Relationships between common rust incidence and severity on a susceptible and a partially resistant sweet corn hybrid

Incidence and severity relationships for common rust on a susceptible (Florida Staysweet) and a partially resistant (Sugar Loaf) sweet corn hybrid were examined for data where incidence was measured as the percentage of leaves per plant with at least one erupted uredinium and severity was assessed as the percentage of the total leaf area infected per plant using the Petersen scale. Relationships were similar for the two hybrids and were explained by a cubic regression of untransformed data with nonuniform variances with an intercept that was not significantly different from zero: Y=0.072(X)-0.00177(X)2+0.0000145(X)3 where Y= predicted rust severity (%) and X = rust incidence (%). Severity was relatively constant at 1% from 20 to 60% incidence but increased from 60 to 100% incidence. Variances also increased above 60% incidence. Severities of 1,2,3, and 4% were predicted from the regression of untransformed data at approximately 52,82,93, and 95% incidence, respectively. For square-root-transformed severity with variances that were more homogenous, the relationship was: Y = 0.218 + 0.0516(X)-0.00104(X)2 + 0.000007(X) where Y= predicted percent severity 1/2 and X = percent incidence. Severities of 1,2,3, and 4% (severity 1/2 of 1, 1.414, 1.732, and 2, respectively) were predicted at 28,86,95, and 100% incidence, respectively, for the regressions of severity 1/2. Slope coefficients of severity 1/2 regressed on incidence for each hybrid at different times (rating dates) were different probably because of the ranges of incidence over which severity 1/2 was regressed. At an 80% incidence action threshold for fungicidal control, predicted rust incidence and severity at 7 days (one latent period) after initiation of control were 99-100% and 3.5-7%, respectively, for Florida Staysweet and 85-95% and 3-4%, respectively, for Sugar Loaf. Considering the rapid development of rust on Florida Staysweet and the proximity of the 80% incidence action threshold to damaging disease levels, a lower threshold may be warranted for susceptible and moderately susceptible hybrids grown in rust-conducive environments.