Influence of plant maturity and plant population density in the epidemiology of gray leaf spot of corn

The role of canopy development in the epidemiology of gray leaf spot (GLS) disease of corn (Zea mays L.) caused by Cercospora zeae-maydis was studied by manipulating plant maturity (planting date), plant population density (PPD). Physiological and environmental changes related to altered plant canopy development, including changes in plant nutrient availability and light penetration were also studied.;Significant differences in epidemic development occurred between timely and late planted corn at the endemic location. Higher area under the disease progress curve (AUDPC) and faster epidemic rates were observed for timely rather than late planted corn, but the epidemic onset was the same. However, no differences in disease severity occurred between planting dates at the non-endemic location, where initial inoculum was introduced. Therefore, plant age did not directly influence disease development, but delayed planting slowed the epidemic rate by affecting the infection cycle components related to initial inoculum level and spore dispersal.;Nitrogen amendments significantly increased leaf nitrogen content (LNC) and AUDPC at low PPD at the endemic location. Thus, increased LNC may have influenced host susceptibility or pathogen reproduction. Changes in other nutrients and water availability may have also occurred and influenced disease development at low PPD.;Increased light penetration at low PPD correlated with disease development. Disease incidence and light penetration were greatest for low PPD-unshaded plots and lowest for high PPD-shaded plots. Intermediate levels of disease were observed in both low PPD-shaded plots and high PPD-unshaded plots. Thus, increased light penetration may increased activation of the toxin cercosporin.;High PPD did not provide a more conducive microenvironment for GLS development. Although periods of high RH greater than 12 hr were more common in high PPD plots, no increase in late season GLS occurred. Separate studies demonstrated that greater disease at low PPD was associated with reduced plant-to-plant resource competition and increased light penetration. Light may have a greater influence on disease than nutrient availability, since the effect of LNC on disease was significant for only one of the 3 years, while differences due to light penetration occurred each year.