Rain every day or every other day in June, temperatures near or above 90 degrees, and so much humidity that your shirt gets sticky wet in the shade is great weather for certain foliar corn diseases. It doesn't take a rocket scientist to come up with that one.
What is trickier is whether you need to do anything about it. Most companies don't recommends spraying fungicide until the corn is much taller. Many labels specify when the product can be applied. Applying fungicide outside of that window can result in damaged ears later in the season.
What you can do now is commit to continuing to scout any field as where you see signs of disease now all season long. Many agronomists still would wait, instead of following the advice of some chemical reps who would recommend to order product now.
Why? Because there is no guarantee what the weather may be like during the next three to four weeks which will be critical for disease development. If the rain stops and the humidity backs off, disease symptoms may stabilize. Perhaps only the most susceptible hybrids will still show damage. That happened in many instances last summer, primarily because it turned cool in July during the critical pollination period. Diseases which had gotten a foothold couldn't compete once conditions shifted enough to shut down the disease.
What you should do is commit to scouting that field every week, other week, or at some appropriate interval. Look for signs of rust, gray leaf spot and other foliar diseases. The farther up the plant you find the disease, the more serious the possible infestation later…if weather conditions still favor the disease!
The best advice, says Dave Nanda, crops consultant, Indianapolis, is following label directions. Be sure to spray fungicides only within the time frame noted on the label. Nanda believes in spraying only if it appears disease on leaves will cause an economic loss.
Some hybrids are more susceptible than others. For example, if a company has a one to nine rating scale with one being best and a hybrid is a six for gray leaf spot susceptibility, that's one you will want to watch. Companies seldom offer hybrids with a higher rating than a six in that situation. So he would recommend keeping a close eye on that hybrid, no matter what the weather is like.