Thinking Fungicide? Remember Pre-harvest Interval

Arrival of cool, wet weather increases risk of wheat disease.

Published on: May 6, 2009

Just when most of Kansas was worrying about dry weather and drought stress problems, the skies opened up and now prolonged cool, wet weather has many wheat farmers thinking fungicide.

Leaf rust, stripe rust, tan spot and powdery mildew are all more likely to be a problem in this kind of weather, says Erick De Wolf, plant pathologist with Kansas State University.

Resistance to disease varies by variety and in recent years some that have had good resistance packages have begun to lose their protection.

The publication, "Wheat Variety Disease and Insect Ratings," available at your Extension office or online at www.plantpath.ksu.edu offers a comparison of how various varieties are against common foliar disease.

For those farmers considering fungicide application, Kansas Wheat has issued a reminder that the labels of many popular fungicides require a "pre-harvest interval" between application time and harvest.

The good news is that one of the most widely used, Quilt, has been re-labeled this year to only require a growth stage application and not a pre-harvest interval. In 2008, wheat harvest in some fields was halted because farmers had not followed the pre-harvest interval.

There are two basic categories of foliar fungicides:
• Strobilurins prevent diseases only and must be applied before symptoms appear. They have somewhat longer residual activity than triazoles.
• Triazoles are a better choice when diseases are already present. They can stop a disease from reproducing and have limited kickback curative ability.
• Combination products contain both triazole and strobilurin modes of action.

Most pre-harvest intervals range between 30 and 45 days; farmers must stay in contact with commercial applicators to learn when the product was applied.

De Wolf says that most fungicide applications should be made between full flag leaf extension and full head emergence. For head scab suppression, apply the appropriate fungicide to the head between the start of flowering and 50% flowering.