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. 

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.

Late Season Corn Foliar Disease Update and Hail-Damaged Corn

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

Scouting by my team and phone calls from extension personnel, consultants, and farmers have made it evident that there are several foliar diseases of corn showing up  in this first part of August. Gray leaf spot (GLS), northern corn leaf blight (NCLB), and tar spot have all been found in various locations over the last week or so. It is becoming very late in the season to try to control GLS or NCLB. Current data on tar spot indicate it likely doesn’t need to be controlled. Thus, there is likely not much to do at this point, but to document which fields have which diseases. This can help in fall scouting to make harvest decisions, as fields with higher levels of leaf disease may not have experienced any yield loss, but might have stalk integrity issues, which could lead to lodging. Determining which fields might be more prone to lodging can help establish harvest order to minimize any losses due to severely lodged plants. Below is more information about each foliar disease.

Gray leaf spot (GLS)

Figure 1. Gray leaf spot on a corn hybrid. Photo Courtesy of Craig Grau, University of Wisconsin-Madison

Gray leaf spot is cause by a fungus named Cercospora zeae-maydis. During times of very warm temperature and high humidity (greater than 90%), GLS can increase rapidly on susceptible hybrids. In fields with large amounts of corn residue (e.g. corn-on-corn rotation, minimal tillage, etc.) GLS may be more prominent due to higher levels of inoculum. Symptoms start as small narrow, blocky lesions that might be tan in the center and have a darker margin (Fig. 1). Lesion can increase in size and number and will typically move from lower leaves to upper leaves. Yield loss is most prominent when lesions reach the ear leaves either 2 weeks before tasseling or two weeks after tasseling. Currently, in Wisconsin, we have seen few fields where lesions have reached the ear leaves prior to brown silk. However, in a small number of fields planted to a susceptible hybrid, there has been rapid increase to the ear leaves prior to tassel. In those fields a fungicide application may result in adequate yield protection to cover the cost of fungicide application. See my previous article on how to make the decision to spray fungicide on corn.

Northern Corn Leaf Blight (NCLB)

Figure 2. Northern corn leaf blight on corn.

Northern corn leaf blight is caused by the fungus Setosphaeria turcica. The fungus is most active when wet weather coincides with temperatures between 65 F and 80 F. During these conditions, the fungus will readily make microscopic spores (called conidia) inside the symptomatic areas of leaves and those spores will get splashed onto more leaves. Therefore, the disease typically moves from the lower canopy, up the corn plant as the season progresses. When temperatures get above 80 F and it is dry, growth and spread of the fungus slows dramatically. This is why little NCLB was observed in July, but is showing up now. It is all about the temperature at which the fungus likes to grow. Lesions initiate as cigar-shaped lesions on lower leaves. When conditions are conducive lesions can expand and increase, moving rapidly up the plant (Fig. 2). Occasionally a gray-to-black fuzzy growth is evident in the center of lesions. This growth is sporulation of the fungus. Like GLS, yield loss is greatest when lesions reach the ear leaf either two weeks before or two weeks after tasseling. Again, consult my previous article on how to make the decision to spray fungicide on corn.

Tar Spot

Tar spot is a relatively new disease in the U.S. and Wisconsin. It is caused by a fungus called Phyllachora maydis. Tar spot causes small tar-like spots on the surface of corn leaves. For great information about tar spot and what it looks like, consult this Purdue Extension fact sheet. Tar spot was first found in the U.S. in 2015. In 2016 and 2017, tar spot was identified in Green, Iowa, Grant, and Lafayette counties in Wisconsin. In 2018 confirmations have been made in these same general areas. In Latin America Phyllachora maydis can be found in a complex with another fungus called Monographella maydis. In areas where the complex occurs significant yield loss has been described. However, in the U.S. Monographella maydis has not been found in complex with Phyllachora maydis. Furthermore, Phyllachora maydis is not known to cause yield loss on corn in the U.S. While it can be a striking disease, fungicide applications are not recommended for tar spot in the U.S. Much more work is needed to characterize this pathogen and understand the disease. We are working with Dr. Nathan Kleczewski at the University of Illinois to improve our understanding of this pathogen in the U.S. If you would like to confirm tar spot on corn, or provide samples for research purposes, you can send samples to the University of Wisconsin Plant Disease Diagnostic Clinic.

What about Spraying Fungicide After Hail Damage?

The best study on this subject was conducted by my colleagues at Iowa State University a couple years back. They found that for the most part application of fungicide after hail does not result in any benefits. Especially after the R2 growth stage. We also had an opportunity to look at a natural hail event in 2014 at Arlington. This happened around VT.  We were also unable to find a significant difference in treating with  a fungicide versus not treating after late season hail-damage. In addition, it isn’t likely that fungal infections will increase after hail. In fact in the Iowa State University study, they found a negative correlation between hail damage and fungal disease. Hail CAN increase Goss’s wilt risk. However, Goss’s wilt is caused by a bacterium. Thus, fungicide application does not work for this disease. For more information on Goss’s Wilt and how to manage it click here. In summary, given the current market prices and the fact that corn is generally through the silking period, fungicide application on hail-damaged corn is not needed.

In-Season Corn Disease Management Decisions – 2017

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

Tasseling has begun on field corn in the southern region 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 less than ideal and create tight profit margins. 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, which has seemed to dominate lately. 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. Incidentally, we did confirm our first NCLB lesions of the year in the diagnostic lab last week. So now is a good time to get out and scout!

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. Brick-red Pustules of the common rust fungus on a corn leaf.

Common Rust: Symptoms of common rust can include chlorotic flecks that eventually rise and break through the epidermis to produce pustules of brick-red spores (Fig. 4). Typically these pustules are sparsely clustered on the leaf. They can also appear on other parts of the plant including the husks and stalks. Conditions that favor the development of common rust are periods of high humidity and nighttime temperatures that remain around 70F with moderate daytime temperatures. This fungus needs very little free moisture for infection to occur. Very hot and dry weather can slow or stop disease development.

Common rust is a extremely common (pun intended) and often results in little yield loss in Wisconsin. Most field corn hybrids planted in Wisconsin are very resistant to the disease. Management for common rust primarily focuses on using these resistant hybrids. Remember resistance is not immunity, so some pustule development can be observed even on the most resistant hybrids. Some inbred corn lines and specialty corn can be highly susceptible to common rust. Under these circumstances a fungicide may be necessary to control common rust. Most of the hybrids I have scouted this season have some pustules, however incidence and severity is relatively low. Therefore, a fungicide application to control common rust isn’t needed for most of these hybrids in Wisconsin. Residue management or rotation is typically not needed for this disease as inoculum (spores) have to be blown up on weather systems from the southern U.S.

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 and GLS. Most hybrids planted in Wisconsin will be resistant to eyespot and common rust.

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 2017 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. 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. 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!

So What About Fungicide Application Timing? We can investigate this questionover 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, DMI only products should be used. Like winter wheat, the application of 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.

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 2017. 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.