AMES, Iowa -- The excessive rains that occurred across north central Iowa last week could increase the risk of stalk rots of corn, according to Alison Robertson, plant pathologist at Iowa State University.  Environmental stress increases the susceptibility of the corn plant to stalk rots, which in turn increase the likelihood of lodging, which complicates harvest.  Flooded fields should be assessed as soon as they have dried to determine the incidence of stalk rot.  Infected fields should be scheduled for an earlier harvest.

It also is possible that stalk rot diseases not normally seen in Iowa could present a problem, for example, Pythium stalk rot and bacterial stalk rot. Both diseases are favored by high temperatures and extremely wet conditions. 

Symptoms of Pythium infection are a wet rot of the internode just above the soil line.  The plants often collapse and may become twisted and distorted. 

The risk of bacterial stalk rot is greatest in fields that remained flooded for more than 12 hours, Robertson said. Symptoms of this disease are tan to dark brown, water-soaked lesions at a single internode.  The pith tissue is soft or slimy and affected stalks suddenly collapse and are usually twisted.  Infected plants usually have a foul odor. 

In addition to stalk rots, flood-damaged fields also should be assessed for ear rots. Since some ear rot fungi produce mycotoxins (Fusarium and Gibberella), any affected grain should be kept separate and evaluated for mycotoxin levels.  Fields with a high incidence of ear rot also should be scheduled for an earlier harvest.

Flooded soybean fields may be at risk for seed decay particularly if plants lodge and the pods are in contact with the soil surface.  In addition, the risk of Phomopsis seed decay may be greater in those fields where plants are at or near physiological maturity (growth stage R7).

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