Studies on dispersal of Septoria tritici pycnidiospores in wheat-clover intercrops

Four experiments studied the effects of a clover understorey on pycnidiospore dispersal of Septoria tritici in a wheat-clover intercrop under simulated rain. Clover significantly reduced the dispersal of spores in a horizontal direction by 33% at a distance of 15 cm from a line inoculum source compared with a wheat monocrop. The clover also reduced the vertical movement of spores from infected leaves at the base of wheat plants by an average of 63 % compared to the monocrop, and this suggests that the main movement of spores was from the base upwards. Splash experiments using blue colour marker showed the vertical decline of splash and the number of drops per cm(2) with height caught on paper adjacent to trays of clover was described by the exponential decline model. The effect of clover in reducing vertical splash approached an asymptote as the leaf area index of the understorey increased. Simulated rain-splash increased the level of disease on the nag leaf and, in one experiment, there was a significant interaction between rain-splash and clover in reducing the number of lesions on the flag leaf. The level of disease resulting from one splash event was low, indicating that subsequent pathogen multiplication is probably required to bring about high severities of disease.