As full season soybeans in Tennessee reach reproductive growth stages, scouting for disease is critical to determine how necessary a fungicide application may be in protecting yield from disease.
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. Some diseases have characteristic symptoms that are easily identified, where others may not be specific and may be mistaken for weather or chemical damage (see previous article Distinguishing Frogeye Leaf Spot from Chemical Injury in Soybean). Entire plant samples should be examined and when needed can be taken to your local extension county agent for identification.
Some common soybean diseases found in Tennessee include Frogeye Leaf Spot, Septoria Brown Spot, Cercospora Blight, Anthracnose, Southern Stem Canker, Charcoal Rot, and Sudden Death Syndrome. Descriptions of symptoms, pictures, and management options for these and other soybean diseases can be found at UTCrops.com (Soybean Disease and Nematode Identification Field Guide). Many diseases can be avoided by planting a disease resistant variety, regularly rotating to nonhost crops, and when warranted fungicide applications can be applied. UT research has previously indicated that under circumstances of high disease pressure and conducive environmental conditions, timing a fungicide application at R3-R5 (beginning pod develop to beginning seed development) often provides the most consistent yield protection.
A foliar fungicide efficacy table (Soybean Fungicide Efficacy Table found at UTCrops.com) was developed by The North Central Regional Committee on Soybean Diseases and the Regional Committee for Soybean Rust Pathology (NCERA-212 and NCERA-208) to provide information on foliar fungicide efficacy for control of major foliar soybean diseases in the U.S. With the identification of Frogeye Leaf Spot disease caused by strains of Cercospora sojina resistant to fungicides in the quinone outside inhibitor (QoI or commonly referred to as strobilurin) group in Tennessee since 2010, it is recommended to use a fungicide that has a mixed mode of action or one that is not a sole QoI/Strobilurin product. Free testing will be conducted this year to determine if Frogeye Leaf Spot disease in a field is caused by QoI/Strobilurin fungicide resistant strain of C. sojina more details are at the end of this article (funding for testing provided by the Tennessee Soybean Promotion Board).
Two diseases that are being monitored across the U.S. due to their importance include Soybean Rust (Phakopsora pachyrhizi) and strains of Frogeye Leaf Spot (Cercospora sojina) resistant to QoI/Strobilurin fungicide group. Updates and locations of these diseases can be found on http://sbr.ipmpipe.org or http://sbrusa.net. In the upper right of the website, in the drop down menu ‘Soybean Rust’ or ‘Frogeye’ can be selected to display data on these diseases.
Frogeye leaf spot has started to develop in some Tennessee soybean sentinel plots and farmers’ fields I’ve visited, but at low levels. No soybean rust has been found in any fields in Tennessee and is still only in the very southeastern area of the U.S.
Free testing will be conducted this year to determine if Frogeye Leaf Spot disease in a field is caused by QoI/Strobilurin fungicide resistant strain of C. sojina. To have a sample tested please send at least 20 symptomatic leaves with sporulating lesions and a sampling form filled out to:
WTREC, Lab C
605 Airways Blvd.
Jackson, TN 38301
Attn: Frogeye Leaf Spot Samples
Leaves with FLS symptoms should be placed in a sealable plastic bag and mailed overnight or as soon as possible. Leaves should be kept in a cool, dark place until mailed. More information and pictures of Frogeye Leaf Spot and how to sample can be found at UTCrops.com (Frogeye Leaf Spot Information and Directions for Frogeye Leaf Spot Samples). If you have any questions contact Heather Kelly (731-425-4713 or youngkelly@utk.edu).
