To Spray or Not to Spray Fungicide on Corn for Grain or Silage?

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

Treating field corn, for grain, with fungicide has become a common practice in the Midwest. With so many fungicide programs and formulations, and the re-emergence of yield-limiting corn diseases, like northern corn leaf blight (NCLB) and gray leaf spot (GLS), foliar fungicide application has demonstrated an ability to reduce foliar disease severity and increase grain yield under some circumstances.

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

Figure 1: A computer simulation of 5% NCLB severity on a corn leaf.

How do I know if disease is active at the time I want to spray?  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 foliar disease symptoms are present in the lower canopy, make a visual estimation of how frequent (percentage of plants with lesions) the disease is in a particular area and how severe (how much leaf area is covered by lesions).  The lower leaves aren’t responsible for much yield accumulation in corn, but spores produced in NCLB and GLS 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. The other consideration you should make while scouting is the resistance rating that the hybrid has for NCLB and/or GLS. If it is rated as resistant, then NCLB or GLS severity might not be predicted to get very severe, while in a susceptible hybrid, NCLB or GLS 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 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 1 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.

What fungicide should I spray and should I spray at all? My question is what are you trying to do? Control a disease or simply boost grain yield? Fungicide should be used as a tool to control a disease and preserve yield. There is no silver bullet fungicide out there for all corn diseases. However, there are many products which work well on a range of diseases. The Corn Fungicide Efficacy table lists products that have been rigorously evaluated in university research trials across the country. You can see there are several products listed that perform well on 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, but in our current market, will this average gain cover the fungicide application?

 

Figure 2. Break-even scenarios for corn when foliar fungicide was applied.

The suggested application rate for Headline® Fungicide is 6 to 12 fl oz/acre. My latest cost sheets indicate that at the 6 fl oz/acre rate, the cost of the product alone would be about $12/acre. Note that this does not include the custom applicator cost. This is a variable expense that would need to be added in to get an accurate ROI for your operation. Today we can estimate that we might sell corn grain somewhere between $3 and $4 per bushel. We can then use the cost of the fungicide product and the price of grain to figure out how many bushels of corn we need to make in the crop that would be treated with pyraclostrobin vs. not treating. Figure 2 is a table with various corn prices along the vertical axis and fungicide costs per acre along the horizontal axis. The cells indicate the bushels of corn per acre needed to break even when using a fungicide at the corresponding cost and corn grain sale price. Using the above scenario, we see that with corn priced between $3 and $4 per acre and a fungicide application cost of $between$10 and $15/acre, we would need to gain 2.5 – 5.0 bushels per acre when using Headline® Fungicide in the current corn market. Obviously these calculations are for just one product, but you can do the same for your farm and fungicide program and use the table to figure out what break-even yield gain you will need to cover your costs.

What are the odds of getting that 2.5 to 5 bushel per acre yield gain when using Headline® Fungicide? Paul et al. went further and calculated the probability of return at various corn prices and fungicide costs. They did separate analyses for foliar disease severity less than 5% and greater than 5%. In our current corn market with around $3/bu corn prices and a cost of Headline® Fungicide at $15/acre (fungicide plus custom application), Paul et al. found that at low foliar disease levels (<5% severity) the odds of a positive ROI using the fungicide would be around 50%. The odds of a positive ROI improve if disease severity is greater than 5%. In their calculations with higher levels of disease (>5% severity), the odds of a positive ROI would be between 60% and 70%. The morale of this story is that if you are going to use fungicides on corn, they should be targeted toward fields that will have, or are at risk, for disease!

So what about fungicide application timing? One of the best times to apply fungicide to maximize any benefits for grain corn is during tasseling (VT) and into the silking (R1-R2) timing. In multiple site-year studies across the corn belt of the U.S., application of fungicide on grain corn at VT resulted in over a 7 bu average yield gain. In Wisconsin, the average at the VT timing is about 5.5 bushels. However, this level of yield gain only materializes when a yield limiting foliar disease is active and moving up the canopy. You can check out results of the fungicide trials and the performance of various products over the last few years in Wisconsin by visiting my Fungicide Test Summaries page.

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.

What about fungicide on corn for silage? This practice has been gaining increased interest over the last several years. Dr. Felipe Cardoso’s animal science laboratory at the University of Illinois has published several peer-reviewed papers describing the physiological changes in the corn plant treated with several fungicides, that result in improved feed quality. In those studies yield was often not directly impacted by the fungicide application, but fibrous changes in the corn plant improved feed conversion to milk production in cows fed silage corn treated with fungicide.

Figure 3. Ear rot of corn.

Another possible benefit to treating both silage corn and grain corn with fungicide is the potential reduction in mycotoxin accumulation. Mycotoxins are secondary metabolites produced by fungi. There are 400-500 different known mycotoxins. In corn in Wisconsin, we typically are most concerned with deoxynivalenol (DON or vomitoxin) and fumonisins. These mycotoxins are produced by fungi in the group Fusarium which can cause ear rot issues (Fig. 3) and also stalk rot issues in corn. Recent studies by colleagues in Ontario, Canada have demonstrated that the triazole containing fungicide Proline (active ingredient: prothioconazole) applied at R1 (silking), or shortly after the beginning of silking, can reduce DON levels in corn grain, compared to not treating. The Field Crops Pathology Laboratory at the University of Wisconsin-Madison set out to determine if Proline, and other fungicides and programs, had a similar effect on silage corn treated with fungicide.

Our trial was established at the Arlington Agricultural Research Station located in Arlington, WI in 2017. The brown midrib (BMR) corn hybrid ‘P0956AMX’ was chosen for this study. Corn was planted on 11 May and chopped on 13 Sep. Single applications of various fungicide products (Table 1) were applied at growth stages V6 (19 Jun), R1 (26 Jul), 5 days post R1 (31 Jul), and 10 days post R1 (4 Aug). Ear rot severity was assessed by visually rating five ears per plot on the day of harvest. Yield was determined by harvesting the center two rows of each plot using a small plot silage chopper with an onboard platform weigh system. Chopped sub-samples were collected from each plot and analyzed for deoxynivalenol (DON) content.

Consistent with other datasets, we found no significant difference in yield across treatments (Table 1). Likewise, little difference in quality could be found among all treatments. This could be due to the fact that we started with a high-quality BMR hybrid, thus not readily responding to physiological changes that correspond to increased feed quality when treated with fungicide.

All fungicide treatments resulted in a significant reduction in DON content compared to the non-treated control, except Delaro applied at V6 and Quilt Xcel applied at R1. Application of the experimental 1 fungicide applied at R1 resulted in the lowest DON content among treatments. Remaining treatments had comparable DON levels to experimental 1 except for Quilt Xcel at R1. These results were consistent with previous data from Canada indicating that there is a “sweet spot” of application timing (especially when using Proline), when the goal is to reduce DON. The window of application begins at R1 (silking) and ends around 10 days after the start of R1.

Additionally, our data have shown that the product Quilt Xcel does not reduce DON levels on par with some other products. In fact, in other trials, Quilt Xcel has resulted in higher levels of DON in grain corn. In grass crops like wheat, it has been shown that products containing the strobilurin fungicide class can increase DON levels over not treating. Therefore, these products are not recommended for application after flowering in wheat. This same phenomenon could be possible in corn. Thus, care should be taken when choosing products and programs specifically aimed at reducing DON levels in corn silage.  

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. Here is what you should consider:

1) Corn hybrid disease resistance score – 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 it is cool and wet, 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 recommended.

4) Consider your costs to apply a fungicide and the price you can sell your corn grain – Will you gain enough 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 at VT when high levels of disease are likely.

6) Dairy farmers should think about the overall goal of using fungicide on silage corn. If the goal is to simply alter the corn plant physiologically to improve feed quality, there are numerous products and application timings that have the potential to provide a benefit over not treating. However, if the goal is to target mycotoxins, specifically DON, certain fungicide products may need to be applied specifically during the short silking stage (R1-R2) of the corn plant, to reduce DON levels.

7) 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. To learn more about fungicide resistance, you can CLICK HERE to download a UW Extension fact sheet.

Other Resources

Wisconsin Field Crops Fungicide Information Page

Applying Fungicides to Corn Early in Iowa

References

Haerr, K.J., Lopes, N.M., Pereira, M.N., Fellows, G.M., and Cardoso, F.C. 2015. Corn Silage from corn treated with foliar fungicide and performance of Holstein cows. J. Dairy Sci. 98:8962-8972.

Kalebich, C.C., Weatherly, M.E., Robinson, K.N., Fellows, G.M., Murphy, M.R., and Cardoso, F.C. 2017. Foliar fungicide (pyraclostrobin) application effects on plant composition of a silage variety corn. Animal Feed Science and Technology. 225:38-53.

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.

White, D.G., editor. 2010. Compendium of Corn Diseases. APS Press.

 

Corn Southern Rust Update – August 19, 2017

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

Counties confirmed to have southern rust on corn – 8.18.17

Southern rust of corn has been confirmed in Wisconsin by the University of Wisconsin-Madison Plant Disease Diagnostics Clinic. The sample came in from Kenosha Co. on August 18, 2017. This find is not entirely surprising considering the rapid movement of the southern rust pathogen from the southern U.S. to the northern U.S. this season. The latest southern rust map can be found at http://ext.ipipe.org.

For more information on southern rust, please see my previous post on the subject. You also should visit the Crop Protection Network Southern Rust information page where you can download a brand new fact sheet about southern rust on corn and management of the disease. Note that late planted corn will be more vulnerable to yield loss from the disease. Corn that is still silking (R1 growth stage) to milk (R3 growth stage) is vulnerable to yield loss by southern rust. Corn that is at the R4 (dough) growth stage or later is not as vulnerable and will likely not respond to a fungicide application. Even if corn is at a vulnerable growth stage, remember that we also have to have conducive weather for the pathogen. Extremely dry weather is not conducive for disease. High humidity and temperatures in the 80s favor disease increase.

Continue to scout and growth-stage your corn. If you find evidence of what you think is southern rust, I would encourage you to send it to the Plant Disease Diagnostics Clinic for confirmation.

Midseason Corn Disease Update

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

Northern Corn Leaf Blight symptoms on a corn leaf.

The Wisconsin Field Crops Pathology crew has scouted corn from the southern portion of Wisconsin, to as far north as Spooner. Overall, disease levels are low. We have run into northern corn leaf blight (NCLB) in fields in the southern and central portions of the state. In most cases incidence was in the 10% or less range, with severity in the 5-10% range on leaves below the ear leaf. We have also had several samples arrive in the diagnostic clinic and confirmed with NCLB. For more information on managing NCLB or other corn diseases in Wisconsin, see my previous post here.

Goss’s wilt has been confirmed in Grant Co. via the diagnostic clinic. Other samples have also been submitted that were suspected for Goss’s wilt. However, these turned out to be NCLB. For assistance in differentiating these two diseases, click here to view a PDF quick diagnostic guide.

Common rust remains super common. I have received several questions about spraying fungicide to control common rust. For field corn hybrids, no fungicide will be needed. In any specialty corn situations (inbreds for seed production, sweet corn, etc.) spraying for  common rust might need to be considered. Most field corn hybrids have excellent resistance to common rust and will yield well, despite finding some pustules on a corn plant.

Southern rust has not yet been found in Wisconsin. However, it has been reported very close to Wisconsin (http://ext.ipipe.org). You should continue to be diligent in scouting for this rust disease. Yield reductions can be substantial if the fungus moves in over the next several weeks. Fortunately, our weather systems have been moving into Wisconsin from Canada and Minnesota. This has likely slowed progress of the southern rust fungus from moving into Wisconsin. Click here to view a great new resource on southern rust by the Crop Protection Network.

 

Corn and Southern Rust

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

Figure 1. Corn Southern Rust Observations as of July 21, 2017 (Map from ext.ipipe.org)

If you are like me, you have been paying attention to reports from the southern U.S. indicating that southern rust of corn is making its way further north again this year. You can follow current southern rust updates on the iPiPE site. The latest reports place southern rust in central portions of Iowa and Illinois (Fig. 1), which means farmers in Wisconsin need to start paying attention to this issue. Scouting over the next several weeks is going to be critical for making in-season management decisions for this disease. Yield reductions in Wisconsin will be greatest if southern rust moves in prior to the “milk” (R3) growth stage in corn. Lets take a closer look at southern rust and its less damaging relative, common rust.

Figure 2. Southern rust pustules on a corn leaf. Photo credit: Department of Plant Pathology., North Carolina State University, Bugwood.org

Southern rust is caused by the fungus Puccinia polysora. Symptoms of southern rust are different from common rust in that they are typically smaller in size and are often a brighter orange color (Fig. 2). Pustules of southern rust also typically only develop on the upper surface and will be be more densely clustered. Favorable conditions for southern rust development include high humidity and temperatures around 80F. However, very little free moisture is need for infection to occur. Southern rust is typically a rare occurrence in Wisconsin. When it does occur, it is usually in the southern and south-western portions of the state, with epidemics initiating late in the season. With that said, southern rust did make it to southern Wisconsin in 2016. However, the arrival was well past R3 and yield reductions caused by southern rust were insignificant in Wisconsin. 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. Therefore close monitoring of forecasts and scouting are needed to make timely in-season management decisions.

Management of Southern Rust

Traditionally resistance was used to manage southern rust. However, in 2008 a resistance-breaking race of the southern rust fungus was confirmed in Georgia. Thus most modern hybrids are considered susceptible to southern rust. Rotation and residue management have no effect on the occurrence of southern rust. The southern rust fungus has to have living corn tissue in order to survive and can not overwinter in Wisconsin. Fungicides are typically used to control southern rust in parts of the U.S. where this is a consistent problem. Efficacy ratings are available for fungicides against southern rust on the Corn Fungicide Efficacy Table. As I said previously, should southern rust make its way to Wisconsin prior to the “milk” (R3) growth stage in corn, it could cause yield reductions. 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.

Figure 3. Brick-red Pustules of the common rust fungus on a corn leaf.

Common rust is caused by the fungus Puccinia sorghi and is extremely common in Wisconsin, but often results in little yield loss. Most field corn hybrids planted in Wisconsin are very resistant to the disease. Symptoms can include chlorotic flecks that eventually rise and break through the epidermis to produce pustules of brick-red spores (Fig. 3). Typically these pustules are sparsely clustered on the leaf. They can also appear on other parts of the plant including the husks and stalks. Management for common rust primarily focuses on using resistant hybrids. Remember resistance is not immunity, so some pustule development can be observed even on the most resistant hybrids. Some inbred corn lines and specialty corn can be highly susceptible to common rust. Under these circumstances a fungicide may be necessary to control common rust. Most of the hybrids I have scouted this season have some pustules, however incidence and severity is relatively low. Therefore, a fungicide application to control common rust 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.

Other Useful Resources about Rusts on Corn

Purdue Extension Fact Sheet – Common and Southern Rusts of Corn

WisCONTEXT Article on Southern Rust

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

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

References

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

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

In-Season Corn Disease Management Decisions – 2017

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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.

2017 Field Crop Fungicide Efficacy Tables Now Posted

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

Northern Corn Leaf Blight symptoms on a corn leaf.

The 2017 fungicide efficacy tables are now posted for foliar diseases of corn, soybeans, and small grains. New this year is an added efficacy table for fungicides effective against seedling diseases of soybean. You can access these tables by clicking directly on the links imbedded in this page or by clicking on the Fungicide Information tab above, and scrolling down the page to find the tables. The efficacy ratings are generated based on independent, University efficacy trial data from across the U.S. If you can’t find a particular product on the table, it is likely that it isn’t commonly used, or there isn’t enough data to confidently generate an efficacy rating. Remember to follow all label recommendations attached to the fungicide container. The label label is the law!

2016 Wisconsin Field Crops Pathology Fungicide Tests Summary Now Available

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

The 2016 Wisconsin Field Crops Pathology Fungicide Tests Summary is now available online as a downloadable PDF. This report is a concise summary of pesticide related research trials conducted in 2016 under the direction of the Wisconsin Field Crops Pathology program in the Department of Plant Pathology at the University of Wisconsin-Madison.  We thank many summer hourlies and research interns for assisting in conducting these trials.  We would also like to thank Carol Groves, Jaime Willbur, Megan McCaghey, Bryan Jensen, John Gaska, Adam Roth and Shawn Conley for technical support.

Mention of specific products in this publication are for your convenience and do represent an endorsement or criticism.  This by no means is a complete test of all products available.  You are responsible for using pesticides according to the manufacturers current label.  Follow all label instructions when using any pesticide.  Remember the label is the law!

To download the current report, or past reports visit the SUMMARIES page by clicking here.

2016 Wisconsin Pest Management Update Tour Slides Now Live!

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

Yet another Wisconsin Pest Management Update Tour is in the books. It was great to see everyone again this year. I hope you found value in the presentations and that information can improve farm productivity.  As promised, I have uploaded the slides from the 2016 tour with some of our preliminary data from 2016. You can download a PDF by CLICKING HERE. Hope to see you at a winter meeting near you!

Corn Stalk Rots and Ear Rots: A Double Whammy for Wisconsin Corn Farmers

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

Figure 1. Anthracnose stalk rot symptoms in a cut corn stalk.

Figure 1. Anthracnose stalk rot symptoms in a cut corn stalk.

The 2016 growing season is going to end with many challenges for Wisconsin farmers. The excessively wet weather has slowed or ended harvest of corn silage and grain harvest has barely started in much of the state. Couple this with warm and wet weather is August and we have a double whammy of stalk rot and ear rot issues to contend with this fall.

What is the Primary Stalk Rot Issue in Wisconsin?

Anthracnose stalk rot (Fig. 1) has been a major concern for Wisconsin corn growers this season. Anthracnose stalk rot is typically worse in fields in a corn-on-corn rotation, and/or no-tilled, and planted to a susceptible hybrid. We have observed several fields with significant lodging and wind damage where anthracnose stalk rot has advanced quickly (Fig. 2). In other fields lodging has been minimal, but some anthracnose stalk rot can be found. In addition, to stalk rot anthracnose, we have also observed Fusarium stalk rot and Gibberella stalk rot. The occurrence of these stalk rots has been much less than that of anthracnose stalk rot.

Figure 2. Corn field with considerable lodging due to anthracnose stalk rot.

Figure 2. Corn field with considerable lodging due to anthracnose stalk rot.

Why did stalk rots start so early in 2016?

The late onset of northern corn leaf blight (NCLB) this season likely contributed to increased stalk rot this season. It has been documented that late season increase in leaf blight diseases, such as NCLB, can cause increased stress that leads to higher levels of stalk rot. Also, ears were large and yield potential appeared high this season. These large yield potentials may have led to increased scavenging of resources from stalks leading to more stalk stress. These stress issues, combined with excessively moist and mild conditions, likely led to the occurrence of higher levels of stalk rot in 2016.

What should I do if I have a field with stalk rot?

In fields were stalk rot is an issue, harvest as early as possible to avoid yield losses from lodging. Delaying harvest will increase the likelihood of lodging which will increase harvesting issues. Once conditions dry enough to allow combines to run, fields with higher levels of stalk rot and/or lodging should be prioritized for harvest.

What should I do about stalk rot for next season?

Management of anthracnose stalk rot (and for any of the stalk rots) is multi-faceted. First, choose hybrids with the best resistance available. Hybrids that also have good resistance to foliar diseases will also offer an advantage when managing stalk rot, as foliar disease can stress corn plants and lead to increased risk of anthracnose stalk rot. Cultural practices such as crop rotation and tillage to manage surface residue can also help. Other practices that reduce plant stress such as balanced fertilization, proper planting population, providing suitable drainage, and using well adapted hybrids for your location will reduce the risk of anthracnose stalk rot.

Fungicides are not recommended for managing anthracnose stalk rot. Attempts to use fungicides to manage anthracnose stalk rot often result in high variability and little translation to a yield advantage. In 2015 we conducted a corn fungicide trial where anthracnose stalk rot was detected at harvest. While higher levels of stalk rot were observed, and some treatments did lead to a significant reduction in stalk rot severity, no differences in lodging or yield were identified among the treatments. To view results of this 2015 trial, click here and scroll down to pages 2 and 3.

What corn ear rots and mycotoxins should I watch out for?

Figure 1. Moldy growth on a corn ear caused by the Diplodia ear rot fungus.

Figure 3. Moldy growth on a corn ear caused by the Diplodia ear rot fungus.

With all the wet weather late in the 2016 season, several ear rots have appeared in corn around much of the state. Ear rots caused by fungi in the groups Diplodia (Fig. 3), Fusarium, and Gibberella (Fig. 4) will be the most likely candidates to watch for as you begin harvest.  Fusarium and Giberrella are typically the most common fungi on corn ears in Wisconsin.  This group of fungi not only damage kernels on ears, but can also produce toxins called mycotoxins.  These toxins (fumonisins and vomitoxin) can threaten livestock that are fed contaminated grain.  Thus grain buyers actively test for mycotoxins in corn grain to monitor mycotoxin levels to be sure they are not above certain action levels established by the U.S. Food and Drug Administration (FDA).

The FDA has established maximum allowable levels of fumonisins in corn and corn products for human consumption ranging from 2-4 parts per million (ppm).  For animal feed, maximum allowable fumonisin levels range from 5 ppm for horses to 100 ppm for poultry. Vomitoxin limits are 5 ppm for cattle and chickens and 1 ppm for human consumption.

Figure 4. Symptoms and signs of Gibberella ear rot of corn.

Figure 4. Symptoms and signs of Gibberella ear rot of corn.

Diplodia ear rot (Fig. 5) is not as common in Wisconsin. However, the weather pattern this season was favorable for occurrence of this disease. This disease is often more severe in years where dry weather precedes silking, followed by wet weather immediately after silking. Diplodia ear rot does not produce mycotoxins. While this disease does not result in mycotoxin accumulation, it can cause grain yield loss and quality issues.

For more information about ear rots and to download a helpful fact sheet produced by a consortium of U.S. corn pathologists, CLICK HERE.

Figure 5. Signs and symptoms of the Diplodia ear rot fungus inside a split corn ear.

Figure 5. Signs and symptoms of the Diplodia ear rot fungus inside a split corn ear.

How do I reduce mycotoxin risks at harvest?

Before harvest, farmers should check their fields to see if moldy corn is present. Sample at least 10-20 ears in five locations of your field. Pull the husks back on those ears and observe how much visible mold is present. If 30% or more of the ears show signs of Gibberella or Fusarium ear rot then testing of harvested grain is definitely advised. If several ears show 50-100% coverage of mold testing should also be done. Observe grain during harvest and occasionally inspect ears as you go. This will also help you determine if mycotoxin testing is needed.

If substantial portions of fields appear to be contaminated with mold, it does not mean that mycotoxins are present and vice versa. Remember, Diplodia ear rot does not produce mycotoxins. However, if you are unsure, then appropriate grain samples should be collected and tested by a reputable lab.  Work with your corn agronomist or local UW Extension agent to ensure proper samples are collected and to identify a reputable lab.  If tests show high levels of mycotoxins in grain, that grain SHOULD NOT BE BLENDED with non-contaminated corn.

For more information on mycotoxins and to download a fact sheet, CLICK HERE.

Helpful information on grain sampling and testing for mycotoxins can be found by CLICKING HERE.

For a list of laboratories that can test corn grain for mycotoxins, consult Table 2-16 in UW Extension publication A3646 – Pest Management in Wisconsin Field Crops.

How should I store corn from fields with ear rots and mold?

If you observe mold in certain areas of the field during harvest, consider harvesting and storing that corn separately, as it can contaminate loads; the fungi causing the moldy appearance can grow on good corn during storage.  Harvest corn in a timely manner, as letting corn stand late into fall promotes Fusarium and Gibberella ear rots.  Avoid kernel damage during harvest, as cracks in kernels can promote fungal growth.  Also, dry corn properly as grain moisture plays a large roll in whether corn ear rot fungi continue to grow and produce mycotoxins.  For short term storage over the winter, drying grain to 15% moisture and keeping grain cool (less than 55F) will slow fungal growth.  For longer term storage and storage in warmer months, grain should be dried to 13% moisture or less. Fast, high-heat drying is preferred over low-heat drying. Some fungi can continue to grow during slow, low-heat drying. Also, keep storage facilities clean.  Finally, mycotoxins are extremely stable compounds: freezing, drying, heating, etc. do not degrade mycotoxins that have already accumulated in grain. While drying helps to stop fungal growth, any mycotoxins that have already accumulated prior to drying will remain in that grain. The addition of acids and reducing pH can reduce fungal growth but will not affect mycotoxins that have already accumulated in harvested grain.

For more information on properly storing grain and to download a fact sheet on the subject, CLICK HERE.

References

This article is a compilation of the following previously written resources:

Smith, D.L. 2016. Wisconsin Late-Season Corn Disease Update. /2016/09/07/wisconsin-late-season-corn-disease-update/.

Smith, D.L. and Mitchell, P. D. 2016. Wet Wisconsin: Moldy Corn and Crop Insurance. http://ipcm.wisc.edu/blog/2016/09/wet-wisconsin-moldy-corn-and-crop-insurance/.

Wisconsin Late-Season Corn Disease Update

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

Figure 1. Anthracnose stalk rot symptoms in a cut corn stalk.

Figure 1. Anthracnose stalk rot symptoms in a cut corn stalk.

NCLB and Anthracnose Stalk Rot

As corn silage harvest has begun and the corn grain crop is finishing, there have been some disease issues of note in Wisconsin. Northern corn leaf blight (NCLB) activity has picked up quickly over the last several weeks. This is due to the fact that the weather has become much cooler and has remained wet. These conditions are favorable for the fungus. You will remember that NCLB was observed very early this season. See my previous post on this topic by clicking here.The hotter and dryer weather we saw mid-season was not only good for corn growth, but it kept the NCLB pressure minimal during the critical time of silking and pollination. As stated in the fourth edition of the “Compendium of Corn Diseases” (Carson, 2016) direct yield losses from NCLB are typically minimal if infection is moderate or delayed until 6 weeks after silking. Therefore, the expected direct yield loss from NCLB in Wisconsin in 2016 is expected to be low, due to its late onset.

Figure 2. Corn field with considerable lodging due to anthracnose stalk rot.

Figure 2. Corn field with considerable lodging due to anthracnose stalk rot.

Certainly, there are other issues to consider with this late onset of NCLB. Dry-down will be accelerated. If you have a considerable epidemic in silage corn, then it would be advisable to try to chop as quickly as possible or consider taking the crop as high-moisture corn. Another issue to consider is the fact that a late-season NCLB epidemic can increase the risk for stalk rot issues. We have observed earlier than typical onset of anthracnose stalk rot this season (Fig. 1). Anthracnose stalk rot has been observed in many fields, with a range in severity dependent on the hybrid resistance and field history. Fields in a corn-on-corn rotation, and/or no-tilled, and planted to a susceptible hybrid are at high risk of severe symptoms. We have observed several fields with significant lodging and wind damage where anthracnose stalk rot has advanced quickly (Fig. 2). In other fields lodging has been minimal, but some anthracnose stalk rot can be found.

Management of anthracnose stalk rot is multi-faceted. First, choose hybrids with best resistance available. Hybrids that also have good resistance to foliar diseases will also offer an advantage when managing stalk rot, as foliar disease can stress corn plants and lead to increased risk of anthracnose stalk rot. Cultural practices such as crop rotation and tillage to manage surface residue can also help. Other practices that reduce plant stress such as balanced fertilization, proper planting population, providing suitable drainage, and using well adapted hybrids for your location will reduce the risk of anthracnose stalk rot. Finally, in fields were stalk rot is an issue, harvest as early as possible to avoid yield losses from lodging.

Fungicides are not recommended for managing anthracnose stalk rot. Attempts to use fungicides to manage anthracnose stalk rot often result in high variability and little translation to a yield advantage. In 2015 we conducted a corn fungicide trial where anthracnose stalk rot was detected at harvest. While higher levels of stalk rot were observed, and some treatments did lead to a significant reduction in stalk rot severity, no differences in lodging or yield were identified among the treatments. To view results of this 2015 trial, click here and scroll down to pages 2 and 3.

Bacterial leaf streak – A new disease of corn in the U.S.

Bacterial leaf streak (BLS) of corn has recently been reported for the first time on corn in the U.S. The first reports were in Nebraska with subsequent reports coming in from other states in the U.S. corn belt including Iowa, Illinois, Colorado, and Kansas. Efforts are underway in Wisconsin to monitor for the disease. As of this writing, BLS has not been found in Wisconsin. However, survey and scouting efforts are continuing, to monitor for this disease.

Bacterial leaf streak is caused by a bacterium named Xanthomonas vasicola pv. vasculorum. Very little is understood about this disease on corn, as it is so new. This pathogen presents no risk to humans or animals and there is little evidence to suggest that it will have an adverse effect on corn yield and quality. You can click here to read the USDA APHIS Statement on Xanthomonas vasicola pv. vasculorum. To learn more about the disease and to watch a video by Dr. Tamra Jackson-Ziems at the University of Nebraska CLICK HERE. Helpful information and hints on initially diagnosing BLS can be found HERE.

If you suspect that you have BLS in your corn crop in Wisconsin, 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.

References

Esker, P. 2016. Anthracnose stalk rot: in “Farmers Guide to Corn Diseases” Edited by: K. Wise, D. Mueller, A. Sisson, D. Smith, C. Bradley, and A. Robertson. APS PRESS.

M.L. Carson. 2016. Northern Corn Leaf Blight: in “Compendium of Corn Diseases, Fourth Edition.” Edited by: G.P. Munkvold and D.G. White. APS PRESS.