Effects of temperature, cultivar and isolate on the incubation period of white leaf spot (Mycosphaerella capsellae) on oilseed rape {Brassica napus)

When leaves of oilseed rape (cv. Cobra) were inoculated with conidial suspensions of Mycosphaerella capsellae (white leaf spot) and incubated in controlled environments, the lag period from inoculation to the appearance of the first lesions decreased, and the total number of lesions produced increased, as temperature increased from 5^oC to 20^oC, although differences between 15^oC and 20^oC were small. With incubation period estimated as the time from inoculation until 5%, 50% or 95% of the lesions were produced, there was a linear relationship between l/(incubation period in days) and temperature over the range 5^oC to 20^oC, from which values at intermediate temperatures could be estimated. Summed mean daily temperatures from inoculation to the production of 5% of the lesions were estimated as 115–130 degree-days in the controlled environment experiments at 5^oC to 20^oC. When pods or leaves of plants in oilseed rape crops (cv. Cobra or cv. Libravo) were inoculated with conidial suspensions of M. capsellae on five occasions from January to October, with variable temperatures during the incubation period, degree-days until the first appearance of lesions were in the range 115–230. The numbers of white leaf spot lesions cm^-2 which developed on inoculated leaves differed greatly between nine oilseed rape cultivars, with most on cv. Tapidor and fewest on cv. Libravo, but the incubation period differed little between cultivars. Similarly, the number of lesions which developed differed between four M. capsellae isolates from different regions but the incubation period did not.