8 - 2021 South Central Ag Lab Field Day - Corn and Soybean Disease Updates

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10/14/2021 Added

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8 - 2021 South Central Ag Lab Field Day - Corn and Soybean Disease Updates - Tamra Jackson-Ziems, Nebraska Extension Plant Pathologist

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[00:00:08.090]Hi, I'm Tamra Jackson
[00:00:09.350]Ziems, Extension Plant Pathologist at the university of Nebraska.
[00:00:13.100]And in this video, I'll summarize some of the content that I covered during the
[00:00:18.080]July 28th field day at the south central ag lab.
[00:00:23.360]One of the things I want to remind you of is to be sure and check out the video
[00:00:27.950]from my student Asha Mane, where she talks about frogeye leaf spot,
[00:00:32.420]because there's some very important information there that I don't want you to
[00:00:35.480]miss about the fungicide resistance that we have confirmed here in Nebraska,
[00:00:40.460]and that's going to change how we manage this disease.
[00:00:43.760]And so be sure and check that out to learn more.
[00:00:47.180]Another disease that we've been hearing more about here in 2021 is Phytophthora
[00:00:51.710]root and stem rot.
[00:00:53.060]And that might surprise you because the fungal like organism causing this
[00:00:56.630]disease does need moisture. And with the dry weather earlier in the season,
[00:01:01.580]we didn't have it until we started having those isolated rain showers. Now,
[00:01:06.260]what people are noticing is that we're seeing patches or single plants that are
[00:01:10.700]beginning to wilt and die in the field.
[00:01:13.400]And so take a closer look at those plants and see if they have that dark lesion
[00:01:17.450]on the side that starts at the soil line and moves up the plant.
[00:01:21.050]And that'll tell you whether you've got Phytophthora or not to misdiagnose it as
[00:01:25.370]soybean gall midge that you can listen to more about in another presentation on
[00:01:29.510]here, if you have Phytophthora root and stem rot,
[00:01:32.540]there's a number of ways that you can manage this disease.
[00:01:35.690]And I recommend using a combination of these management strategies first,
[00:01:40.310]be sure to select seed treatment,
[00:01:42.110]fungicides that work specifically for these Oomycete diseases,
[00:01:46.340]some of the older chemistries contain matalaxyl or mefenoxam.
[00:01:50.570]And if you select one of those active ingredients,
[00:01:52.820]you have to use the higher rate for it to work against Phytophthora.
[00:01:57.470]Next, when you're selecting soybean varieties,
[00:01:59.930]there's a couple of things to pay attention,
[00:02:01.640]to make sure you're selecting varieties that have RPS genes in them.
[00:02:06.530]This provides race specific resistance to some of the common
[00:02:11.240]races. Unfortunately, some of the more common resistance genes that we have,
[00:02:16.100]like RPS 1c and RPS
[00:02:18.470]1K are not as effective as they used to be. In a recently published paper,
[00:02:24.020]coming out of
[00:02:25.580]Nebraska and Iowa labs are telling us that 70% of
[00:02:30.530]the isolates of this Phytophthora species are virulent on those
[00:02:34.940]RPS genes. And in spite of that, though,
[00:02:38.150]we do have another RPS gene that is more effective that RPS gene is
[00:02:42.860]RPS 3a00:02:44.270 --> 00:02:49.250 unfortunately 3a is only in about 10% of
[00:02:49.250]the commercially available varieties that we have right now,
[00:02:52.580]but it's much more effective only about 10% of Phytophthora isolates are
[00:02:57.140]virulent on it. And so we recommend selecting varietiesIf you can,
[00:03:01.480]that have RPS 3a
[00:03:02.860]or trying to get stacked varieties that have multiple RPS
[00:03:07.720]genes in them for the best control.
[00:03:09.910]The second part of variety selection is to also select a variety that has a high
[00:03:14.860]rating for partial resistance or field tolerance.
[00:03:19.030]And you can often get that in a combination with the RPS genes,
[00:03:22.360]and it'll give you the best effects for management of that disease.
[00:03:27.850]Another pathogen I want to talk about that is important.
[00:03:30.820]This is soybean cyst nematode, and surely you've heard a lot about this already,
[00:03:35.470]but this is still a critically important pathogen.
[00:03:38.620]And we should continue to monitor the population densities in our fields at
[00:03:42.730]least every two or three years,
[00:03:44.830]to make sure that we're keeping those at low to moderate levels.
[00:03:47.980]If we allow them to get too high,
[00:03:49.480]it's almost impossible to knock them back to manageable levels.
[00:03:53.410]The good news is we do have a new strategy for you.
[00:03:56.920]We always recommend crop rotation with a non host. And for most of us,
[00:04:00.820]that's going to be with corn. But when you plant soybean varieties,
[00:04:04.300]make sure you plant resistant varieties to soybean cyst.
[00:04:08.230]Most of those varieties have PI 88788 background genetics
[00:04:13.060]in them. Unfortunately,
[00:04:15.400]that background genetics is becoming less effective as the nematodes are
[00:04:19.120]adapting to that resistance. And so even in Nebraska,
[00:04:23.440]over 50% of our populations are virulent on 88788.
[00:04:28.780]We recommend rotating the source of resistance when you do come back to soybean
[00:04:33.400]every year. And so rotating it with a Peking is a good idea.
[00:04:38.470]Now we have an additional source of resistance available in commercially
[00:04:42.280]available varieties, and that is PI 89772.
[00:04:47.260]And this is one that has now become available in at least two varieties from
[00:04:51.400]Syngenta through their Golden Harvest brand.
[00:04:54.370]This is maturity group 2.3. So it may not fit for everyone's rotation,
[00:04:59.620]but this gives us another option for rotation that will help us prevent
[00:05:03.670]selection,
[00:05:04.210]pressure on the nematodes and hopefully manage them for a longer period of time.
[00:05:08.500]There's additional research being conducted in other states like in Georgia,
[00:05:12.580]where they're also beginning to combine resistance types to provide more
[00:05:16.750]durability as well. And so we have that to look forward to in the future.
[00:05:22.450]Another disease,
[00:05:23.260]that's got a lot of attention this year in 2021 is Southern rust.
[00:05:27.130]And just like in most years, we've now confirmed Southern rust here in Nebraska.
[00:05:31.930]And that was back on July 20th and Nemaha county. Since then,
[00:05:35.590]we've confirmed it in 00:05:37.090 --> 00:05:41.950 a number of counties in the Southern part of the state and even up into central
[00:05:41.950]Nebraska in Greeley county.
[00:05:44.260]And so at this time we recommend people get out and scout intensively
[00:05:49.270]to watch for monitor this disease and it spread within fields.
[00:05:53.470]We're still pretty early in this at this time.
[00:05:56.350]And so it's not going to mean that everyone needs a fungicide application.
[00:06:01.520]It might mean watching to see disease development and spread and that it might
[00:06:05.840]be necessary later to make an application,
[00:06:08.690]especially on vulnerable fields where you might see more disease such as those
[00:06:13.040]that are late planted or under pivot irrigation.
[00:06:16.370]Be sure to also look at the rating for your hybrid for Southern rust,
[00:06:19.780]because now we do have some resistance to Southern rust out there,
[00:06:24.590]and we might see some control of that disease there.
[00:06:28.250]Be sure and look for the orange to tan pustules and spore development on the
[00:06:33.080]upper side of the leaf. Sometimes on the bottom,
[00:06:35.930]you'll see the pustule pass through too if it's near the midrib.
[00:06:40.130]And so flip those over and if you can rub off orange to tan spores,
[00:06:43.460]you've probably got Southern rust,
[00:06:45.500]please submit questionable samples to the UNL Plant and Pest Diagnostic Lab,
[00:06:49.820]Or contact one of us in Nebraska Extension.
[00:06:53.930]We've conducted a number of fungicide trials on Southern rust and
[00:06:58.850]timing trials to try to help us get a better idea when is best to spray.
[00:07:03.710]And we did that in 2020 as well and had some Southern rust development in our
[00:07:07.970]experiments.
[00:07:09.260]And so I want to summarize some of the results from that experiment now.
[00:07:13.460]And so when we look at disease severity,
[00:07:16.130]you'll notice that every product that we used reduced disease
[00:07:20.630]severity,
[00:07:21.800]and that was compared to the non-treated control on the left side of that graph.
[00:07:26.630]You'll notice though, in the bars that I've got blocked off,
[00:07:29.210]that we made applications at R1 and at R3 (stages).
[00:07:32.540]And so you can compare how well they worked. And in general,
[00:07:35.840]you'll notice that they all work to reduce disease,
[00:07:39.350]but even that R1 application,
[00:07:41.960]which was before we saw rust development here on the research farm,
[00:07:46.640]did a good job, and had the consistently lowest disease severity.
[00:07:51.800]And this is in some contrast to what we've shown in the past here.
[00:07:55.910]And so be sure and take a look at these data, look through there.
[00:07:59.360]Also notice we have a comparative treatment of Xyway that we applied
[00:08:04.190]in furrow at planting that's right next to the non-treated control on the
[00:08:09.110]left side of this graph, you'll notice it did not control Southern rust,
[00:08:13.580]but where we used that product and a follow-up application of Lucento at R1,
[00:08:18.200]did a great job of controlling Southern rust.
[00:08:20.240]So you've got a number of options that can control the disease.
[00:08:24.710]When we look at the yield data thereafter, we see a little bit of variability.
[00:08:29.510]And so when you look at these,
[00:08:31.340]check out the pairs of treatments that R1 versus R3,
[00:08:35.150]the letters above them show statistical significance and any two bars that have
[00:08:39.590]any of the letters that are the same above them,
[00:08:42.510]mean they're not different statistically (different) or they weren't consistently (so).
[00:08:46.130]So that means that in most of these cases,
[00:08:49.610]the yield didn't matter last year, whether we sprayed at R1 or R3.
[00:08:54.470]And so that might give you some confidence that you can wait a little bit later
[00:08:58.470]and make those applications without any disadvantages.
[00:09:02.850]Do notice too, that the Lucento application is Iowa did yield
[00:09:07.170]as well as many of those applications as well. Moving forward.
[00:09:11.790]I also want to remind you that this season we're also seeing bacterial leaf
[00:09:15.420]streak again, and this one's going to be with us year after year. And so
[00:09:19.890]it started early this year. Most often,
[00:09:21.870]starting in the lower leaves working its way up.
[00:09:24.360]But when we have severe weather with high wind move through,
[00:09:27.690]you're often going to see that in the upper leaves. And when you look at them,
[00:09:30.990]make sure they're backlit. And if you see that bright yellow discoloration,
[00:09:35.130]there's good chance that you're looking at bacterial leaf streak because those
[00:09:38.070]bacteria are bright yellow. So it can start at any stage,
[00:09:42.240]but just remember it's important that we identify the disease correctly because
[00:09:46.560]fungicides will not control this bacterial disease.
[00:09:50.640]And so when you look to see where the disease is located,
[00:09:54.570]it is pretty much through most of the corn growing areas of the state. In fact,
[00:09:58.650]confirmed in 75 of our Nebraska counties.
[00:10:01.680]Now my main concern about this disease,
[00:10:05.490]although we believe it's causing some yield loss when it's severe,
[00:10:09.210]my concern is,
[00:10:10.050]is that we still have people confusing it with the common fungal disease,
[00:10:14.580]gray leaf spot. And so on this slide,
[00:10:17.130]you can see them side by side with gray leaf spot and compare the lesions.
[00:10:22.350]It's important to notice bacterial leaf streak or bacterial lesions in general,
[00:10:26.850]are wavy and irregular in shape.
[00:10:29.490]They're similar in size to those of gray leaf spot,
[00:10:32.070]but gray leaf spot lesions are smooth and rectangular.
[00:10:36.180]And so hopefully that'll help you tell them apart,
[00:10:38.580]but when there's a lot of it, or both on on the leaves, both diseases,
[00:10:43.470]it gets hard to differentiate between them.
[00:10:45.930]So let us know if you need help doing that. Finally,
[00:10:49.740]the last disease I want to share with you is tar spot.
[00:10:53.340]We have not confirmed tar spot in Nebraska yet,
[00:10:57.180]but if you're in some of those areas of extreme Eastern Nebraska,
[00:11:00.270]we want to make sure you're watching for it this summer or in the future.
[00:11:04.350]As we expect it to move into the state, sometime soon,
[00:11:08.100]this is caused by a fungus and it produces reproductive structures that look
[00:11:12.090]like black dots that are embedded in the corn leaf.
[00:11:16.290]And so if you see black dots, try to rub them or wash them off.
[00:11:19.380]And if you can't rub or wash it off, there's a chance that we have tar spot.
[00:11:23.640]And we really want to know where this pathogen and disease has moved into the
[00:11:27.720]state. And so watch out for those on this map,
[00:11:31.710]you can see historically in the gray shaded counties where the disease has been
[00:11:36.660]reported and confirmed in the past,
[00:11:38.970]you'll notice it has moved all the way across Iowa into the Western most
[00:11:43.200]counties.
[00:11:44.280]And so in that Northwest part (of Iowa) or north east part of Nebraska,
[00:11:48.450]I expect that's probably where we're going to see it first. And so,
[00:11:52.530]inevitably if weather conditions are favorable,
[00:11:54.850]it will continue to spread into the state.
[00:11:57.640]This fungus likes cool wet conditions.
[00:12:00.250]So it might mean we don't see it until later in the season, if at all this year,
[00:12:04.870]because of the dry weather. So be on the lookout for that disease.
[00:12:08.530]And you can check out the map at the same place that you do,
[00:12:12.190]our Southern corn rust, the tar spot map.
[00:12:14.590]This is all on the IPMPipe website. And it's easiest to just Google that,
[00:12:19.150]to find it. This is what you're going to be looking for though.
[00:12:22.630]Those black flecks that are on those leaves and they can become quite severe
[00:12:27.190]under favorable conditions, but it takes about 20,
[00:12:30.100]25% of the leaf covered before we see yield (loss). In a few years,
[00:12:35.040]probably we'll begin to need to think about fungicide applications to manage
[00:12:39.480]this disease.
[00:12:40.710]And they do a good job based on the results from some of our summaries,
[00:12:44.520]from colleagues to the east.
[00:12:46.350]If you want more information about any of this or any of these diseases,
[00:12:50.460]please check out our website and newsletter on CropWatch or any of the other
[00:12:54.780]Nebraska extension content. In addition,
[00:12:57.600]you can also look on the Crop Protection Network,
[00:13:01.350]and this is an effort from multiple universities’ pathologists providing
[00:13:05.820]content and that's free. And so check that out as well. And thank you
[00:13:14.940]if you need more help.

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8 - 2021 South Central Ag Lab Field Day - Corn and Soybean Disease Updates

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