Wisconsin Corn Tar Spot Update – July 29, 2020

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

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

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

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

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

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

The Recommendation

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

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

More Tar Spot Information

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

Wisconsin Corn Tar Spot Update – July 23, 2020

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

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

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

 

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

 

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

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

The Recommendation

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

More Tar Spot Information

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

Wisconsin Corn Tar Spot Update – July 14, 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

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

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

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

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

The Recommendation

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

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

More Tar Spot Information

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

Wisconsin Corn Tar Spot Update – July 9, 2020

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

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

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

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

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

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

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

The Recommendation

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

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

More Tar Spot Information

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

Corn Disease and Nutritive Value Considerations for the 2019 Silage Harvest

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

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

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

Foliar Disease of Silage Corn in 2019 

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

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

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

 

Figure 2. Northern corn leaf blight on corn.

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

Ear rots and mycotoxins of silage corn in 2019 

Figure 3. Gibberella ear rot on corn.

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

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

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

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

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

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

The Take-Home

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

Tar Spot Now Confirmed in Wisconsin in 2019

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

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

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

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

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

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

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

What does this mean for you?

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

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

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

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

The Tar Spot Situation

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

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

Other Corn Diseases To Watch in Wisconsin

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

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

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

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

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

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

Management of Southern Rust

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

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

Other Useful Resources about Rusts on Corn

Purdue Extension Fact Sheet – Common and Southern Rusts of Corn

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

Wisconsin Corn Tar Spot and General Disease Update – July 18, 2019

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

Figure 1. Tarspotter risk predictions for the state of Wisconsin on July 19, 2019.

Figure 1 shows the calculated risk from Tarspotter for July 19, 2019, for various locations in Wisconsin. As you can see, the present risk has dropped substantially over the past week, leaving much of the state at low risk. The drop is due to the high temperatures and drier conditions. Tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period. We have also scouted fields in southern and southwestern WI and have not found tar spot in our travels.

Gray Leaf Spot, Common Rust, and Northern Corn Leaf Blight, Oh My!

Figure 2. Gray leaf spot on a corn leaf.

While scouting we have observed other foliar diseases of corn, including gray leaf spot (GLS: Fig. 2), common rust (Fig. 3), and northern corn leaf blight (NCLB; Fig. 4). Out fo these three, GLS has been the most consistent to find in fields we have visited. In Grant Co., GLS has made its way to the mid-canopy of some corn planted no-till in a field that had corn last season. It will be important to keep an eye on GLS and NCLB over the next couple of weeks. These two disease can become yield limiting if they reach the ear leaf of corn at high severity levels before the R3 corn growth stage. Scouting to determine the number of plants showing symptoms and the severity will be important in determining if a fungicide application at the tasseling growth stage is needed. Right now I’m most concerned about GLS and NCLB in field corn in Wisconsin, while keeping an eye out for tar spot.

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 2019 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 both NCLB and GLS along with efficacy against 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.

Figure 3. Common rust on a corn leaf.

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. But in our current market, will this average gain cover the fungicide application? Today’s corn future price for September has a bushel of corn at $3.76.

Let’s Take a Closer Look at Corn Fungicide Return on Investment (ROI): 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) and triazole (DMI) fungicides in the same jug. Products such as Headline AMP® or Quilt Xcel® would fall into this 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 product alone ranges from $15/acre (Quit Xcel®) to $22/acre (Headline AMP®). If the fungicide will be flown on with an aircraft, that cost will likely add nearly $15/acre to the application. Thus, fungicide plus application would range from $30/acre to $37/acre. If we can sell corn at $3.76 per bushel then we would need to preserve 8 bu/acre to nearly 10 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 across the entire corn belt, the average yield preservation over not treating was 7.20 bu/a. This average projection is short of the 8 bu/a minimum we would need in the scenario above. However, the probability of preserving yield in the 8-10 bu/a range in this range is estimated to be 25% – 50%. This means that if we apply Quit Xcel® at 10.5 fl oz or Headline AMP® at 10.0 fl oz aerially, we will only break even 25% – 50% of the time with corn priced at $3.76 per bushel. If we can sell our corn for a better price or make the applications cheaper, then the odds will improve, but probably not climb above 70% even under the best case scenario. We do know that in Wisconsin, the odds of breaking even do improve if NCLB or GLS are active and increasing during the tasseling period. Get out there and scout!

Figure 4. Northern corn leaf blight on a corn leaf.

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. The VT application resulted in nearly 8 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 with 5 days after silking starts, can be beneficial. Note though 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 should be used. Like winter wheat, the application of some QoI-containing fungicides can increase DON accumulation in the grain portion of corn plants. Some work has been done using Proline® to control Fusarium ear rot. This DMI only product has shown promise in reducing ear rot and DON accumulation in the grain portion of the corn plant and has a label for suppressing Fusarium ear rot in Wisconsin. Performance of some additional products in Wisconsin in a 2018 silage corn trial can be viewed by CLICKING HERE and scrolling down to pages 4 and 5.

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.

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 2019. Here is what you should consider:

1) Corn hybrid disease resistance score for NCLB and GLS – 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 or GLS, 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?

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

Video: Disease Management in Low-Margin Years (fast forward to 10:00 for corn information)

Fact Sheet: A4137 – Grain Management Considerations in Low-Margin Years

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

References

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.

Wisconsin Corn Tar Spot Update for July 11, 2019 – Frass Happens!

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

Figure 1. Tarspotter risk predictions for the state of Wisconsin on July 11, 2019.

Figure 1 shows the calculated risk from Tarspotter for July 11, 2019, for various locations in Wisconsin. Figure 2 shows the risk for locations in southern and south-central Wisconsin. As you can see, the present risk remains generally high for tar spot development in all locations examined. However, risk percentages have dropped over the last week, for the most part. The continued elevated risk is due to the fact that the humidity was high over the last weekend, despite warmer temperatures. As temperatures climb and conditions remain dry, expect risk to decline. Tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period.

Frass Happens!

Figure 2. Tarspotter risk predictions for southern Wisconsin on July 11, 2019.

That’s right folks, frass or bug poop is a common reality in corn fields. With all of the hype concerning tar spot, it has gotten folks out scouting corn. This is a good thing. However, we have received many samples in our diagnostic clinic that were submitted for tar spot confirmation. So far, these have all been confirmed to be insect frass or bug poop (Fig. 3). We have also scouted fields in Dane co., Walworth Co., Grant Co., and Lafayette Co. and have not observed tar spot in any fields visited. We have seen lots of insect frass though.

How can you tell if it is bug poop and not tar spot? Use a little spit or some water from a water bottle. Wet the leaf and lightly rub.  Bug poop will come off, tar spot will not. In addition, tar spot spots will be raised and can be felt when you rub your finger across the black spot. Frass generally won’t feel like it is raised. Continue to scout and send samples. It is always better to get a confirmation, before you spray. If it is nothing to worry about, you can save the trip and the money!

The Current Recommendation

Figure 3. Insect frass (a.k.a bug poop) on the surface of a corn leaf. Photo Credit: Roger Schmidt.

While weather continues to be mostly conducive for tar spot, evaluate the likelihood that tar spot might develop in your field. Remember, if you have no history of the disease, then the likelihood of local inoculum being present is low. Saving the fungicide application for as as close to tassel or white silking periods will be the best option resulting in the highest return on the fungicide investment. If you have a history and you know you have a susceptible hybrid coupled with a no-till situation, then the risk is higher and you need to evaluate the economics of doing an application of fungicide as early as V6. Remember, if you do a V6-V8 application of fungicide, conditions could stay conducive later in the season for tar spot. Those early applications will “burn out” by the time the tasseling period rolls around. So if you do put a fungicide spray on at V6, you might have to come back at VT with another application to protect plants during the reproductive phase, should we stay in favorable conditions for tar spot. Keep an eye on the weather and keep scouting!

More Tar Spot Information

  1. Tar spot Fact sheet
  2. Short Tar Spot Video
  3. Tar Spot Webinar 
  4. Corn Fungicide Efficacy Table

Wisconsin Corn Tar Spot Update – July 3, 2019

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

Figure 1. Tarspotter risk predictions for the state of Wisconsin on July 3, 2019.

Figure 1 shows the calculated risk from Tarspotter for July 3, 2019, for various locations in Wisconsin. Figure 2 shows the risk for locations in southern and south-central Wisconsin. As you can see, the present risk remains generally high for tar spot development in all locations examined. This is due to the fact that the weather continues to be relatively wet and humid for the past 30 days across the entire state. Tar spot is favored by persistent temperatures between 60 and 70 F and high relative humidity averaging above 75% for a 30-day period. We have been right in the zone for conducive conditions for this disease.

We have also spent the last few days scouting corn in Walworth Co., Grant Co., and Lafayette Co. Some of these fields are planted to known tar spot-susceptible hybrids and planted in fields with a history of the disease. We have been unable to find any symptoms of tar spot. Fields generally look disease free with just a bit of Anthracnose, which is common this time of year.

The Recommendation

Figure 2. Tarspotter risk predictions for southern Wisconsin on July 3, 2019.

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

More Tar Spot Information

  1. Tar spot Fact sheet
  2. Short Tar Spot Video
  3. Tar Spot Webinar 
  4. Corn Fungicide Efficacy Table