Splash dispersal of spores of Pseudocercosporella capsellae (white leaf spot) from oilseed rape leaves of different inclination, flexibility and age

Spore dispersal gradients were fitted well by exponential models in three experiments when simulated rain fell on to oilseed rape leaves infected by Pseudocercosporella capsellae (white leaf spot). Regressions of number (ln) of spores on distance from the source generally accounted for > 90 % of the variance. Gradients were steepest, with the smallest half-distances (5.0-5.5 cm), when the sources were younger, detached leaves placed horizontally on nylon mesh. Half-distances were 6.9 cm for younger leaves inclined at 30-degrees to the horizontal, 7.5 cm for older detached leaves placed horizontally and 8.5-10 cm for lower or upper leaves remaining attached to plants. Droplet size categories with the greatest numbers of spore-carrying droplets dispersed from a source were 0-200-mu-m diameter (43 % of the droplets) and 200-400-mu-m (30 %), respectively, for the younger and older horizontal detached leaves. The number of spores per droplet increased similarly with increasing droplet size for both sources, from 1.5 spores per droplet for the 0-200-mu-m droplets to c. 22 spores per droplet for droplets > 1 000-mu-m in diameter. The number of spores dispersed decreased exponentially with time, with a half-life of 2-5 min., estimated by regression of number (ln) of spores on time. The efficiency of dispersal, as estimated by the proportion of the spores present in a source which were dispersed in splash droplets, was greater for younger (50 % dispersed) than for older (9 %) horizontal detached leaves, greater for upper (43 %) than lower (2 %) attached leaves and greater for horizontal (78 %) than inclined (49 %) or flexible (39 %) detached leaves. In field experiments, white leaf spot spread from source plants, both vertically and horizontally, but epidemic progress was stopped by hot dry spring weather.