This past week Wayne County IPM scouts noted increased incidence of leaf diseases in soybean fields. Most of this can be attributed to our rainy weather. According to Anne Dorrance, OSU Extension soybean pathologist, foliar pathogens have the most impact on soybeans at the later growth stages (R3 to R6) by reducing the photosynthetic area of the leaves that contribute to pod development and seed growth. So, does this mean you should go out and spray a fungicide? The best defense against disease remains planting varieties that have a good disease resistance package, but that does not always happen. The decision to use a fungicide should be made based on scouting to determine what, if any, disease may be affecting a soybean field, the treatment threshold for a disease, the yield potential of the field, the expected selling price of soybeans and the cost of the fungicide application.
Diseases that should be scouted for once soybeans reach reproductive growth stage include septoria brown spot, frogeye leaf spot and sclerotinia stem rot or white mold. Septoria brown spot is a lower canopy disease and it is only rarely feasible to apply a fungicide to treat this disease. Average losses are in the 2-3 bushel/acre range. Frogeye leaf spot is potentially more troublesome because infections where 5 to 12% of the leaf is affected can result in yield losses of 5 to 10 bushels/acre and because there are reports of populations that are no longer managed by the strobilurin class of fungicides. White mold can cause early plant death but an OSU Extension fact sheet on the disease says that soybeans can withstand a substantial amount of white mold before significant yield losses occur.
In a recent OSU Extension newsletter, Anne Dorrance said that with bean price below $10, fields where yields have already been reduced by late planting date, late weed control and/or flooding injury are not fields that will pay back a fungicide application. Make sure that field yield potential is evaluated realistically and accurately before any fungicide is applied.
Rory Lewandowski is an OSU Extension Agriculture & Natural Resources educator and may be reached at 330-264-8722.