Effects of Temperature on Germination of Peronospora parasitica Conidia and Infection of Brassica oleracea

The effect of temperature on germination of a South African isolate of Peronospora parasitica, and infection of Brassica oleracea was studied. The optimum condition for germination was 2OʹC at 100% relative humidity. The percentage germination obtained was 80-98% and 70- 80% between 15 and 25 C at 100% relative humidity, after a 12 and 6h incubation period, respectively. Optimum temperature for germ tube growth was also 20 C. The temperature range for maximum infection of seedlings of a highly susceptible cabbage cultivar and subsequent disease development in vitro was 15-25 C and 90-100% infection was achieved after 48 h of incubation. At < 15 C and 26-30 C infection percentage was decreased to 40-50% and 35-40%, respectively. No disease incidence was recorded at temperatures above 35 C. A scanning electron microscope study of the infection process showed that penetration of cotyledons by germ tubes was mostly via stomata and occasionally directly throtigh the cuticle. Results are discussed in relation to the need for future studies of P. parasitica in South Africa.