Influence of infected corn residue as a source of primary inoculum of Cercospora zeae-maydis (Tehon and Daniels) the cause of gray leaf spot of corn

The influence of infected corn residue on the epidemiology of gray leaf spot was studied in Ohio. The overwinter survival of the pathogen in infected tissue was evaluated in samples maintained on the soil surface, or buried 5 to 10 cm, during winter and spring of 1990 and 1991. Survival was measured as the ability of the pathogen to sporulate on infected tissues following exposure in the field for different periods of time from December to May, or June. Sporulation declined to zero for samples collected in May on buried infected tissues. Samples of conidia from leaf blades in May 1990 and June 1991 had 50-80% germination. The effect of the amount of soil-surface residue (0, 10, 35, and 85%soil coverage) on the progress of gray leaf spot of corn over time was also studied during 1990 and 1991. The relationship between disease severity and amount of residue on the soil surface was positive, significant at P<0.001 in 1990, but not significant (P=0.096) in 191. Final severity for 1990 and 1991 was 12.6-118.3 and 0.4-3.2 lesions per leaf, respectively. Area under the disease progress curves and r-values for the 85% residue level were clearly higher than the other levels under favorable conditions. The r-values ranged from 0.12-0.19/day and 0.01-0.04/day in 1990 and 1991, respectively. Spread of gray leaf spot from infected corn residue on the soil surface and disease progress over time were studied under low, medium, and high corn plant densities in 1990 (favorable) and 1991 (unfavorable environmental conditions). Disease spread was described by the exponential model, and disease progress over time could be described by the exponential, Gompertz, or logistic model. The main effect of plant density was not significant for the intercept (a) and slope (b) of the linearized disease gradient (P>0.20), and for the mean disease severity per direction (Ym) (P>0.10), in both years. In 1990 the time x plant density and time x direction interaction effects were highly significant (P<0.001) for Ym. In 1991, gradients became steeper at the end of the season. There was no significant effect of direction (P>0.15) on the mean disease severity and intercept, and a marginal significance (P=0.052) for the slope.