South Carolina Soybean Rust Update – 8/11

August 11, 2023

FROM:  Jonathan Croft, Clemson Extension Agronomic Agent, Joe Varn, Clemson Extension Agronomic Agent, Rogan Gibson, Clemson Extension Agronomic Agent, and Dr. John Mueller, Extension Soybean Pathologist, Edisto Research and Education Center.

Currently, Asian Soybean Rust has NOT been identified in South Carolina.

Samples were collected and examined from Allendale, Barnwell, Bamberg, Colleton, Dorchester and Orangeburg Counties. Soybean growth stages ranged from early bloom to R5.6.

Dr. Mueller spoke with several Agronomic Row Crop Agents, consultants, and growers in the last week that have seen very low levels of target spot in some scattered soybean fields.  If you have forgotten what this disease looks like we have attached two photos of typical target spot.  The first photo shows a typical large lesion. Lesions are mostly round and consist of a circular area of dead, brown tissue often surrounded by a bright yellow halo caused by the toxin the fungus produces.  The size of the halo may vary considerably.  The defining trait is the concentric circles in the brown area that make it look like an archery target.  Often these rings are very distinct.  This year we have seen some examples where the rings were less pronounced.  This has been especially true on cotton.  The second photo shows multiple lesions on one leaf.  This is not uncommon.

If you see target spot in your field it is probably a good idea to spray a fungicide even if you are not to R-3, our normal recommended time for spraying a foliar fungicide on soybean.   Target spot can spread quickly and cause severe defoliation if we have several rainy or overcast days in a row.  It is probably best to spray a fungicide with more than 1 active ingredient. We recommend that 2 weeks after your first spray you scout the field and determine if a second fungicide spray is needed.   Keep in mind that we want as much leaf area on plants as possible going into R5 (seed set) to maximize yield, so we want to protect leaf area.  A second spray will also help to protect pods and seeds from infection by pod diseases, especially if we get a hurricane or extended wet period.”  

If you think you have rust or other foliar diseases in a field collect 25 to 50 of the leaves you are “suspicious” of.  Place them in a “ziplock” type bag.  Keep bag out of direct sunlight and in a cool place if possible.  Do not allow it to freeze or get close to freezing.  Contact your local county agronomic agent for evaluation.  

A list of fungicides available for use on soybean can be found in the “2023 Pest Management Handbook” or on the Crop Protection Network. This publication lists the fungicides and rates them from poor to excellent for Brown Spot, Cercospora Leaf Blight, Frogeye Leaf Spot, Pod and Stem Blight, Rust, Target Spot and White Mold.