Wisconsin Soybean and Corn Disease Update – July 7, 2021

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

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

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

Soybean White Mold Update

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

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

Current White Mold Management Recommendation

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

Corn Tar Spot Update

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

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

Current Tar Spot Management Recommendation

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

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

Fireworks Fly! Time to Think about White Mold Management in Soybeans in Wisconsin

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

Shawn Conley, Extension Soybean and Small Grains Agronomist, Department of Agronomy, University of Wisconsin-Madison

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

Paul Mitchell, Extension Economist, Agricultural and Applied Economics, University of Wisconsin-Madison

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

In Wisconsin, the first week of July brings us a heightened awareness of white mold in soybeans, and its management. The 2020 season in Wisconsin resulted in just pockets of white mold in the state. However, now is not the time to be complacent. If the weather becomes conducive in 2021, the situation could be much different than last year.

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.

While conditions have been hot and dry in parts of the state, we are seeing cooler and wetter conditions over the last week. The white mold situation can change rapidly based on weather, thus anticipating favorable conditions for white mold, can help you protect your soybean crop.

Predicting White Mold

The flowering growth stages are a critical time to manage white mold in-season. You can view a fact sheet and new 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 Sporecaster smartphone application for Android and iPhone.

Figure 2. Sporecaster predictions for selected non-irrigated locations in Wisconsin for June 29, 2021

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.

Sporecaster uses university research to turn a few simple taps on a smartphone screen into an instant forecast of the risk of apothecia being present in a soybean field, which helps growers predict the best timing for white mold treatment during the flowering period.

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 2021, 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 1.4 of the  Sporecaster. If you want to learn more about the science of Sporecaster, check out the embedded video below.

Not only can users run predictions of risk during the soybean bloom period for any field, 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 June 29, 2021 for non-irrigated soybeans. Currently, if soybeans are flowering, risk is moderate to low in much of Wisconsin for non-irrigated soybeans, due to the recent hot and dry weather. In the north-eastern portions of the state, risk for flowering soybeans is higher due to more frequent rain events that have occurred there. Check back to this blog regularly as I will post maps like these with interpretation of risk for Wisconsin as we move through the season.

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 between the R1 and R2 growth stage performs well. We have also observed that the fungicide Aproach applied at 9 fl oz at R1 and again at R3 also performs comparably to the Endura treatment. 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. 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.

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 and download a fact sheet on white mold 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 sclerotiorumapothecial 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.

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

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

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

Figure 1. NCLB Lesions on a corn leaf

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

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

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

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

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

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

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

Figure 4. Tar Spot Signs and Symptoms on Corn Leaves

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

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

Figure 5. Eyespot symptoms on a corn leaf.

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

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

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

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

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

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

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

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

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

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

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

Summary

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

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

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

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

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

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

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

Other Resources

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

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

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

References

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

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

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

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

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

Wisconsin Winter Wheat Disease Update – May 28, 2021

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

Figure 1. Fusarium head blight risk for susceptible winter wheat varieties for Wisconsin as of May 28, 2021.

Winter wheat in Southern and South-central Wisconsin is quickly approaching anthesis. By early next week the window of opportunity to apply fungicide for Fusarium head blight (FHB; scab) will be here. Currently the risk for FHB is variable and ranges from low to high depending on where you are in the state (Figure 1). Given the recent rain events and rising temperatures, I think the risk is there for FHB next week. This situation should be monitored closely and a timely fungicide application decision should be made. In my previous post, I talk about how to manage FHB.  There are essentially three options for products in Wisconsin for control of FHB. These include Prosaro, Caramba, and Miravis Ace. All also have efficacy against other foliar diseases too. Remember, your window of opportunity to spray for FHB ranges from the start of anthesis (flowering) to about 7 days after the start of anthesis.

Stripe rust still remains unidentified in the state. Confirmed reports of stripe rust are only as close as central Illinois (Figure 2). Continue to remain diligent in scouting for this disease. Remember that fungicides for control of FHB will also be efficacious for stripe rust. Thus, we should be able to “kill two birds with one stone” when spraying for FHB.

Figure 2. Confirmed stripe rust reports for the U.S. as of May 28, 2021.

Reports of powdery mildew continue to come in.  Remember that the FHB fungicide treatments will control this disease. So at this point, two applications of fungicide are not needed. A well-timed FHB-focused fungicide app should help slow powdery mildew.

Get out there and scout, scout, scout!

Managing Winter Wheat Diseases in Wisconsin During the 2021 Field Season

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

Up until this week diseases of winter wheat have been basically non-existent. Dry weather leading to a moderate drought in much of the state has meant that the environment has not been favorable for most wheat pathogens of concern for us in the upper Midwest. There is an exception to that rule, and we have observed one disease, and gotten reports of that disease this week. That disease is powdery mildew and does tend to follow a few different “rules” when it comes to fungal diseases. Let’s discuss this disease further and then dig in a bit on what you should do for disease management as we move through the rest of the 2021 winter wheat season.

So, what’s up with powdery mildew?

Figure 1. Signs and symptoms associated with powdery mildew on a wheat leaf.

Powdery mildew of winter wheat is caused by the fungus Blumeria graminis f. sp. tritici. The most notable sign of powdery mildew is the white, fluffy fungal growth that occurs on the surface of leaves (Fig. 1). Yellow spots may be present on the underside of the leaf. The white “tufts” might also have very small black pepper-like structures in them. Generally, the disease will start in the lower canopy, and if weather is favorable, will move up the canopy eventually reaching the flag leaf and even infecting heads on susceptible varieties.

The reason that powdery mildew has been an issue this year, despite the dry weather, is that it happens to like cool conditions combined with dew events. Warm days and cool nights often lead to dew and extended periods of leaf wetness (think semi-arid climates). This combined with temperatures less than 80 F, means the fungus can thrive on susceptible varieties. Excessive rain events actually deter this particular fungus, as heavy rain events can wash spores from the leaf. So, it isn’t surprising that we are seeing powdery mildew right now given the weather we have had.

Should you spray fungicide for powdery mildew?

Most of the time I would say no. Often in Wisconsin, the weather begins to turn much warmer as we approach heading and the fungus will stop spreading and remain a novelty in the lower canopy. Remember, once daytime temperatures get above 80 F, the fungus will stop or slow in progression. The key in making the fungicide spray decision is to know the susceptibility of the variety you planted and watch the weather. If the weather remains conducive (temps below 80 F, no rain, but dew) and the variety is ranked susceptible, then spraying around flag leaf emergence might be warranted. You can consult the Fungicide Efficacy for Control of Wheat Diseases table for products rated with the best efficacy for powdery mildew. Note that most of the higher rated products are triazole compounds, or compounds with a triazole in their mix. No need to be fancy here, find something that fits your budget and has good efficacy. There should be ample choices.

I don’t care about powdery mildew, but what disease should I keep an eye on next?

The last few seasons, two diseases have been our primary focus on winter wheat in Wisconsin. These diseases are stripe rust and Fusarium head blight (scab or FHB). In 2019 and 2020, only FHB was yield limiting. However, in 2016 and 2017, we dealt with both stripe rust and FHB in the same season. Obviously, this year could be different, but the stripe rust situation is starting to “heat up” a bit to our south. The “Wheat Ag Pest Monitor for Stripe Rust”  indicates that there are confirmed cases of stripe rust in central Illinois (Fig. 2). This situation needs to be watched carefully over the next 1-2 weeks in Wisconsin.

 

Figure 2. Confirmed stripe rust observations in the U.S. as of May 19, 2021.

 

We are rapidly approaching the first growth stage where the application of fungicide might be warranted if stripe rust is found in your locale or is nearby. Based on recent published research, in years where stripe rust was active at the emerging flag leaf growth stage (Feekes 8), we obtained excellent control of stripe rust and a positive return on investment where we sprayed fungicide at this growth stage with a second application of fungicide for FHB control around anthesis.

Currently there is no stripe rust confirmed in Wisconsin but given the rainy weather recently and the positive confirmations in Illinois, this situation should be closely monitored, and a fungicide might be warranted on susceptible varieties if stripe rust is found prior to heading.

If you decide you need to apply a fungicide, there are many choices with decent efficacy against stripe rust. Again, consult the Fungicide Efficacy for Control of Wheat Diseases table for the best ratings for the disease. Find a product that is efficacious and fits your farm budget.

So, what should I do about Fusarium head blight?

Figure 3. Fusarium head blight of winter wheat

Fusarium head blight (Fig. 3) has been a perennial problem for us in Wisconsin over the last 5 years. Not only have we seen significant damage and yield reductions due to the disease, but we have seen significant discounts at the elevator for levels of deoxynivalenol (DON or Vomitoxin) above 2 ppm. It is important to manage this disease actively here in Wisconsin.

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. In 2019 we evaluated the susceptible variety, Hopewell, against the resistant variety, Harpoon. Figure 4 shows the FHB levels for the two varieties which were also subjected to a fungicide application. Clearly variety resistance works.

When it comes to fungicides for FHB, there are really just three products to choose from. These are Caramba, Prosaro, and Miravis Ace. Again the Fungicide Efficacy for Control of Wheat Diseases table shows the efficacy ratings of these products against FHB. 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. Also, these fungicides are effective against stripe rust, so if that disease happens to move in later, a single application of fungicide at the anthesis timing should take care of both problems.

 

Figure 4. Fusarium head blight index (FHB Index) from a 2019 integrated management trial where the susceptible variety, Hopewell, and the resistant variety, Harpoon, were both treated with various fungicide programs or not treated with fungicide.

 

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. While most wheat as of May 19, 2021 is not near heading yet in Wisconsin, you can see in figure 5 that for susceptible FHB varieties, the risk is currently low. Again, dry weather leading up to this week has not been favorable for FHB.

 

Figure 5. Fusarium head blight risk as of May 19, 2021 in the U.S.

 

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

This can be cooked down to two main points. Here they are:

  • In some years applying fungicides for stripe rust on susceptible cultivars around Feekes 8 will be needed. Make this decision based on the following:
    1. The known susceptibility of the variety
    2. Local presence of strip rust and/or confirmed reports nearby
    3. Favorable weather (frequent rains and temperatures less than 80 F)
  • Plan to apply an FHB fungicide application – especially on susceptible varieties
    1. Shoot for Anthesis or up to 5-days after the start of anthesis for Prosaro and Caramba fungicides
    2. 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
    3. Watch the “Scab Alerts” – it isn’t perfect, but can help you make a decision (http://www.wheatscab.psu.edu)

 

What’s the Skinny on Foliar Fungicides for Alfalfa in Wisconsin?

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

Figure 1. An alfalfa foliar fungicide trial in Arlington, WI.

With high commodity prices this spring and some freeze events affecting alfalfa in Wisconsin several weeks ago, I have been getting many questions about using fungicide on alfalfa. Should you do it? Is it economically viable? Does fungicide help to heal the freeze injury? The answers as you might expect are not always straight forward. Ain’t that just the way…..

What do we know about fungicides?

 Before I start into this topic, I’ll mention that you can find more extensive information about fungicides and how they work on the “Field Crops Fungicide Information” page. For all of the products labeled for alfalfa in Wisconsin, they should be applied when the stem have reached a length of 6-8 inches of growth. Applying at this time allows for preventative application (see below) and limits the amount of damage from wheel traffic. In addition, most of these products have at least a 14-day pre-harvest interval. To find information on the fungicides labeled for use in alfalfa in Wisconsin, download A3646-Pest Management in Wisconsin Field Crops.

 

Figure 2. Results of a large-plot foliar fungicide trial on 55V50 alfalfa at the Arlington Agricultural Research Station Located in Arlington, WI in 2016.

Remember that mode of action and mobility in the plant play a big role in how a certain fungicide should be used. As a rule of thumb remember that fungicides should mostly be used as protectants, applied prior to disease development to protect healthy plant tissue. Applying fungicide after extensive disease is observed almost never benefits the plant and is not economically viable. This means that you need to anticipate when disease might show up. In alfalfa, there are numerous foliar diseases that can affect the crop during the season, including common leaf spot, Letosphaerulina leaf spot, and Stemphylium leaf spot to name just a few. Most of these leaf spots are promoted by wet weather. Keeping good history of the diseases you have seen in the past on your farm, and tracking weather events (especially those where the weather is wetter than average) can help you anticipate the onset of these diseases. In alfalfa, we also must consider what our target cutting-duration is to be. We know from our more recent research (Figure 2) that cutting duration plays a significant role in the success of fungicide applications on alfalfa. If cutting duration is to be longer than 28-30 days, then the likelihood of an economic return is higher when fungicide is used. If the intent is to continue to cut using a shorter duration of 25-30 days, then the likelihood goes down in achieving an economic return when applying fungicide. Regardless of cutting duration, weather can also influence success. Abnormally wet weather can push pathogens faster. Dryer weather may mean that no fungicide is needed as the pathogen can’t increase as readily. So, pay attention to the rainfall as you make the decision to apply.

Why does cutting duration impact economic return when using fungicide on alfalfa?

Well, it simply comes down to the biology and epidemiology of the pathogens that affect the foliage of alfalfa. These pathogens don’t show up until about 14-21 days after cutting (or greenup). It takes them time to appear and start to get to a level that they cause the plant to defoliate. In the 1990s Duthie and Campbell reported that longer durations of cut between alfalfa crops allowed the pathogens to grow to levels that were more likely to be damaging. If we cut on shorter durations, we are physically removing the pathogen with the plant material during harvest, prior to when significant defoliation would happen. The application of fungicide in this scenario has little effect, because the pathogen was not causing any damage. Compare this to longer durations of cut, where the pathogens can rise to levels where they might cause defoliation. The application of fungicide in this scenario has a higher likelihood of success as it can slow the pathogen down and limit defoliation. Figure 2 shows data from 2016 in large alfalfa plots, where we gradually increased the duration of cut. Notice as the duration increased there was a corresponding increase in the return on investment (ROI) either as dry hay or in milk production.

Does fungicide help to heal freeze injury?

The short answer is no. Fungicides work to prevent and deter fungi. They work as primarily protectants on the plant and do not facilitate any healing of the plant. Can they protect an open injury from a pathogen? Perhaps, but data are weak at best, with this being a viable tactic. Thus, if you plan to use fungicide as a foliar application on alfalfa, make sure your target is indeed a foliar fungus.

Does alfalfa variety affect success when using a fungicide?

Directly, variety can play a small role in success with fungicides. Some varieties can be more susceptible to certain pathogens. However, the modern varieties I have looked at in recent years tend to respond similarly to these various foliar pathogens, with decent levels of partial resistance. We don’t see much separation in fungicide product on the various varieties.

The one caveat to consider though is if you are using reduced- or low-lignin varieties. If this is the case and your intent is to widen the cutting duration (longer than 30 days) then the likelihood of economic success when using a fungicide goes up. Note however, that this phenomenon is the same even if you are using a longer cutting duration on a conventional variety. Remember, cutting duration is really the important driver here.

Does fungicide product impact ROI on alfalfa?

Not really. You can find results from fungicide efficacy trials in Wisconsin on the “Wisconsin Fungicide Tests Summaries” page. Most of the fungicides labeled for alfalfa in Wisconsin have decent efficacy toward the major pathogen we run into. So, it simply comes down to that cutting duration again and what your best deal is on price for the fungicide of interest. Obviously, the lower that the fungicide cost is, the higher the ROI could be. But remember to get the timing of application correct and try to anticipate those potentially longer durations of cut.

Is the first crop the crop with the most potential for success when using foliar fungicide?

Perhaps. It makes the most sense. This crop has the highest yield potential and likely will sit out there the longest, exposed to pathogens and experiencing rain events. However, when we looked at this scientifically in some of our older work, the odds of success with a foliar fungicide are equal on all crops. What this means is that using foliar fungicide on any of the cuttings during the season could result in success. It simply comes down to that cutting duration and if the weather is wet enough to allow the pathogen to reach levels that cause defoliation on the plants, prior to harvest.

The Take-Home

In alfalfa, application of fungicide should be reserved when the risk of foliar disease is high. Wet weather leading to increased durations between cuttings can lead to higher risk of disease and warrant application of fungicides. Dry weather or short durations between cutting intervals lead to low risk of foliar disease development. Fungicides are not warranted under these conditions and quality is not readily affected by fungicide applications under short cutting durations.  

Please join us for the 2020 Wisconsin Pest Management Update Virtual Meetings! 

 

Registration Link: https://go.wisc.edu/78t2bd  

The format for the Wisconsin Pest Management Update will be different this year because of COVID-19. Presentations will include pest management information for Wisconsin field and forage crops. Speakers will include Mark Renz, Nick Arneson, and Rodrigo Werle, Weed Scientists, Damon Smith, Plant Pathologist, and Bryan Jensen, Entomologist. Virtual meetings with the same content will be offered multiple times the week of November 9 and will run for 2 hours each. The dates and times are listed below, please choose the option that best fits your schedule. There are also 2 CCA CEUs in the area of Pest Management available for this meeting.

Register by November 2, 2020. Register here:  https://go.wisc.edu/78t2bd  

Thanks to Kimberly Schmidt, Shawano County Agriculture Educator, Dan Marzu, Lincoln and Langlade Counties Agriculture Educator, and Mimi Broeske, Nutrient and Pest Management Editor, for their assistance with registration and promotion of our event.

This program is sponsored by the University of Wisconsin-Madison Division of Extension and University of Wisconsin-Madison College of Agricultural and Life Sciences. An EEO/AA employer, University of Wisconsin-Madison Division of Extension provides equal opportunities in employment and programming, including Title VI, Title IX, the Americans with Disabilities Act (ADA) and Section 504 of the Rehabilitation Act requirements.

Crop Protection Network (CPN) Needs Your Input!

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

Help the CPN determine how to serve you best through this short survey.

The Crop Protection Network (CPN) develops tools to help farmers, ag industry, and researchers with crop protection decisions. The CPN is a partnership of university Extension specialists providing unbiased, research-based information at no cost.

The CPN is conducting a survey to help determine how to best serve agricultural clientele now and in the future. You can help by taking a short survey at: https://iastate.qualtrics.com/jfe/form/SV_1FUAhZZ9YZmDva5

Since 2015, the CPN has developed free extension tools including publications, foliar fungicide efficacy guides, annual disease loss estimates, training for field scouts, and a tool for Certified Crop Adviser’s (CCAs) to earn continuing education units. See cropprotectionnetwork.org to access these resources.

 

 

 

Wisconsin Soybean White Mold Update – July 29, 2020

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

Shawn Conley, Extension Soybean and Small Grains Agronomist, Department of Agronomy, University of Wisconsin-Madison

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

Figure 1. Sporecaster predictions for selected non-irrigated locations in Wisconsin for July 29, 2020.

Figure 1 illustrates the calculated risk of white mold for select Wisconsin locations for non-irrigated soybeans, as determined by Sporecaster for July 29, 2020. This means that if soybeans are flowering and the area between rows is filled in more than 50%, risk is just moderate in most locations of the state, with the exception of the far northeast portions of the state and Door County. This moderate risk indicates that there may not be apothecia present in fields in these locations at this time, with reduced risk of subsequent white mold development due to hot and dry conditions. Remember that this season, you have the ability to change the action threshold for each field in the app. Last season the action threshold was locked at 40%, which is still a reasonable threshold for Wisconsin. Thus, figure 1 risk is calculated based on 40%. You can tailor this threshold to your liking based on your prior knowledge of a field, or your acceptable risk level. Further tailored predictions for irrigated locations and locations planted to narrower row-spacing can be run by downloading the Sporecaster app to your smartphone.

As we move toward the end of the fungicide spray window at R3, a fungicide application might not be warranted at this time on non-irrigated fields. In irrigated fields, we are seeing higher risk and finding apothecia in irrigated fields in central locations. A fungicide spray might be warranted in this situation.

I’m Ready To Spray, What Should I use?

If the canopy has met threshold, soybeans are flowering, and your Sporecaster risk is high, then a fungicide might be warranted. If you have decided to spray soybeans for white mold, what are the best products to use? I have written extensively about this in a previous post which you can find HERE. 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 at 8 oz once between the R1 and R3 growth stages performs well. We have also observed that the fungicide Aproach applied at 9 fl oz at R1 and again at R3 also performs comparably to the Endura treatment. 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.

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. Here is a helpful video if you would like some tips on how to use Sporecaster.

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 White Mold Resources

  1. To watch an in-depth video on white mold management, CLICK HERE.
  2. To find more information and download a fact sheet on white mold from the Crop Protection Network, CLICK HERE.

Wisconsin Corn Tar Spot Update – July 29, 2020

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

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

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

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

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

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

The Recommendation

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

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

More Tar Spot Information

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