Let's Talk About Tar Spot Management With Dr. Matthew Helm

Let's Talk About Tar Spot Management With Dr. Matthew Helm


First identified in 2015, tar spot has continued to ravage crops, with 2020 and 2021 seeing the brunt of its ever-evolving attack. A lot of research is now being done on tar spot management to mitigate its impacts. Joining Todd Steinacher is Dr. Matthew Helm, a research molecular biologist with the Agricultural Research Service arm of the United States Department of Agriculture. Matthew explains the origins, cause, and manifestation of tar spot to crops as it evolves with the Midwest climate. They also touch on where the research on tar spot in corn is headed, from corn varieties to identifying underlying genes and what farmers can do to prevent the spread on their crops. Keep your ears glued and tune in to this educational conversation.

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Let’s Talk About Tar Spot Management With Dr. Matthew Helm

In this episode, I’d like to take a deeper dive into a pest that has been creating a lot of challenges and attention this harvest, which is tar spot in corn. With that, I would like to introduce our guest, Matt Helm. He is a PhD in Research Molecular Biologist. Welcome to the show, Matt.

Thank you for having me, Todd. It’s a pleasure being with you.

I always liked to get a little background where we’re folks come from as far as our guests, if you could take a moment and tell us about your agronomic journey and what’s led you to study tar spot.

My educational background starts essentially in Northern Central Indiana. I graduated high school from Elwood Community High School. It’s a small town in Indiana. I then went to Manchester University and there, I started my journey as a plant scientist by taking two very influential classes. One of them being Ethnobotany and the other being Plant Physiology.

With Ethnobotany, that’s essentially the study of how plants have influenced society and how society has influenced plants and that synergistic interaction between the two. That was eye-opening and I love that class. It fundamentally changed my trajectory to becoming more interested in Plant Science. After taking Ethnobotany, I then took Plant Physiology and that’s where you study the molecular and genetic basis of how plants function.

That sealed the deal with respect to earning and doing the PhD in Plant Science Research. Following my Bachelor’s Degree at Manchester, I then went to Southern Indiana at Indiana University in Bloomington, Indiana. That’s where I joined the lab of Dr. Roger Innes and that specific lab focuses on plant disease resistance. Specifically, how plants are able to recognize pathogens and how they mount on defense response.

During my PhD, I study soybean primarily, and I showed that you could tailor the immune responses of plants so that they can confer disease resistance against a multitude of pathogens. You’re not relying on fungicide applications. You’re using the endogenous immune system of plants and we’re tweaking that a little bit.

I graduated with my PhD in 2019, and then performed a Postdoctoral Research Assistantship with the United States Department of Agricultural Research Service or, for short, the USDA-ARS. In that particular USDA ARS research unit, was housed at Purdue University. During my Postdoc research, I continued to work on disease resistance and crop plants but I transitioned mostly to working on wheat and corn. That Postdoc research provided a platform for studying tar spot in corn and allowed me to be selected to have a permanent position with the USDA ARS as a Permanent Research Scientist. I began that role in March of 2021, and continue to love my position, and it’s exciting to study tar spot in corn.

You’ve got a good depth of knowledge and background to bring you to the forefront to study in tar spot. I know you said you enjoy studying tar spot and probably, most of our readers that are reading cringe on it because we don’t want to know, see or hear about it. We need folks like you out there that get up every day and enjoy that to give good agronomic insight to not only me, the world or the AgriGold agronomist team but also growers, so we can all make better agronomic decisions as we’re on this path for higher yield that we can bring some consistency to it. If we have diseases coming in throughout 2021, that’s what’s going to knock our consistency in lower our APHs long-term. I do appreciate you being the one that goes down the path to study tar spot because I don’t know if I could look at one particular disease my whole life. I do appreciate your efforts in that realm.

Tar spot was established in the Midwest. It is slowly moving throughout all major corn production areas and in fact, 2021 is arguably one of the worst years for tar spot disease.

I’m very passionate about what I do. I love what I do. It’s a fungal pathogen of corn that essentially no one knows a whole lot about. There are so many varieties of research topics that you can choose from. It’s an exciting time for me as a molecular research biologist to study this pathogen but once you hear the word tar spots, the farming community cringes a little bit.

My goal on this episode is to understand as much tar spot as we can and share it with our readers that you and your colleagues have learned or found some correlations to. There are people in the Countryside who have been battling this since 2015. There are people who heard about it and there are people who did not spray or did and took a 50-bushel hit and having a sustainability issue. Everywhere in between is where a lot of our people are coming from. If you could give me a little bit of the history of what’s led to this almost outbreak of tar spot?

We’ve known about tar spot since the early-1900s. Tar spot was primarily limited to Central America and Northern Latin America. That pathogen was primarily limited in that area. It wasn’t a significant yield loss agent but in the year 2015 and early-2016 growing seasons, we started receiving reports of tar spot around the counties that surround Lake Michigan.

At that time, we wouldn’t have considered tar spot a major fungal pathogen of corn. It didn’t look like it was going to reduce corn yields significantly. In fact, it wasn’t those two growing seasons of what wasn’t a significant player. However, in the year 2018, even this 2021, those were what we call the epidemic years for tar spot. Since the year 2015, tar spot have grown and spread beyond the County surrounding Lake Michigan. Have spread to the majority of counties in Indiana. Most in Illinois, Wisconsin, Minnesota and Iowa. There are reports of tar spot in Pennsylvania, Ontario, Canada, as well as some counties in Florida.

We’re not sure exactly how tar spot came to the Continental United States from Central and Latin America. There are two major hypotheses that are running now. First, it could have been some major weather event such as a hurricane or a storm that basically blew the spores into the Midwest, and because we have several commercial varieties or hybrids of corn that are susceptible, it created a favorable environment for fungal colonization.

The second major hypothesis is that it was introduced through some type of human movement, where you took corn debris from Central America and Latin America, and then introduced it into the Midwest, where we had these susceptible hybrids, and that allowed the establishment of the fungus essentially. There’s no robust and reliable evidence that supports hypotheses. Those are the two major schools of thought essentially but what we do know is that tar spot is established in the Midwest.

It is slowly moving throughout all major corn production areas. In fact, 2021, arguably is one of the worst years for tar spot disease. It’s in majority of Midwest counties and the one piece of advice for farmers that I had is, just because you haven’t found it in your field this season, does not necessarily mean that you’re not going to have it next season.

It’s great if you could continually scalp, and especially during harvest season where you’re harvesting corn and looking through the plant debris. If you see any spots of tar on the leaf, that’s a good indication that you might have to prepare certain management strategies for the next growing season. What I would also consider devastating about this pathogen is that it is able to overwinter, meaning it survives during the cold of the Midwest, which we didn’t appreciate when we were reading studies about tar spots in Latin America and Central America. That overwintering serves as a rich and dense resource of spore inoculum for the following year.

Tar Spot Management: When you’re harvesting corn and looking through plant debris, if you see any of the tar spot or spots of tar on the leaf, that’s a good indication that you might have to prepare certain management strategies for the next growing season.


You’ve got a whole host of things to break down into. On my end, I’m sitting back saying, “Where do I start?” Where I do want to start is the fact that this thing has aggressively spread since 2015. It started in those microenvironments. In my territory, it was always a Northern Illinois type of deal. A few years ago, it started creeping down closer to Central Illinois.

I could find it in some spots and 2021, it blew right past and went clear to Kentucky into Florida. What allows this pest to move that aggressively? To me, we’ve got Southern Rust that comes from the South and it can aggressively move up and annihilate yield. What allows tar spot to be that aggressive and move that bountiful?

From what we’ve known in studies in Central America, we thought tar spot required a good level of moisture conditions. Humidity levels are certainly important. We also thought at the time that cool temperatures were conducive to the spread of tar spot. If you look at the counties that surround Lake Michigan, they receive a lot of lake-effect weather. Those are typically, cooler counties and they have higher humidity conditions.

However, in 2018 and 2021 essentially, it was a hot and dry growing season, so we are considered in a drought for the majority of the growing seasons here. Our running school of thought here is that tar spot isn’t requiring the same environmental conditions that we find in Central America. It’s somehow has adapted to the environmental and growing conditions in the Midwest.

We have anecdotal evidence from researchers here at Purdue University that shows that moisture conditions and moisture levels in humidity play a very important role in the spread of tar spot. Facilitating the aggressive movement of tar spots is the high humidity and the high moisture levels in allowing it to spread beyond the counties that surround Lake Michigan.

Beyond the environmental conditions here, hybrid genetics also plays a very important role. Historically, plant breeders had not bred corn varieties for resistance to tar spot because that’s never been a significant fungal pathogen. Through certain plant breeding pipelines, we could have introduced one or several susceptibility genes in these commercial hybrid varieties that multiple companies grow and produce year after year. It could have been this perfect environment where you have the pathogen evolving to grow in different environmental conditions, as well as it favoring and exploiting the susceptibility of these corn hybrids that we’re growing.

Would you say that if for some reason, let’s say, a field had a problem this 2021, we could physically find that the tar spots chunk some leaves, and in 2022, it’s going to go in the corn-corn. The corn stocks at fall and plant corn next spring. Those spores are going to attack that crop but as it grows within that crop, is it going to release spores to where they are now airborne and going to carry it throughout the field and to other areas? Is that how it’s moving?

Yes. If you have had a tar spot in your field this past growing season, we know the fungus can overwinter and it overwinters. Historically, Midwest winters are freezing cold. We know that those spores over winter and serve as that source of inoculum for the following year. If a farmer is growing a corn-corn rotation. I would strongly advise that the grower begins scaling for tar spot next growing season because there’s a strong likelihood that there will be a high tar spot disease pressure within that field and perhaps neighboring fields as well.

Tar spot overwinters, meaning it survives during the cold of the Midwest.

Let’s say it is a corn-corn situation, and there was some tar spot, whether the populations were low, medium or high, and it can overwhelm the soils in some other pests through residue management and some tillage bearing some of that residue and breaking it down. If it’s on the leaves, we size it appropriately from the combine to maybe a VT tool, then incorporated some soil, get a lot of that residue broke down. Does that eliminate the survival of it or does it need to live on our residue? Can it jump to the soil and live in the soil then?

What we do know is, it’s more than likely requires leaf tissue. It is probably not going to be a soil-borne pathogen. I can’t confidently state that we haven’t ruled that hypothesis out but certainly, we do know that tar spot is what we often refer to as a biotrophic pathogen. That’s a scientific term meaning that this fungus requires a living host in order to be produced, replicate and spread.

To answer your question more directly, it’s probably going to be limited to the leaf tissue. It’s not going to make that jump into the soil and survive in the soil or soil debris. Personally, I’m not a farmer but if I had tar spot on the leaves and this growing season, I would do everything to basically bury that source of inoculum as much as I can. You’re not going to get rid of it but if you can at least suppress the inoculum to lower levels, that will benefit the US farmer greatly.

Let’s say we’re in a corn-bean-corn rotation. We had corn issues that had some problems, whichever degree of pressure. Next year, we go to soybeans. We’re going to no-till soybeans into it. The following year, we’re going to put corn back into it. Can that inoculum live on the dead tissue next year and jump in the next cycle or could it, by not having that susceptible host of live corn plants, cause those spores that are on the residue to not produce and spread?

I don’t know the answer to that one. I honestly, don’t think we’ve tested that, whether you’ve done corn-bean-corn rotation to see if you have tar spots on one year and beans next year. Will you get tar spot the following year? Most of the research has been concerned with it overwintering one season, and then you plant corn the next season instead of corn-bean-corn.

Knowing the virulence of this pathogen and the yield loss associated with this, I would not be surprised if the fungus could overwinter multiple times. It seems it’s a very hardy pathogen when it comes to overwintering. To be completely honest, I don’t think we’ve ever tested that, so we can’t attest to that specifically. That’d be interesting.

Not a problem, as I was thinking of what you’re saying about how that pest requires leaf tissue to survive. Even though over that soybean year, it is leaf tissue but it’s dead tissue. I didn’t know if it would signal it to stay dormant for another year or signal it to, “Go ahead and create your spores, and let’s get up in the air, move and go find a field.”

We know that this tar spot pathogen, it doesn’t infect soybean leaf tissue. It seems to be pretty specific to corn tissue, so I don’t know what type of environmental cue is required for this fungus to transition from this dormant state to more of these inducing spores to infect more corn. We’re not sure what the environmental cue is for that.

You had referenced that it primarily attacks corn but don’t really see much from soybean. There are some diseases that may follow attacking all grasses. From a corn standpoint, we know what attacks corn, can it attack wheat or even ditch grasses and can overwinter in weeds in our ditches, and then flourish the next year?

YMP 23 | Tar Spot Management
Tar Spot Management: Tar spot is what we often refer to as biotrophic pathogen, meaning that it requires a living host to be produced, replicated, and spread. It’s probably going to be limited to the leaf tissue. It’s not going to make that jump into the soil and survive.


We don’t know the specific answer to that question but we do know that corn is the primary host. We don’t know of any alternative host. Anecdotal evidence from our research plots in 2021 is we found tar spot only on corn, and we had to the South of us a wheat field, and then to the West of us is a field of sorghum. We had fairly high tar spot disease pressure in our cornfield.

One day I was curious, scouting through the wheat fields and sorghum fields, and I wasn’t able to find any tar spot symptoms specifically on wheat or sorghum. I can’t attest to barley or any alternative ditch wheat grasses. It looks like this is fairly specific to corn, and so I wouldn’t be necessarily concerned for tar spot making a host jump to wheat or sorghum even though they’re both types of grass. It looks like this is specifically evolve to infect corn leaf tissue.

I did little non-scientific studies myself. There are a lot of fields like Johnsongrass and shatter canes that pop back up in the Countryside, and I was not able to find any tar spot in either of those two grass weed species, so that’s a good sign maybe.

From the more biological side of me and being the researcher, it’s corn, wheat and sorghum are all grasses and they’re very similar but the evolutionary history between corn and wheat is pretty drastic. The most closely related cereal grain to corn is it is indeed sorghum. They’re similarly related to this tree of life. They’re fairly distinct in the diseases that they get between the two major cereal types of grass, sorghum and corn. I’d be surprised if tar spot were able to infect sorghum or if it was able to infect wheat or barley.

I’ve had a lot of questions from growers asking, “Why did one field get attacked and not the other or why did one over the other not get attacked?” To me, it depends on how severe 2020 came. It was there but we just didn’t know it or two, once it got up and was in the air from spores, move around in 2021.

My thought is earworms. Earworm moth is floating from the South, and it’s wherever that moth lands regionally, that’s going to say, “These are going to be the fields that probably have some earworm issues. Not every field has it.” It’s wherever the weather pattern or the jet streams move it. Is that a good way of thinking about it?

Just because it’s found in one field and the next field doesn’t, having a high disease pressure doesn’t mean that the fungal pathogen isn’t present in that field. It could be present at extremely low levels or low disease pressure. Environmental conditions drive the expansion of this fungal pathogen to a certain cornfield more so than others.

I’d also say, look at, whether your neighbors sprayed fungicide applications, whether the different hybrid genetics that you’re growing. We do know from our research with the USDA ARS, that certain varieties of corn are more susceptible than others, and others are more tolerant or resistant than others. What we do know from research is every corn variety is susceptible to tar spot but there are varying degrees of susceptibility. Hybrid genetics and corn hybrids do play a specific role and a very important role in the spread of tar spot.

I can remember walking a lot of plots and looking at fields. There might be some hybrid rudis. You physically see a little bit more out there but it did nothing to yield. You have another field that had a little bit of pressure out there but you see a 50 to 60-bushel hit. Does it matter when it attacks the plant and what gross stage within the plant? For some reason, if the plant was already under Southern rust stress and nitrogen stress versus another one that’s been called high managed because it can be the same hybrid and you see some differences, if you have a healthier, more robust plant, can it handle that a little bit better?

No single fungicide is going to be the one key answer to tar spot. You might have to use different modes of action or fungicides to really dampen the tar spot pressure.

I would argue that having a plant or a hybrid variety that is undergoing very little stress at the time as tar spot infection will allow that hybrid to attack or defend itself against tar spot more effectively. Plants certainly have an immune system that’s capable of mounting defense responses against a variety of pathogens and pests.

We do know from research that if you introduce external stressors, whether it’s biotic or abiotic, so nitrogen deficiency. If you’re in the field that’s undergoing a drought response or a flooded field, we do know that those external stressors do influence the deadliness of tar spot. If you do grow the same hybrid variety in two different environmental conditions, and then one hybrid isn’t experiencing the same external or environmental stressors as the other, certainly tar spot will take advantage of that, just like any other fungal pathogen.

We have anecdotal evidence suggesting that maybe there’s some type of interaction between tar spot and common rust, at least in some of our research plots. That we’re in areas where we find a whole lot of common rust, we find a whole lot of tar spot. That probably is some type of synergistic interaction, where either tar spot or rust are working together to feed off that living tissue and creating an environment on the leaf that promotes, either tar spot or rust infection.

To back up a little bit when we talk about overwintering and you’d reference out in Canada. I know people that are in Kentucky and Florida. Are those environments conducive for it to overwinter in those regional pockets? If somebody in Kentucky or Florida, had it in their field this 2021, so they anticipate that it can overwinter and be there in 2022.

We don’t know the question to, whether more of the Southern states or counties their overwintering period, would that be more or less conducive to tar spots pressure? We don’t have a good answer to that. A word of caution here is that, if it overwinters in hardier weather conditions for wintering environments, I would argue that it can probably survive overwintering in the Southern United States as well.

It does survive in very humid conditions and very hot growing conditions. If I were a farmer in the Southern United States, and I had tar spots disease pressure, I would consider that a major fungal pathogen for the next growing season. Even the overwintering might be less harsh than the Northern half of the United States is wintering period.

When we think about when tar spot attack so that can lead in a little bit from a treatment standpoint, what makes it favor to attack a plant reproduce the stages on? Why don’t we see much on the front side? Maybe we do. We just don’t physically see the spots yet. Do you have any thoughts?

This is quite interesting with corn, and this has sparked my interest in working with corn because we don’t see this with other grasses or other prop plants. In other cases, soybean, for example, we do see pathogens infecting seedlings, especially viral pathogens. They affect seedlings and continually affect them throughout the growing stage and reproductive life cycle of that crop plant.

With corn, it’s a little bit different. It seems that the seedlings are more resistant to pathogen colonization. Once you hit that reproductive stage where you start developing the ear, and the silks and tassels come out, and you transition from a vegetative growth stage to reproductive to growth stage, that’s where we start seeing a lot of the fungal pathogen pressures increase. A lot of the viral pathogens are able to take hold and reproduce in a lot of the living tissue.

YMP 23 | Tar Spot Management
Tar Spot Management: From our research at USDA-ARS, certain varieties of corn are more susceptible than others and are more tolerant or resistant than other. Every corn variety is susceptible to tar spot but there’s varying degrees of susceptibility.


Talking with a few of the maize or corn geneticists at Purdue, the running hypothesis is that there’s a big transition from the vegetative growth stage or the pre-reproductive growth stages. The transition to reproduction is where a lot of those photosynthetic leaf tissues are now basically putting all of their photosynthates and all the sugars into that ear and developing that ear.

It’s creating an environment in the leaf tissue where a lot of these fungal pathogens can easily colonize that leaf tissue, especially right after the transition to the reproductive stage of corn. With tar spot, in particular, we haven’t noticed that it’s able to infect seedlings. We have noticed that it tends to be infecting leaf tissue after silk emergence and after the tassels have emerged.

If it’s in the reproductive stages, there are a lot more sugars and everything else going from the ear leaf above. When it attacks, is it trying to feed on the cells and leaves or is it trying to extrapolate a lot of those sugars and all the good juices within the plant during those times? What is it taking away?

It’s mostly taking away the nutrients that are left within that leaf tissue itself. It’s a biotrophic pathogen. It requires living tissue in order to form its spores and spread from corn plants to corn plants. It’s extracting the nutrients from the corn cells within that leaf tissue. Interestingly enough, we know tar spot overwinters on corn residue and so somehow it surviving on dead leaf tissue. We often refer to those types of pathogens that are able to survive both dead leaf tissue as necrotrophic pathogens.

The field is pretty consistent that tar spot are indeed a biotroph. Since it can survive off corn residue in the winter and survive off dead leaf tissue that it’s also a necrotroph, it lives these two different lifestyles. A lot of fungal pathogens are able to be both biotrophs as well as necrotrophs. That can also lead to tar spot being easily spread from field to field because it has both these two different lifestyles.

If it comes into to the latter part of the grain field, and some of our studies, we’ve seen that, even at three quarter milk line, there’s 24% yield left to go, whether it is Southern rust attack, lack of nitrogen, respiration issues or tar spot coming in, and basically, annihilating the cells that still needed to be used for photosynthesis. There are nutrients coming out of it, and that’s why there’s that hard crash there at the end. Would you agree to that?

Yes, I would agree to that. I would say that from our research plots in 2021, we first noticed tar spot around the middle of July 2021. The disease pressure itself then was pretty low. As a research plant pathologist and more molecular biologists have been disheartened to know that tar spot wasn’t in our research plots.

It started to take off throughout the month of August, as well as September 2021. It went from low disease pressure, weeks later to extremely high disease pressure, and the majority of our corn lines that were growing in our research plots are indeed susceptible to this fungal pathogen. If you’re going out to a field, you’re scouting and you see it, I would strongly advise that you continually go out there to manage and assess how it is spreading, and what varieties it’s spreading to. It’s almost like an exponential curve where it was no disease pressure, then skyrocketed the disease pressure, and that hammers down on the yields significantly.

Transitioning to topics that are around treatments and control. I’ve got a lot of questions in this category from observations that I made and some other growers. There were people who sprayed a fungicide and still had a major problem. My question to you is, is there a good timing? Maybe in those situations, the fungicide got put on too late. There’s probably also a consideration for what type of fungicide. Not all the AIs within a fungicide are going to go after that. What are your thoughts there?

Use your time wisely. Do your research. If you have questions or concerns, reach out to extension specialists in your area, and gather all the information that you can about tar spot.

I can’t speak to a whole lot of the fungicide applications. That’s not my research background but I do know from talking with our extension pathologists here at Purdue University and anecdotal observations, the timing of the fungicide application is very important, as well as the type of fungicide that you are spraying on your corn crop.

It’s hard because you don’t want to spray fungicide if you don’t observe or find tar spot in the field but you don’t want to wait until you find tar spots in the field to spray fungicide application. It’s this difficult question to answer. What we do know is that even if you do multiple fungicide applications, it’s only suppressing tar spot spread. It’s not completely stopping or preventing it.

I can’t speak to the efficacy of fungicides. I don’t know all the fungicides on the market but I don’t think there’s one fungicide that is 100% effective against tar spot. We do know that they only slow the spread of tar spots and maybe how tar spots form its spores. I don’t think any fungicide is going to be the one key answer here. You might have to use different modes of action fungicides to damp in the tar spot pressure.

I agree with that because there’s been a lot of new fungicide brands. I’m not going to reference any of them because that’s not what I want to go down to. When you do the research on them and look at the AI in their labels, there are several fungicides that do not have tar spot on their label. If somebody is concerned about tar spot, make sure we’re using a product that specifically going after on its label.

A lot of times, you can say, “I sprayed. I checked the box.” You put a fungicide out there but was the right box checked, and did you do your homework to make sure it is going to go after it? That is a very important piece of homework to do when you’re evaluating your fungicide products in which companies you’re going to partner with. If you’re worried about it, make sure it’s on that label.

To follow up on that point, extension plant pathologists that many of the land-grant universities are indeed working on, which commercially available fungicides seem to be a little bit more effective than other fungicide applications. I know the extension plant pathologist at Purdue, Dr. Darcy Telenko. She and her team are specifically looking at this question of, which fungicides seem to be a little bit more effective than others at controlling tar spot and limiting tar spot disease pressure.

Again, because this is a new pathogen, this fungal pathogen, there’s very limited information about, which fungicides tend to be more effective and maybe which corn hybrids are a little bit more effective. Darcy’s lab is actively working on this. They should have some data coming out of her lab and from this past growing season.

Some great resources there to follow up on. The other one that I want to run past you. There are some growers that were working with some fungicide AIs in-furrow and then came back at tasseling. Maybe there are too when they sprayed, and they could see a lot of positive responses there versus physically going out and flying a plane out two times. What are your thoughts on a system like that? Granting there are not a lot of info systems out in the marketplace but there are growing because of other nutritional demands but is that a good option to look at?

I can’t attest to the info systems. That’s not my research background so much but I’d say measurement that you can do to suppress that tar spot disease pressure is going to help. Different modes of action on fungicides are certainly important, as well as different methodologies for controlling tar spot. It’s going to be all-hands-on-deck approach to try to minimize the disease pressure and populations as much as we can.

YMP 23 | Tar Spot Management
Tar Spot Management: Forecasting app are going to be very important in the near future to try to strategically design a management plan that’s going to be most effective against tar spot.


There have been two apps that have risen to my awareness. It seems like there’s one out of Wisconsin. There’s another one more broadly and you can put in your location, and then you can set some threshold parameters into it. It’ll give you an indication of based on humidity, air temperatures, moisture, and what weeks there might be heavier threats to where you can maybe be a little bit more strategic when you apply your fungicide. What are your thoughts on systems like that? What do we need to be watching as tasseling is coming and we’re watching these apps or weather outlook? What are things that would say, “We’ve got a problem coming in the next week.”

I would argue looking at the past environmental conditions, especially about 2 to 3 weeks after you check the app or before you check the app maybe. 1) Look at those environmental conditions to see if they may be conducive to tar spot disease development. 2) Looking at the apps developed from the University of Wisconsin, that’s what you’re referencing here.

Those apps are very informative because they can help strategically apply your fungicides if you need them. They’re going to be very important for US farmers and growers in the near future. There are other apps for your phone or tablet. They have one for fusarium head blight for wheat, as well as tar spot for corn.

They’re going to play a significant role in the near future because fungicide application is costly or expensive. To the US farmer and grower, they don’t want to apply a fungicide if they don’t have to. If you look at these apps and say, “I have all of the criteria for pretty high tar spots disease pressure.” I would strongly consider a fungicide application if you can. Those forecasting apps are going to be very important in the future to try to strategically design a management plan, that’s going to be most effective against tar spot.

Everybody probably reading could contest that getting the fungicide applied is probably the most challenging part within the recommended windows. There are so many acres that are getting sprayed with the fungicide, whether it be guys with the ground rigs and haggis, airplanes or helicopters going out there.

Fungicides do help preserve yield. We know that across the board, so there’s more adoption to it to increase that a little bit more. You’d said that when maybe we use a fungicide, we don’t necessarily kill it. We potentially control the spread. If we are worried about it, are we better to go on the front side to where we minimize that spread or take it to the backside because we could get it again? It could take off and run maybe because fungicides only last a handful of weeks, and we get Southern rust that comes in late. Are we better tile on that backside to potentially hit both in protecting three quarter milk line? What are your thoughts there?

That’s an interesting question that we haven’t addressed, specifically in our research plots. Not by need more of an appropriate question for Dr. Telenko’s and her research team, since they’re actively looking at that fungicide applications. Looking at it from a person, not in fungicide research or looking at more of a general scientist background, the earlier applications might be a little bit more advantageous.

YMP 23 | Tar Spot Management


It can prevent the establishment and the beginning systemic spread of tar spot. If it is somehow interacting with common rust, rust, leaf blight or gray leaf spot, perhaps that could have some type of negative interaction if you applied that fungicide early with those other fungal pathogens. To be completely honest, I don’t have a good answer to that.

Think about some other options, as far as, control methods that we can do. Managing residue, it’s going to be a big piece. Consider rotation at this point. Let’s do everything that we know might work based on other diseases. We’d rather find out that it didn’t work and do it versus not do it and found out we should have done it.

Sometimes we got to use our historical trends, what we know. What are your thoughts on planting dates? In some environments, we are saying people plant earlier, which can speed up that whole process. If we’re going through flowering or speeding up grain field maybe, versus planting later, would that alter when that attack happens and how it attacks the plant?

It does influence when tar spot is able to attack a lot of the major corn hybrids that we have. Planting earlier does have some advantages because you’re speeding up the process and you’re limiting the favorable environmental window for tar spot to thrive. We do know that we see a lot of the tar spot pressure around at the very beginning of July. We start to notice it in the middle or end of July, and that’s where we start to see more of the tassel. The ear development and the silks coming out.

It’s also the environmental conditions during that section of the growing period is favorable for tar spot development. I’m fairly confident, Dr. Telenko and her group were looking at that specifically. It’d be interesting to see if you plant earlier, does that play the same corn hybrid at an earlier planting date, and then a later planting date, to see if the corn hybrid at the later gets more tar spot disease pressure than the ones earlier plants in. We do know that tar spot thrive and at least in Northern Indiana in July and August specifically, and those are usually the hot humid conditions of our growing season.

Thanks, Matt, for all this great insight on this pest that we’re still trying to learn. It feels like we’re building the ship as it’s sailing out into sea here with this pest. It is that complicated because we get a lot of questions this Fall. Everybody is wanting to know questions and literally, we’re still building the ship as it’s floating there because it hasn’t bloomed this aggressively in years. You’d reference 2021 is probably the biggest spread we’ve seen since 2015, and prior to that wasn’t here, a lot to be learned, we’re scratching our heads a lot.

It’s because this pathogen is so new to the United States and 2018 was a bad year, 2020 is certainly on pace to be considered the worst year for tar spot. There’s not a whole lot of effective advice I can give US farmers or US growers, other than we’re continually investing in research funds to study all aspects of tar spot disease resistance, whether it is introducing more genetic-based resistance, which is the focus of my lab specifically. Looking at different varieties on how they hold up against tar spot.

Not only tar spots but other significant diseases such as gray leaf spots or Northern corn leaf blight or rust. This pathogen is so new, and there’s very limited information that we have on how to control tar spot specifically. That’s the unfortunate thing. It’s pretty much limited to cultural practices bearing that corn residue overwintering fungicide applications but until we can introduce good hybrid genetics, and get it a more genetic-based tool against tar spot. We’re going to be studying this fungal pathogen extensively for the next several years.

As we wind down this episode, I do greatly appreciate the insight that you have brought to the table that we can understand why this pest is a very aggressive pest. It compromises a lot of potential dollars on the table but yet there’s not a lot of long-term data that supports a lot of stuff with this, and why it’s important for your type of research, the funding that goes into your camp as well as your counterparts across the country, wherever corn is grown.

YMP 23 | Tar Spot Management


I’m thankful that the network is there. At the end of the day, I’m very thankful that your network allows you to come to visit with me so I can learn, and then we can share with our growers. At the end of the day, we’re going to overcome this challenge like any other challenge it’s doing what we know but we got winter coming. You spend a lot of your time going to meetings, asking questions, and figuring out what options do work. We do have a narrow window into the harvest to first planting next spring. Use your time wisely would be my best recommendation.

Use your time wisely and do your research. If you have questions or concerns, reach out to your extension specialists in your area. Gather all the information that you can about tar spot. On our end of things from the USDA, we are actively researching this pathogen to look at new sources of resistance. My team here in the Crop Production and Pest Control Research Unit at the USDA, we’re screening different corn varieties used in major plant breeding programs to look at their levels of resistance, to try to identify the underlying genes in corn that could help confer resistance or even tolerance against tar spots.

We’re also looking at growing mutant varieties of corn. These are corn varieties that, either have no defense responses or they have extremely elevated defense responses in fields to test, whether those mutant varieties can be new sources of resistance to get tar spot. We’re all hands-on deck approach to try to tackle this tar spot pathogen but it is going to be in the next few years down the road.

We’re slowly making progress on all fronts. There are only so many people who are working on so many different aspects of tar spot, and because it’s a new pathogen that we know very little about it so far. The USDA is very much actively involved in researching tar spot biology and help to spread the management strategies we can use to control this disease.

Matt, I do appreciate you coming on and share knowledge. I’ll extend an invitation out in the future as you learn more. When it comes back on, you have an open invitation to chat with our group because collectively, this show is the sheer agronomic insights. Offset some of these challenges. We can still obtain higher yield.

With that, I do greatly appreciate you jumping on this with us, educating our growers and farmers across the landscape. People out there reading, stay tuned. Spend some time this winter, educating yourself on tar spot. If there are meetings out there, webinars, and YouTube videos, take advantage of all these opportunities to learn. We’ve got a narrow window when the combine goes in the shed and the planner comes out next spring. We’ve got to use that time very wisely. Thanks everybody for tuning in. Matt, thanks again. Have a great one, everyone.

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About Dr. Matthew Helm

Dr. Matthew Helm is a research molecular biologist with the Agricultural Research Service arm of the United States Department of Agriculture (USDA-ARS) and is stationed at Purdue University.