As if this season needed more problems, assume you have some decent corn that wasn't wiped out by flooding and ponding. The only problem is it's full of disease right now. It's primarily gray leaf spot in many fields, or northern corn leaf blight in others, particularly in more northern counties.
Suppose the lower leaves are drying up and there are multiple lesions all the way up to the very top leaf under the tassel. If the plant checks out and shuts down early due to this infection, how much yield is at stake?
Bob Nielsen, Purdue University Extension corn specialist, says that if corn is at half milk-line inside the kernels when the plant shuts down, it can still accumulate 88% of the dry matter it would have accumulated if allowed to grow the final part of the season. Many of the studies which determined these numbers are based on frost situations, but Nielsen says the concept would be the same if the plant shut down and died early due to leaf diseases wiping out its photosynthesis factory.
"That means there is a possible yield loss of 12% in this case," Nielsen says.
Overall, he believes yield losses from foliar disease could range from 10% to 50%. The larger losses would be only in extreme situations, and would be more likely where corn wasn't as mature when the plant was shut down and could no longer produce sugars.
Stage of maturity of the corn is the key as to how much potential yield will be lost in these late-season situations, Nielsen adds. It's all about how much dry matter the plant can stuff into the kernels before systems shut down.
Black layer or physiological maturity doesn't happen until moisture levels reach about 30 to 35%. Once black layers develop on kernels, materials can't be added or removed from kernels.