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Keep scouting corn for diseases until harvest

Tom J. Bechman ear mold on corn
SEE EAR MOLD: Notice ear mold is beginning on the tip of this ear. Corn experts believe it will be crucial to scout for ear molds and stalk rots this year.
Corn Illustrated: Conditions this year are favorable for diseases in some locations.

We have a chance to study the effect of conventional vs. no-till on the development of corn ear and stalk rot diseases this year. Both conventional and no-till corn looked surprisingly clean in mid-July as pollination occurred. However, there is still plenty of time for disease organisms that cause leaf diseases, and later stalk rots and ear rots, to invade.

Tar spot was already documented in Indiana by Purdue University plant pathologists in mid-July. Corn exhibited at county fairs in July sometimes showed a moderate to minimal amount of gray leaf spot lesions on lower leaves.

To protect this year’s crop, be proactive in spraying foliar fungicides. There are several disease organisms waiting in the wings. The disease triangle says you need three things to be present for a disease epidemic: the pathogen, a susceptible host and an environment suitable to spread the disease.

The rusts must blow into the Corn Belt from Southern states each year, but most of the other pathogens survive on corn residue and are already here. Wet, warm, humid weather in July provided an environment favorable for several diseases, such as gray leaf spot.

In addition, Bob Nielsen, Purdue Extension corn specialist, notes that cloudy, wet weather during active pollination can set the stage for ear rots because the fungi that cause ear molds infect silks near the end of pollination. He also notes that stalk rots will be more of a function of plant stress during the last half of grain fill, and not affected by cloudy weather during pollination.

Diseases to watch for

Northern corn leaf blight develops in cooler and humid weather. Hot, humid weather tips the scales toward gray leaf spot, so it would appear to have the edge this year, along with tar spot. We didn’t see much anthracnose leaf blight last year, but it shows up in hotter weather.

Anthracnose stalk rot has become the dominant stalk rot. This pathogen attacks roots and stalks weakened by leaf diseases. The spores of this fungus can stay alive in the soil for a long time, especially in corn-after-corn and no-till situations. It can infect stalks by splashing spores from the soil to lower internodes of stalks. It makes shiny black spots on stalks that look like shoe polish. Stalks become black inside and can go down.

If conditions are conducive to diplodia and gibberella ear and stalk rots, look for them. Fungi survive in the soil for many years. Diplodia has a whitish color. This disease can infect ears, usually starting at the base and spreading to the whole ear. Infected kernels are light in test weight but don’t produce any toxins.

Gibberella ear rot is produced by a pink fungus that usually starts at ear tips. It can produce a mycotoxin. Grain should be tested before feeding livestock. Both diplodia and gibberella cause a lot of yield loss if corn isn’t harvested on time.

Crop scouting must start at emergence and should not end until the crop is in the bins. Scout for ear and stalk rots by pulling back husks, pushing or pinching the stalks, and planning which fields should be harvested first to reduce field losses.

Nanda is director of genetics for Seed Genetics Direct, Jeffersonville, Ohio. Email [email protected] or call 317-910-9876 and leave a message.

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