All but a small handful of the more than 100 corn stalks I judged at the Jackson County fair a few days ago showed definite lesions of gray leaf spot. It tended to be prevalent on lower leaves, and in several cases, was on the leaf below, or even the ear leaf. If stalks people brought to the fair looked like that, what do their fields look like?
At the same time a farmer 40 miles farther north says his fields are clean. He figured to have popcorn fields sprayed at a minimum, but when his dealer checked the fields, he said they didn't need spraying, at least not yet.
So what's going on? Dave Nanda, consultant for Seed Consultants, Inc., says that it's all about disease inoculum, how susceptible the hybrid is and whether or not the weather conditions are favorable. Cooler and damp weather favors northern corn leaf blight. Warmer and damp weather factors gray leaf spot, he says. Both are caused by fungal organisms. They're going to be most prevalent in corn after corn and no-till fields, but you can find them anywhere.
Whether or not you spray and when depends upon when stage of corn growth and whether or not you find lesions, he says. Stick to the label on when it says to spray a product. Some fudge the line and don't wait as long as labels say so they can spray insecticides and fungicides together. Nanda advises against it, citing cases of ear damage where a fungicide was sprayed too early.
The bottom line is that it would pay you to be scouting it now, and to know which fields might be more susceptible, based upon past crop rotation, tillage method and hybrid choice. That's where you will likely want to look first – in fields that are most at risk.