Host resistance and pathogen conidial characteristics across location of Philippine corn downy mildew

Twenty-five corn inbred lines from the Centro Internacional de Mijoramiento de Maizd y Trigo (CIMMYT) (International Maize and Wheat Improvement Center) and three aditional lines from the Institute of Plant Breeding (IPB), University of the Philippines Los Baños (UPLB) were evaluated for downy mildew reaction in disease nurseries located at UPLB, Laguna and Koronadal, South Cotabato. The study determined the response of corn inbred lines to different mildew pathogen populations. The source of pathogen spores to infect the plants was susceptible sweetcorn variety planted around blocks of entries 2 weeks ahead of the test entries and naturally or artificially infected with the local corn downy mildew inoculum. Based on percent disease incidence 21 days after plant emergence, three plants were found resistant, four moderately resistant and 16 susceptible in UPLB while two were resistant, seven were moderately resistant and 19 were susceptible in Koronadal, South Cotabato. The two most resistant lines identified in both locations were Nei 9008 and P345C4S2B46-2-2-1-2-B-B-B. All components of resistance that were evaluated such as percent disease incidence, percent disease severity, sporulation capacity and the rate of disease development showed significant differences among entries. Resistant lines had lower downy mildew incidence, reduced severity of infection and decreased rate of disease development than the susceptible inbreds. Susceptible lines sporulated more than the resistant lines. Spore germination was higher in Cotabato than in UPLB. Spore production was found to be affected not only by genotype and moisture but also by the number of hours the inflected plants were exposed to daylight. Differences in conidial size and shape were detected. At different locations in Luzon and Mindanao using susceptible trap lines.