2.1.7
THE EFFECTS OF RAINFALL ON NUMBERS OF AIR-BOURNE ASCOSPORES OF LEPTOSPHAERIA MACULANS AND THE DEVELOPMENT OF PHOMA LEAF LESIONS ON WINTER OILSEED RAPE

J WEST, P LEECH and B FITT

IACR, Rothamsted, Harpenden, UK

Background and objectives

Stem canker, caused by the fungus Leptosphaeria maculans, is an important disease of brassica crops including winter oilseed rape [1]. In the UK, most disease epidemics are initiated by ascospores, which are produced on crop residues and may be transported by wind up to several kilometres. Under favourable conditions, ascospores are able to infect leaves to produce phoma leaf lesions from which the fungus may spread to infect the stem of the plant. These stem infections eventually develop into the characteristic stem cankers. Leaf lesions may also produce pycnidia containing asexual spores (conidia) but there is little evidence that conidia cause a secondary cycle of leaf lesions in the UK. Currently, fungicides are effective against stem canker only when they are applied before or during early stages of leaf infection [2]. Therefore, the accurate prediction of peaks in numbers of ascospores released, would be an important tool to enhance the timing of fungicide applications.

Materials and methods
Air-borne ascospores of L. maculans were collected over several seasons using a Burkard spore sampler and daily counts of spore numbers were made. The periodicity in numbers of air-borne spores with time was compared with meteorological data, particularly rainfall, recorded over the same periods. In each season, the relationship between rainfall and spore numbers was examined. The incidence and severity of leaf lesions in untreated plots of winter oilseed rape was then related to the numbers of air-borne ascospores, recorded each day, from autumn to spring.

Results and conclusions
The overall numbers of air-borne ascospores collected, varied from season to season, probably owing to differences in the quantity of inoculum and its proximity to the spore sampler. However, the patterns of spore release clearly demonstrated that, for most of the season, spore release occurred after rainfall. In the autumn, however, rainfall did not cause spores to be released from new crop residue (from the last crop) while the spores were still developing. However, some minor spore releases from older inoculum sources did occur at this time. The period of time required for ascospore development, before substantial releases of spores occurred, was also related to climatic factors. The incidence of phoma leaf lesions in untreated field plots was related to spore numbers in the air. A mathematical model linking climate to leaf lesion incidence is currently under development.

References
[1] Biddulph JE, West JS, 1998 Leptosphaeria maculans In: Crop Disease Compendium. Wallingford: CABI (in press) [2] Gladders P, Symonds BV, Hardwick NV, Sansford CE, 1998. Integrated Control in Oilseed Crops: IOBC Bulletin (In press)