SYSTEMIC INFECTION OF DOWNY MILDEW ON MAIZE IN THAILAND AND THE EFFECT OF WEATHER FACTORS

Seven epidemics of downy mildew on maize were monitored in holistic fi eld trials on the cultivars 'CM 109' and 'SSDMR' in Thailand during 19 83-1985. In the susceptible cv. 'CM 109', practically all epidemics fo llowed a monomolecular pattern reaching 100 % systemic disease inciden ce. For the resistant cv. 'SSDMR', the curves are similar but at lower intensity levels. In addition, in a sequential planting experiment th e final incidence of systemic disease 6 weeks after planting was monit ored for 34 epidemics. Their results show that daily mean temperature had the greatest influence on systemic infection. High relative humidi ty was ranked as the second most important factor for the epidemics. A Gompertz function with two independent variables, 'daily mean tempera ture' and 'mean daily number of hours with relative humidity above 90 %', was fitted to the incidence data of the two cultivars. For the mor e susceptible cv. 'CM109', the coefficient of determination was 0.71, temperature alone explained 64 % of the variation. In experiments unde r controlled conditions, the incubation periods of systemics infection s were determined. Under more or less constant temperatures, duration of leaf wetness after infection was negatively correlated with the inc ubation period of systemic lesions which ranged from 14.4 to 11.5 days .