IACR-Rothamsted, Harpenden, Herts, UK

Background and objectives
Stem canker (Leptosphaeria maculans/i>) is one of the most damaging diseases of winter oilseed rape in the UK. Epidemics are initiated in autumn by airborne ascospores released from infected debris [1]. These ascospores infect leaves to produce phoma leaf spots and the pathogen then grows down the leaf petioles into the stem. However, few experiments on the disease have used ascospores as inoculum for infecting oilseed rape leaves. In most experiments, conidia have been used to inoculate leaves, which have had to be wounded beforehand because conidia cannot usually infect healthy leaves. Furthermore, there is little evidence that conidia have any role in the epidemiology of stem canker in winter oilseed rape crops in the UK. The severity of stem canker epidemics in the summer may depend on when leaf spots appear in the autumn, which is affected by the occurrence of weather favourable for infection by the ascospores. The effects of temperature and leaf wetness duration on infection of winter oilseed rape leaves by ascospores of L. ;maculans will be discussed.

Materials and methods
Oilseed rape plants (cv. Nickel) were grown in pots filled with a peat-based compost. Mature pseudothecia of L. ;maculans were excised from naturally infected stem debris then crushed in water with a pestle and mortar to release the ascospores. Plants with four expanded leaves (GS 1,4) were inoculated by spraying with an ascospore suspension (1000 spores/ml). Treatments were temperature (growth cabinets at 5, 8, 12, 16, 20 or 24C) and leaf wetness duration (4, 8, 16, 20, 24, 30 or 48 ;h after inoculation, maintained by covering plants with polyethylene bags). Experiments were arranged in a split-plot design with cabinets at different temperatures as the main plots replicated in time (two replicates) and leaf wetness duration treatments (six plants per treatment) arranged randomly in each cabinet. Numbers of new phoma leaf spot lesions which appeared were counted daily until no new lesions appeared. Data were ln-transformed and analysed by residual maximum likelihood analysis to investigate the effects of temperature and wetness duration on the maximum number of lesions and on the leaf spot incubation period (estimated as time from inoculation to the appearance of one lesion or 50% of the lesions) [2].

Results and conclusions
Both temperature and leaf wetness duration affected the maximum number of lesions which developed, but the interaction between them was not significant. Most lesions developed at 20C with a 48-h leaf wetness duration. Numbers of lesions were less at 24 than at 20C and decreased with decreasing temperature to 8C. Only three lesions developed at 5C.The numbers of lesions decreased as leaf wetness duration decreased from 48 to 4 ;h; with a leaf wetness duration of only 4 ;h very few lesions developed. Temperature, leaf wetness duration and their interaction all affected the length of the incubation period. The incubation period decreased greatly with increasing temperature from 8 to 24C and decreased slightly with increasing leaf wetness duration from 4 to 20 ;h but was not affected by increasing leaf wetness duration after 20 ;h.

This information about criteria for infection of oilseed rape leaves by ascospores of L. ;maculans can be used, in combination with information about the development and release of ascospores, in the construction of schemes for forecasting the development of phoma leaf spot in winter oilseed rape crops in the autumn and for predicting the severity of stem canker epidemics in the summer in the UK.

1. Gladders P, Musa TM, 1980. Plant Pathology 29, 28-37.
2. Hong CX, Fitt BDL, Welham SJ, 1996. Plant Pathology 45, 1077-1089.