Many Iowa cornfields are now reaching the black layer stage of development, which means it is time to scout for stalk and ear rots - both of which are being reported in Iowa fields. This timely advice comes from Alison Robertson, Iowa State University Extension plant pathologist.
When corn reaches black layer, the crop should be scouted for stalk and ear rots, she says. Anthracnose stalk rot is apparent in many fields across the state and Diplodia ear rot has also been reported. For information and photos of these diseases go to www.extension.iastate.edu/CropNews/2008/0902robertson.htm
Decide which fields you should harvest first
The incidence of corn ear rot should be determined before harvest since ear rot diseases can reduce yield and quality of the corn harvest. Furthermore, some of the fungi that infect corn ears may produce mycotoxins, which are harmful and can be fatal, to livestock.
At least 100 plants, scattered throughout the field, should be assessed. Look for visible symptoms of ear rot by stripping back the husks. For a description and photographs of ear rots you might encounter in the field see corn ear rots at www.ipm.iastate.edu/ipm/icm/2004/10-4-2004/earrot.htm.
If more than 10% of the ears have significant mold that is greater than 25% of the ear, the corn should be harvested in a timely manner and dried to below 15% moisture as quickly as possible to prevent further mold growth and, in some cases, mycotoxin accumulation.
To scout for stalk rot, at least 100 plants scattered throughout the field should be assessed. Test the stalk firmness at the lower internodes by squeezing the stalk with your thumb and forefinger. If more than 15% of the stalks are rotted, schedule for the earliest possible harvest because significant lodging is possible. Scout different hybrids and fields with different tillage, rotation, or fertilization histories separately.