Holy Tar Spot, Batman!

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

Figure 1. Tar Spot on Corn From the Arlington Agriculture Experiment station in 2018.

The phone has been ringing off the hook over the past week. In fact, this morning I had to dump my voicemail as it wouldn’t take any more messages. Why is that? For those of you who have been traveling the southern Wisconsin and Northern Illinois countryside, you know why. Holy tar spot, Batman!

Tar spot (Fig. 1) is no longer a cosmetic leaf disease in Wisconsin and Illinois. We have seen epic levels this season, resulting in severe damage in some fields and early dry-down of corn. 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, but also has expanded to include reports from as far north as Columbia Co. over to the eastern side of the state, including Fond Du lac and other areas of the cron belt. Severity has ranged from simple cosmetic damage to complete death of entire fields. Unfortunately, it is hard to discern why the epidemic is so significant here in the upper Midwest in 2018. Some anecdotal thoughts include hybrid susceptibility, environmental impacts, and synergistic reactions between multiple organisms.

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. In addition, a third organism, Coniothyrium phyllachorae, has also been found to be associated with the complex. This organisms has also not been implicated in the epidemics in the Midwest. So, our epidemic of 2018 is a real head-scratcher.

What is known about tar spot?

Figure 2. Tar spot with “Fisheyes” on the upper surface of corn leaves.

Tar spot is favored by cool conditions (60-70 F) and high relative humidity (averages above 75%). It is generally accepted that when Phyllachora maydis occurs by itself, damage is cosmetic in nature and does not result in significant yield reductions. This has been most of the case since 2016 when tar spot was first identified in Wisconsin. In Latin America where Phyllachora maydis can form complexes with Monographella maydis and/or Coniothyrium phyllachorae, “fisheye” symptoms along with severe necrosis and early dry down can be observed. In Latin America, it has been reported that when in complex, damage from tar spot can result in yield loss as much as 30%. The main method to manage tar spot in Latin America is to use resistant hybrids. Little is known about effective fungicides, especially fungicides that might be available in the U.S.

So is tar spot behaving the same in the Midwest U.S. as in Latin America?

The simple answer is, I don’t know. We can easily find the presence of Phyllachora maydis on affected corn in the Midwest. However, we aren’t able to easily find the other organisms that have been implicated. However, we do find “fisheyes” and significant necrosis on corn leaves (Figs. 2 and 3). It could be that Phyllachora maydis is forming a complex with other organisms in the Midwest. Things don’t have to be the same as in Latin America. One simple observation is that in areas where gray leaf spot (GLS) got an early foothold on corn, we have seen more severe epidemics of tar spot. Is that the complex in Wisconsin? Not sure, but definitely a lot of damage in some fields by both organisms. This is why we are working hard right now to identify organisms implicated in causing this damage. We need to know what the complex might be here in Wisconsin and the Midwest.

What is the impact on yield and do I need to worry about mycotoxins?

Figure 3. Tar spot with “Fisheyes” on the lower surface of a corn leaf.

We are not sure what the impact on yield will be yet. This is a hard question to answer and will depend on when the epidemic started in a field and how severe symptoms will be. If I had to make an educated guess, on grain corn, damage will likely range from none, to low test weights on moderately affected fields, to some yield loss in fields that were hit early. On silage corn, it might be difficult to pack the bunker as dry-down is occurring rapidly and moisture might be too low to make quality silage. Mycotoxins have not been implicated in the organisms reported to cause tar spot in Latin America. However, that doesn’t mean that other organisms that cause mycotoxins might not be present on harvested grain or silage. While we don’t expect there to be an issue with mycotoxins, I would encourage you to continue to test grain and feed to be sure mycotoxins are not present.

What should I do with fields affected by tar spot?

Scout these fields. It will be important to make timely harvest decisions. For fields that will be taken for grain, consider harvesting these fields early. One observation we have made is that in fields affected by early epidemics and dry-down, stalks have been scavenged for nutrients to finish filling the ear. This is leaving stalk integrity in some of these fields as highly questionable, with various stalk rots moving in. Timely harvest will be key in these fields before excessive lodging occurs. For silage, other considerations, such as high-moisture corn, might be needed where corn is too dry to effectively pack the bunker. Also, be sure to continue to test for mycotoxins just to be sure there aren’t other organisms capitalizing on already stressed corn plants. DO NOT apply fungicides at this point. Fungicides will not be effective after the milk growth stage and most fungicides have a pre-harvest interval that has already passed for both silage and grain.

What is the Wisconsin Field Crops Pathology program doing to understand tar spot?

While frustrating for those of you who have to deal with this disease, we do need to take our time to try to figure out what the cause of the epidemic is. If we don’t know what to control, how do we develop a management plan? You wouldn’t just take antibiotics, if you didn’t know what caused your sore throat, would you?

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. If we can get a better handle on what is causing the the significant damage on corn, we can make informed decisions about how to manage it. Our program is also rating and taking notes in fungicide trials where we have tar spot. We aren’t sure we will find differences, but taking notes on timing of application and products will inform our recommendations. We also are looking for hybrid trials where we could take notes on resistant hybrids. Finally, we have initiated a late-season field trial to begin to understand the epidemiology of this disease. If we can understand when it moves in and how long it takes to see symptoms after infection, we might be able to better apply in-season management strategies. In summary, we are working on trying to find some answers, but research and acquiring relevant data will take a little time.

Other Resources

Invasive Species Compendium – Phyllochora maydis

USDA-ARS Fact Sheet – Tar Spot of Corn

University of Illinois Pest Bulletin

Late Season Corn Foliar Disease Update and Hail-Damaged Corn

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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.