Soybean and corn diseases in late summer and what to plan for next year
Welcome everyone to the second episode of the U of M's IPM podcast for field crops. I'm doctor Anthony Hanson hosting today. As we get into August, we're thinking about crop diseases a lot. So today we've invited Doctor. Dean Malvik, professor at the Department of Plant Pathology to talk to us about corn and soybean diseases.
Anthony Hanson:So welcome Dean.
Dean Malvick:Thank you. I'm glad to be here with you today.
Anthony Hanson:Yeah. We've So had you on before, but do want to give just a brief introduction of who you are and then what you've been doing for research at the University of Minnesota?
Dean Malvick:Yes. As a research and extension plant pathologist, I'm based in the department of plant pathology on the St. Paul campus. Although my work, both the extension and research, extends well beyond campus through many parts of the state. So as as Anthony mentioned, I my work focuses on diseases of corn and soybeans, and I've I've been doing work on a number of different diseases over the past few years, including corn.
Dean Malvick:We've been doing work on bacterial leaf streak and doing some survey work with the newer disease tar spot. We've done some work with Northern corn leaf blight in the past. We've worked on seed treatments for various aspects of corn improvement and establishment. In soybeans, a lot of the work is focused on, well, many different diseases, but especially in sudden death syndrome, white mold, rhizoctonia, brown stem rot. More recently, we've been doing some more work with frog eyed leaf spot, which is a relatively new and increasing disease in Minnesota.
Dean Malvick:So
Anthony Hanson:looking specifically, what diseases are popping up about now? We're August right now. I know white mold, I'm starting to hear some mention of that popping up in fields. Do you have an idea of how prevalent that is in the state so far or is it kind of just starting to show up in terms of symptoms?
Dean Malvick:Yes, this is the time white mold really typically starts to show up and be fairly obvious in some fields. You know, we're a little bit advanced in our normal crop stage development this year. So we'll probably see white mold a little earlier depending on where we are, of course. This is a disease that's strongly driven by cool temperatures and wet weather, and some places have been fairly dry recently. Many places have actually, and that doesn't really favor white mold.
Dean Malvick:That doesn't mean the infection isn't there and we get a little rain or it will take off again. So that that can and and will likely happen some places. Question about how widespread it is this year? I I don't have a good, I guess, feel for that at this point. I haven't heard many reports yet.
Dean Malvick:Certainly, we've had temperatures that are favorable for development. And, where it's been raining and now our our canopies and our pod set is is a good time for the disease and not good for the crop if the disease is coming, of course. But this is a prime time for the disease to really start to develop where it develops. So it's a good time to at least scout for it and find out where it is.
Anthony Hanson:So especially if I have a field where I fairly consistently get white mold, what options do I have either early in the year when I'm planning or right now when I'm obviously seeing symptoms? Is there anything I can do at this time of year as well?
Dean Malvick:Yes. Very good question. At this time of the year, no, there's really nothing we can do. I mean, the best thing we can do is to get more information on where the highest risk is, where the disease develops. The next year or the next time soybeans are planted in that field, we have target our treatments better if we have that capability.
Dean Malvick:So options we have for a future crop would be number one, we need to plant the most resistant soybean variety we can find. Now, are no soybean varieties that are completely resistant to white mold, but some are much more resistant than others, and they do hold up much better. Another option is to look at potential for fungicides. Now, we're past the window right now, the time for application of fungicides this year in most fields. The best application timing is between R1 and R3, so very first flowering to up to the first pod set development.
Dean Malvick:And so the optimal time, if we had to pick one time, is when the rows are billing and the plants are flowering. And so in in true crop growth staging, that might be late r one, r two, again, depending on the year. And so that's an option. I know fungicide is complete control, but they can greatly suppress the disease if they're applied in a in a timely fashion. And if they're applied before the disease gets established, which is one of the big challenges with applying fungicides.
Dean Malvick:They need to be put on before the disease really gets established. Therefore, we're trying to guess in many ways. There are some risk assessment models out there to help us, I think, but in the cool wet conditions, crop stage, that's what drives those decisions. The other option we have is something to consider, well, it's not as proven, I guess, as is fungicides, would be a product called Contans. It's a soil applied, essentially, biocontrol for the white mold fungus.
Dean Malvick:It truly does kill the white mold fungus as it survives in the soil. So we have some data to say that it has potential from various places around the Midwest. We don't have the data set we'd like to say that. Definitely, it's gonna provide a strong benefit, but that is another option that's been shown to be very effective in some other crops.
Anthony Hanson:So, you mentioned having some partial resistance in soybean to white mold. Is it easier to find certain varieties that have that resistance? It seems like or they kind of more difficult to find?
Dean Malvick:Well, again, giving that it's a partial resistance and that it's a scale. There's nothing that's completely resistant, like I said. Most companies have varieties that are more resistant than others. So if we look at their scale, you know, between zero and ten, whichever direction it goes from high to low resistance, nearly every company has some, you know, well above the average or the mil of that scale, and and those can really be beneficial. So so again, I guess it's they're pretty widely available, but again, none of them provide a complete control.
Anthony Hanson:So you mentioned sudden death syndrome for soybean, and this isn't a disease that, you know, some people have heard that much about yet. Others may have experienced themselves. Do you want to give a little more information on what the disease is and where you've been looking for it so far in the state?
Dean Malvick:Yes. It's sudden death syndrome. That's a disease that was first found in Minnesota in two thousand and two in or around Owatonna in that area. And it's a soil borne fungal disease of soybean. It's one of the more important soybean diseases across the Midwest.
Dean Malvick:And, again, it's been, like I said, Minnesota for about fifteen years or so, eighteen years. And it's been spreading across the Midwest, across the country since the eighties. Initially, when it was found in in Minnesota was in that Southeastern part of the state. It's spread fairly widely now. So we know that it's in most counties in the Southern half of Minnesota, and we've been finding it a little bit north of that as well.
Dean Malvick:And it was confirmed in North Dakota as well in 2018. So it is spreading. So it infects the roots, this fungus that causes SDS, and then, it happens early in the season, probably in the first in the first two or three weeks after germination. Then disease develops as the season progresses, and we don't typically see symptoms until this time of the year, the August. And then we start to see interveinal sclerosis, yellow spots between the veins on the leaves, and then the leaves start to turn brown and die.
Dean Malvick:And so that's that's a scenario of how the disease develops. So it it can cause pretty significant yield loss. And in in options for managing that, again, most companies now have pretty good levels of resistance for that. Nothing completely resistant, but again, the most resistant varieties hold up against SDS much better than the most susceptible ones. And the other option we have now are some seed treatments, and those can be chosen also, obviously, in the winter and spring.
Dean Malvick:There's nothing we can do for SDS this time of the year when it typically starts to show up, but we can use seed treatments in those varieties if we choose them early enough. They are very promising, although they are fairly expensive. Those are the key things to think about, I think, in terms of SDS.
Anthony Hanson:So we'll move along to another disease in soybean you mentioned was frog eye leaf spot. And you mentioned this is kind of a new development for the state as well.
Dean Malvick:Yes. Probably, at least, probably, it's an interesting one. It's been a problem in the central and southern part of The US for a long time. It's not that it's totally new for Minnesota or states nearby, but it's definitely been increasing in the last few years, maybe in response to increasing rain in the summertime. But it is favored by wet, warm conditions, and therefore, we've had last couple summers in some parts of the state, we've had very favorable conditions for it.
Dean Malvick:So it causes these small circular spots on leaves that are surrounded by a darker halo or ring, and hence the name frog out leaf spot comes from those spots. And again, it can be a pretty significant disease. In fact, farther south, fungicides are often sprayed to manage it. And we've been finding it, as I said, in increasing levels the last two years, 2019, 2018. And I know of at least one field in in South Central Minnesota that was sprayed forth with fungicides specifically to manage it, so it reached levels of of significant concern.
Dean Malvick:And what we've been finding here is what has been found in the other parts of the country is that fungus that causes it is is resistant to certain kinds of fungicides. This has been widely reported across much of the country, and we confirmed it in Minnesota as well that the frogye leaf spot fungus is resistant to the QOI, the strabellium class of fungicides. So those are not alone going to be effective for managing it. So that's been something that that we need to keep in mind too if this disease increases and becomes a concern.
Anthony Hanson:So is this a disease that is continuing to spread in the state that you want people to report it to you potentially or others if they see it in their fields?
Dean Malvick:Yes. I'm glad you asked that question. We we do want to know where it's developing. We've had reports, you know, as far and confirmed reports last summer as far north as Stearns County. And I've talked with folks that have seen it west of there and maybe a bit north as well.
Dean Malvick:We didn't have confirmation though. So there's no reason it can't develop anywhere in the state, but we do want to know where it's developing and if the fungus is widely resistant to this widely used class of fungicides. So if somebody has samples and sees the disease, I'd appreciate getting either photographs and ultimately samples so we can confirm it, and ultimately, in many places, test for the fungicide resistance. So we'd appreciate any contacts or samples that we could get to help us understand this disease a bit better.
Anthony Hanson:And when we post this podcast, we also post the Minnesota Crop News blog. So we'll include some pictures of what these diseases so far look like when we put the podcast up. For one last question in soybean here, how about soybean cyst nematode? What's been going on with that lately? Have there been any changes with that?
Anthony Hanson:That's kind of a different pest, so to say, in terms of it's not your typical diseases, but it's also not an insect like the entomologists deal with. It sounds like it's kind of the pathologists who end up dealing with soybean cyst nematode mostly. Are there outbreaks you've heard of at all, or are there any different management tactics that have been either not working as well or new developments in that area?
Dean Malvick:Yes. And and the first thing I guess would be to mention that the disease or the nematode continues to spread. So everybody that plants soybeans, especially if they don't receive the yield that they're getting, pays to sample regularly to confirm the levels of this nematode in their fields, and if they have it, if they haven't confirmed it before. So that's that's a key thing, the sampling and confirmation. And in terms of the management, we've relied on particular source of resistance in most of our soybean varieties for a long time.
Dean Malvick:It's called the p I eighty eight seven eighty eight source of resistance. And that is not working as effectively as it has in the past in many places. Although in many places, it still is an effective source of resistance. But the companies that produce soybeans are developing other sources of resistance, such as one called pecan, and that is effective in some fields where the other type of resistance has been losing efficacy. So that's another thing to keep in mind.
Dean Malvick:And it's always been true with this nematode rotation is important. The longer we can stay away from soybeans, more likely the nematode population will drop in the soil, creating a little less risk for for soybean planted there again in the future. So those those are a a few things to keep in mind, and this is a sort of insidious problem in a way. It's not always that obvious that it's a problem, but we need to keep that in mind. Keep trying to manage it.
Dean Malvick:Keep trying to monitor it.
Anthony Hanson:So I'm sure some people are excited to hear about corn instead of soybeans here. So how about corn diseases? What are the main ones that you're concerned about right now or that you're watching in the state?
Dean Malvick:Yes, that's a good question. Number one would be tar spot of corn. Now it hasn't caused any notable yield loss in Minnesota yet, but I think most folks that are planting corn and reading about corn issues, challenges, the last couple of years have read about this disease, tar spot, in other states. And we did confirm it for the first time in Minnesota in some Southeastern counties, in in 2019. So tar spot is a fungal disease of corn leaves, and it causes very small can head to couple millimeter size black spots on the leaves that cannot be rubbed off easily.
Dean Malvick:In fact, they're embedded in the tissue. So that's a that's a diagnostic feature. So it's been spreading across The US since about 2015, 2016. And, again, like I said, it finally reached Minnesota as far as for our first find in at least last fall. After it was found last year, we certainly think there's a greater chance of it developing in many areas.
Dean Malvick:And as of last week, tar spot was confirmed in Minnesota in the Southeastern part of the state in some fields near areas where it was found last year. So the disease, tar spot, is now confirmed in Minnesota in 2020, although at low levels. And even those fields, the levels are low, as I said, and they're not developing very fast because of dry conditions. I know survey efforts have been made in other fields and areas where it was found last year, and tar spot has not been found. And without much rain and with dry conditions, we don't expect tar spot to develop very quickly, but that could change if weather conditions change.
Dean Malvick:So this is definitely a disease we're watching because it can cause significant yield loss. So we need to know where this disease is developing and and really scalp for it. Fungicides can be effective to suppress it for sure. We need more data about timing and which products work the best, but we do have evidence from other states where trials have been done that fungicides can effectively suppress tar spot. So this is a disease we're watching, and if anybody sees any suspected samples of this, you know, please contact me or or contact local extension educator or local agronomist because we really wanna know where this is developing.
Dean Malvick:So that's been a the marquee, the most prominent disease that that we've been looking for. Other diseases are out there that aren't new, but certainly can be problematic. Northern corn leaf blight is widespread across Southern Minnesota this year, although not severe in most many cases, at least not yet. It's pretty prevalent. Gray leaf spots have been developing in some of the rainier areas, especially in the South.
Dean Malvick:Common rust is out there. We don't normally have much concern about that, and I don't think we will this year either. It's not developed to the point where we need to be concerned, but it is there. Another disease that is spreading into the Upper Midwest now is southern rust, which is a more dangerous rust disease on corn than is common rust. So we're gonna be watching for that a bit too.
Dean Malvick:So for some of these diseases that I mentioned, there's a national map that's available that's updated regularly, and one can look for it. It's called the Corn IPM PIPE, Pest information platform for extension. That's what the PIPE stands for. Again, Corn IPM PIPE. And one can look at a map of the Midwest, in fact, much of the country, showing counties where some of these diseases have been found.
Dean Malvick:It's updated very regularly. So those are a few things that we're looking at and watching for. And this is a key time of the year right now where the a lot of these start to develop.
Anthony Hanson:So another disease that I've heard you've been looking at is also bacterial leaf streak. By the name suggests, it's not a fungus, so it's kind of a different type of disease to work with, and most of these tend to be more fungal diseases. So you wanna talk a little bit about how this disease works and how it's managed?
Dean Malvick:Yes, I should back up. In the bigger picture of fungal versus bacterial diseases, you know, most of the important diseases on corn and soybean are definitely fungal or or oomycete fungal related. We have a few that are caused by bacteria. Now on soybeans, a very common disease is bacterial blight, and that's been that's common across much of the state many years, although it really becomes severe. Just a superficial problem.
Dean Malvick:But on corn, we have two diseases caused by bacteria that are pretty significant. One that we've all seen and dealt with, I think, in the last or at least most of us over the past ten years is Goss's wilt. Now that's subsided a little bit, actually quite a lot recently, I think partly because of better and more resistant hybrids, we still have outbreaks of that disease as well, and we had some outbreaks in Goss's wilt in 2019. And that is, again, a bacterial disease. Fungicides have no clear effect on controlling that at all.
Dean Malvick:The other newer one is bacterial leaf streak. It's caused by different bacterium. It's a different one that causes bacterial leaf streak on wheat. It's not the same bacterial pathogen. It causes brown linear lesions on leaves that can look a lot like gray leaf spot, which is a fungal disease.
Dean Malvick:So this is also a relatively new disease, first confirmed in Minnesota and much of the Midwest in 2016, or maybe going back to 2015. So this is mostly in the southern half of the state from what we've seen so far. It can cause significant lesions and be pretty obvious at times on on field or dead corn, although we haven't don't have any evidence of yield loss in that. It does seem to be a bigger problem on sweet corn. So we've been doing some research on that, trying to figure out how damaging it can be, if there are different levels of resistance among hybrids, if there are treatments that might be effective.
Dean Malvick:So that research being done at the Southern Research and Outreach Center in Waseca with, cooperators there is is in the early stages, and and we'll hopefully, we'll learn more from that this year. That is another disease to watch for, and certainly has been reported in a number of fields this year.
Anthony Hanson:So I'll give you a kind of more broad question since this is the IPM podcast. My question is kind of dealing with all of these different diseases you've listed. Me speaking more as kind of an entomologist in training, a lot of times we're focusing on just one pest and deciding are we going to treat that one? But with all of these different diseases, how does IPM vary for looking at diseases? Are you usually focusing on one disease as well, or are you finding multiple diseases in a field?
Dean Malvick:I'd say in most fields, we have more than one disease, even if we don't always notice it. You know, we talked about in soybeans, soybean cyst nematode. We talked about sudden death syndrome. We didn't talk about brown stem rot, which is another important disease in Minnesota, but I mentioned that now because soybean cyst nematode can make both SDS and brown stem rot worse. In fact, most places where we see severe SDS, we have high levels of nematodes in the soil.
Dean Malvick:Likewise, when we get one kind of root rot, we often see another kind of root rot. Usually, we have one prominent player, I would say, that's causing the greatest damage to the plant. But oftentimes, there's more than one thing going on. Sometimes it's difficult to figure out exactly which is the prominent player causing the most damage. You know, we look at crops.
Dean Malvick:I mentioned on corn, all these different leaf diseases. Usually, it's one in a particular time that kind of rises up and is the most concerning. Although two years ago, 2018, when tar spot became a really clear significant problem in the Midwest and states to our East, many of those fields where tar spot was severe, there was also a lot of other foliar diseases, including northern horn leaf blight, that were pretty significant at times. So the same conditions seem to favor both in some fields at least. So there there's no real simple answer to this.
Dean Malvick:Many of these fungi that cause these diseases are out in most fields a lot of the time or the bacteria. They're really waiting for the right conditions to come along to really cause a problem. Fortunately, most of the time, we don't have those conditions, which is why we have to keep scouting. We don't always know when they're gonna pop up and become severe.
Anthony Hanson:Are there any other diseases you're keeping an eye out for on the horizon that are either in nearby states or diseases that you consider that would be invasive that you're worried about coming to the state?
Dean Malvick:At this point, I am not aware of anything that isn't already here. You know, a year ago, tar spot would have been the number one on that list. We've been watching it develop in other states to our South and and a little bit to our East. It hadn't come in Minnesota. But but last year, again, like I said, tar spot was found in Minnesota.
Dean Malvick:So our key question there is how well it will become established, how far will it spread, how much damage can it cause in our climate and cropping systems, and we don't know yet. So we're watching that one for sure. We have a lot to learn. And bacterial leaf streak, it's another new disease. Again, we have a lot to learn about it.
Dean Malvick:It doesn't seem to pose the same risk as it does tar spot, but it does pop up and develop in ways that are a little bit unexplainable in some ways based on what we know, although weather is probably a primary driver there as well. There are always other disease that the USDA has on their list of diseases to watch for, and the Minnesota Department of Agriculture has some of those on their list, and they are scouting and looking for those. None of those seem to be a real high risk at this point based on what we know in terms of their developments in The US, and they're not really worth mentioning. But certainly, we are looking and others are looking for other diseases, and, fortunately, they're not being found. So I think we have enough to deal with right now.
Dean Malvick:Hopefully, we can stop, not develop and and get anything new at this point in time, but we have to be watchful and wary and and always be looking to see what's happening. Do you have any research you wanna highlight that's been either exciting for you or in the pipeline that you're interested in seeing what will happen in the coming years? Well, I mentioned one is this is spectral leaf streak. It's very interesting to see how that will develop. And and, trying to assess what the potential is for that to be a significant problem that reduces yield.
Dean Malvick:We're we're looking forward to getting more information on that as well as looking for ways to manage it with a full air applied application and things to see if any of those might be affected. That that's certainly one. We have a number of different avenues of research going on with SDS, looking at seed treatments and varieties and combinations and other things related to the pathogen population. And I think all that will lead some some interesting results over the years, and hopefully in the near future. Brown stem rod, another widespread significant disease on soybean, And we're trying to find out a little bit more about how rotation and number of years away from soybean affect the population of that fungal pathogen.
Dean Malvick:In corn too, we're really looking forward to finding out if tar spot will become a significant problem. Now I I hope we don't have to do much more research on it, and I hope it just doesn't ever become a problem. But, you know, we certainly will increase our research effort on that if it shows any level of of increase in severity like it has in other states. So we have a number of different kinds of things happening, all which could be beneficial, we hope, in the short and long term.
Anthony Hanson:I guess one last resource to point out is at the university, we have the plant disease diagnostic clinic. Do you want to give a little outline on how that works? We've talked about it in previous podcasts, but this year, I want to highlight that it sounds like it is still open and taking samples if they want to send those in.
Dean Malvick:Yes. In the department of plant pathology on the St. Paul campus, we have a plant disease clinic that is running, staffed with expertise on diagnosing plant diseases, and they welcome samples at any point in time. So it it's a great resource to keep in mind. They have expertise in many different diseases and different kinds of systems all the way from not just field crops, but all kinds of ornamentals and trees and and seed health as well.
Dean Malvick:So that's a great service, a great facility to keep in mind. And again, you know, we have COVID nineteen problems all over. That is a lab clinic is fully operational and running a 100%.
Anthony Hanson:Alright. Well, thank you, doctor Melvik, and thanks to everyone else who's been listening so far to all the different IPM topics we've been talking about over these last three years and into this year as well. So thank you, Dean.
Dean Malvick:Alright. Thank you. And it's good to talk with you, and keep watching for problems in those fields.
