Mid-season Corn Diseases

Michael Sindelar, Tamra Jackson-Ziems Author

07/08/2020 Added

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The Nebraska CropWatch Podcast returns for another season. In this episode Michael and Tamra Jackson-Ziems take a look at corn diseases. Both fungal and bacteria diseases are discussed along with treatment options.

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[00:00:00.304](joyful music)
[00:00:02.960]Welcome to CropWatch Podcast,
[00:00:05.410]a production of Nebraska Extension.
[00:00:12.270]Welcome to the CropWatch Podcast.
[00:00:14.130]I'm Michael Sindelar, cropping systems extension educator.
[00:00:17.390]Today I will be joined by Tamra Jackson-Ziems,
[00:00:21.460]a disease extension specialist
[00:00:24.160]at the University of Nebraska.
[00:00:26.250]Hi, Tamra, how are you doing today?
[00:00:28.080]I'm good. How are you?
[00:00:29.710]Oh, I'm doing great.
[00:00:31.360]Wish we could get a little bit more rain here
[00:00:33.040]in the south central part of the state.
[00:00:35.390]Me too.
[00:00:36.788]We'll get through it.
[00:00:39.280]So today we're gonna start by talking about corn diseases.
[00:00:44.540]Looking back, what are we worried about
[00:00:48.100]for seedling diseases?
[00:00:49.930]And are they affecting corn now,
[00:00:51.960]even though we've kind of moved
[00:00:53.260]into the V7, V8 stage for corn?
[00:00:57.500]It's a good question.
[00:00:58.940]You know, we did, in those early weeks,
[00:01:01.050]start to see a few seedling diseases.
[00:01:04.130]I was getting some reports,
[00:01:05.760]especially out in Southwest Nebraska,
[00:01:08.000]people seeing a few of those and wondering,
[00:01:12.120]you know, what's going on out here.
[00:01:14.600]But you're right, as corn gets out of those seedling stages,
[00:01:17.880]gets up past V6, most of those aren't a threat anymore,
[00:01:22.060]and so I wouldn't worry about seedling diseases.
[00:01:24.640]There's plenty of other things, though,
[00:01:26.240]that we should be watching out for.
[00:01:29.260]Speaking of other things to watch out for,
[00:01:32.450]one of your big things for research
[00:01:34.340]is bacterial leaf streak.
[00:01:36.480]What is that looking like this year?
[00:01:39.620]Well, it's definitely out there again this year.
[00:01:42.480]We have had several confirmed reports
[00:01:45.360]of bacterial leaf streak in a number of counties,
[00:01:48.470]scattered all over the state.
[00:01:50.800]And, you know, that's not unusual this time of year.
[00:01:54.620]We know that that disease has been confirmed
[00:01:56.920]in 75 of our counties now,
[00:02:00.210]and so, some of the pictures I'm getting too,
[00:02:04.820]it looks like some of those lower leaves,
[00:02:06.543]some fields, it's pretty severe.
[00:02:09.560]Others, it's just barely there.
[00:02:12.300]And so, a lot of that depends on
[00:02:14.280]the hybrid that's out there,
[00:02:16.310]and some of the other conditions.
[00:02:17.980]And so, if you've had bacterial leaf streak before,
[00:02:20.690]it's likely you may be dealing with it again.
[00:02:24.140]If you haven't had it, you know,
[00:02:26.140]and you're seeing some, I guess,
[00:02:29.280]chlorosis or necrosis striping between the veins
[00:02:32.880]and lesions on those lower leaves,
[00:02:35.150]and you're not sure what it is,
[00:02:36.420]let's get a sample into the diagnostic clinic
[00:02:38.650]and we can confirm that for you.
[00:02:41.360]So when you're looking for bacterial leaf streak,
[00:02:43.240]what are you looking for?
[00:02:44.160]Does it start at the bottom leaves and move up
[00:02:46.330]or does it start at the top
[00:02:47.610]or does it just not really care
[00:02:49.440]and it starts anywhere on the plant
[00:02:50.770]and spreads throughout?
[00:02:52.330]Well, I hate to say, but it depends.
[00:02:55.000]And so, early in the season, like right now,
[00:02:58.460]we usually see it in the lower leaves.
[00:03:00.790]And it behaves very much
[00:03:03.810]like what we see with gray leaf spots,
[00:03:05.960]starting in those lower leaves, moving up the plant.
[00:03:09.380]And in fact, one of the big concerns
[00:03:11.880]about bacterial leaf streak
[00:03:13.660]is that sometimes those lesions
[00:03:16.150]can be confused with gray leaf spot lesions.
[00:03:20.160]And on some hybrids, maybe it looks more similar
[00:03:23.750]to gray leaf spot, but those lesions
[00:03:26.520]are gonna start out between the veins
[00:03:29.090]and they can't cross the veins easily.
[00:03:32.230]And the thing that you have to think about
[00:03:34.760]is that bacterial diseases
[00:03:36.380]tend to have more wavy or irregular shaped lesions.
[00:03:40.370]And that's a way to tell it apart from gray leaf spot.
[00:03:43.620]Early in the season, we usually don't see gray leaf spot,
[00:03:46.980]that fungal disease, and so,
[00:03:48.480]if you're seeing something show up early,
[00:03:49.940]that looks similar, it's probably not gray leaf spot,
[00:03:53.280]until we have a lot more heat and humidity.
[00:03:56.300]And so, the other thing about bacterial leaf streak
[00:03:59.590]that's been pretty consistent
[00:04:02.100]is that this bacterium produces a yellow pigment.
[00:04:05.090]And when you hold those leaves,
[00:04:07.010]those infected leaves up to the light,
[00:04:09.660]and look at 'em, it can make the lesions appear
[00:04:12.070]much more yellow.
[00:04:13.790]And that can be a real good symptom for you to look for
[00:04:17.440]in trying to identify it.
[00:04:19.780]Later in the season, we have seen
[00:04:24.050]the lesions develop very suddenly
[00:04:26.360]in the upper canopy though,
[00:04:28.080]and so, sometimes after we've had
[00:04:30.400]a major storm move through an area
[00:04:33.020]with pounding rain and a lot of wind,
[00:04:37.930]sometimes we do see sudden development
[00:04:40.590]of lesions in that upper canopy.
[00:04:42.660]And so, that's just something to watch out for.
[00:04:45.110]And again, we've got a lot of resources online,
[00:04:47.600]lots of pictures, and the diagnostic clinic.
[00:04:50.570]And so, if you need help,
[00:04:52.040]there's a lot of places to get help,
[00:04:54.080]including your county extension educators
[00:04:56.820]and specialists like me.
[00:04:59.550]All right.
[00:05:01.820]What are our treatment options for bacterial leaf streak?
[00:05:05.270]I know fungicide is not an option
[00:05:08.180]'cause it is bacteria, and fungicides don't kill bacteria.
[00:05:12.040]Exactly.
[00:05:13.610]And that's a limiting factor, right?
[00:05:15.840]And Nebraska has been dealing
[00:05:17.790]with bacterial diseases for decades.
[00:05:20.550]And we have not had consistent benefits
[00:05:23.500]from applications of even bactericides.
[00:05:27.370]One of the problems is that,
[00:05:29.970]although bactericides, they do kill bacteria,
[00:05:33.520]but there are limitations,
[00:05:34.770]and many of those are, they're not absorbed into the leaf
[00:05:38.520]and they're not systemic like
[00:05:41.170]the way that our fungicides work,
[00:05:43.130]and so, some of those products
[00:05:45.030]need to be reapplied, and frequently.
[00:05:48.650]And sometimes that becomes uneconomical,
[00:05:51.480]of course, for our dead corn,
[00:05:53.550]at the prices that we've had.
[00:05:55.860]And so, managing bacterial diseases
[00:05:58.730]is best done with hybrid selection and crop rotation
[00:06:03.070]and using some of those other methods
[00:06:05.960]that are more consistently effective.
[00:06:08.410]But bacterial leaf streak is certainly a challenge
[00:06:10.820]and I would work closely
[00:06:12.000]with my seed company agronomist
[00:06:14.460]trying to select the most appropriate hybrids I could
[00:06:17.800]that maybe performed better with bacterial leaf streak
[00:06:21.650]in those fields where you've had it in the past.
[00:06:25.410]Right, well talking about new diseases,
[00:06:27.960]another one that's on the horizon is tar spot.
[00:06:30.580]What can you tell us about tar spot?
[00:06:33.820]Wow, well, I'm pleased to report
[00:06:35.760]that we have not reported tar spot
[00:06:39.070]in Nebraska at this point.
[00:06:41.790]But I really do wanna emphasize
[00:06:44.070]that this is one we should be watching for.
[00:06:47.080]And if you were paying attention last year,
[00:06:49.140]you may have noticed that on the map
[00:06:53.920]where we monitor this disease,
[00:06:56.690]tar spot moved across Iowa pretty rapidly in 2019.
[00:07:03.360]And so, that's something that we wanna watch closely,
[00:07:07.060]because right now, that disease has been confirmed
[00:07:10.990]as far west as Western Iowa,
[00:07:13.560]it's within one county of Nebraska.
[00:07:16.460]And that means there's a good chance it's already here
[00:07:19.360]and we just haven't confirmed it.
[00:07:21.530]And so, this disease is one that actually
[00:07:25.040]is caused by a fungus, Phyllachora maydis is the fungus,
[00:07:29.300]and that fungus likes cool and damp conditions.
[00:07:34.340]So temperatures in the 60s and lower 70s
[00:07:37.500]are just perfect for it.
[00:07:39.190]And if you think about last fall, in September,
[00:07:42.490]it was wet and cool and it slowed down grain drying,
[00:07:46.575]it delayed harvest.
[00:07:48.480]Well, during that time period,
[00:07:49.840]when we were getting repeated rain events,
[00:07:51.890]it was also pushing that pathogen
[00:07:54.300]closer and closer to Nebraska.
[00:07:56.740]And so, people who are in
[00:07:58.730]the eastern-most counties of the state,
[00:08:01.430]we hope you'll be watching.
[00:08:02.840]And if you run across corn leaves
[00:08:06.810]that have black dots on them, that's what it looks like.
[00:08:11.460]It looks like black paint or tar
[00:08:14.290]has been splattered across the leaf.
[00:08:16.510]And those are the fungal reproductive structures
[00:08:19.720]of tar spot.
[00:08:20.740]And if you can't rub those off,
[00:08:23.130]there's a chance that that might be what it is,
[00:08:25.100]please contact me, submit a sample to our diagnostic clinic,
[00:08:30.010]so that we can look at it under the microscope
[00:08:33.230]and confirm if that's what it is.
[00:08:36.390]All right.
[00:08:37.223]So it's not confirmed in Nebraska,
[00:08:38.740]but it's something we definitely need to be looking at,
[00:08:40.961]especially people in the eastern counties.
[00:08:43.290]Yes, thank you.
[00:08:46.350]Other diseases.
[00:08:47.300]You talked about gray leaf spot.
[00:08:49.450]What is our gray leaf spot forecast
[00:08:52.900]for this year looking so far
[00:08:54.340]with the conditions that we've had?
[00:08:56.460]I know when you look across a state,
[00:08:58.400]it's not easy to say 'cause the eastern half
[00:09:00.910]is a lot wetter and more humid,
[00:09:02.970]compared to the central part,
[00:09:04.440]compared to the western part.
[00:09:07.770]That's absolutely right.
[00:09:09.540]And we've seen tar spot historically,
[00:09:12.680]just about everywhere in the state on corn.
[00:09:15.810]And so, it's consistently one that we wanna be mindful of.
[00:09:20.970]The thing about gray leaf spot is that fungus,
[00:09:24.220]it prefers warm conditions, humid conditions.
[00:09:28.330]And so, what we see at a high humidity in a cornfield
[00:09:33.560]is perfect for it in most of our summer times.
[00:09:36.530]And so, you don't generally see it
[00:09:38.680]early in the season though.
[00:09:41.330]We typically have the best weather
[00:09:43.960]around tasseling and afterwards
[00:09:45.920]when we usually begin to see tar spot.
[00:09:49.040]This pattern of a little bit of rain in Eastern Nebraska
[00:09:52.720]could start to drive development
[00:09:54.960]of gray leaf spot here during the next few weeks.
[00:09:58.070]So we wanna watch for that.
[00:10:00.360]And so, gray leaf spot being a fungal disease,
[00:10:04.380]it can be managed with a fungicide if you need it.
[00:10:08.390]And so, that's something to keep in mind.
[00:10:11.470]But also hybrid selection
[00:10:13.210]can help you slow down development of that disease.
[00:10:16.250]But you're at higher risk for it
[00:10:17.900]in irrigated corn and continuous corn
[00:10:22.080]or even in minimum tillage systems.
[00:10:24.440]And so, it's not to say not to use those systems,
[00:10:29.150]but be aware that hybrid selection
[00:10:31.750]and potential fungicide use may be more important to you
[00:10:35.240]in those higher risk situations.
[00:10:38.040]That fungus needs at least 95% humidity
[00:10:41.720]for 11 or 12 hours to produce spores.
[00:10:45.000]And it always always starts in the lower leaves,
[00:10:47.630]when you see those little graded tan rectangles
[00:10:50.530]between the leaf veins, and it moves up the plant.
[00:10:55.240]But because it's pretty slow,
[00:10:57.450]can take 14 or even 21 days to create a lesion
[00:11:01.130]and reproduce to produce more spores.
[00:11:04.870]And so, that means the highest leaf
[00:11:07.210]on the plant where you find it,
[00:11:08.850]the fungus may have actually already infected
[00:11:10.890]one or two leaves above that.
[00:11:13.490]And so, keep that in mind when you're looking
[00:11:16.340]at susceptible hybrids and making fungicide decisions.
[00:11:21.420]So looking at making fungicide decisions,
[00:11:23.450]especially maybe for people
[00:11:24.820]that are in a no-till irrigated system,
[00:11:30.230]what are your recommendations?
[00:11:31.450]Is there a threshold?
[00:11:33.850]Are you reactive or proactive
[00:11:37.010]trying to combat this disease when applying fungicides?
[00:11:43.570]I guess, what would your fungicide recommendations be?
[00:11:47.800]Good question, Michael, thank you.
[00:11:49.510]So, fungicides do a great job controlling fungal diseases.
[00:11:54.700]So there's no doubt about that.
[00:11:57.030]And so, as many people know,
[00:11:59.810]we can conduct research every summer
[00:12:02.020]looking at fungicide products,
[00:12:04.570]different active ingredients applied to corn
[00:12:06.930]at various stages and in various ways,
[00:12:09.890]and so to our counterparts across the United States.
[00:12:14.290]And so, I think one of the most important things we're doing
[00:12:17.760]is we're collaborating better on that now and sharing data.
[00:12:22.100]And when you look at the averages across the country,
[00:12:25.800]you'll see that if you look at timing, for instance,
[00:12:30.720]the most effective timing as far as yield improvements
[00:12:36.720]are when we make applications
[00:12:38.690]at tasseling or shortly thereafter.
[00:12:42.460]But in most cases, you get the best response
[00:12:45.730]from a fungicide, almost always,
[00:12:48.260]when there's disease present.
[00:12:50.600]And so, we recommend applying fungicides
[00:12:55.190]after you've scouted and confirmed disease
[00:12:58.060]that needs the application.
[00:13:00.220]The unfortunate thing is
[00:13:01.340]we don't have thresholds for diseases.
[00:13:04.940]And much of that is because of how many other factors
[00:13:09.610]affect disease development.
[00:13:11.390]And, for example, the weather conditions we talked about.
[00:13:14.800]And so, if you know it's gonna be
[00:13:16.540]very humid and very warm,
[00:13:18.720]and you also consider the genetics
[00:13:20.700]of the hybrid in that field,
[00:13:22.560]if it's one that's been sensitive in the past
[00:13:25.010]and could benefit from some help,
[00:13:28.510]that would be a scenario where,
[00:13:30.040]you know, a fungicide might be a good idea.
[00:13:32.810]Maybe it's in continuous corn or minimum tillage,
[00:13:35.450]so it's at higher risk for disease,
[00:13:37.880]and you've seen that moving up the plant.
[00:13:40.870]But in other areas, you know,
[00:13:42.600]we don't always see gray leaf spot that early,
[00:13:46.080]and we know fungicides can still be effective
[00:13:48.770]if we wait a little longer,
[00:13:50.720]a little bit past tasseling, two, three, four weeks.
[00:13:55.230]And sometimes, fields don't need fungicides.
[00:13:59.240]And the potential benefits
[00:14:02.040]or potential risks of applying a fungicide
[00:14:05.590]are that we could help promote resistance
[00:14:09.170]in some of these pathogens.
[00:14:11.020]With that said, in corn, we have not got pathogens
[00:14:13.800]right now that we're dealing with in our area
[00:14:17.270]that are resistant on corn.
[00:14:20.310]It's a bigger problem in soybean, we'll talk about later.
[00:14:23.150]But consider that and know that
[00:14:27.820]there's always the possibility
[00:14:29.370]that a late season flush of disease
[00:14:31.880]may require a fungicide.
[00:14:34.160]And if you apply too early, now or at tasseling,
[00:14:38.980]before disease develops, that product might be worn off
[00:14:42.670]and not able to protect you later on when you need it.
[00:14:47.360]Right, thank you.
[00:14:48.530]It's some other good information.
[00:14:51.830]Especially right now with corn prices being low.
[00:14:54.590]Your recommendation to hold off
[00:14:56.300]to make sure it's in the field, makes a lot of sense.
[00:15:01.150]Thank you.
[00:15:02.600]So, looking at some of the current events
[00:15:05.360]over the past few weeks, we've had a lot of diseases.
[00:15:08.560]What does high wind and hail that can damage the plant,
[00:15:15.190]how does that change our game
[00:15:17.310]for when it comes to looking for diseases for corn?
[00:15:20.910]What might that introduce that we haven't had to look for
[00:15:23.540]in the past few years?
[00:15:25.160]Oh, that's a good question.
[00:15:26.650]You know, the wind, especially that we've had
[00:15:30.520]these last couple or three weeks has been brutal
[00:15:34.430]here across the whole state.
[00:15:36.730]And we have had some hail events
[00:15:40.330]and sandblasting as a result of the wind,
[00:15:43.300]and there's just been a lot of nasty weather going on,
[00:15:47.260]and the consequences, of course,
[00:15:50.330]we have the direct impacts of wounding in corn,
[00:15:54.120]and the damage to those leaves.
[00:15:56.950]The secondary potential problems, though,
[00:15:59.580]that will begin to show up now
[00:16:01.850]and in the coming weeks are
[00:16:03.780]that those wounds can be open opportunities
[00:16:07.260]for pathogens to infect.
[00:16:08.900]And usually those are bacterial diseases,
[00:16:11.980]especially the one causing Goss's bacterial wilt and blight.
[00:16:15.960]And so, that disease is still around.
[00:16:19.010]And so, people who've seen Goss's wilt in their own fields
[00:16:24.120]or in nearby fields should know that it's still alive.
[00:16:27.900]Even if you haven't seen it in a couple or three years,
[00:16:30.470]that pathogen still overwinters in that residue.
[00:16:34.010]And so, every year we should be diligent
[00:16:36.830]about selecting Goss's wilt resistant hybrids.
[00:16:40.760]But even when we have got a good resistant hybrid,
[00:16:44.220]after events like we've seen the last few weeks,
[00:16:47.080]we can see Goss's wilt development,
[00:16:51.010]and sometimes becoming pretty significant.
[00:16:53.920]Resistant hybrids are not immune to diseases.
[00:16:56.780]They tend to have less disease and so,
[00:17:00.530]I would keep watching for Goss's wilt.
[00:17:03.220]Sometimes, you know, we see an increase
[00:17:05.070]in bacterial leaf streak too,
[00:17:07.260]but it doesn't need a wound to infect the plant.
[00:17:10.960]So those are the bacterial things.
[00:17:13.350]Also, maybe bacterial stock rot could be an issue
[00:17:17.740]if we have those hot conditions
[00:17:20.470]like we've had a little bit of lately.
[00:17:23.170]Fungal wise, common smuts, probably the big one
[00:17:27.110]you might see after a wounding event.
[00:17:30.070]And you might recognize common smut
[00:17:33.280]being that mushroom-like yucky growth on corn ears.
[00:17:38.680]Well, it can grow anywhere on the plant.
[00:17:40.780]You know, sometimes those small plants
[00:17:42.430]get whipped around and the lower stalk gets damaged,
[00:17:46.200]so you'll see what looks like a tumor develop on there.
[00:17:49.720]That's common smut too.
[00:17:51.160]Or even on the leaves, it looks like
[00:17:54.330]kind of warty and bumpy.
[00:17:56.730]And the unfortunate thing is,
[00:17:58.620]there's really nothing we can do about common smut.
[00:18:00.830]But it doesn't spread.
[00:18:02.720]It's only gonna be right where it infects,
[00:18:05.290]it's not gonna move from plant to plant to plant.
[00:18:07.910]But it is very common and in every field,
[00:18:10.670]so you might see some of that too.
[00:18:15.545]All right.
[00:18:17.290]A lot of information to the think about
[00:18:20.290]and process for this year from this podcast.
[00:18:24.620]Any other general thoughts
[00:18:26.140]before we wrap it up for the day?
[00:18:29.950]Well, you know, I always talk about
[00:18:32.640]the bad stuff, I guess.
[00:18:33.870]I wanna say most of our corn crop is looking fantastic.
[00:18:37.280]And it's nice to see some great things
[00:18:39.710]out there in the field.
[00:18:40.740]But if people have some things going on,
[00:18:43.420]and they need some help,
[00:18:44.810]that's what we're here for in Nebraska Extension.
[00:18:47.320]So, thanks to everyone tuning in.
[00:18:50.730]And please reach out to us.
[00:18:52.280]We have a lot of resources and people here to help
[00:18:54.560]if you need it.
[00:18:56.630]All right.
[00:18:57.730]Well, thank you for joining us, Tamra.
[00:18:59.770]Thank you. And have a good day,
[00:19:00.603]and hopefully we have a great growing season.
[00:19:03.230]I sure hope so, thank you.
[00:19:05.508](joyful music)

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Mid-season Corn Diseases

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