EARLY DEVELOPMENT OF LIGHT LEAF-SPOT (PYRENOPEZIZA-BRASSICAE) ON WINTER OILSEED RAPE (BRASSICA-NAPUS) IN RELATION TO TEMPERATURE AND LEAF WETNESS

In controlled environment experiments to study early development of light leaf spot, lesions developed with leaf wetness durations of 16 to 48 h after inoculation of oilseed rape with conidial suspensions of Pyrenopeziza brassicae at 12 or 18 degrees C, but not with leaf wetness durations of 0 to 13h. The incubation period was 21 to 22 days at 12 degrees C and 14 to 18 days at 18 degrees C for leaf wetness durations of 16 to 48 h. The latent period was 21 to 23 days at 12 degrees C and 18 to 19 days at 18 degrees C, and the total number of lesions increased with increasing leaf wetness duration at both temperatures. In field experiments, light leaf spot always developed on oilseed rape with a leaf wetness duration of 48 h after inoculation in both 1990/1991 and 1991/1992, but the percentage leaf area affected was less an plants placed in an oilseed rape crop than on those placed in a glasshouse. Plants moved to an oilseed rape crop immediately after inoculation nearly always developed light leaf spot symptoms when they were inoculated between 19 October 1990 and 1 March 1991 or between 27 September 1991 and 14 February 1992, but plants inoculated between 31 August and 16 October 1990 or on 20 September 1991, when estimated leaf wetness duration was less than 16 h for several days after they were placed in crops, did not develop symptoms. The latent period of light leaf spot on plants transferred to the oilseed rape crop was 15 to 40 days, and there was an approximately linear relationship between 1/(latent period) and mean temperature during this period. The accumulated temperature during the latent period ranged from c. 150 to 250 day-degrees. The severity of lesions on these plants increased with increasing temperature from 5 to 15 degrees C.