STUDIES ON DISPERSAL OF SEPTORIA-TRITICI PYCNIDIOSPORES IN WHEAT-CLOVER INTERCROPS

Four experiments studied the effects of a clover understorey on pycnid iospore dispersal of Septoria tritici in a wheat-clover intercrop unde r simulated rain. Clover significantly reduced the dispersal of spores in a horizontal direction by 33% at a distance of 15 cm from a line i noculum source compared with a wheat monocrop. The clover also reduced the vertical movement of spores from infected leaves at the base of w heat plants by an average of 63 % compared to the monocrop, and this s uggests that the main movement of spores was from the base upwards. Sp lash experiments using blue colour marker showed the vertical decline of splash and the number of drops per cm(2) with height caught on pape r 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. Simula ted rain-splash increased the level of disease on the nag leaf and, in one experiment, there was a significant interaction between rain-spla sh and clover in reducing the number of lesions on the flag leaf. The level of disease resulting from one splash event was low, indicating t hat subsequent pathogen multiplication is probably required to bring a bout high severities of disease.