2011 Field Crops Rust Symposium:
Modeling Southern Corn Rust in a Search for Potential Spray Thresholds
Presenting Author: P. VINCELLI
Affiliation: University of Kentucky, KY, USA
Most corn hybrids grown in Kentucky are susceptible to southern rust, caused by Puccinia polysora. Because of the destructive potential of the disease, producers may consider applying fungicide if the disease is detected early. Unfortunately, spray thresholds that might serve as a decision guide have not been published. We obtained estimates of apparent infection rates (r) for the disease from published literature. We then used the logistic equation to model disease progress using the following parameters: initial disease estimates of 25, 125, and 250 pustules per 50 plants, r values of 0.1–0.3 logits/day, and growth stages at initial detection from R1 through R6. We found that the r value was the most influential parameter on disease progress. The timing of first detection was also important, with predicted AUDPC values for detection at R1 (anthesis) being more than double those at R2 ("blister" stage). The number of pustules at initial detection was the least influential, as compared to r value and timing of initial detection. Our analysis highlights the challenges of developing a useful action threshold for southern rust. Those challenges include 1) the primacy of the r value in driving disease progress, a parameter that cannot be accurately forecasted for a given field; and 2) the difficulty of associating forecasted AUDPC values with economically significant yield losses.