"Boy, wait until that salesman for Brand X comes by. It might be this week. He may bring me a Christmas present. I'm going to accept the present, then nail him. He will want to sell me more of the hybrid he sold me last year, and I'm going to let him have it. He told me it was resistant to northern corn leaf blight, and I still had a lot of lesions show up."

Crop Watch 12/14: Variable soil types elicit different hybrid response

Obviously, you wouldn't think like this. But maybe you've talked to a neighbor that fits this scenario. Northern corn leaf blight was severe in some parts of Indiana last year. Gray leaf spot was prevalent in other parts, especially counties more to the south.

Crop Watch 2015: Don't be shocked if you saw disease lesions in hybrids rated high for resistance last season. By Tom J. Bechman

"Weather conditions were right for the disease and there was plenty of inoculum," says Dave Nanda, crops consultant for Seed Consultants, Inc. 

Gray leaf spot showed up in both hybrids in the Crop Watch '15 field. Both were said to have 'good' resistance.

That's the first place you may have gone awry, says Kiersten Wise, Purdue University Extension plant pathologist. You need to know whether good is a 6 or a 9 on the company's rating scale, if the company uses high numbers as less disease, or a 3 or a 1 if the company uses a scale where low numbers mean less disease.

Even so, Wise says lesions can still appear on resistant hybrids, even ones with higher ratings. In many cases they won't be as severe, particularly if it is northern corn leaf blight, she notes. They might not form the center of the lesion where spores develop that can spread the disease.

Crop Watch 12/4: Two ears per stalk isn't the answer to 500 bushel corn

Minimal lesions: These northern corn leaf blight lesions, while small, developed on corn rated as resistant by the company for NCLB.

However, presence of the disease is still possible, especially if there is plenty of residue and weather conditions are favorable, she adds.

Both those conditions were met in many areas this year. Unless you had a check strip, what you don't know is if the lesions that developed on resistant corn caused yield loss, she adds. It's possible that yield loss, if any, was minimal.