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!

2023 Wisconsin Fungicide Test and Disease Management Summary Now Available!

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Brian Mueller, Researcher II, UW-Madison, Plant Pathology

Damon Smith, Professor and Extension Specialist, UW-Madison, Plant Pathology

Each year the Wisconsin Field Crops Pathology Program conducts a wide array of fungicide and disease management tests on alfalfa, corn, soybeans, and wheat. These tests help inform researchers, practitioners, and farmers about the efficacy of certain fungicide products on specific diseases and how to pair them with other disease management strategies. We hope you find this report useful in making decisions for the 2024 field season.

The 2023 Wisconsin Field Crops Fungicide Test and Disease Management Summary is available by clicking here. These tests are by no means an exhaustive evaluation of all products available, but can be used to understand the general performance of a particular fungicide in a particular environment. Keep in mind that the best data to make an informed decision, come from multiple years and environments. To find fungicide performance data from Wisconsin in other years, visit the Wisconsin Fungicide Test Summaries page. You can also consult publication A3646 – Pest Management in Wisconsin Field Crops to find information on products labeled for specific crops and efficacy ratings for particular products. Additional efficacy ratings for some fungicide products for corn foliar fungicides, soybean foliar and seed-applied fungicides, and wheat foliar fungicides can be found on the Crop Protection Network website.

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

Wisconsin Field Crops Disease Update, August 9, 2023

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

Figure 1. A Screen shot of a map developed in the Field Prophet app showing risk for tar spot development in Wisconsin as of August 9, 2023.

Well, it was going to happen sometime soon, tar spot has been confirmed in two counties in Wisconsin. You can track tar spot confirmations in realtime HERE. Both Lafayette and Rock counties were confirmed to have low levels of tar spot in several fields this week. The good news is that these finds are a month behind the initial confirmations in the state for the last two seasons. What does this mean? Well, it means that the tar spot impact on grain yield will likely not be has significant as it has been over the last couple of seasons. Exceptions to this statement will include late-planted corn where the current growth stages are around VT (tasseling) and susceptible silage hybrids. You should scout and track these situations carefully. Be prepared to chop silage early if tar spot really starts to move. You will want to watch moisture carefully in these situations.

The current risk for tar spot development remains moderate to high across much of the state (Fig. 1). Cooler weather and dewy evenings and mornings are keeping the risk elevated. Fungicide applications for much of the corn in the state should have happened already. Remember the optimal time to apply fungicides to control tar spot (and most other foliar corn diseases) is between the VT (tasseling) and R3 (milk) growth stages. Spraying fungicide after R3 has not yielded much of a return on investment. If you find tar spot, please don’t hesitate to send a high-quality photo to damon.smith@wisc.edu. We don’t disclose exact locations, but do like to track the county-level tar spot information. If you would like to learn more about tar spot and managing it, see my previous post HERE.

Figure 2. Sporecaster predictions for selected non-irrigated locations in Wisconsin for August 9, 2023.

In other news, white mold risk ranges from low in the southern portion of the state to moderate and high in the mid and upper portions of the state, respectively (Fig. 2). Most soybeans are probably headed toward the R4 or R5 growth stage. This means fungicide applications will no longer yield positive returns on investment. If soybeans were planted late and they are still in the R1 to R3 growth stages and you are in a moderate to high risk area, a fungicide should be applied at this time. If you would like to learn more about white mold management, see my previous post HERE. We have observed active white mold on susceptible varieties under irrigation already this season. I am anticipating pockets of white mold in the state, especially in our typical areas of concern in the central and northeastern quadrants.

As always, make sure you are out and scouting to be prepared for what is coming ahead!

Wisconsin Field Crops Disease Update, July 27, 2023

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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

Rains continue to fall around Wisconsin. While we still have a moderate drought in much of the state, some of that drought is being alleviated. However, with these timely rains, come disease concerns. Here are our thoughts on what is happening.

Phytophthora Root and Stem Rot of Soybean

Figure 1. Stem rot symptoms of Phythophthora rot and stem rot on soybeans.

It has been a couple of years since we have seen a significant epidemic of Phytophthora root and stem rot (PRSR) of soybean. However, since it has started raining, we have got a good look at how susceptible many of our soybean varieties are here in Wisconsin. PRSR is primarily cause by the fungal-like organism, Phytophthora sojae. PRSR is usually worse in fields that are no-till and/or are slow to drain. The PRSR pathogen likes to survive in old soybean residue and can also persist as a long-term survival structure in the soil itself. The organisms that causes PRSR becomes active if the soil temperatures are over 60 F and the soil becomes saturated. We had those conditions occur back in early to mid-July. Once the soil dried out a bit and a bit of environmental stress kicked in, we can readily observe the damage the organism caused in early July. Primarily what we are seeing right now is the stem rot phase (Fig. 1), with the symptoms including wilting of the plant and a distinct purple-brown lesion extending from the soil surface upward. If plants are pulled from the ground, you will also see poor root systems which is where the organisms typically first infects and causes damage.

At this point in the season, there is nothing that can be done.  DO NOT spray foliar fungicides for this problem. This will not be effective. You will want to check on the variety with the symptoms and consult the tech sheet to see what type of “Phytophthora gene” may have been included in the variety. These genes are called Rps genes and provide race-level resistance. The population of the PRSR pathogen can be a single race or mixed races in the field. The last time a survey of Phytophthora races was done in Wisconsin, it was noted that the Rps 1-k resistance gene should be effective on about 99% of the acres in the state. However, that survey was done over 15 years ago. Due to heavy use of the Rps 1-k resistance gene, we believe that the population in the state has shifted. We are seeing that resistance readily overcome. Unfortunately, most of the varieties currently grown in the state have this resistance. A recent check of the soybean variety trials 2022 show that out of 265 varieties tested 25% had no PRSR resistance gene, 2% had Rps 1-a, 29% Rps 1-c, 26% Rps 1-k, 9% Rps 3-a, and 9% had multi-genes. We are actively working with the Wisconsin Soybean Marketing Board to understand what the current population looks like. However, it is too early to tell what the races are primarily in our fields. Moving forward. perhaps choosing Rps 3-a or mixed gene varieties could help, but that is a shot in the dark for now.

Other things you can do for PRSR are to open up the rotation between soybean crops, and improve drainage in fields that are typically saturated for long periods of time. Like I said above, adjusting variety choice can help too. Seed treatment fungicides can also be used. However remember that the seed treatment is only going to be effective for the first 30 days or so after planting. After that we have to rely on varietal resistance to manage this problem. If you would like to find efficacy data on the seed treatments you can find that HERE.

We are looking for samples of PRSR from around Wisconsin. So feel free to reach out (damon.smith@wisc.edu) and we can coordinate getting samples sent to us. This will help with our survey efforts and eventual varietal recommendations.

Tar Spot Update

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 27, 2023.

You can find the most recent updates on tar spot confirmations across the U.S. here: https://corn.ipmpipe.org/tarspot/.  Tarspotter is also showing mostly moderate to high risk across the state of Wisconsin (Fig. 2). This means you should be actively scouting for tar spot at this time. The risk is likely that you will find it across much of the state. If the corn growth stage is between VT/R1 and R3, then you might go ahead and consider a fungicide application. 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.

White Mold Update

Figure 3. Sporecaster predictions for selected non-irrigated locations in Wisconsin for July 27, 2023.

The risk for white mold according to Sporecaster is a bit more spotty, compared to tar spot. Mostly the northern tier of the state is at high risk while central and southern Wisconsin varies from moderate to low (Fig. 3). 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. The recent rains have made the risk in these areas generally stay high or increase. 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 the crop. Scout, scout, scout!