The roles of ascospores and conidia of Pyrenopeziza brassicae in light leaf spot epidemics on winter oilseed rape (Brassica napus) in the UK

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 conidi a 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 p etioles with apothecia decreased with increasing distance into the crop fro m the edge of pathways. Air-borne ascospores of P. brassicae were first col lected above debris of oilseed rape affected with light leaf spot on 5 Octo ber 1998 and 18 September 1999, 12 or 23 days, respectively, after the debr is had been exposed outdoors. P. brassicae conidia were first observed on l eaves of winter oilseed rape on 6 January 1999 and 15 February 2000, respec tively, after plots had been inoculated with debris in November 1998 and Oc tober 1999. In 1991/92, numbers of ascospores above a naturally infected cr op were small from January to April and increased in June and July. P. bras sicae conidia were first observed in February and the percentage plants wit h leaves, stems or pods with light leaf spot increased greatly in May and J une. In 1992/93, in a crop inoculated with debris, numbers of airborne asco spores were small from October to January and increased from April to June. P. brassicae conidia were first observed on leaves in late November and li ght leaf spot was seen on stems and pods in March and June 1993, respective ly.