CHARACTERIZATION OF GRAY LEAF-SPOT EPIDEMICS OF MAIZE

Spread of gray leaf spec caused by Cercospora zeae-maydis, from infect ed maize residue on the soil surface and progress over time were studi ed in Ohio (U.S.A.) under low, medium, and high maize plant density du ring 1 year which was favorable for disease development and another ye ar that was less favorable. In both years, disease spread was adequate ly described by the exponential model, corrected for multiple infectio ns, whereas disease progress over time was equally well represented by the exponential, Gompertz, and logistic models. The main effect of pl ant 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 (Y-m) (P > 0.10), in both years. Under favorabl e environmental conditions (1990), disease gradients were steeper, and the time x plant density and time x direction interaction effects wer e highly significant (P < 0.001) for Y-m. In general, Y-m was inversel y related to plant density, but the effect changed during the season. A direction effect on b and Y-m was due to a dominant prevailing wind direction. Disease gradients remained fairly constant over time and we re relatively shallow, suggesting potential long distance spread. Unde r less favorable environmental conditions (1991), disease gradients we re shallow at the beginning, but tended to become stepper at the end o f the season, after supplemental irrigation. There was no significant effect of direction (P > 0.15) on Y-m and a, and a marginal significan ce (P = 0.05) on b. Under favorable and unfavorable weather conditions , the b's ranged from -0.15 ro -0.56/m and -0.04 to -0.38/m, respectiv ely. The intercept (y(o)) and slope (r) of the linearized logistic di sease progress model did not differ (P > 0.03) among plant densities, in both years. Under favorable conditions, direction affected both y(o ) and r where the disease progressed slowest to the south and fastest to the north side of the inoculum source, an indirect association wit h wind direction. The r-values declined with distance from the inoculu m source, suggesting that the infected residue was the major source of inoculum during the epidemic. The rate of disease progress ranged fro m 0.13-0.17/day and 0.02-0.06/day under favorable and unfavorable cond itions, and the final mean number of lesions per leaf (Y-m) ranged fro m 24.4-3.8 and 0.7-1.5, respectively.