Late Season Corn Diseases

Michael Sindelar Author

10/01/2020 Added

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Is it normal for the top of corn to die before the rest of the plant? Join Michael and Tamra for the answer to this question and more. This week’s podcast is on late season corn diseases.

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[00:00:00.500](soft music)
[00:00:02.950]Welcome to CropWatch podcast,
[00:00:05.400]a production of Nebraska extension.
[00:00:12.160]Welcome to the CropWatch podcast.
[00:00:13.570]I'm Michael Sindelar, cropping systems extension educator.
[00:00:16.640]Today, I'm gonna be joined by, well not by
[00:00:19.970]but with Tamra Jackson-Ziems
[00:00:21.920]and we're going to be talking about corn.
[00:00:24.330]Specifically, kind of more of the late season corn diseases.
[00:00:27.920]How are you doing this afternoon Tamra?
[00:00:30.090]I'm doing great, thank you.
[00:00:31.570]How are you?
[00:00:32.680]Oh, I'm doing great, doing great.
[00:00:35.040]Especially since we get to talk about corn here so,
[00:00:38.189]That's right.
[00:00:39.910]When we look out,
[00:00:41.700]it's definitely been a very interesting season
[00:00:44.180]especially when we look statewide.
[00:00:46.050]We've had so much difference and it could be even
[00:00:49.290]within the same county.
[00:00:50.520]Difference in rainfall, some parts have drought,
[00:00:54.450]some parts are fine,
[00:00:55.840]some parts are gonna have maybe even record yields
[00:00:59.110]because they just got lucky.
[00:01:00.910]What are we looking for?
[00:01:03.190]Late season diseases, and the things I'm thinking
[00:01:06.490]off the top of my mind,
[00:01:07.800]we've had hail storms come through.
[00:01:09.170]We've had wind storms come through.
[00:01:11.060]We have areas under drought.
[00:01:12.730]How's that gonna affect us here
[00:01:14.150]in the last part of our season?
[00:01:15.750]What do we need to be looking for?
[00:01:17.740]Do we need to be focusing on stalks?
[00:01:19.600]Do we need to be focusing on shanks?
[00:01:21.790]Do we need to be focusing on ears?
[00:01:23.380]And does that change
[00:01:24.650]by maybe what's occurred through the year?
[00:01:26.540]I know that was a long question. (Tamra laughs)
[00:01:29.420]Well, there's a lot of stuff there Michael
[00:01:31.960]and you know, this is complicated
[00:01:34.550]because we do have so many varying conditions
[00:01:37.590]around the state and we're finishing up
[00:01:40.630]kind of on the downhill side of a lot of diseases.
[00:01:43.810]Some of which have been pretty important to us this year.
[00:01:46.690]And the weather conditions have really impacted
[00:01:51.060]what is showing up and where
[00:01:52.890]and it's gonna continue to do so as we approach harvest.
[00:01:56.690]And so, kind of wrapping up on those diseases,
[00:02:00.430]probably the biggest disease news I think of this year
[00:02:03.380]for much of the eastern half of the state is southern rust.
[00:02:08.270]And it's not that we don't see southern rust
[00:02:11.140]at least a little bit almost every year but honestly
[00:02:14.150]I haven't seen this much southern rust since 2006.
[00:02:18.140]And it came in right on time
[00:02:20.880]but a lot more than maybe people were prepared for.
[00:02:25.560]And we watched that develop like it normally does, patchy.
[00:02:30.800]And in some fields it became severe in a few of those areas
[00:02:35.340]but not in others.
[00:02:36.870]And here at the end now
[00:02:38.500]we're seeing a blast of southern rust developing
[00:02:42.340]in especially the lower canopy,
[00:02:44.620]a lot of these fields as they start to finish up.
[00:02:47.070]And that's because of the increased humidity
[00:02:49.870]in that lower half of the canopy.
[00:02:51.960]And southern rust just doesn't need as much moisture
[00:02:54.900]for as long as other fungi,
[00:02:57.350]like the fungus causing gray leaf spot.
[00:03:00.020]And so people that are used to
[00:03:02.730]seeing a lot more gray leaf spot,
[00:03:04.470]those little rectangular lesions
[00:03:06.660]may be surprised that they didn't see as much
[00:03:09.300]as they usually do or may have been expecting.
[00:03:13.180]And so drought conditions of course have held that one back.
[00:03:17.600]And the temperatures, the nighttime temperatures
[00:03:20.370]have been in general cooler I think,
[00:03:22.580]in a few of these weeks and that certainly could have
[00:03:25.620]slowed down both gray leaf spot and southern rust too.
[00:03:30.830]And a lot of that depends on where you're at.
[00:03:34.950]I was most concerned because
[00:03:37.791]you know we see southern rust in Southern Nebraska a lot
[00:03:40.490]and we're used to looking for it down there and watching.
[00:03:43.630]But when southern rust moved into Northern Nebraska,
[00:03:46.480]that became a bigger concern.
[00:03:48.210]We don't watch for southern rust
[00:03:50.060]in this part of the state up North
[00:03:51.870]and may have taken people by surprise.
[00:03:54.670]And so that's why we had a lot of information
[00:03:56.980]pouring out about it, letting people know.
[00:03:59.780]And few of those fields needed to be treated
[00:04:02.820]and a lot got treated
[00:04:03.980]so hopefully they saw the benefits from that.
[00:04:07.270]So with southern rust, What is somebody looking for
[00:04:09.830]when they walk out into the field?
[00:04:12.340]Or what, you know,
[00:04:13.820]key things maybe they should key on
[00:04:15.810]to take a sample to take it to somebody who knows more?
[00:04:19.760]That's a good question,
[00:04:21.228]and right now we're seeing a lot of orange spores
[00:04:25.120]and that's the key.
[00:04:27.250]Earlier in the season you see little patches of spores
[00:04:31.370]and pustules on these leaves and we need those leaves
[00:04:36.210]so that in specific we can look under the microscope
[00:04:39.750]at the spore shape.
[00:04:41.630]Because we do have common rust too
[00:04:43.570]and we don't worry about common rust
[00:04:45.430]because our contemporary hybrids have resistance to that.
[00:04:48.980]Southern rust is different though.
[00:04:50.630]We don't have much resistance to it
[00:04:52.900]and that pathogen can be quite aggressive
[00:04:56.100]under favorable conditions.
[00:04:58.220]We didn't even have the most favorable conditions this year
[00:05:01.390]but we did have a lot of inoculum coming from the South.
[00:05:04.940]There was a lot of southern rust down South.
[00:05:07.910]And right now walking through a lot of these fields,
[00:05:11.040]people are turning orange because of the volume of spores
[00:05:15.520]that are getting on them,
[00:05:16.520]rubbing off the leaves and blowing around.
[00:05:19.500]And the good news about that is this does not overwinter.
[00:05:23.590]And so having southern rust this year
[00:05:25.440]doesn't mean you're going to have it next year.
[00:05:28.030]And now you're going to see it change too.
[00:05:32.410]They're starting to notice black spores out there
[00:05:35.570]and on these same leaves.
[00:05:37.960]And that's also southern rust.
[00:05:40.130]We don't always see this stage of the fungus
[00:05:42.780]but as the season goes on and that fungus in the leaf ages,
[00:05:47.160]we see the fungus transition producing telia spores.
[00:05:51.570]These are the black spores
[00:05:52.860]that it's using to try to overwinter.
[00:05:55.270]They've got thick walls but it's still not gonna overwinter.
[00:05:59.640]But it's alarming because it looks different.
[00:06:02.300]It's black spores and it may alarm people
[00:06:06.210]because we've been warning everyone to be watching
[00:06:09.440]for tar spot and those black spots.
[00:06:12.310]And that's not what this is
[00:06:13.920]and so we just wanna make sure
[00:06:15.990]that if people have any questions they can let us know,
[00:06:18.760]send in samples and we can help them out.
[00:06:21.970]Alright, Is there anything,
[00:06:23.700]I know what some diseases we're looking top and bottom
[00:06:26.670]of the leaves can kind of help us know if we need something.
[00:06:30.020]So it's southern rust, is that one where we're looking for
[00:06:32.640]pustules on both sides or just one side?
[00:06:36.350]That's a good question too.
[00:06:37.700]Well early on, you know we were telling people to look
[00:06:40.490]for the orange to tan pustules on the top side of the leaf.
[00:06:45.620]Well, there's exceptions to that and it's confusing to say
[00:06:49.530]it always does this except for when it doesn't.(chuckles)
[00:06:53.190]And although southern rust tends to generally produce spores
[00:06:56.390]on top of the leaf, if it's near the midrib,
[00:07:00.190]we see it on the bottom too.
[00:07:02.340]Or the other thing that's been confusing this year
[00:07:05.680]is in a lot of the fields that were impacted by wind
[00:07:08.230]and the green snap.
[00:07:09.570]And when it broke those tops out
[00:07:12.830]and the ones that were still connected and hanging on,
[00:07:15.840]suddenly those leaves are twisted
[00:07:18.720]and they may be upside down.
[00:07:20.820]Well, now the bottom of the leaf
[00:07:22.520]becomes the top of the leaf
[00:07:23.950]and it surprised people to see southern rust
[00:07:26.310]on the bottom of the leaf.
[00:07:28.270]But it's just about where those spores
[00:07:30.220]are landing on the leaf.
[00:07:31.580]And so we should be more clear about that.
[00:07:34.950]You can get southern rust
[00:07:36.310]on the bottom of the leaf more commonly,
[00:07:38.620]if they're upside down.
[00:07:40.560]That sounds really strange
[00:07:41.850]but there's been a lot of that,
[00:07:43.900]especially in South Central Nebraska this year.
[00:07:46.980]Yes there has.
[00:07:50.050]You mentioned it briefly, tar spot.
[00:07:52.150]Is that something that we need to be worrying about
[00:07:53.890]on the eastern part of the state
[00:07:55.390]or has the conditions just been too dry this year?
[00:08:00.100]Well, that's probably a loaded question.
[00:08:03.160]We have not confirmed tar spot in Nebraska yet.
[00:08:07.780]And if people were paying attention
[00:08:10.030]and watching in the media,
[00:08:11.290]they know that in western Iowa last year
[00:08:14.680]that we confirmed tar spot
[00:08:16.496]or my colleagues did over at Iowa State University,
[00:08:20.150]one county away from the Eastern Nebraska border.
[00:08:23.740]So it's possible we even have some that made it
[00:08:26.880]across the river into Nebraska.
[00:08:29.230]But you're absolutely right,
[00:08:30.640]it's probably not become an issue
[00:08:33.250]and hasn't moved much if any
[00:08:36.060]because of the drought conditions that we've observed.
[00:08:39.820]It's important though that we do watch for it.
[00:08:43.100]And it's not that if it moves here now
[00:08:45.670]that I'm worried about it causing yield loss,
[00:08:48.010]it's that we know the fungus will overwinter
[00:08:50.520]once it is here.
[00:08:51.680]And that means that those same fields
[00:08:53.680]in those same counties
[00:08:55.240]are gonna continue to see that disease
[00:08:57.230]and it'll spread from there.
[00:08:59.210]And so in those other states where they've had it longer,
[00:09:04.131]they don't even usually spray a fungicide for it,
[00:09:09.500]but they do observe it getting worse quickly
[00:09:11.688]under certain conditions.
[00:09:13.470]Usually those cool wet conditions.
[00:09:16.010]Temperatures in the sixties and seventies
[00:09:18.280]are very favorable.
[00:09:19.970]And that's what we had last fall in September.
[00:09:23.180]It was cool and wet
[00:09:24.970]and we even had some storm systems that moved those spores
[00:09:29.500]and blew from East to West
[00:09:32.710]in contrast over where the weather normally moves
[00:09:35.670]from West to East.
[00:09:37.110]That probably helped move it across Iowa was so quickly.
[00:09:40.590]And you know that kind of thing can move it
[00:09:42.610]into Nebraska too but we'll see it here eventually,
[00:09:46.300]if not this fall and if anyone sees black dots on leaves
[00:09:51.310]that they can't rub off
[00:09:53.340]and it's not where they're having southern rust especially,
[00:09:57.050]let's get a look at that.
[00:09:58.380]And so normally black dots are a presence left by insects,
[00:10:04.440]the insect frass but that should rub off.
[00:10:06.730]And fungal structures don't though.
[00:10:09.190]They're embedded in the surface of the leaf
[00:10:11.080]and you can't rub them off.
[00:10:13.150]You can't spit on it and rub it off.
[00:10:16.220]So let us know if you're finding something like that
[00:10:19.300]because we want to make sure and let people know
[00:10:21.500]it's in the area when it does move here.
[00:10:24.679]Right, that sounds great.
[00:10:27.280]I know another thing on my mind
[00:10:28.680]is it's nice to know what standability issues I may have
[00:10:32.620]in the future, because I've either gotten enough rain
[00:10:35.450]that it's going to cause an issue
[00:10:36.650]or I wasn't expecting it, I have drought.
[00:10:39.800]I should not have any fungal diseases.
[00:10:43.070]You know, what should we be looking for?
[00:10:45.620]Stalk rots, whether we're in drought
[00:10:47.930]or we've had a good season.
[00:10:51.360]Excellent question, so looking ahead
[00:10:54.210]that would be a concern for us.
[00:10:56.980]And anywhere that you had a lot of leaf diseases
[00:11:01.090]is where you may see
[00:11:03.300]the plants cannibalize stalk tissue inside.
[00:11:07.440]Anywhere you had wounding,
[00:11:08.870]you mentioned hail earlier
[00:11:10.492]or maybe some of the conventional fields
[00:11:13.050]where we've got more insect pressure.
[00:11:17.757]Any kind of stress, the drought, or too much rain.
[00:11:21.570]Any kind of stress can lead to stalk rot
[00:11:25.010]or standability issues later on.
[00:11:30.110]I know there's a little bit of harvest already going on
[00:11:32.510]in extreme southeast Nebraska.
[00:11:35.070]But as we approach harvest in the rest of the state,
[00:11:38.650]I think it would be a good idea to go out
[00:11:41.010]and test stalk strength.
[00:11:43.140]And you can do that easily with a push or a pinch test.
[00:11:46.750]I like the push test cause I can just walk through the field
[00:11:49.470]at normal speed and push these stalks at arms length.
[00:11:54.020]And if those stocks bend and they don't snap back up at you
[00:11:59.130]and especially if they bend below the ear.
[00:12:03.340]I would count that and you're gonna wanna push
[00:12:05.970]on at least 100 different stalks,
[00:12:07.930]maybe in a couple three parts of the field
[00:12:10.280]to really get an idea if you're gonna have a stalk issue.
[00:12:15.241]The fields that have the highest incidence
[00:12:17.820]of stalks that bend over and lodge
[00:12:21.260]might be ones you want to harvest first.
[00:12:24.570]Some people depending on how bad it is,
[00:12:26.750]you might even wanna get them a little earlier
[00:12:28.470]than you normally would
[00:12:30.210]before you have some of the fall winds
[00:12:32.700]we often do that may knock some of that over
[00:12:35.080]and really create a problem at harvest.
[00:12:39.597]The other thing related to stalk rot in general
[00:12:43.660]is you can see areas of the field that die early.
[00:12:49.260]And just like in the soybeans we talked about,
[00:12:53.100]there's a lot of reasons the plants can die.
[00:12:55.560]What we might think is prematurely.
[00:12:57.890]Whether it's some of the field conditions,
[00:12:59.930]whether it's compaction
[00:13:01.120]or on some of those side hills or disease.
[00:13:04.670]And so we're seeing a few fields
[00:13:08.850]where we're seeing the top dieback,
[00:13:10.990]which is often caused by anthracnose.
[00:13:13.910]But in other parts there's just some plants turning early
[00:13:18.340]and it's pretty deceptive and sometimes it's very dramatic
[00:13:21.890]on the sides of these hills.
[00:13:23.960]And you won't know what's going on till you get out there
[00:13:26.680]and I'd take a shovel with me too.
[00:13:28.920]If plants died early, you can get out, dig them up,
[00:13:33.270]use your pocket knife to carefully split those stems,
[00:13:37.410]those stalks open all the way down through the root ball
[00:13:41.180]because we've also seen something called,
[00:13:43.610]what we're calling a crown rot that's caused by fusarium.
[00:13:46.920]And we've seen a little bit of that already
[00:13:49.320]especially in some of the irrigated fields
[00:13:52.130]or ones that got the rain.
[00:13:54.000]So there's quite a few things out there.
[00:13:55.990]You know, while they're out checking stalk strength
[00:13:58.540]go ahead and husk a few ears back.
[00:14:00.630]Check to see if you've got any evidence of ear rot.
[00:14:03.810]If you've had any 'specially insect damage or feeding
[00:14:07.320]that can be a precursor to some of that.
[00:14:12.110]So I know you mentioned anthracnose
[00:14:13.780]as a cause for a top dieback.
[00:14:16.480]That's also a stalk rot if I remember right.
[00:14:19.280]So if you have a top dieback
[00:14:21.430]are you expecting to also have that as a stalk rot?
[00:14:25.790]Ironically, no.
[00:14:27.450]So it's the same fungus and it's just able to cause disease
[00:14:31.770]in three different ways.
[00:14:33.700]And so anthracnose caused by a fungus called colletotrichum.
[00:14:38.268]And earlier in the season even during the seedling phase,
[00:14:42.720]it can cause a leaf disease
[00:14:44.450]with big reddish brown lesions on there.
[00:14:47.500]Later in the season whether or not you had leaf disease,
[00:14:50.640]you might that top dieback.
[00:14:53.380]And so what happens is you get spores
[00:14:56.330]that they are washed down the leaf sheath and they collect
[00:15:00.040]around the node where it's nice and moist inside
[00:15:03.450]and then they infected a node or two.
[00:15:06.270]And those nodes become weak points and filled with fungus
[00:15:11.160]and they stop water movement up the plant
[00:15:13.270]and so then the top of the plant dies.
[00:15:15.660]And so you can see that if you carefully split that open.
[00:15:19.260]You may see one or more nodes at one point
[00:15:22.090]and above that are really brown and discolored.
[00:15:25.940]Otherwise the other part of this disease,
[00:15:29.460]the actual stalk rot.
[00:15:31.550]Usually in the lower part of the stem.
[00:15:34.260]and you see those same things.
[00:15:36.670]The discoloration when you split them open, decay,
[00:15:40.490]it may even smell a little musty and be damp inside.
[00:15:46.330]And you've seen this too after harvest.
[00:15:49.730]When you see black shiny lesions on the outside,
[00:15:53.450]even before harvest too, but they're visible on the outside,
[00:15:57.200]these black shiny lesions.
[00:15:58.540]That's all anthracnose.
[00:16:00.290]Pretty complex disease.
[00:16:02.340]And we do have resistance in some hybrids
[00:16:04.560]so if people see this year after year, you might work
[00:16:08.240]with your seed company agronomist
[00:16:09.780]to position resistant hybrids in some of those fields.
[00:16:14.000]So top dieback is not something that just happens
[00:16:16.340]'cause it's dry and hot.
[00:16:18.040]There may be other issues leading to it.
[00:16:20.530]Probably both.
[00:16:21.363]There's probably multiple things, but yes
[00:16:23.160]it's not always disease, but environmental conditions too.
[00:16:27.535]This is a tough time of year.
[00:16:30.670]It is, is there anything else we need to worry about?
[00:16:34.760]You know especially in my mind,
[00:16:37.680]I'm thinking is there anything
[00:16:39.190]that's affecting ear shanks right now
[00:16:41.140]that we need to think of?
[00:16:42.357]'Cause I hate to get to this point and just, you know,
[00:16:46.420]the ears are heavy and bam, the plant drops the ear.
[00:16:51.630]Well you know, we've heard about that
[00:16:54.510]in a few isolated areas so far
[00:16:57.590]so it's unclear if that's going to be a big issue for us.
[00:17:02.310]And so maybe that's something we should be looking for
[00:17:05.380]when we're out testing stalk strength.
[00:17:07.840]And if you see ears that have a husk that discolored
[00:17:12.710]before the rest of the field or a spot,
[00:17:16.610]those are the ones I think I'd look at first.
[00:17:18.550]Or if the ear leaf turns,
[00:17:20.720]but none of the rest of the plant turns.
[00:17:23.460]That ear, I'd have sat back and take a look at it.
[00:17:26.040]Sometimes we can have a fungus
[00:17:28.310]like one causing Diplodia or what we call Stenocarpella.
[00:17:31.870]Now it infects that shank.
[00:17:34.610]It makes it weak and it can continue on up
[00:17:38.360]into the butt of the ear.
[00:17:41.460]But you know, there's some other things, some genetic
[00:17:43.700]by environmental interactions that seem to also impact
[00:17:47.620]some of those things.
[00:17:48.500]So lot of reasons to still get out in the field
[00:17:51.540]and look at what's going on so you don't have any surprises.
[00:17:55.490]Right, so definitely the takeaway
[00:17:57.810]is to be looking in your field
[00:17:59.315]to make sure that you're not surprised.
[00:18:02.060]I think so, absolutely.
[00:18:04.740]Alright.
[00:18:05.573]Well once again, thank you for joining me today.
[00:18:08.330]I hope you have a great season.
[00:18:10.670]Thank you and likewise.
[00:18:12.090]And I encourage people if they have any questions or doubts
[00:18:15.080]or want to see some pictures,
[00:18:16.210]we've got a lot of resources on the cropwatch.unl.edu
[00:18:20.550]and on the Crop Protection Network.
[00:18:23.310]And please contact us if you have questions.
[00:18:26.382](soft music)

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Late Season Corn Diseases

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