Experiments in controlled environments were carried out to determine the effects 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 208C, but not at 248C; spore pustules developed when the leaf wetness duration after inoculation was longer than or equal to approximately 6 h at 12±208C, 10 h at 88C, 16 h at 68C or 24 h at 48C. On leaves of cvs. Capricorn or Cobra, light leaf spot symptoms developed at 8 and 168C when the leaf wetness duration after inoculation was greater than 3 or 24 h, respectively. The latent period (the time period from inoculation to first spore pustules) of P. brassicae on cv. Bristol was, on average, approximately 10 days at 168C when leaf wetness duration was 24 h, and increased to approximately 12 days as temperature increased to 208C and to 26 days as temperature decreased to 48C. At 88C, an increase in leaf wetness duration from 10 to 72 h decreased the latent period from approximately 25 to 16 days; at 68C, an increase in leaf wetness duration from 16 to 72 h decreased the latent period from approximately 23 to 17 days. The numbers of conidia produced were greatest at 12±168C, and decreased as temperature decreased to 88C or increased to 208C. At temperatures from 8 to 208C, an increase in leaf wetness duration from 6 to 24 h increased the production of conidia. There were linear relationships between the number of conidia produced on a leaf and the proportion of the leaf area covered by`lesions' (both log 10 -transformed) at different temperatures.
Ascospores of Pyrenopeziza brassicae were produced in apothecia (cup-shaped ascomata) on oilseed rape debris. The conidia, which were morphologically identical to the ascospores, were produced in acervular conidiomata on the surface of living oilseed rape tissues. Ascospores were more infective than conidia on oilseed rape leaves. The proportion of lesions caused by conidia located on leaf veins was greater than for lesions caused by ascospores. In June 2000, on the ground under a crop with light leaf spot, numbers of petioles with apothecia decreased with increasing distance into the crop from the edge of pathways. Air-borne ascospores of P. brassicae were first collected above debris of oilseed rape affected with light leaf spot on 5 October 1998 and 18 September 1999, 12 or 23 days, respectively, after the debris had been exposed outdoors. P. brassicae conidia were first observed on leaves of winter oilseed rape on 6 January 1999 and 15 February 2000, respectively, after plots had been inoculated with debris in November 1998 and October 1999. In 1991/92, numbers of ascospores above a naturally infected crop were small from January to April and increased in June and July. P. brassicae conidia were first observed in February and the percentage plants with leaves, stems or pods with light leaf spot increased greatly in May and June. In 1992/93, in a crop inoculated with debris, numbers of airborne ascospores were small from October to January and increased from April to June. P. brassicae conidia were first observed on leaves in late November and light leaf spot was seen on stems and pods in March and June 1993, respectively
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