Although late summer rains have reduced the risk of stalk rot in much of Iowa this year, cornfields that experienced stress around pollination time need to be checked now for stalk strength. "The chance of stalk rot becoming a problem increases when plants experience stress during the growing season," says Alison Robertson, an Iowa State University Extension plant pathologist.
Stress includes too much or too little rain. Hail, wind and insect damage also stress corn plants. Stalk rot fungi infect plants through roots and insect wounds. The fungi first attack the pith tissue inside the stalk; the outer rind of the cornstalk will also eventually decay. Diseased stalks may have only half or less of their original strength. If harvest is delayed or high winds hit the field, diseased plants easily lodge and as lodging increases harvest losses increase dramatically.
Give stalks the pinch test
Check for stalk rot by pinching the lowest internode, right above the ground. Do this on at least 100 plants scattered throughout the field. If more than 15% of the stalks are rotted (stalks easily crushed) harvest the field as early as possible to reduce yield loss. "There can be economic tradeoffs with increased drying costs for early harvested grain," says Robertson. "But field scouting and checking for stalk rot is the best guide for potential losses."
All corn plants start to decay after maturity as part of the natural process of recycling organic matter and nutrients. The fungi that do this recycling are present in every cornfield. "However, when the decaying process occurs before physiological maturity - as stalk rot - it represents a disease problem with potential economic effects," she notes.
Ideally, you should start scouting cornfields for signs of stalk rot about 40 to 50 days after the silking date, especially targeting areas that remained dry in August.
Additional information can be found in the Iowa State University Extension publication "Corn Stalk Rot in Iowa" (IPM 50). Copies are available at county ISU Extension offices or call the Extension Distribution Office (515) 294-5247.