Tan Spot Prompts Early Fungicide Applications

Make sure there is disease present before spraying to maintain efficacy of product.

Published on: Jul 8, 2013

With growers and crop consultants reporting more tan spot in small grains this season, farmers have made early fungicide applications.

In the recent past, even if they have not seen signs of disease, growers have sprayed. In fact, it's become common practice to spray fungicides in small grains, along with early herbicide applications around the five-leaf stage, even when there are no signs of disease.

Madeleine Smith, a University of Minnesota small grains plant pathology specialist, cautions farmers against spraying if there is no need to do so.

"Every time a fungicide, or any pesticide, is applied, the fungi or pest must adapt or die," she said. "Most organisms will try to adapt rather than die. It's survival of the fittest. The time it takes to adapt can vary and depends on a number of factors such as host resistance, dose rates and environmental conditions."

Tan Spot Prompts Early Fungicide Applications
Tan Spot Prompts Early Fungicide Applications

She added that there have been cases where fungi adapted and became resistant to fungicides in commodity crops, including small grains.

"The idea that there will always be a new, more effective fungicide is not necessarily the case," she said. "Chemical companies must work harder to identify and optimize new chemistries. They are often more expensive than current market leaders because of the additional investment in bringing these products to market."

To maintain long-term fungicide efficacy, Smith suggests:

-Scout the crop once a week, especially around the critical spraying decision-making periods.

-Use disease-forecasting models to assess risk of diseases developing in your area.

-Know the disease in your crop. If in doubt, consult your local Extension educator. Send plant samples to the University Plant Disease Clinic.

-If no disease is present, weigh the risks. Does this early stage merit fungicide? If not, don't apply. Remember, you can usually spray later if necessary.

-Choose the right product, rate and dose for the disease. Use tank mixes with different modes of action of fungicide to reduce the selection pressure.

To learn more about small grains from Extension, visit www.extension.umn.edu/agriculture/crops/small-grains