If you’re a grower here in the mid-Atlantic, you’ve seen your share of corn diseases. This season has brought many of us timely rains, high humidity and moderate temperatures, all of which can favor the development of foliar diseases like gray leaf spot (Figure 1) and northern corn leaf blight (Figure 2).
The question is: what do you do about it? Our best data points to a specific window of time in which to apply a fungicide in order to get consistently good economic return. For corn, this is from tasseling (VT) to brown silk/blister (R2).
Here are the factors you’ll want to consider:
Scout your fields by tassel time. Is there fungal disease (like Gray Leaf Spot or Northern Corn Leaf Blight) on the third leaf below the ear or higher on 50% or more of your plants?
This is the tipping point at which spraying becomes an option. But see below…
Are you are growing a hybrid with little to no built-in resistance to GLS or NCLB?
You are far more likely to see an economic benefit from fungicide if you are growing a hybrid without disease resistance. If you are growing a resistant hybrid, you will likely knock down disease levels, but it is unlikely to translate to yield.
Are you growing corn following corn? No-till?
These fungi survive from year to year in the old leaves and stover from the previous year. The more corn residue you have on the surface of the field, the more inoculum is present.
Do you suspect humid weather or frequent rains to continue now and through August?
It’s nearly impossible to predict the weather, but try anyway. It seems like the weather patterns that start us off in the beginning of the season continue all summer, so if you’ve been wet, you’ll probably stay wet, and might benefit from an application.
Are you growing for grain or silage?
The fungicide work done has mostly been on corn for grain. A few years ago some studies were conducted in Wisconsin and Minnesota to determine the benefit of fungicides for silage. The researchers found no clear benefit of fungicides at any timing for silage, and suggested the most important factor is still proper hybrid selection in this case.
When are you hoping to harvest?
Always read your labels carefully. There will be increasing concerns about post-harvest interval and delayed dry down the further we go into the season.
What price will you get for your corn?
Today’s spot prices on corn are hovering around $4.50 to $4.75, so think about what you’ll be getting paid at delivery this year. The cost in product varies from about $12-$24/A, and custom application costs may drive you up to $40/A to buy the materials and get them on your crop.
You’ll need to realize a benefit of at least 3-5 bu/A to cover your costs. You can probably get that if you have several of the previously mentioned factors working against you, but the later in the season after brown silk you apply, the less the benefit will be.
High humidity combined with canopy closure in full season beans at this time creates conditions that favor many diseases. The most common ones you will begin to see include downy mildew, brown spot, and white mold.
Downy mildew can probably be found in many flowering fields right now. It is rarely yield limiting, so fungicide applications are not typically recommended. The University of Kentucky has a helpful fact sheet with nice pictures for identification.
Brown spot is our most common foliar disease in PA. This fungal disease first affects the lower leaves, progressing upwards through the canopy via rain splash. Figure 3 shows some first trifoliates infected with brown spot—this is not uncommon. Notice the soil splash present on the leaves. If confirmed brown spot infection is unusually high in the mid-canopy around R1, a fungicide application by R3 may be warranted.
Another disease to keep your eye out for is frogeye leaf spot. If it shows up early in a field (like R1 or before), it may pay you to spray at R3.
Finally, in fields with a history of white mold, a suppressive fungicide application should be applied at R1. The fungus that causes this disease enters through dying flowers and thrives when the environment is humid with a closed canopy that prevents air flow.
The stem and lower leaves become infected, and then the disease moves up the plant resulting in yellowing and defoliation. As it progresses, white tufts of fungus are visible on the tissues of the lower canopy, and ultimately hard, black overwintering structures (sclerotia) form that can survive in residue and in the soil for many years.
Current conditions and timing make the situation highly favorable for the pathogen, so if you are growing in a field where white mold has been a problem in the past, an application of Aproach or Endura can absolutely help limit the yield loss. A second application may be necessary depending on timing.
Remember not to get your expectations too high for control of this disease, it is a very tough nut to crack once you have it in a field—take care not to spread it to clean fields.
It’s always a tough call on foliar sprays for these crops, but these are the factors you should be taking into consideration before spending the money. If you choose to spray, leave a few random passes in the same field untreated if possible (this won’t be an option with aerial application), and harvest them separately. This will help you see for yourself if you got any real benefit from the treatment.