When you find disease lesions on lower leaves at this time of the season, it's cause for concern. And it may send you back to your seed corn notes to see how susceptible that hybrid is to the disease you've identified. If it's susceptible and you expect humid conditions to continue, you may want to consider a fungicide.
However, it's when you find lesions above the ear leaf that bells and sirens should go off. That's when the factory that will wind up making most of the yield is under attack. In fact, Dave Nanda, a crops consultant, indicates as much as 90% of the final yield may come from those few top leaves.
Recently, Bob Hooten of FMC pulled the top few leaves out of corn at a field day FMC hosted for dealers and the media to make a point. "This is where your yield comes from," he emphasized. "If you're after 240 bushel corn, then you need to protect these top leaves from damage."
Fungicides are only one of the two main pests that can reduce effectiveness of top leaves, Hooten notes. The other major pest that can cause serious issues are insects. Several types of insects, from Japanese beetles early in the season to rootworm beetles, can do damage. However, there is one that people often overlook.
"Aphids in those type leaves are often missed," he notes. "If you pull the leaves back and look, many times you will find aphids feeding in those places. If you're after top yields and not just average yields, you need to think about controlling those insects too."
Working for a company that sells insecticides, Hooten's answer is to spray an insecticide. What works well, he says, may be to include an insecticide in a fungicide application, assuming you are spraying a fungicide as well.
Hooten emphasized his comments were directed to those seeking high yields. Past trial data shows that an insecticide will usually more than pay for itself when added at that stage, especially with current corn prices. If you're already making a trip to apply fungicides, then there would be no extra application cost directly for the fungicide.
His main message was to consider the health of the top leaves of the plant, and to consider more than just disease pressure. He wants farmers to realize insects can also do damage that takes the top end off yields.