Dr. Damon Smith, or @badgercropdoc, is an extension specialist and associate professor at the University of Wisconsin, Madison. We’re talking about different agronomic issues farmers have faced this year, including a newer one: tar spot. You can listen to the Podcast here.
Damon L. Smith, Ph.D., Associate Professor and Extension Specialist, Department of Plant Pathology, University of Wisconsin-Madison
Brian Mueller, M.S., Assistant Researcher, Department of Plant Pathology, University of Wisconsin-Madison
John Goeser, Ph.D., Adjunct Assistant Professor, Department of Dairy Science, University of Wisconsin-Madison and Animal Nutrition and R&I, Rock River Labs
The 2018 corn growing season can be summed up by saying it has been challenging. In Wisconsin, corn diseases have kept producers and agronomists moving to try to manage multiple issues for much of the summer. Early in the season gray leaf spot (GLS) moved in quick, followed by onset of a new disease called tar spot (TS), then northern corn leaf blight (NCLB) reared its head in August. This was then followed by high levels of ear rot in some fields. This is the first season that I have had to rate four diseases in one research trial. While GLS, NCLB, and TS create their own unique management challenges they can also stress the corn plant which results in secondary issues such as loss in stalk integrity or a plant that is left susceptible to other pathogens. I have written a previous article describing some of these issues, and how to deal with them.
To manage some of these foliar issues, increasing interest has been placed on using fungicides, especially in grain corn. Over the last several seasons, interest is growing from dairy producers who are also treating corn silage hybrids with fungicide. Why all the interest in treating silage corn with fungicides? Some of the intent is to improve feed digestibility. 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 fungicides, that result in improved feed digestibility. 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 (Haerr et al., J. Dairy Sci., 2015; Kalebich et al., J. Animal Feed Sci., 2017). Our laboratory at the University of Wisconsin-Madison is also interested in the effects that fungicides might have on mycotoxin accumulation in silage corn hybrids. We are especially interested in the accumulation of deoxynivalenol (DON or vomitoxin). In corn, DON is primarily produced by a fungus called Fusarium graminearum. Fusarium graminearum can cause Gibberella ear rot (Fig. 1) and also Gibberella stalk and crown rot of corn (Fig. 2).
Mycotoxins are secondary metabolites produced by fungi, that can be toxic to plants and/or animals (including humans). These are termed secondary metabolites as they are not produced by primary metabolism in fungi. We understand some things about secondary metabolism in fungi, but there is much left to learn. Stressors on fungi can be responsible for inducing secondary metabolism, but other environmental and substrate cues might also be important. The inconsistency in induction of secondary metabolism in fungi, might be one reason that we see no linear relationship between ear rot in corn and vomitoxin accumulation. It is not uncommon to find low levels of ear rot, yet find high levels of vomitoxin in finished corn grain.
Animal nutritionists have observed many impacts of mycotoxins on animals, including dairy cattle. These can range from simple reductions in milk production all the way on up to feed refusal, 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 DON, the suggested 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.
So how do fungicides affect DON in corn plants at harvest? Let’s look at some data from Wisconsin in 2017 and 2018.
Results of 2017 Silage Corn Fungicide Trials
During the 2017 growing season, we conducted a small-plot research trial at the Arlington Agricultural Research Station located in Arlington, WI. 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 (Fig. 3) 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 (Fig. 3). Unlike other studies, little difference in digestible fiber (TTNDFd) 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 (Fig. 4). 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 narrow window 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.
Results of 2018 Silage Corn Fungicide Trials
In 2018, we repeated trials that we began in 2017. However, we made several modifications, including adding treatments and adjusting application timings. We also added another hybrid, in addition to repeating trials on PO956AMX. The new hybrid was another BMR, F2F627. We also made plots wider. We increased the width so that we could harvest our center two rows of plot using the small-plot chopper and also destructively sample plants on rows 2 and 5. We cut 5 plants from these rows in a subset of fungicide-treated, or non-treated plots. We then removed the ears (husks attached to ear) from the stalks. We ground the stalk portion and the ear portion and kept these samples separate, so that we had paired-samples for each of these plots. We wanted to know where DON was accumulating in the corn plant. Remember that F. graminearum can cause a stalk rot AND an ear rot. Plots were planted on May 1 in 2018. Growth stage V6 applications were made on June 15, V12 on July 11, R1 on July 18, and R2 on August 1. Plots were chopped on Sep 12. All data acquisition and sample testing was the same as in 2017, with the exception of adding the paired-samples for DON testing. Data from each hybrid were analyzed separately as these hybrids were planted in two separate blocks of the same field.
Figure 5 shows mean foliar disease, ear rot, yield, forage quality, and DON levels for both hybrids. Data are sorted by DON level and red highlights of certain products/timings are included to show which were consistent across both hybrids. Foliar disease and ear rot were higher in 2018 than 2017. Each hybrid responded a bit differently to each disease. Similar to 2018 there was little effect of fungicide on yield and forage quality. For PO956AMX, fungicide application also did not significantly affect DON levels. For F2F627 there were some marginally significant differences in DON as a result of fungicide application. Regardless, DON levels were extremely high in this field. This was a natural epidemic, indicating that the environment was very conducive for the fungus in 2018. Some products were consistent across trial in giving some reduction relative to the non-treated. These included Proline applied at R1, Delaro applied at R2, and Miravis Neo applied at V6. With that said, levels of DON were still over 7 ppm, even in better performing treatments. In years where the environment is conducive for F. graminearum, fungicide alone won’t be enough to reduce DON levels to acceptable limits. Incorporation of other techniques such as resistant hybrids will be needed.
What Part of the Plant is DON Accumulating?
Results of our paired-sample analysis were interesting. We chose to sample plots from the non-treated and two fungicide treatments. The details of which fungicide treatments are not important here, as treatment had little effect on DON level in this analysis (Fig. 6). The part of the plant we sampled was significant, as was the interaction of hybrid and plant part. Interestingly, we were able to find high levels of DON in stalks and ears, with PO956AMX actually having twice the level of DON in stalks compared to ears. F2F627 responded differently with higher DON in ears than stalks.
We also dug a little deeper into this relationship, and conducted some correlation analysis with several parameters from these plots. Bissonette et al. (2018) reported that in wheat, DON levels in grain were positively correlated with straw DON levels. They hypothesized that this correlation was due to the fact that DON is water soluble and can be washed from the grain to the straw, in the field, during rain events. We wanted to see if stalk DON was correlated with ear DON in our corn trials, as a similar phenomenon might exist in corn.
Figure 7 shows that while ear DON levels are correlated (either positively or negatively) with some other parameters, ear DON was not significantly (alpha=0.05) correlated with stalk DON. In fact, the relationship (while not significant at alpha=0.05) was actually negatively correlated. This suggested that stalk DON levels and ear DON levels might be originating from independent events in these corn trials. Remember that F. graminearum can cause a stalk and crown rot AND/OR an ear rot. These different diseases can occur independent of each other. Thus, it is plausible that the stalk DON levels might be due to stalk infection and subsequent rot, that isn’t necessarily related to ear rot in corn.
What does all of this data mean? Well, first it means the Badger Crop Docs have some more work ahead of them. However, we have some interesting data that suggests the following:
-Farmers should test stalks for DON level if planning to feed
-Think crown and stalk infection vs. ear infection by F. graminearum
-Different than wheat, where water-leaching of DON may be leading to straw DON
-It might be hard to get fungicide into stalks to reduce stalk infection; Thus, DON still accumulates in the stalk portion, independent of ear infection control by fungicide applied at R1
-Could V6 application timings be reducing stalk infection and subsequent stalk DON accumulation? More work needs to be done.
A Farmer’s Guide to Corn Diseases. 2016. Wise, K., Mueller, D., Sisson, A., Smith, D., Bradley, C., and Robertson, A., eds. APS Press, St. Paul, MN. 161 pp.
Bissonnette, K.m., Kolb, F.L., Ames, K.A., and Bradley, C.A. 2018. Effect of Fusarium head blight management practices on mycotoxin contamination of wheat. Plant Disease 102:1141-1147.
Compendium of Corn Diseases, 4thEdition. Munkvold, G. and White, D., eds. APS Press, St. Paul, MN. 165 pp.
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. Journal of Diary Science 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.
Mycotoxins: Risk in Plant, Animal, and Human Systems. 2003. Richard, J.L., Payne, G.A., Desjardins, A.E., Maragos, C., Norred, III, W. et al., eds. Council for Agricultural Science and Technology, Ames, IA. 199 pp.
Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison
The 2018 corn growing season has been met with numerous disease challenges this season. From typical foliar disease issues like gray leaf spot and northern corn leaf blight, to new diseases like tar spot and bacterial leaf streak, the season has not been easy. As we have started to chop silage, ear rot and mycotoxin issues are also readily apparent.
In an effort to address the new disease, tar spot, we have put together a new video on what we know and don’t know. You can view that new video on YouTube, by CLICKING HERE. We have also assembled a second video on ear rots and mycotoxin issues in silage corn. That video can be found on YouTube by CLICKING HERE.
We hope you find these videos informative and help you gain ideas to manage these issues in your operation.
Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison
Carol Groves, Associate Researcher, University of Wisconsin-Madison
Brian Hudelson, Plant Disease Diagnostician, University of Wisconsin-Madison
Sue Lueloff, Assistant Plant Disease Diagnostician, University of Wisconsin-Madison
The 2018 corn production season in Wisconsin has been challenging to say the least. We had what looked to be some of the best corn production we ever had, and then the diseases started to move in. We have observed numerous foliar disease issues and have spent a lot of time trying to understand the tar spot epidemic in Wisconsin and surrounding states.
To add insult to injury, we have now confirmed bacterial leaf streak (BLS) of corn. You may remember that we have been on the lookout for this disease over the past several seasons, but have not confirmed it officially in the state until now. A corn sample was received in our Plant Disease Diagnostic Clinic this season from Pierce County with symptoms consistent with those for BLS (Fig. 1). The sample was confirmed positive in our clinic through multiple tests, including bacterial streaming and PCR. Subsequently, the sample has been confirmed positive by multiple laboratories, including the CPHST-Beltsville Laboratory.
Bacterial leaf streak (BLS) of corn was reported for the first time on corn in the U.S. in 2016, but was likely present in Nebraska since 2014. The first report was in Nebraska with subsequent reports coming in from other states in the U.S. corn belt. Other states where the disease has been confirmed include Iowa, Illinois, Colorado, Kansas, Minnesota, Oklahoma, South Dakota, Texas, and now Wisconsin.
What causes bacterial leaf streak and what are the symptoms?
Bacterial leaf streak is caused by a bacterium named Xanthomonas vasicola pv. vasculorum. It causes wavy narrow leaf lesions with wavy edges that are often brown in color. Lesions can appear translucent and have halos when backlit. Symptoms on corn have been observed as early as V7, starting in the lower canopy and moving up the canopy if weather conditions are favorable (wet weather, with hot temperatures). Little is known about the disease cycle, but researcher believe it can overwinter on corn residue. The bacterium is presumed to be spread by irrigation, splashing rain, or wind-driven rain. No injury is needed for the bacterium to enter the plant. It is unknown if the bacterium can be spread on, or in, seed and if there are alternative weed hosts.
Does bacterial leaf streak cause yield loss?
Little is actually known about the disease on corn in the U.S. Most researchers believe that yield loss is minimal if the disease moves in late in the season. If the disease moves in earlier and causes extensive leaf blighting during grain fill, then yield losses could be more substantial. Little is known about the effect of BLS on grain quality.
How do I manage bacterial leaf streak of corn?
Some corn hybrids appear to have better resistance to BLS than others. Work with your seed dealer to find a hybrid that is rated as resistant and fits your environment. Hybrid resistance will be key to manage this disease. BLS is caused by a bacterium, thus, fungicides are not effective in controlling this disease. Withholding irrigation has also been shown to not be effective as the disease can occur in drylands and irrigated fields. Managing corn residue through rotation may be helpful. Tillage and burying residue might also be an option, but managing soil erosion should be placed as a higher priority.
Other Resources about bacterial leaf streak
- 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.
- To learn how BLS is diagnosed in the lab, CLICK HERE to watch a video from Iowa State University.
- CLICK HERE to download a fact sheet on BLS of corn, by a team of U.S. plant pathologists.
How do I get a diagnosis if I suspect bacterial leaf streak?
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.
Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison
Corn is looking pretty rugged in many areas of the Wisconsin corn belt. Areas in southern, southwestern, and south-central Wisconsin have experienced major foliar disease epidemics including the new disease, tar spot. Areas in eastern, east-central, and south-central Wisconsin have also seen heavy flooding and storm damage in corn fields. We have seen fields severely diseased, experiencing stalk rot, lodged, flooded – you name it, it has been a challenging finish to a season that had much promise.
How is tar spot affecting stalk integrity?
For corn foliar diseases such as northern corn leaf blight (NCLB) and gray leaf spot (GLS), it is well known that high severity can lead to stalk integrity issues. As foliage is damaged, less photosynthetic capacity is available from the leaves to produce carbohydrates for the plant. To fill an ear of corn, carbohydrates are needed from somewhere. In corn where the foliage is significantly damaged, the stalks become a considerable source to fill out the ear (a sink for nutrients). This leaves the stalk tissues devoid of carbohydrates leading to cell death and subsequent colonization of the stalk by fungal pathogens who are taking the opportunity to feed on a weak stalks. Thus, it isn’t uncommon to see stalk rots like Gibberella stalk rot, Fusarium stalk rot or Anthracnose stalk rot at higher incidence where high foliar disease pressure was observed (Fig.1). Where you find stalk rots, you often find root rots caused by the same pathogens. Root rot and stalk rot often go hand-in-hand.
Other causes for loss in stalk integrity can include large ears (nutrient sinks) that the plant can’t fill out, without using some of the stalk resources. In 2018 we saw many fields where the crop was moving through growth stages quickly and setting what appeared to be good yields. However, weather conditions changed midseason, with wet weather and more cloud cover, combined with nitrogen issues in some fields. This led to large ears that needed to be filled out, with again, limited photosynthetic capacity. The stalks were scavenged for carbohydrate, leaving them, again, with limited integrity.
Now throw in some tar spot. Yet, another foliar disease that can limit photosynthetic capacity of the corn plant. We have observed many fields with significant stalk integrity issues. Whether just tar spot, or tar spot combined with GLS, NLCB, and/or stalk scavenging just for carbohydrates – stalks are in bad shape in many areas of Wisconsin. This is resulting in significant lodging issues in many fields, especially those hit with bad storms over the last several weeks (Fig. 2). Harvesting fields with low stalk integrity early will be key to protect yield potential. Conduct a “pinch” test or “push” test to determine which field have lower stalk integrity. Simply pinch stalks or push stalks to a 30 degree angle. Those plant that are soft and easily pinch or don’t pop back up after pushing, have stalk integrity issues. If 30-50% or more of these plants are identified with stalk integrity problems, they should be harvested first, to prevent yield losses from lodging.
What about tar spot, lodged corn, and mycotoxins?
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. As plants dry down they can no longer actively fight fungal infection. We have looked at many brown and drying leaf samples from corn plants with tar spot. We do find many other fungal organisms, including Fusarium-organisms, which can produce mycotoxins. So while tar spot itself may not lead to mycotoxins, opportunistic fungi that colonize secondarily may result in elevated mycotoxin levels.
In addition, corn that has lodged and is in contact with the wet and saturated ground is at risk of being colonized by organisms that produce mycotoxins. Many of the known mycotoxin-producing fungi are found in the soil and on residue on the surface of the soil. If lodged corn is in contact with the ground and there is good moisture, it is possible that the ear and plant are being colonized and mycotoxins are being produced. So while your combine might be able to pick a plant up and harvest the ear, beware that it might be heavily colonized with organisms that produce mycotoxins. If taking corn for silage, lodged plants run the risk of significant hygiene issues in the bunker, including mycotoxins issues.
Where else can mycotoxins come from?
Corn ears don’t have to touch the ground to be infected with ear-rot fungi, they can also be colonized by ear-rot fungi through the silks. Given the kind of crazy year we have had, ear rot might be a significant concern in fields that saw erratic weather this season. Ear rots caused by fungi in the groups Diplodia (Fig. 3), Fusarium, and Gibberella 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 mycotoxins. The toxins of main concern produced by these organisms are fumonisins and vomitoxin and can threaten livestock that are fed contaminated grain. Thus grain buyers actively test for mycotoxins in corn grain, and feed managers monitor silage for 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.
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. For example, 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.
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.
Munkvold, G.P. and White, D.G. Compendium of Corn Diseases, 4th Edition. APS Press.
In addition, This article is a compilation of the following previously written resources:
Smith, D.L. 2016. Wisconsin Late-Season Corn Disease Update. http://fyi.uwex.edu/fieldcroppathology/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/.
Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison
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?
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?
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.
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)
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)
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 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.
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.
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?
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.
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.
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.
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.
Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison
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.
Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin-Madison
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.