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What’s Up With Crop Diseases

This
year’s soggy weather in many locations has set the stage for future foliar
diseases this growing season. Mark Carlton, an Iowa State University Extension
agronomist, outlined the situation in Iowa at the ISU Southeast Iowa Research
Farm near Crawfordsville, earlier this month.

Soybeans

In
soybeans, watch for these foliar diseases: 

1.    Brown spot

2.    Frogeye leaf spot

3.    Cerospora leaf spot

Fungicides
can help protect soybeans from these diseases. However, there are no thresholds
developed. Start scouting at R1 (beginning flowering) and watch for disease
movement and rainy cold weather. These factors can increase the odds of a
positive payback from fungicides. Carlton notes there were cases last year
where timely fungicide applications at R3 (beginning pod) had an 8 to 10 bushel
per acre payoff.

Fields
with continuous no-till beans, particularly with frogeye leafspot, also favor
disease development.

Corn

In
corn, the decision is more clear-cut. “We need to get out in the fields and
scout,” he says. When corn is at the V14 to V15 stage, when corn is up around
eye height, scout up to tasseling.”

In the case of
gray leaf spot (GLS), the threshold developed in the mid-1990s are lesions on
the third leaf below the ear leaf or higher on 50% of the plants at tasseling.
This threshold is being reevaluated due to new hybrids and fungicide products
that have been developed since them.

For now, though,
the threshold stands. It’s also important to consider high-risk cases, such as
fields in continuous corn with high residue in river bottom areas. Hybrid
ratings for GLS resistance should also be taken into account.

“Timing is important,” says
Carlton. The optimum time to apply fungicides is between tasseling and the R1
(silking) stage. 

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