Mid-season Corn Diseases
Michael Sindelar, Tamra Jackson-Ziems
Author
07/08/2020
Added
1
Plays
Description
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.
Searchable Transcript
Toggle between list and paragraph view.
- [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)
The screen size you are trying to search captions on is too small!
You can always jump over to MediaHub and check it out there.
Log in to post comments
Embed
Copy the following code into your page
HTML
<div style="height: 5.62em; max-width: 56.12rem; overflow: hidden; position:relative; -webkit-box-flex: 1; flex-grow: 1;"> <iframe style="bottom: 0; left: 0; position: absolute; right: 0; top: 0; border: 0; height: 100%; width: 100%;" src="https://mediahub.unl.edu/media/13545?format=iframe&autoplay=0" title="Audio Player: Mid-season Corn Diseases" allowfullscreen ></iframe> </div>
Comments
0 Comments