A wetter-than-normal summer in many parts of the Corn Belt has set the stage for disease. The question is whether conditions the rest of the summer will allow one disease or another to get enough of a foothold to take off and affect yields, or will the weather keep things in check.
In the Eastern Corn Belt, gray leaf spot starts out as small lesions, usually on lower leaves, and if conditions are right, lesions form higher up on the plant. Since agronomists say a good portion of the energy is captured by the top four leaves on the plant, it becomes more critical if the disease gets to the ear leaf or higher in enough concentration to lead to early leaf loss. If the factory is destroyed, production shuts down, and you won't get maximum yield.
In other parts of the Corn Belt, and to a lesser extent in the Eastern Corn Belt, northern corn leaf blight has been spotted in some fields. It produces longer, wider lesions. If favorable weather conditions come together they can cover the surface of a leaf fairly quickly. The result is the same – they shut down production of sugars since photosynthesis is limited by foliar leaf damage.
If it stays cooler than normal, agronomists say that's good for northern corn leaf blight. If the weather shifts and it goes warmer than normal, it favors gray leaf spot. If it doesn't do one or the other fairly soon, neither may get to the point that they cause widespread yield loss in any part of the Corn Belt.
If you're in an area where you could still spray fungicides because corn isn't as far along, you should be scouting, agronomists say. Check labels and spray according to label directions. Don't sue adjuvants if the label doesn't specify it's OK to use them. Make sure corn is at the stage noted on the label before spraying.
If that window has passed, you may still want to scout so you know what to expect this fall. If a disease takes off this month, it could set the plant up for secondary infections, like stalk rots. That could mean it might pay to mark that field for early harvest.