Update on Soybean Rust

Carl A. Bradley
Extension Plant Pathologist

 With soybean rust being observed in late August in Giles County, Tennessee (south-central TN), and with Hurricanes Harvey and Irma potentially carrying and depositing spores of the soybean rust fungus in Kentucky, the detection of soybean rust in Kentucky this year is eminent.  Although the soybean crop is well on its way to reaching maturity in Kentucky, there could potentially be some late-planted or late-maturing fields at risk.  At this point, soybean growers can be cautious about soybean rust, but it is likely that no action is needed.  For late-planted or late-maturing fields, growers should consider yield potential and disease scouting observations when making a decision on if a foliar fungicide should be applied.  For fields with soybean plants at developmental stage R5 (beginning seed) or beyond, it would be highly unlikely that soybean rust would cause any yield loss at this time.

Monitoring the distribution and northwardly progress of soybean rust in the U.S. can be done by going to the Soybean Rust IPM-PIPE website at: http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi.  If soybean plants with suspicious symptoms are observed, it is very important to work with your local county agent to submit a sample to the University of Kentucky Plant Disease Diagnostic Lab for diagnosis.  Soybean rust can be difficult to identify, especially when it is at a low incidence and severity.  Small, slightly-raised pustules can be observed on the underside of affected soybean leaflets.  These pustules often are observed near a leaf vein, where dew or moisture may be retained (Fig. 1.).  The use of a hand lens or a dissecting microscope makes it easier to observe these pustules (Fig. 2). 

Fig. 1. Soybean rust pustules observed on the underside of a soybean leaflet (inside the “box” drawn with a pen) (Photo: Carl Bradley, UK)

 Fig. 2. A close-up view of soybean rust pustules on the underside of a soybean leaflet (Photo: Carl Bradley, UK)