Abstract :
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.
