Leaf samples from sentinel plots in Bryan and Sequoyah Counties, and from a commercial field in Sequoyah Co. were positive for rust this week. Both of the sentinel plots were at the R4 growth stage. This is the first report of soybean rust in Oklahoma in 2009. Rust incidence, the % of leaves with rust, ranged from 3% at the Sequoyah Co. sentinel plot to 24% at the Bryan Co. sentinel plot. Disease severity, the percentage of leaf area with rust, was less than 1% at each location. However, several of the leaves from the Bryan Co. location had over 100 pustules.
Based on past experiences with rust in 2007, this is a rapid jump from no disease the previous week. Weather conditions have been very favorable for disease development in recent weeks. Several counties in Arkansas along the Oklahoma border have also recently become positive and rust is very active in our neighboring state. It is likely that additional counties in eastern OK will become positive shortly.
Many fields in Oklahoma have advanced past growth stages at which soybean rust causes yield loss. Fields at R6 (a full pod at one of the top four nodes) are no longer vulnerable to rust damage. Fields at R4 or earlier are still vulnerable. Fields at R5