Follow these steps to arrive at the most suitable determination.
You can bet 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. 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). However, the question then becomes: what about later applications? 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 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 price on corn is hovering around $3.80, 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 6-8 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 you apply, the less the benefit will be.
It's always a tough call, but these are the factors you should be taking into consideration before spending the money. If you choose to spray at R3 or later, leave a pass in the same field untreated if possible (this won't be an option with aerial application). This will help you see for yourself if you got any benefit from the treatment.
As long as you're at it, this is a great time to research resistant hybrids when you go to your seed field days. Look at the differences in disease levels, and ask if the plots have been sprayed with fungicide. We know we'll always have disease problems in this region, because more likely than not, it will always be warm and humid here in PA. It might be worth investing some of your money up front in the kind of help a seed breeder can offer!