While many full season soybean fields are past the growth stage of spraying a fungicide (R6), later planted soybean may still benefit from a fungicide application.

As I have mentioned in previous articles the amount and severity of a disease and effect on yield are closely associated with environmental conditions, management practices, growth stage and susceptibility of soybean variety planted. The main diseases that have been reported in Tennessee this season include frogeye leaf spot (FLS) and sudden death syndrome (SDS). FLS is caused by a fungus, Cercospora sojina, and creates tan to grey circular to angular lesions, with a dark brown to purple margin (Picture 1). Symptoms can be confused with chemical injury – read previous article for more info, Distinguishing Frogeye Leaf Spot from Chemical Injury in Soybean. The spores of the fungus survive in soybean debris and are wind and rain splash dispersed. Optimum conditions for disease infection include high humidity (>90%) and temperatures between 77 and 86 °F, though disease can develop outside of these conditions. A timely fungicide during soybean reproductive stages (usually R3, pod development) can protect yield loss from this disease. I have on-going research projects to better define the economic disease threshold to better guide fungicide applications in the future.

SDS is also caused by a fungus, Fusarium solani f. sp. glycines, but it is in the soil and infects the roots of soybeanplants earlier in the season. The fungus produces a toxin that is translocated to the leaves resulting in scattered, interveinal cholorotic (yellow) spots

(Picture 2). These spots may become necrotic (brown, dead tissue) (Picture 3). A foliar fungicide will NOT control SDS. Management options for SDS include cultural practices (rotation, improving drainage, etc.), variety selection and new fungicide seed treatments have begun to show control of the disease, mainly ILeVo fungicide seed treatment (active ingredient fluopyram) by Bayer, which is anticipated to be available for the 2015 growing season. For more information on SDS read the previous article, Sudden Death Syndrome (SDS) in Soybean. Symptoms of SDS can look similar to stem

canker and triazole or surfactant injury. An easy way to determine if you are dealing with chemical injury or a disease is to look at the newest growth on the plant (upper most leaves), if these leaves are symptomless then you are most likely dealing with chemical injury. If the upper, newest leaves are also showing symptoms you are most likely dealing with a disease (Picture 4, new growth with SDS symptoms). There are nutrition deficiencies and other disorders that can look like disease, for more information on issues that have been reported this season read Dr. Angela McClure’s article on Yellow Tops in Soybean.

Other diseases that can be found this season include Septoria brown spot, target spot, and stem canker. Soybean rust has not been found in Tennessee, and the furthest north it has been reported is in Prattville, AL, just northwest of Montgomery. Septoria brown spot (Picture 5) and target spot (Picture 6) do not usually pose a great threat to yield, although some varieties can be more susceptible than others. Stem canker symptoms can look very similar to SDS, although there will usually be a lesion on one side of the stem (Picture 7). One can also split the stem and look at the center pith – if SDS the center pith would still be white, while it will be brown if stem canker (Picture 8). Although if Dectes stem borer has gotten into the plant the center pith will be brown (Picture 9) and cannot be used to determine the disease. While stem canker is usually more prevalent during hot, dry years and SDS favors cool, wet years, I have found small areas of stem canker out in my research plots this year; although much more SDS is present this year than stem canker. Similar to SDS, stem canker cannot be controlled with a foliar fungicide, only variety selection and rotation will help manage it. More information on soybean diseases can be found at UTcrops.com (Soybean Disease and Nematode Identification Field Guide) as well as a foliar fungicide efficacy table (Soybean Fungicide Efficacy Table). This winter a soybean fungicide seed treatment efficacy table will be finalized and available on the UTcrops.com website.

Picture 5. Septoria brown spot