Tar spot and late-season corn and soybean disease updates
Good
Speaker 2:morning, everyone, and welcome to today's strategic farming field notes program brought to you by the University of Minnesota Extension. I'm Eric Yu, a crops educator with, over at the Farmington Regional Office. We'd like to thank our sponsors before we begin. These sessions are made possible, through the generous support of Minnesota farm families from the Minnesota Soybean Research and Promotion Council and Minnesota Corn Research and Promotion Council. We're glad you could, join us for today's session, which will focus on, corn and soybean disease updates for the season.
Speaker 2:Our speaker today is doctor Dean Melvick, University of Minnesota Extension Plant Pathology state specialist. Before we begin, just a quick reminder, there is a q and a box at the bottom of your screen, so please feel free to submit questions throughout the webinar. And with that, I'll turn it over to doctor Malvik to share his updates on disease issues across the state.
Speaker 1:Good morning, everybody. Glad to be here and glad to know you're all out there. So I'm going to cover a few of the highlights and some topics that have been coming up recently via email and text and other ways. And there's, as was stated, there's also time for questions as we go along here. So this this time of the season is the time we start to see problems disease problems often when they if they develop in fields, as we all know.
Speaker 1:We're we're we're still in the early stage of most disease development. You know, most of our most significant diseases on corn and soybeans tend to maybe really start to be showing up in the next, you know, week or two, but some are showing up now. And we'll address some of those as we go along. And again, if you have questions as we go along, we can address those at the time or at the end here. So just a few highlights as we move through this.
Speaker 1:So first of all, we'll talk about a few soybean diseases that are concerned with all the rain we've been having in many parts of the state. One of the diseases that we know is brought on by rain at this time of flowering is white mold, which can as we all know be a very damaging disease on soybeans. Now we're still these days like today it's awfully warm for white mold to develop, but the conditions are appropriate in many places still. At night it's getting cool enough, at least it was, not last night, it was very warm. But it is a disease certainly we have to watch for.
Speaker 1:And fungicide applications certainly can be effective. I know other things we can do are already done, but it would be useful where white mold develops to take a look at whether or not, you know, partially resistant varieties, you know, stand density, and any reduced fertility may be making a difference in how much white mold you see in fields. Now as we we talked about and everybody knows, fungicides can reduce white mold damage and increase yield under severe white mold pressure. And the time to get those on is before the disease develops to any great extent. So early flowering stage is when the rows are filling.
Speaker 1:So right about now, we're reaching the end, I'd say, a lot of fields when would be optimal for a soybean white mold management with fungicides, but I think the opportunity is still there in a lot of fields. So something to consider as we see more rain and especially if we see cool weather in the forecast. Another disease that I will just mention, I have not seen it this year nor heard reports, but a few years ago it was increasing quite commonly across the state and that's frog eye leaf spot of soybean. Causes the small brown elliptical to round spots with the darker margin. It's a very significant disease in the Southern Half of The US.
Speaker 1:And we had significant amounts of it a few years ago. Since that time, we haven't seen a lot in most fields, but it is still something to watch for. It can be a yield reducing problem. So again, that's something to think about, this time of the year, we've having lots of moisture. Beans are flowering, it's an optimal time for this disease to start to develop.
Speaker 1:And again, that's frog eyed leaf spot or soybean. The other one that of course we've dealt with for a long time in most parts of the state is sudden death syndrome. And that disease, I suspect we've had good conditions for developing that disease in many parts of the state. Since it starts the infection process when the radical, the new root is just developing in the first two to three weeks, probably after planting. The soil is moist, pathogen is there, that infection can start and this is about the time of the year when we might start to see it.
Speaker 1:It's a little early, we often don't see it till the August or so, but there have been times when certainly it has developed by the July. That is a disease to watch for. There are more seed treatments now available for managing it. So it is something to take note of if you use those seed treatments to take note of how effective they were. And I just want to bring up another disease that causes similar symptoms as we progress through the season.
Speaker 1:We usually don't see that. Those symptoms develop until later in August would be brown stem rot. That's a widespread disease in Minnesota. The leaf symptoms can look very similar. Of course, if you split the stems, look in the pith, the brown stem rot results in that brown discoloration, whereas the SDS doesn't.
Speaker 1:SDS has been found across most of the Southern half of Minnesota now, And as far Northwest as roughly Clay County on the southern part of the Red River Valley. We have no reason to think it wouldn't be developing further in the Red River Valley. So if any of you see the disease further north, and if you know of anybody working up there that might be out seeing it, we'd really appreciate getting samples from River Valley to try to confirm how far sudden death syndrome is spreading. In North Dakota, they have confirmed it all the way up to the Canadian border. We have not confirmed it that far yet here.
Speaker 1:And it may not be, but it could be also because could have missed it. So again, if anybody sees samples in the far Northwest part of the state, we'd appreciate getting samples so we can confirm whether or not it's SDS. And one of the last diseases I'll mention, with all the rainfall, we've had very good conditions in many fields for Phytophthora rot of soybean, root and stem rot. It causes a dark brown discoloration of the lower stem and kill plants anytime throughout the season. It often kills plants and patches, erratic patches and fields, sometimes much more.
Speaker 1:And we're finding more and more cases where resistance genes that are bred into varieties for resistance to Phytophthora are no longer effective. So, again, take note of that disease. You see plants dying in the field. It's quite possibly this is one of the one of the potential causes. Now moving on to corn diseases, and these are starting to develop.
Speaker 1:First, I'll mention corn rusts. I think most of us know that there was a a large amount of southern rust across much of Minnesota last year, 2,024. And common rust is the other rust that we often see in Minnesota, and is here nearly every year, but rarely causes any significant damage or is any great concern on most corn hybrids. Some of the sweet corn is more susceptible. Southern rust is more concerning because it like all rusts, these corn rusts have to move up in the Southern US.
Speaker 1:They don't overwinter here. By the time they get here, the temperatures are usually too warm for common rust to be a significant issue. That's why common rust is usually more prevalent in cooler summers. We have weather like we're having today and yesterday in the nineties, a lot of weather days in the eighties lately, that's ideal conditions for southern rust. Heard a couple of people say they've seen it.
Speaker 1:I think I've seen saw it in one field. They're reporting it across Iowa, which is no surprise. It would come up from the South, you know, before it gets here. So again, this is something to watch for. After a major outbreak of Southern rust in Minnesota last year, I think we are wondering what's the likelihood it might happen again.
Speaker 1:We don't know, but it gives something to watch for, and it could be a recurring situation. And how do you tell them apart? The common rust tends to be darker and brown, pustules common on both surface, top and bottom. Whereas the southern rust has orange to light brown pustules, mostly on the upper leaf surface. So those are a couple things that that we can look for to help us determine if it's southern rust or if it's common rust.
Speaker 1:One other corn disease that I will mention is bacterial leaf streak of corn. Now this disease is another relatively new disease in Minnesota and the Midwest. First found here just about eleven years ago. Now it has not been a major problem. We saw a lot more of it six or seven years ago, but suddenly this year, I got an email with questions about it from Southern Minnesota a couple days ago, and I saw it in the field yesterday.
Speaker 1:Not at levels that are really concerning, at least not in the field I saw, but again, if you notice it, pay attention as it is something that's out there and can reach damaging levels sometimes under some hybrids. The other thing to note is that it looks very much like gray leaf spot, a fungal disease. The margins of these long linear lesions on leaves tend to be straighter and more square for gray leaf spot, more wavy for bacterial leaf strip. So there are two ways you can think about how to distinguish those two. Isn't it?
Speaker 1:Of course, fungicides work well for gray leaves, but they do not work for bacterial leaf streak. So, again, something to pay attention to. I think we may see more of it this year with our frequent rainfalls. And again, not usually a major concern, but something to pay attention to as well. And it's been mostly found in the Southern part of Minnesota so far, with very few exceptions.
Speaker 1:Again, gray leaf spot tends to have more square elongated spots with very straight margins bordered by veins. So the other one I got a message about just yesterday was Goss's leaf blight and wilt. I think most of us know, many of us know ten, fifteen years ago, this was a significant issue in Minnesota. It's declined such that we rarely see it or hear much about it anymore. You know, there are outbreaks at small levels, mostly in West Central Minnesota from what I've heard, but, you know, it has not gone away.
Speaker 1:I've heard a case of significant levels in a field not too far away. So it's gone to a level that's almost insignificant because most hybrids now have a good level of resistance to it. If we plant hybrids that are susceptible to Goss's wilt, I wouldn't be too surprised to see significant levels of this disease again. So this causes this long linear oval lesions on leaves that tend to be bordered by freckles, lighter green tissue with darker spots. That's sort of a characteristic symptom of Goss's leaflet.
Speaker 1:And in the lab, it can be diagnosed pretty easy with lab tests and including by cutting open that lesion and looking at the bacterial ooze flow right out of the leaf tissue. So last disease that I'll mention that many questions come up about of course is corn tar spot. New in Minnesota since 2019 caused by the fungus Phyllachrommatis produces these small raised black spots on leaves. The spots don't rub off. They go through the leaf, and they're usually somewhat elongated.
Speaker 1:And so I've been getting a number of questions and calls about that and texts with with photos as well as some samples. And just just to back up, where it was confirmed as of 2024 in the fall, excuse me, across all of Southern Minnesota, all the way up into the Southern Red River Valley, as well as over into South Dakota and into North Dakota. So the disease spread quite a bit last year. Fortunately, most of that spread was only at low levels of disease severity in fields. Now, as of yesterday, it had been confirmed in three counties in Minnesota, all along the southern edge.
Speaker 1:And that can be found, the distribution of tar spot can be found on a website called Corn IPM Pipe. Corn IPM pipe. And so you can look at that map whenever you wish and see where it has been confirmed and reported in Minnesota, as well as counties around us. And again, said it's been found and confirmed in these three counties in Minnesota. I've heard reports of it being found in other places, although I have not actually seen confirmation that it was.
Speaker 1:But I I have no suspicion, no no doubts that it's occurring in other places as well. So it is developing in Minnesota in various places, so certainly something to watch for. It's a little later than we found it last year, but still not too late to cause damage if it continues to develop. And so one of the concerns, especially in the early stages is identifying tar spot. Once it fully develops, it's pretty easy to identify.
Speaker 1:Again, using those characteristics that I just described, but in the very earliest stages, the small spots can be kind of hard to tell for sure, and some of the spots we see in the fields are round black spots that can be easily wiped off the leaf. I received one sample that I absolutely could not tell for sure unless I put it under the microscope and I saw the characteristic spores of the fungus come right out of that stroma, that tar spot. So again, early spots can be just look scattered and kind of hard to identify in the very early stages. But as the disease develops more and more, the spots become prevalent, and in some cases we even see that the fisheye type lesion, which is a black spot surrounded by a chlorophorotic or lighter halo. Now, just a bit of review about tar spot development.
Speaker 1:Now, so overwinters on residue, you know, that's been infected last year. It also can spread by the wind. So those are the two sources of spores. Now knowing that the disease was widespread across much of Minnesota, especially the southern half of the state in 2024, means of course most fields have a high level or some level of inoculum in them from previous years. But that doesn't really dictate how much we see.
Speaker 1:We often see severe levels in fields that did not have corn last year. In fact, I've talked to some individuals in the Southeast and they say they see more of it in fields that didn't have corn last year. An observation, we can't quite explain that, but the point is that the rotational history of a field can't be used to predict how much tar spot we see. And the disease develops in cycles. We see a few spots initially, those develop into larger spots, they produce spores and then a few weeks later, spores produced by the large spots spread and infect the leaf to result in a large number of infections typically.
Speaker 1:What we've seen so far in Minnesota has been very low levels of infection, both in terms of incidence in fields and severity on leaves. But again, it's early stages and we will wait to see what happens. And what we found is with some of our work and others is that there's an incubation latent period of about, well, nineteen to twenty days, almost three weeks from the time the spore lands on a leaf, and then we can actually see the black spots. So that's really important considering when the disease is developing and how much damage we might incur at the end of the season. So a real significant part of that is if we don't see tar spot develop early, let's say before August 10 or some date like that, the chances of it developing to a severe enough level after that point to cause yield loss, it diminishes pretty quickly.
Speaker 1:Still have a lot to learn about this, but that is one general observation that's being developed across the Midwest as well as here in Minnesota. So moderate temperatures favor the disease and moisture favors it to a point. So some studies have been done showing that really high levels of relative humidity such as greater than 90% is not associated with more tar spot. Similarly, when average temperatures are greater than mid seventies, tar spot tends to be less severe. So that doesn't mean we can't have warm days, but if we have cool nights and we average the temperature back in the seventies, the disease can probably still develop.
Speaker 1:So the risk factors for tar spot are still many unknowns we have. It likes periods of cooler temperatures, 60 to 70s, moisture is favorable to a point, but too much might actually result in less tar spot. I'd say that's what the results say that have been published, but I think there's still a lot of questioning, like, what what is the role of moisture here? How much is too much? Again, an early start for the disease seems to be important to really result in high levels that result in in a lot of yield loss.
Speaker 1:And fungicides are effective. The later we can delay them, the better. Otherwise, we might have to put on two applications if the disease keeps developing. And, you know, VT to r two is probably the optimal time for most applications. Again, the r two would probably be better if we can wait that long.
Speaker 1:And there are number of different fungicides available that are effective for tar spot. And that just concludes the the summary of information that I have here. And I think we have a few questions, and we can try to address those now.
Speaker 2:Yeah. So thank you, Dean. Just just to go off on the corn tar spot topic, you mentioned that there's low incidences in Minnesota. And just a question on how do you scout for it? Because you can't always expect it to be on the outer edges of the fields.
Speaker 2:More often than not, you might see it in the middle of a large field. So what is the most effective way we can kinda scout for this disease?
Speaker 1:Yeah. Scouting for tar spot certainly can be a challenge. Unfortunately, it's often found near the edges of the fields. So I think in in many cases, we can have some level of confidence if we can't find it anywhere near the edge that there probably isn't a lot of tar spot in the area. Now that is not a a given.
Speaker 1:Right? It could develop in the middle of the field first, and we've seen aerial photos, and we've had experiences here in Minnesota where that has happened. But, of course, this time of the year, when corn is as tall as it is, trying to thoroughly scout a field is a real challenge. So I think we need to have some level of confidence, and I think we can. Now if we can't find it, you know, near the edges of the fields, I'd say it's worth walking in some distance, but certainly we want to cross the middle of fields to to have a lot of confidence of whether it's there or not.
Speaker 2:And then you mentioned that it tends to be better to spray kind of later into the season with for corn tar spot. And, I guess, at what point as far as the disease pressure of corn tar spot does it pay to spray?
Speaker 1:Yeah. Let me let me back up. It pays to wait if we can't. We're seeing a lot of disease at the tasselum stage. I don't think we are in Minnesota right now.
Speaker 1:Better to wait a little longer. Okay. The the question is, right, when does it pay to spray? What is the threshold? You know, I I'd say there's there's there's some debate on that for sure.
Speaker 1:But if we see a pretty consistent level of, you know, five to 10 lesions, tar spots across a large number of the plants in the field by r one about now, and we see favorable weather ahead of us, you know, that the decision might lean towards spring for it, especially in the fields that I've had before. Because we know some areas seem more prone to it than others, and that's a little challenging to figure out where that is still based on our knowledge.
Speaker 2:That's and then with the many unknowns regarding corn tar spot development and kind of, I guess, the progression of it throughout the year to year, is there a way to forecast what it might look like this year with the later, findings this year and the Loma County's reporting it?
Speaker 1:I think I think we're too early
Speaker 2:Too early.
Speaker 1:To say to predict anything, personally. And if we were in the August, I'd say the likelihood of high levels across a large swath of the state is pretty low. We're still early enough that it it could develop. And it's very hard to predict. You know, of you heard me talk about this before and know the exact situation, but we've been watching fields near Rochester for a number of years.
Speaker 1:2023 the disease actually I guess 2022 and it was in a drought year, very severe down there. And last year it was raining at first in that part of the state. And we saw very low levels of tar spot until very, very late in the season. So we expected to develop, we thought conditions were pretty favorable, it didn't develop. Other fields up near Cannon Falls developed very high levels even though they hadn't seen anything like that in the past.
Speaker 1:So it's it's hard to predict. And and we're right in the cusp, I think, of the point where not finding it is not a a predictor of how risk how much risk there is.
Speaker 2:Yep. And I know that there's been efforts within the university to monitor for corn tar spot across the state. Do you wanna kinda just talk about a little bit about those monitoring efforts?
Speaker 1:Yeah. That monitoring that distribution of of tar spot is something I've been doing. And those of you that have looked at this, corn IPM tar spot national distribution map, see how counties get highlighted down there. I I'm one of them that do that for Minnesota and a couple of people that can do that too. You know, we need confirmation to put counties on the map.
Speaker 1:So we need good photos, you know, often very good photos or or an actual sample. So we would like help from folks across the state again to help us determine where tar spot is developing. Not only we can, again, know that for the help of others in the state who who want to know, but because it will ultimately, I think, help us understand what are the risk factors, what are the conditions that favorites development. So I guess I'll leave it at that.
Speaker 2:Yeah. Yeah. And, yeah, we can continue to check those monitoring sites, as you mentioned, with the pipe IPM.
Speaker 1:Mhmm.
Speaker 2:But it it's not gonna reflect what's really out there if we're not able to get help with other counties reporting these and scouting for these. Maybe one last question regarding do you wanna mention kind of what are the fungicide options for corn tar spot?
Speaker 1:Yes. There there are many. And so the crop protection network and the Crop Protection Network, that's a website that publishes a lot of very good solid information on management of crop diseases. It publishes a table every year and updates it in the winter showing relative efficacy of different products for tar spot and other corn foliar diseases as well. And so that would be something that would be a very good source.
Speaker 1:The good thing is we have a large number of fungicides that are effective against tar spot, and that table gives relative efficacy of of a a large range of different fungicides.
Speaker 2:Alright. Well, thanks, Dean. I think we're kinda nearing that end of the webinar. So, again, thank you, doctor Dean Malvik, for joining us for today's field notes program. And we wanna thank everyone for joining, and, we do have a link for that crop protection network in that chat, so please feel free to open that up.
Speaker 2:And, we want wanna thank everyone for joining today's University of Minnesota extension field notes program. We'd like to also thank our sponsors again, the Minnesota Soybean Research and Soybean Promotion Council and the Minnesota Corn Research and Promotion Council for their ongoing support. Before you log off, please take a moment to complete the brief three minute question three question survey. Your feedback is going to be very helpful for us when we plan and improve future programs. And just a quick reminder, we do not have a, field note session next week due to field school.
Speaker 2:So July 30
