Fungicides and Fungicide Resistance  - Daren Mueller will be back up to talk about fungicide resistance and he is an assistant professor and Extension plant pathologist at Iowa State University. Daren's responsibilities include the management of soybean diseases. Daren is originally from Clinton, Wisconsin and he earned his Bachelor of Science in Meat and Animal Science at the University of Wisconsin Madison and his Masters and PhD in Plant Pathology from the University of Illinois. So welcome Daren back.  (applause)  - You won't be clapping afterwards so don't worry, get them over with now.  All right, yeah, well thanks for the introduction. Like she said, my name is Daren Mueller and I am at Iowa State. And thank you for inviting me to this conference. It's been great. This talk is on fungicide resistance in field crops and it will be followed by some stuff on corn fungicides by Damon, and so there will be a little bit... 

A couple of slides where I'm gonna just go through them pretty quickly and let Damon fully explain some of that. And so, if you have questions, just please hold them until the end. All right, so fungicides, I think that this is probably not surprising for most of you in the audience. Usually when we talk about fungicides with graduate students or others, they're a little bit surprised that in the field crop world, there has been a big uptick in use of these. But, you know that's why we have an uptick or why we're talking about these.  It doesn't really matter the reasons but it's pretty cool that we have new products that are available in all of our field crops. I think we just heard a talk in wheat and fungicides by Brian and we have some stuff in soybeans and we have, you know stuff in corn, and so there's a lot of research, a lot of stuff that's being done. The thing that's gonna be tricky is there's gonna be a lot of these products that we sort of know as Quadris or Headline, or sort of the common things and they're gonna be, they're now coming off patent. And now we have a lot of products that are gonna be following that are generics that have the same active ingredient. And so it's not...it's gonna be... 

We're gonna have to spend a little more time making sure that as we deal with fungicide, deal with resistance issues, that we make sure that everyone's on the same page as far as what's going on. Back in the soybean rust days-- I know there was a question about soy bean rust earlier. You know, a lot of the talk was preventative versus curative. Those terms are sort of marketing terms that sort of emerged out of the soybean rust era. Trying to just sort of distinguish between Strobilurins and Triazoles. You know, the...in today's day and age, most of the products are premixes. Most of the products, it's not as simple as preventative and curative. And so it's sort of a.. it continues to be a sort of a growing idea or continuing to be evolved as far as the use of fungicides in field crops. So this is my soybean leaf. 

As I was looking at it when I was looking at my slides earlier. I did get my education in Wisconsin. We spent a lot of time having milk breaks, not necessarily art classes so you're gonna have to excuse the soybean leaf or whatever that is defined as a leaf there. But-- and those little blue dots on it, that's the fungal spores, and so... I spent a lot more time drinking milk and playing football and stuff at recess, not necessarily in art class. But, so what we have are the blue spores are the ones that are gonna be killed by the fungicide when they're sprayed, when the fungicide is sprayed. And the red one is...naturally exist out in the world, out in that field and when you spray the fungicide, it may be insensitive to that, to that fungicide. And then, to continue that, eventually over time, each one of these may reproduce and you'll end up with some blue and red spores and then you spray again. Eventually, over time, you may end up with a population of the red spores. These will be the ones that have some kind of mutation or have an insensitivity to the fungicides, and once you have this red spore population, this would be when you would have a fungicide resistance issue. 

And so you can... you probably are well aware of sort of the issues we have in weed science, and herbicide resistant weeds. It's the same concept but just using a different organism.  So, what leads to fungicide resistance? As far as...  What are the risk factors? And it really comes down to, a lot of it is in the fungus. And so, as you look at the different pathogens across there, they're going to vary quite a bit as far as the genetic diversity, within that group of pathogens and then also how quickly they reproduce. And so, are they going to reproduce, are they going to have several cycles during the season, are they going to have just one cycle during the season. And then, the other half is the fungicide and so you're going to have different modes of actions. Most of the one that we have available in the field crops are going to be sort of that, the single site or a more narrow Mode of Action compared to the multi site ones. And, so the inherent risk of resistance developing in these fungicides is going to vary across the fungicides and across the classes. So, when you start looking at, just to compare it back to weeds, I described a couple of herbicide resistant weeds that are out there. 

And, sort of give you perspective as far as how much... the differences as far as reproductive ability of these organisms. If you look at Kochia and Wild oats, as far as how many seeds are being produced, you're looking at thousands or maybe hundreds of seeds depending on sort of health or the size of the plant. And so, if you compare that to plant pathogens, you know, you're looking at a lot more numbers. And so to give you an example of sort of take one rust pustule from a wheat plant or from a soybean plant and you're looking at thousands of spores that are being produced from one pustule. And then, the one example that I could find in the literature, if you estimate one barberry plant. This is sort of an alternate host for the wheat stem rust, you're looking at, you know, 50-60 billions of spores being produced just from one plant. And so, if you have a pathogen that has a lot of diversity, and is producing a lot of different spores, you can imagine that there's going to be naturally be something that is resistant to the fungicide out in nature.  And so you can also have, and sort of speaking of the diversity... This is probably not information that is readily available. 

You probably have to dork out a little bit and get into some of the literature that is not really as fun to read. Unless you're a plant pathologist or maybe I'll couch that, I mean Damon up here is cringing a little bit. But in general, there's some pathogens that have a lot of diversity, and a lot of these are going to be like, as an example, the Cercosporin, Cercospora pathogens. And these would be Cercospora zae-maydis, that's Gray leaf spot in corn, Cercospora sojina is Frogeye leaf spot in soybean. There's probably a Cercospora pathogen in most of the plants that exist out there. These have a lot of diversity, as far as the natural, makeup of this group of population. And so, just to show the key take home point is that as you look across a field and you look at the pathogens, there's probably existing ones out there that already are resistant to the fungicides, they naturally exist. And so, you take that and you combine it with really having a limited number of fungicides that are available. If you look at sort of the foliar fungicides, sort of the ones that we talked about, the ones that Brian covered in wheat. And for corn and soybean, you're looking at Strobilurins, Triazoles and SDHI fungicides. 

And so, there's really not a lot of options out there as far as rotating or going to a different class of fungicides. You get into the white mold and Sclerotinia, there are going to be a few more classes. Omega from Syngenta is in a different class. In the seed treatment world as well, there's a couple of classes that we'll be targeting OMRI seeds. And so , there are a few additional ones when you get very specific uses. But, in general, you're looking at, for field crops, you're looking at three classes of fungicides. And these three classes of fungicides, unfortunately, they all have a medium or high risk for developing fungicide resistance. So for the Triazoles that are medium risk, we already have resistance being documented in a lot of different pathogens. Usually this resistance is gonna be developing fairly slow and so it's usually more of a gradual resistance. SDHIs are medium to high risk and this is going to be a single mutation that's gonna confer resistance. 

And then, probably the one where you have the most common reports of fungicide resistance, are gonna be with Strobilurin fungicides. And these, we have many different pathogens that have already been announced. And really this-- there is one major mutation. That's the G143A mutation, that's up there. And when you have that one, again, it's an all or nothing thing. It goes from being sensitive to the fungicide to being resistant to that fungicide. And that mutation unfortunately is...  Is very stable. And so once that pathogen population has experience that and you've selected for it, it won't go back to being sensitive to that fungicide. Now, out of the good news out of that, there are some pathogens when they get that mutation... 

 it will be a lethal mutation for that pathogen and so these will have a lower risk for developing resistance.  There's also a term called cross resistance. What this means is if you have resistance developing in one fungicide in a group of pathogens.. Or in a group of fungicides, So, let's take Strobilurin, if you have ones developing resistance to Azoxystrobin. The other Strobilurins will experience that resistance as well. And so, so, all of the fungicides in that class, will be insensitive or will not work on that pathogen. Now, I'm not going to go into a lot of details between the difference between Strobilurins and SDHIs but these are both targeting fairly close systems within that cell. And although they do, they are different FRAC codes or there different codes or classes of fungicides and so... There's no cross-resistance between these two.  All right, this is probably the most important slide. 

And so, this is, as you sort of think through, fungicide resistance and identifying it, or what's going on. Really, it comes down to there are a lot of different things that will make a fungicide not work and don't immediately blame fungicide resistance as being it, and so make sure you go through your options. Make sure you sort of think through what happened out in that field before you sort of point the finger at fungicide resistance. And so, one would be and these are sort of all things that I've definitely experienced in Iowa. One would be incorrect disease diagnosis. If you think you have a fungus and you sprayed fungicide to control Goss's Wilt on corn, which I've seen, it's not gonna work. Goss's Wilt is caused by a bacteria, and so sort of having the wrong diagnosis, you though you had Northern corn leaf blight and you had Goss's Wilt, that's not gonna work. Improper mixing of the fungicides or the sprayer not being calibrated. These are sort of user error, that's certainly something. And then also application would be another one. 

So, if you get the fungicide on too early, and then disease develops a little bit later. We had a lot of applications in Iowa go out at v5 this year in corn. And by the time Northern corn leaf blight really took off, that product wasn't really at an effective level out in that field. And so, the application timing might be an issue as far as efficacy. It also might be that the fungicides just don't work. And so there are some pathogens out there that the fungicides just aren't that good a fungicide on that group for that disease. And so, sort of keep that in mind as well. And then the last case is fungicide resistance. So there are a lot of different pathogens that have already been listed as fungicide resistant. If you go to the committee that sort of monitors this, it's called FRAC. 

They will...you can find the publication. It's probably 20-30 pages long. These are all the ones that are in field crops. These are all for wheat and then if you get into corn and soybean, you can see there's not really anything in corn. There's something in corn smut but these are all done in the lab. There's nothing in the field in corn. And then if you get into soybeans, there's recently we've been using fungicides in soybeans now in the US for at least in these classes of fungicides for last 10 or so years. We're already starting to see some soybean pathogens develop resistance to the Strobilurin fungicides.  And so, I'm going to sort of outline Frogeye Leaf Spot. It's not a, certainly not one of the more pressing diseases that you guys see in Iowa or in Wisconsin. 

But it's one that you could possibly get. And so, this is usually found wherever soybeans are growing but usually in a little bit warmer conditions. And so, finding it in the upper midwest, is not overly common but it's not unheard of. Management, if you look at the literature, is usually... you know they say crop rotation. This survives in the crop residue. Plant high quality seed, this can be seed transmitted. And then foliar fungicides... Just as a side note, in Iowa this last year, we had another disease that came in. We had a disease called Fila Sticta Leaf Spot. And there was a lot of fields where they thought they had Frogeye Leaf Spot, and so again, go back to...make sure you have the diagnosis of the disease down right. 

So a lot of the fields that we had in Iowa this last year where it was "Frogeye Leaf Spot" it was actually a different fungal disease. So where have resistance strains of this pathogen been reported?  It's been really all the way through the the Mississippi River following that. And so it's been reported up in Illinois, Indiana, all the way throughout a lot of the southern states. Really...  Wherever we're testing for this is usually where we find it. Now, we've been looking for it in Iowa, at least for the last several years, and we just haven't had a lot of Frogeye. And so, We continue to look for it and eventually, I'm sure we'll find it. But, it's really, it's fairly common being able to find resistant strains of this pathogen. So, how do we do that? Just real quickly, just sort of think through it. It's not that easy of a process. 

So if you have the leaf you have to incubate the leaf. You pick off that second image is the fungus growing inside that lesion. You take that and transfer it to a petri dish that has some agar on it and you grow that out, make sure it's pure. And then eventually you pick out, and then you have to get that to sporulate. You pick out the spores, you put that onto media that..or at different levels of the fungicide in that media and then you test for germination. You can see a picture of the germinating spore. And so, this is sort of a painful process to do. You can also go through and just look for the G143A mutation, sort of molecularly go through and just look for mutations. But that doesn't really capture all of it. And so, it's sort of a high input process here. 

 So, what does that look like in the field or what are the consequences if you do end up with a resistant strain? This was in 2012 and 13 in Illinois looking at a resistant strain to the pathogen for Frogeye Leaf Spot. And then they were looking at solo applications of Strobilurin, Triazoles, premixes and then they also had Topsin M, which is in a different class of fungicides. And if you look at the top line, this is the untreated control, you can see the yield was 50 bushels an acre and the Frogeye Leaf Spot, the second to last column, was at 40. Now if you sort of scroll down and you look at the last two lines, Quadris and Headline, this would be the Strobilurins. And you could see the Frogeye Leaf Spot were at very, very similar levels. And the yields were at similar levels. And then, you look at really the other fungicides in between those and you can see that there was less disease and higher yields where you had other classes of fungicides. And so, where you had the Strobilurin alone, it essentially did not affect Frogeye Leaf Spot out in the field at all.  So, if you have a resistant strain to this or if you have a lot of Frogeye and you're using fungicides, You know, they say-- Some of the management strategies would be in addition to the crop rotation and the other things we mentioned earlier. We plant a resistant variety, make sure you're reducing the risk of disease, is one of the key factors. 

And then make sure you're applying products that have more than one active ingredient. And this is, industry has largely taken care of this. Now if you get into alfalfa and into some other crops you can still get some straight Strobilurins. But for the most part, in corn, soybean, most of the products that are available are now premixes. And then also, think about a fungicide resistant management plan. And so, there's a lot of text here, so I'm going to leave it up here for a little bit, but-- This is going to involve minimizing the risk of disease. So, you're going to try to for whatever disease it is, try to manage it using other options. So, if it's resistant varieties or crop rotation or whatever it is. But try to minimize the selection pressure and that's going to help sort of prolong the effectiveness of fungicides.  Make sure you use fungicides early in the development of the disease in each season. 

And this becomes a little bit tricky because we always tell people to go out and scout make sure you have disease. But sometimes once you start seeing disease that might be a little too late. Take White Mold for example, so if you start seeing White Mold out in the field, you probably already missed your opportunity. And so, so there's a little bit of a risk as far as waiting and wait for disease to develop. But we want there to be, you don't want to be spraying when there's a full blown epidemic. That's really going to put a lot of selection pressure on that population. You also want to be avoiding single applications, we talked about that. And then also, think through the... If you have fungicides that are premixed, don't let that sort of lure you into a false sense of security. What I mean by that is , is that, some of these products, like.. 

I'm gonna just pick Stratego yield as an example. It has prothiaconazole and it has trifloxystrobin, so it has a Strobilurin and it has a Triazole. There might be a disease out there that is not.. That Prothioconazole doesn't work that well against and so, if you, if that's the case then with that product you would have a lot of selection pressure on that Strobilurin side of the fungicide. So, sort of be doing your homework as far as what fungicides work and don't work on different pathogens. And then also, know the pathogens, know what's out there, sort of if you need to. You have to sort of get in to some of the literature or at least be asking some of the extension specialists and stuff about these pathogens. Now we try to provide some materials for you. So you you're not starting with a blank slate. Each year, all of the state specialists get together, we go through all of our fungicide data. 

We try to develop tables looking at efficacy for--  I think we have them for wheat, corn and soybeans. And if you get online, if you look, if you just Googled fungicide efficacy tables, you'd probably come across these. And you can see that there are different ratings for the different fungicides and the different classes of fungicides. So this would be a place to start, at least, with your fungicides. I would also encourage you to get out and to be testing these and evaluating them in your fields and in your farms. So...  Also I mentioned this earlier but that G143A mutation, it may be lethal. So Cercospora zeae-maydis, Gray leaf spot in corn, this is a pathogen that's an example of that. So, this is sort of know your pathogen. You know, so if you end up with... 

So for Gray leaf spot, the chances of having resistance in Gray leaf spot is going to be much lower than it will be for Northern corn leaf blight or some of the other corn pathogens, as an example. So again, just sort of be doing your homework as far as what pathogens are out there. And then the kissing cousin of this is Cercospora sojina and this one's not lethal. And this, so this is sort of held up as the standard for fungicide resistance in the soybean world. I'm not going to to talking about this a lot. I think Damon is going to cover this in the corn fungicide section. But diseases do matter. So when you're out and you're out scouting, we really try to encourage people to be paying attention to diseases. If diseases are present, you have more of chance to have an economic yield response and have fungicides be worth spraying.  And then if you do spray get out and check afterwards. 

And so if you are evaluating your field a couple of weeks after you spray, you should know if the fungus or the pathogens or the lesions continue to grow and continue to reproduce like you haven't sprayed anything, this should be raising red flags. Instead of going out and spraying a second time. This is sort of how the Frogeye leaf spot epidemic resistant pathogen took off as a farmer sprayed four times before it dawned on him to ask somebody about, "hey what's going on, why isn't this working?" And so, get out and pay attention in your field after you spray that fungicide to see if it's working. And then the last thing, just sort of think through just sort of big picture when you are thinking about spraying a fungicide. Again, start with diseases and this is something that you can determine out in that field during the season. And then, if you don't want to go out and scout a field, you can sort of... The second option is to sort of to think about what are the risk factors that would sort of set you up for more disease. And so, this would be if you have a susceptible hybrid or susceptible variety. If you're going corn on corn or if the previous crop has some of that residue in it that might be providing it. And your tillage is similar and then if you irrigate, if you're adding moisture back into that system. 

Most of the fungi do like that. And so, the other things that you, that may affect the fungicide efficacy would be the products and the additives that are being added to the products. And the timing of the application, so be thinking about that. And there is going to be a little bit of data on v5 or SAR1 applications in the next talk. And then other things to weigh in, sort of I say from the air conditioning and also I encourage you to be scouting from the air conditioning. And what I mean by that is do your homework. And so, look at the economics, look at the weather conditions, look at the plant pathology in the news. There's newsletters that are being written in the state, in the region by your seed companies. If you start reading a lot about Northern corn leaf blight in corn, this might...this should be... Or you sprayed something or you planted something that was susceptible, this should be triggering maybe some momentum towards spraying a fungicide. 

So with that, I welcome any questions. (applause)