Effects of temperature and wetness duration on conidial infection, latent period and asexual sporulation of Pyrenopeziza brassicae on leaves of oilseed rape

Experiments in controlled environments were carried out to determine the ef fects of temperature and leaf wetness duration on infection of oilseed rape leaves by conidia of the light leaf spot pathogen, Pyrenopeziza brassicae. Visible spore pustules developed on leaves of cv. Bristol inoculated with P. brassicae conidia at temperatures from 4 to 20 degrees C, but not at 24 degrees C; spore pustules developed when the leaf wetness duration after in oculation was longer than or equal to approximately 6 h at 12-20 degrees C, 10 h at 8 degrees C, 16 h at 6 degrees C or 24 h at 4 degrees C. On leaves of cvs. Capricorn or Cobra, light leaf spot symptoms developed at 8 and 16 degrees C when the leaf wetness duration after inoculation was greater tha n 3 or 24 h, respectively. The latent period (the time period from inoculat ion to first spore pustules) of P. brassicae on cv. Bristol was, on average , approximately 10 days at 16 degrees C when leaf wetness duration was 24 h , and increased to approximately 12 days as temperature increased to 20 deg rees C and to 26 days as temperature decreased to 4 degrees C. At 8 degrees C, an increase in leaf wetness duration from 10 to 72 h decreased the late nt period from approximately 25 to 16 days; at 6 degrees C, an increase in leaf wetness duration from 16 to 72 h decreased the latent period from appr oximately 23 to 17 days. The numbers of conidia produced were greatest at 1 2-16 degrees C, and decreased as temperature decreased to 8 degrees C or in creased to 20 degrees C. At temperatures from 8 to 20 degrees C, an increas e in leaf wetness duration from 6 to 24 h increased the production of conid ia. There were linear relationships between the number of conidia produced on a leaf and the proportion of the leaf area covered by 'lesions' (both lo g(10)-transformed) at different temperatures.