Tar spot symptoms, conditions, and solutions

Kathryn: 00:08

Hello, and welcome to the Talking Crop podcast. My name is Kathryn Seebruck, and I'm a commercial agriculture educator with University of Illinois Extension serving Jo Daviess, Stephenson, and Winnebago Counties. Talking Crop is a row crop production podcast with episodes occurring every other week during the growing season between the months of May and September. This episode marks the beginning of season two of Talking Crop, and I am very excited for you all to hear from this season's lineup of guests. We'll be covering so many great topics from Data Intensive Farm Management to updated corn nitrogen recommendations and a whole lot more in between.

Kathryn: 00:45

I hope that you find something of value to your operation in any and all of these episodes, and please don't hesitate to reach out to me with any comments or suggestions on who you want to hear from or what you might want to hear about on the podcast. I can be reached directly via email at seebruck@illinois.edu which is s e e b r u c k at illinois dot edu, or you can fill out a brief survey at go.illinois.edu/tcsurvey. And this survey is linked in the episode description along with my email. In today's episode, I talk with Steven Brand, a commercial agriculture specialist with University of Illinois Extension. We talk all about tar spot, a disease I'm sure everyone is familiar with, that has been infecting Illinois cornfields since 2015.

Kathryn: 01:32

Steven gives us the rundown on tar spot, including how to identify the disease, what conditions favor disease development, and how best to prevent and control tar spot. You can find Steven's contact information in the episode description, as well as some additional information that we didn't quite get to in the episode. And I also wanted to mention that in the episode we do talk about an app called FieldProfit, which I was originally going to link in the episode description for you all. But in doing some further research, I found that FieldProfit is unfortunately no longer available. However, there is an alternative app called TarSpotter that does essentially the same thing, so that will be linked in the episode description instead.

Kathryn: 02:11

On the next episode of Talking Crop, I'll be speaking with Dr. David Bullock with the University of Illinois Department of Agricultural and Consumer Economics about the Data Intensive Farm Management program that he leads. DIFM enables farmers to conduct large scale agronomic field trials on their own fields, which gives them the unique ability to answer research questions tailored specifically to their own fields that they manage every day, which is really invaluable. That episode will air on Wednesday, May 28. So without further ado, please enjoy this episode of Talking Crop, Talking Crop, Tar Spot Symptoms, Conditions, and Solutions with Steven Brand.

Kathryn: 02:49

Hello, Steven, and welcome to the Talking Crop Podcast. I'm really glad that you're here today, and I'm excited to talk to you about tar spot.

Steven: 02:58

Thanks for having me, Kathryn. Happy to be here and happy to bring some expertise to this topic.

Kathryn: 03:03

Perfect. Yeah. That is exactly what we're looking for. So as usual, we are just going to jump right in and kinda start with some basics on tar spot. So could you talk us through how to identify tar spot?

Kathryn: 03:18

Because I know identification with diseases can be pretty tricky, and there I know are some tips and tricks with different diseases, and I think tar spot has one in particular. So can you talk to us about how to properly identify tar spot?

Steven: 03:31

Yeah. Tar spot's a pretty straightforward disease to identify. As you're going through the corn plants, you're gonna see small black lesions on the surface of the leaf. It's called tar spot because they literally look like someone was just ringing tar through a field in little specks all over the corn plants. You cannot scratch these lesions off.

Steven: 03:53

And then as the disease advances and the leaf necrosis kicks in, it has a distinct chocolatey color. So if you're familiar with a chocolate lab, that's gonna be the color you look for starting at the base of the plant and working up.

Kathryn: 04:06

Okay. So it starts out at the as those distinct black marks and then later on as the disease progresses, is that chocolatey color?

Steven: 04:13

Correct.

Kathryn: 04:14

Awesome. So, yeah, that sounds like it's pretty pretty easy to identify, which is good. And so how bad can tar spot actually get? Like, it a disease that you know, if you have it, you don't need to be too worried about it, or is it something that we need to kinda be cognizant of and, you know, wanna prevent any significant potential yield loss if that's an issue with this disease?

Steven: 04:39

Yeah. This disease, it can be very frustrating. It is very weather dependent. And if the disease is endemic, if it starts early on in the season and continues to ramp without any fungicide or any resistance in the corn plants, it can lead to 30 to 50 bushel yield loss. So we're looking at

Kathryn: 05:02

Oh, wow.

Steven: 05:02

Close to twenty five percent. So it can be a very damaging, yield reducing disease.

Kathryn: 05:10

Yeah. That is pretty significant. I would definitely wanna make sure that I'm doing preventative control for this one for sure. And you mentioned, you know, when we first start seeing it. So when typically during the season does tar spot start to appear?

Steven: 05:26

The earliest you might see it is gonna be V9 , V10. That's when it could really start to ramp up in damage. Previous to this position, I was working at Purdue and we had a variety of planting date trials and we had some corn go in very late into June. And at V4, we started noticing little specks of tar spot on these very small corn plants. We saw the same thing with some of the early plantings but as the season went on, it grew out of those as the lower leaves dropped off and we didn't see the disease come back until later.

Steven: 06:01

V10 is really the time where you need to be very cognizant of it and start scouting to make sure it's not in your fields. V timings are mostly gonna be from V10 to R3.

Kathryn: 06:16

Okay. Wow. So it could it can kinda vary.

Steven: 06:18

Yeah.

Kathryn: 06:20

So kinda looking at the historical aspect of this disease, I remember hearing about it, oh, gosh, a few years ago, probably after, like, 2019 or so.

Kathryn: 06:34

So when did we first really start seeing tar spot in Illinois? Like, when did it first become, like, a prevalent issue?

Steven: 06:41

So the first year we saw it in Illinois was 2015. This disease historically has been in the very far South in Mexico. And over the years, it's traveled north. Since 2015, it's spread very rapidly. It's in almost every county in Illinois or has been confirmed in almost every county.

Steven: 07:02

My gut feeling it's in every single county in Illinois at this point. Iowa is completely covered. Indiana, it's been found in every single county. Southern Wisconsin, it's very prevalent and is in all likelihood in your field.

Kathryn: 07:17

Okay. Yeah. And so what then what did last year, what did 2024 look like? Was it, like, a bad heavy tar spot year?

Kathryn: 07:27

Was it light? Can you kinda talk about that a little bit?

Steven: 07:31

Yeah. I can speak to Northern Illinois at least. It's very, again, very weather dependent, so different parts of the states will have different levels of tar spot. Up here in the North, it came in late. It didn't really show up until R1 and then didn't start to ramp until R2, R3.

Steven: 07:52

So we didn't really see any yield loss at the end of the season as you're starting to get into R4, R5, R6, you are gonna be seeing the disease, but your fill had already completed by then, so we didn't see any yield loss. Illinois actually set records for corn yields last year.

Kathryn: 08:11

Yeah. Thankfully. Right? So it's always nice when we have a low disease year. So due to that, are you thinking that 2025 might be also a lower disease prevalence year or not?

Steven: 08:26

So it's very weather dependent, so it's, it's impossible to say, this disease is very frustrating. If the weather is cool and wet early on in the season, it continues that way, we could see an endemic season. If it's hot, drought conditions, high humidity, that all suppresses the disease. So we may not see it come on until late. Honestly, the only way is to wait, see what the weather does, and to scout.

Steven: 08:56

That's gonna be the biggest key to identifying it.

Kathryn: 08:59

Okay. Perfect. Yeah. So weather dependent, which, like you said, makes it very frustrating and hard to kind of pin down and predict. So related to that, can you talk about, I know researchers talk about the disease triangle.

Kathryn: 09:14

And so can you talk about that and how it helps us understand this disease in general, but obviously tar spot specifically, and consequently, how we can control it using the tools provided to us with that disease triangle?

Steven: 09:32

Yeah. So as you know, the disease triangle is a pathogen, a susceptible host, and environmental factors. For pretty much all farmers in Illinois at this point, if you're planting corn, the disease is gonna be in your field and you're gonna have the corn so we're gonna have two of the three factors there and it's just all gonna depend on the environment. We have seen through research that the disease will overwinter for multiple seasons. They've gotten to four and five years and the disease is still creating new infections through overwintering.

Steven: 10:05

So if you're trying to out rotate it by doing multiple years of beans or fallow, even if you were to outpace the disease until it's no longer able to replicate again, it can just blow in from a neighbor's field. So rotation isn't really gonna be a good option for trying to suppress this disease. Variety resistance is one of the bigger up and coming ways to combat the disease. They have shown that resistant diseases have a significantly lower amount of lesions on the plants and does have some good resistance to it. Outside of that, scouting and getting timely applications out with your fungicides are gonna be the biggest combatants.

Kathryn: 10:53

Okay. That's really great information. With the resistant hybrids, is there, like, a wide selection of resistant hybrids, or is that not as widespread quite yet?

Steven: 11:08

There's more and more coming up every single year. I would speak to your seed dealer and your seed provider. They're gonna have the best information on that. And, yeah, that that'd be the way I'd go with that.

Kathryn: 11:19

Okay. Awesome. And you mentioned the disease overwintering. So can you talk about tar spot's disease cycle? And then you kind of already touched on this a little bit, but with it overwintering and how you can't necessarily kinda outrotate it, so to speak.

Kathryn: 11:38

So can you first talk about its disease cycle and any other implications besides rotation that we can learn from the disease cycle of tar spot and how we can control it or at least better understand it?

Steven: 11:50

Yeah. So the disease overwinters in the soil as stromata, those are the little black specks on the corn plants with ascospores inside of those. That's what's gonna spread the disease, and that's overwintering on plant residue. During the new season, the ascospores as you get to the milder temperatures with some rain, they're gonna release those spores and infect new plants. From there, it's gonna take about twelve to fourteen days for the new lesions to appear on the plants.

Steven: 12:23

So if you see those specks that means that plant was infected two weeks ago. So those are going to keep replicating, it has multiple cycles within the disease and it'll eventually hit a exponential growth. The longer the disease is able to reproduce and it starts earlier, the worse the disease will get later on and the faster you're gonna start to see those plants die off. Besides resistance, getting timely applications out for your fungicides is gonna be the best way to combat this, give your plants some protection. We say to scout because we want the plants to be protected during the reproductive cycle of the plant.

Steven: 13:10

So you want to have the best coverage through ear fill. If you're not gonna scout, the general rule of thumb is, it'd be spraying at VT-R1 and give that plant some coverage through R3 , give it the best chance to slow that disease down, knock it back before it goes into exponential growth. And, yeah, name of the game is try to keep that top half of the plant from the ear leaf and up green so it can complete that ear fill and lead to the lesser yield losses.

Kathryn: 13:44

Okay. Yeah. Perfect. And so you mentioned scouting a couple times, which that's something I always kind of harp on with folks because scouting is kind of that first step in integrated pest management. It helps you understand if you have a pest present and if you do, its prevalence.

Kathryn: 14:02

And so with scouting for tar spot, can you walk us through kind of your suggested methodology for scouting through a field?

Steven: 14:13

Yeah. You're gonna go a handful of rows in, six, eight, ten rows in. You don't wanna be right on the field edge. Start at the base of the plant and start working your way up leaf to leaf. You're gonna start to see those lesions.

Steven: 14:27

If the disease is there, it'll start from the bottom and work its way up. When the plant gets to 1% leaf coverage at the ear leaf of the, stromata, the little black specks, that is your key where you need to spray. You probably needed to spray the day before. It may not seem like a lot, but 15 to 20% coverage of a leaf will be a completely dead leaf because they're gonna start to form those necrotic rings and work out and kill the leaf from there. So that's the easy way to scout them.

Steven: 15:03

It's just work your way up through the plants and if you start seeing the disease, keep note of it and get ready to have a spray go out.

Kathryn: 15:10

Okay. And so, you know, we talked about scouting, utilizing resistant hybrids are a really great tool. Obviously, fungicides as well. So with the the hybrid resistance, going back to that, are you said, you know, more and more resistant hybrids are coming out every year. Do they, do you know, like, how much like, how strong are they in their resistance?

Kathryn: 15:40

So are they like, if it's a really heavy disease year, are those resistant hybrids still gonna get infected, or is it are they do they do a good enough job that you have pretty decent protection even in a year where you have those, like you talked about, those appropriate weather conditions for the disease to proliferate?

Steven: 16:01

Yeah. You will still have the disease. It's not gonna be complete immunity with these resistances, but it will significantly slow the disease down. And through that, it'll help to lessen that exponential growth. That's the biggest thing you're trying to avoid or at least push that part of the disease cycle further into the season until after your fill and you get those yields.

Steven: 16:27

I can't speak to how potent they are, that I'm not entirely sure of, but it's they're enough where if you have eight rows of a resistant variety and a non resistant variety, you will see significant differences.

Kathryn: 16:43

Okay. Awesome.

Steven: 16:44

Or noticeable differences. I don't wanna say significant.

Kathryn: 16:47

Right. Yeah. You answered that perfectly. Thank you for bearing with me as I sputtered my way through that question.

Steven: 16:54

No. It's fine.

Kathryn: 16:58

And so with fungicides, you were mentioning that it's best to apply - can you actually repeat when it's best to apply fungicides with and without scouting? Because I think this is really important. This is what a lot of growers are gonna be like, the information that they're actually really gonna wanna know is when should I spray and, again, the difference between if you scouted or did not get around to scouting for it.

Steven: 17:20

Yeah. If you're scouting, you're gonna be looking for that 1% leaf coverage in the stromata, the little black specks. That's gonna be the key indicator to, you know, get a spray out immediately because that's already two weeks into the infection, and those spores are reproducing more ascospores at that point. So in the next two weeks, it's going to rapidly increase the disease. If you're gonna be spraying in the vegetative states, check with an extension specialist, with FS, somebody who knows the disease, a CCA, to make sure to put the to get that spray out correctly.

Kathryn: 18:04

Mhmm.

Steven: 18:05

VT-R1 is gonna be the most common time for sprays. That's the one I personally would recommend if you're not gonna scout to try and get the best coverage from R1 to R3, during ear fill. If the disease is endemic and it's a very bad year, you may need to have a second spray at R3 . After R3 , it's not generally recommended that you spray, ear fills coming up on completion and you'll probably do more harm driving a sprayer through the field if you're getting to R5, especially R6, then it's not gonna stop the spread of the disease at that point.

Kathryn: 18:52

Right. Yeah. At that point, it's probably not very economical.

Steven: 18:55

Yeah.

Kathryn: 18:55

Because, like you're saying, the damage has already been done, and so you don't wanna waste money on something that's probably not gonna have much of a positive effect.

Steven: 19:04

Yeah. So the the big take home from that is scout, know what's in your fields, you make an educated decision on your sprays from there because if it's a year where the disease comes in late and you're scouting and you're not seeing it come in until R2, you'll most likely be fine. The disease won't start to ramp until after ear fill until it's later in the season and you won't necessarily have to worry about that. And you could save yourself a generic VT-R1 fungicide spray. But if you're not gonna scout or don't have the ability to scout and you're just gonna put a blanket coverage out there, VT-R1 is probably the best time to spray it.

Kathryn: 19:43

Alright. That's really good information to know. And for those listening, as Extension educators and specialists, we very, very highly recommend scouting. Please try to get there and do so as best you can. It's just like I said, kind of that initial aspect of your integrated pest management system, and it's gonna help you in the long run.

Kathryn: 20:06

It's one of those things where you you wanna do the work upfront, that hard work upfront because it's gonna help you on the back end of things. So I wanna put that little tidbit in there for you all. So, Steven, one last question about this. You know, with agriculture technology always, always enhancing and improving, you know, researchers and the like develop lots of tools that help us to better track and identify and control our pests. So is there anything of that nature for tar spot that folks can look into that can help them in this endeavor of controlling and preventing this disease?

Steven: 20:48

Yeah. I got a couple things for this. As far as spray technology with tar spot, coverage is gonna be very important for stopping this disease, contact fungicides are going to prevent that disease from spreading for a couple of weeks as long as the fungicide is active on that plant. So when you're looking at sprays, making sure you get the best coverage is going to be ideal. I say focus on the top of the plant because as you're going through and spraying, as you go further down that plant, more of that fungicide is gonna be collected on leaves and the further down is gonna get less and less and less.

Steven: 21:30

That's why we think ear leaf and up is gonna be the best areas to at least protect for that ear fill.

Kathryn: 21:36

Mhmm.

Steven: 21:37

On top of that, there's an app in a website. It's called FieldProfit. It's formerly known as the TarSpotter app, and it's a really great tool to help identify weather conditions that are conducive to tar spot's spreading in the field. So it'll give you over the course of all the days, it'll show you the weather, the temperature, the humidity, and then the, I get, "danger factor" of tar spot spreading in the field. Of course, with this, you're gonna wanna go out and scout because this can tell you when the weather is conducive to this spreading, but also don't just go spray to spray because, you know, it's that the weather is conducive.

Steven: 22:22

Go out and actually see if you have it in your field.

Kathryn: 22:24

Mhmm.

Steven: 22:25

Yeah. FieldProfit's a very good tool for helping with tar spot.

Kathryn: 22:29

Okay. Yeah. That sounds awesome. And I will link that tool to this episode. So for those that are interested, you will be very easily able to find it.

Kathryn: 22:41

And so, Steven, I think that's all that I have for you. Is there anything else that you wanted to mention or recommend when it comes to tar spot prevention and control, identification, any of the like?

Steven: 22:57

Really, it's scouting, knowing what you're looking for. If you're having issues identifying tar spot, feel free to reach out to anyone within Extension. We'll be more than happy to help. Scouting, knowing the disease and knowing when to spray or when you can delay a spray is paramount with this disease, so just stay informed.

Kathryn: 23:21

Awesome. Well, thank you so much, Steven. This is, I think, a really important topic. I know the disease can be a little bit of a scary concept because it's always you know, you don't know if you're gonna have it. And like you're saying with this one, it's so weather dependent, so it can be a little bit worrisome.

Kathryn: 23:35

But you've provided a lot of really great expertise and information that I'm sure folks will take a lot away with. And so, again, I appreciate you being on today.

Steven: 23:45

Thank you for having me. I really appreciate it.