Development of greenhouse inoculation procedures for evaluation of partial resistance to Cercospora zeae-maydis in maize inbreds

Greenhouse experiments were conducted to determine the effects of inoculation methods on incubation period, lesion length, percentage leaf area affected and sporulation of Cercospora zeae-maydis on young maize (Zea mays L.) plants inoculated at V3 growth stage. Seedling plants were inoculated by four methods: (i) application of conidial suspension while puncturing the leaves within the whorl several times, (ii) spraying conidial suspension on leaves, (iii) placing colonized agar into lateral slits in leaves and (iv) placing colonized agar into whorls. Analysis of variance revealed a significant effect of genotype and inoculation method on several components of resistance and overall disease severity. Application of conidial suspension while puncturing the whorl was found to be the least laborious method, and it produced characteristic symptoms of gray leaf spot. Consistent trends were observed in classification of inbreds to resistant, susceptible and intermediate classes. Increasing the duration of exposure to high humidity by placing plastic bags over plants for 5 days significantly increased disease severity (P <= 0.001). Cercospora zeae-maydis produced conidia in all the lesions examined. Spore production was generally most abundant in lesions on susceptible inbreds that displayed necrotic lesion types (LT) and least abundant in lesions on resistant inbreds that were characterized by chlorotic and fleck LTs. The results demonstrated that inoculations in the greenhouse can provide an indication of inbred responses to C. zeae-maydis and may be useful in evaluating resistance and in studies of host-pathogen interactions.