Tips For Using Of Foliar Fungicides For Late Season Soybean Diseases

JACKSON, TENN.

Many soybean producers are considering spraying their soybeans with a foliar fungicide. The best time to spray for late season diseases is at the R3 stage of growth. Under heavy disease conditions, and when soybean prices are really good, a second application might be beneficial at the R5 growth 5 stage. The recent rains have made prospects look good for excellent yields, but that good moisture also creates the conditions for more late-season foliar diseases.

   Moisture is the number one factor needed for disease development, but along with moisture, continuous soybeans, susceptible varieties, high cyst nematode counts and low soil fertility (especially low potassium and low pH) also increase the chances for yield loss from disease. Producers should consider spraying fields that are subject to these conditions. However, if there is little hope for a good yield (at least 30 + bushels per acre) spraying a fungicide might not be as economical as one that has a high yield potential.

   Fungicide coverage of as many leaves and stems as possible is essential for good foliar disease control. If spraying with ground equipment, use at least 15 to 20 gallons of water per acre along with a fine droplet size (about 250 to 300 microns) and high pressure. If spraying with an airplane use at least 5 gallons water per acre and fly 6 to 12 feet above the soybean canopy for good coverage. Mixing a herbicide with the fungicide application is not recommended because application and timing is usually not compatible.

   Soybean rust is not a threat in Tennessee at this time. Therefore, all that is needed for control of late season diseases is an EPA approved Strobilurin fungicide such as Headline or Quadris with a surfactant such as NIS or a COC. Evito (fluoxastrobin, a strobilurin similar to Quadris and Headline) and Topguard (flutriafol) have recently received an EPA label for soybeans. Topguard has performed well in soybean rust tests in Quincy, FL and controlled late season diseases in our tests at Milan.

   A premix or tank mix of a Strobilurin plus a Triazole fungicide is also effective as long as there is an adequate amount of Strobilurin in the mix. If making a second application, it is wise to use a tank mix or a premix of Strobilurin plus a Triazole. This would help reduce the chances of a built-up of resistant fungi to the Strobilurin fungicides and help reduce the risk of soybean rust.

   Using just a Triazole for late season diseases such as Brown Spot, Frogeye Leaf Spot and Anthracnose would not be the best choice for maximum control. Basically, the Triazoles are best used for soybean rust control. Triazoles are necessary when there is a high risk for soybean rust. The risk of rust is very low at this time. We are continuing to monitor the rust situation by scouting sentinel plots and checking spore traps.

   For more information on soybean diseases, disease resistant varieties and fungicides check the utcrops.com web site under soybean diseases. Δ

DR. MELVIN NEWMAN: Extension Plant Pathologist, University of Tennessee