supply in-season management.

Stripe rust of wheat is caused by the fungus Puccinia striiformis. Stripe rust can be identified by orange/yellow pustules that typically occur in a striped pattern on the surface of the wheat leaf. However, under low severity, single, or very few sparsely spaced pustules may be observed. Subsequent infections can arise from a single pustule. Disease is favored by prolonged periods of rain (or dew), high relative humidity, and cool temperatures ranging from 50 to 60 ºF.

Management of stripe rust includes using resistant cultivars and applying fungicides. Although it is too late to make decisions on a cultivar, scouting should be prioritized to fields where you know there was a susceptible cultivar planted. Considering the early start to the stripe rust epidemic to our south, careful and frequent scouting will be critical this season. If stripe rust pustules are observed, consider sending samples to the University of Wisconsin Plant Disease Diagnostic Clinic for positive identification. If stripe rust is confirmed and it appears to be active, a fungicide application might be necessary.

In recent years in Wisconsin, we have not needed to apply a fungicide before the Fusarium head blight timing of Feekes 10.5.1. However, in years when stripe rust starts early, research has demonstrated that an application at the flag leaf emergence timing (Feekes 8) helps to protect grain yield. For more information on growth-staging wheat, check out the “Visual Guide to Winter Wheat Development and Growth Staging.”

In our work titled “Wheat grain and straw yield, grain quality, and disease benefits associated with increased management intensity” we found that years with intensive stripe rust epidemics (2016 and 2017) a fungicide application at Feekes 8, in addition to a second application of fungicide at Feekes 10.5.1, helped to protect yield at the end of the season. In years where there was no stripe rust, a Feekes 8 application of fungicide was not needed, but an application at Feekes 10.5.1 almost always provided a positive return on investment.

If you find stripe rust and are considering an application of fungicide at Feekes 8, you have lots of options of products. Be sure to consult the “Fungicide Efficacy for Control of Wheat Diseases” table (Fig. 2) to find products that provide excellent control of stripe rust. Be sure to check your local recommendations and also the label to verify the use of all products in your area. You can also check out our fungicide test reports HERE. Be sure to go back to the 2016 and 2017 era reports to find data on stripe rust, as those were the last years of epidemics suitable to obtain efficacy data on stripe rust.

As always, SCOUT, SCOUT, SCOUT!