Soybean Diseases and Fungicides

New information about soybean diseases and their control, and fungicide resistance in fungal populations, has recently been posted in various venues. Links to and a summary of the contents of each of these resources appears below.

Dr. Heather Kelly with UT Extension posted Scouting for Soybean Diseases and Deciding on Fungicides on the utcrops.com website. In this article, she provides a link to her Soybean Disease and Nematode Identification Guide that is available from UT Extension. This guide describes disease symptoms (with accompanying pictures) and management options for soybean diseases.

She also presents the following table that contains summarized data from 2013-2015 Frogeye Leaf Spot (FLS–caused by Cercospora sojina) field trials using FLS-susceptible varieties.

Using the above data set, it was determined that there is a strong correlation between percent FLS control and seed yield (R² = 0.8923). She iterates the importance of using fungicides with a mixed mode of action or a mix that is not solely a QoI/strobilurin product.

Two new items have been added to PMN’s Soybean Fungicide Resistance Hub.

Fungicide Resistance in the Cercospora Leaf Blight and Purple Seed Stain Pathogen of Soybean is a webcast supported in part by the United Soybean Board and presented by Dr. Trey Price with the LSU AgCenter. In his presentation, Dr. Price discusses the symptoms of the disease caused by Cercospora kikuchii pathogen, and how the symptoms are manifested in the foliar and mature seed stages. Severe infections in soybean can result in defoliation (Cercospora leaf blight) and poor seed quality (purple seed stain) that will result in dockage at the elevator. He also shows how fungicide efficacy against this pathogen has declined over the years to the point that the various classes of fungicides that are available now provide only limited efficacy against the pathogen, and no yield protection. Finally, he states that control measures consist of choosing tolerant varieties based on ratings taken in field trials (there are no resistant varieties) and early planting.

Principles of Fungicide Resistance is a new publication authored by Dr. Carl Bradley (Univ. Of Kentucky), Dr. Clayton Hollier (LSU AgCenter), and Dr. Heather Kelly (UT Extension). The authors define fungicide resistance, describe how disease resistance to fungicides develops followed by the subsequent loss of fungicide efficacy, and the factors associated with fungal pathogens and fungicides that affect resistance development. They discuss the primary chemical classes of fungicides commonly applied to soybean, and how the FRAC code can be used to distinguish these different fungicide classes. And finally, they present management practices that will prevent or delay development of fungicide resistance in order to retain fungicide efficacy over a long period of time. The authors compiled the below table that shows the FRAC determination of the risk level of fungicide resistance developing to each of the fungicide groups.

Links to the above resources have been added to the Guidelines for Managing Soybean Diseases White Paper on this website.

Composed by Larry G. Heatherly, July 2016, larryheatherly@bellsouth.net