Corn stripe rust

More corn diseases showing up in Iowa fields this year

ISU agronomists are getting an increasing number of calls about leaf diseases on corn.

Each Monday morning Iowa State University Extension field agronomists hold a conference call with the crop specialists on campus at Ames, to discuss current crop problems and share information about what's happening in fields around the state. "We are getting an increasing number of calls from farmers and crop consultants finding northern corn leaf blight and other corn diseases," says Clarke McGrath, an ISU field agronomist based at Harlan in western Iowa.

There is a lot of discussion about whether fields should be sprayed with a fungicide, and when they should be sprayed, and what products to use. Here's how McGrath is answering those questions:

  • Should you scout all your corn fields for foliar diseases, even if you have "resistant" corn hybrids planted in those fields? Yes
  • Should you spray fungicide if you see northern corn leaf blight? It depends
  • What fungicide products and rates should you use? It depends
  • If you treat around the VT growth stage of corn (tasseling stage), will you need to spray the fungicide again later? It depends

Yes, a lot of these decisions are "it depends" -- on a number of factors, says McGrath, who writes a monthly agronomy column in Wallaces Farmer magazine. "So let's dig deeper for more information here, so we can make an informed decision."

Should we scout all of our fields for foliar diseases?
Absolutely, and for good reason, he says. Even "resistant" corn genetics are not bulletproof against pathogens, and northern corn leaf blight resistance is no exception. Here's some information on northern corn leaf blight that discusses resistance: pioneer.com/home/site/us/agronomy/library/managing-nclb. It's a company link that ISU Extension plant pathologist Dr. Alison Robertson had in her Integrated Crop Management newsletter article, which has other good information on managing this disease: extension.iastate.edu/CropNews/2014/0714Robertson.htm.

Keep these key points in mind as you scout for foliar diseases
"Not only are we seeing northern corn leaf blight out there, but another disease, gray leaf spot, is also showing up as well in more fields," says McGrath. "And we never know when some other pest could pop up, so regular scouting is always a good idea and it's critical now, with this disease pressure."

Scout weekly, as northern corn leaf blight can go from a few random lesions seen on some corn plants in a field to a real mess in short order. New northern corn leaf blight lesions can produce spores in as little as a week and can easily disperse via wind, so this disease can spread much faster than many other corn leaf diseases.

While most of the time the corn hybrid ratings are pretty accurate, sometimes the genetics just haven't yet been exposed to a high enough disease load to know exactly how resistant they may be. "So, even if you have genetics planted that are rated pretty well against northern corn leaf blight, keep an eye on that corn. Last year we had a few cases of resistant genetics that buckled under the pathogen load," he says.

Should you spray fungicide if you see northern corn leaf blight?
"This is a really, really difficult question to answer because there are so many factors to consider," notes McGrath. He gives this advice to help illustrate the issues. You should picture in mind, the disease triangle. "We need the following three things," he says, "for northern corn leaf blight to really to take off and attack corn plants."

  • A host, which is your corn field. But how resistant, or susceptible, is the corn hybrid you have planted there?
  • The pathogen, which is obviously here in Iowa fields, but what races of the disease are present? Are they races of the disease that your corn's genetics can handle?
  • The right weather: it has been perfect for northern corn leaf blight development this summer in most of Iowa. While we've had periods lasting several days of warmer weather, much of this summer has been wet and cooler than usual, which is favorable for diseases like northern corn leaf blight to develop and spread.
  • Now you can see that even the three main factors in the disease triangle can create a lot of "it depends", says McGrath.

High risk factors for northern corn leaf blight developing
There are some high risk factors for northern corn leaf blight, says McGrath, which might make it more likely that you should consider spraying with a fungicide:

  • Corn hybrids with poor ratings for disease resistance. If you see northern corn leaf blight now in the field, you'll probably see a lot more in short order if you don't treat the field with a fungicide.
  • Early disease development, especially during pre-tassel growth stages, which we've seen in some areas of the state this year.
  • Corn on corn, if you grow corn following corn, you need to really watch those fields closely.
  • History of severe NCLB is important. If you have a history of severe infestations of northern corn leaf blight in your fields or in your area, watch out. "We had quite a bit of this disease in fields last year in many parts of the state," he says.
  • If you are in a low till or no-till system and have a lot of corn residue on the top of the soil surface, even after a soybean year…you need to really watch out for foliar diseases, if the weather forecast calls for humid/wet weather and moderate temperatures.

What is the disease threshold to make a spraying decision?
"After laying out all this information, we still haven't discussed a disease threshold," notes McGrath. "That's because there really aren't any rock-solid thresholds, but experience from the last several years has many agronomists and researchers in the following ballpark."

If you see lesions on around 50% of the plants (even a single lesion on every second or third plant is likely enough to trigger the need for a fungicide application unless your genetics are solidly resistant to the disease) at tasseling time, odds are you will need to pull the trigger and make a fungicide application, says McGrath.

What about corn hybrids rated as resistant to NCLB?
"This year we are hearing more reports of northern corn leaf blight on hybrids that have good resistance scores for the pathogen," says McGrath. "In the discussion above, I mentioned if you have lesions on around 50% of the plants, that even a single lesion on every second or third plant is likely enough to trigger a fungicide application unless your genetics are solidly resistant at tasseling, odds are you will need to pull the trigger on a fungicide application."

McGrath adds, "However, ISU Extension plant pathologists Alison Robertson and Daren Mueller are hearing and seeing enough about northern corn leaf blight race shifts and/or it overcoming genetic resistance that this is what they are thinking: if you are scouting and you see that many lesions, then it does not matter if you have a solid resistance package the pathogen has figured out a way around it (i.e., race of NCLB pathogen can cause disease to this hybrid)."

"Their thinking was that we didn't want growers to check their hybrid rating scores and not scout hybrids with good resistance scores; or scout them initially, see a lesion or two on every other plant and not scout again thinking that the resistance genes had done their job. Scout all your hybrids, and if the NCLB is on hybrids that are rated resistant, if you see this much disease – come back a week later to see if the disease has developed further – if it has, you may want to consider spraying since the background resistance may be low."

Consider the economics of making the treatment decision
What about the economics of product cost and application cost? Of course, that's part of your decision, says McGrath. Also, expected production, corn price and crop insurance all can factor into the decision of whether or not to apply a foliar fungicide, and can make that decision more complex. For more on disease thresholds, he refers farmers to read extension.iastate.edu/CropNews/2014/0717Mueller.htm.

He notes that Alison Robertson makes another good point: "One thing to think about is that northern corn leaf blight can result in 30% to 50% yield loss, and that's 60 to 100 bushel per acre on a field with 200 bushel per acre yield potential. Protecting that yield potential with a $25 or $50 input could make sense. Remember, too, however, that fungicides protect yield by reducing disease risk. They don't replace nitrogen or improve plant vigor in drowned out areas of the field. Back to considering economics, choose the fields you are going to spray prudently; a fungicide application should be made to those fields with the highest yield potential first; thereafter you can spray fields with problems as your pocketbook allows."

What fungicide product and rates should you use?
You have a lot of choices. McGrath suggests you take a look at the "Fungicide Efficacy Table" put together by the Corn Disease Working Group—made up of Extension plant pathologists and researchers in the Midwest. Go online and look for "Management of Corn Diseases: Fungicide Efficacy for Control of Corn Diseases, April 2015".

That table shows which fungicide products are very good at controlling northern corn leaf blight. Application rates and timing depend on your field-by-field situations, he adds. For example, some applications were flown on last week or are ready to go on shortly:

  • Combination strobilurin/triazole products at normal rates with some extra strobilurin thrown in to extend the residual.
  • Combo products at the upper end of the label rate to get more strobilurin and triazole on the field to extend the residual.
  • Straight or combo products applied at normal rates, sometimes with discussion about a potential follow-up application if needed.

"No doubt there are some other tactics used," he says. "I just provide these examples to show the diversity of northern corn leaf blight management plans available.

"Your local ag chem retailer is a great source of information on product choices, rates, mixtures and timing," adds McGrath. "They know the rebate programs and what other products you used in case there's a tie-in that can reduce your costs. They work directly with chemical company reps to understand the rate/mixture strategies that will likely perform the best for your area and parameters. They'll be able to go through the do's and don'ts of applying fungicides ahead of the VT growth stage. For more on this topic, visit cropwatch.unl.edu/corn-fungicide-precautions.

If you spray now (around VT), will you have to spray again?
"It's impossible to know for sure, but usually we get by with one application," says McGrath. "On the other hand, this season northern corn leaf blight showed up earlier than expected and the long term weather forecast looks like it could favor the disease triangle this summer. Conventional wisdom is that we need to protect against foliar diseases through brown silk or R2 growth stage, depending on your source."

How long is that you ask? It depends! "The time between VT and R1 or between any growth stages can vary with different hybrids and environmental conditions," says McGrath. "So we have to 'ball park' it." That means:

  • From VT or tasseling stage through brown silk or R2 stage of corn growth might be around 2 to 3 weeks.
  • Chemical representatives and university plant pathologists say that most of the fungicides available for use today have a two to three week residual.

"This lines up pretty well most years for making a single application of fungicide," says McGrath. "But you can see that if we apply it earlier, or our crop stages go longer, or if we have some other abnormality in the disease triangle then a second application could be a possibility. If you have to make an initial application, keep up with your scouting schedule so you can watch for new disease symptoms that may develop."

Would susceptible genetics need a second application?
Regarding the question of whether corn hybrids that are susceptible to northern corn leaf blight would potentially need a second application of fungicide, ISU's Alison Robertson offers this insight: "It is possible that a susceptible corn hybrid that is sprayed at tasseling, may need a second application," she says. "We have never been in this situation before so we are all learning. Most fungicide products should be effective for approximately three weeks. If after four to five weeks, the disease is beginning to develop within the upper canopy, we may need to spray again."

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