INITIAL SPREAD OF BEAN RUST CLOSE TO AN INOCULATED BEAN LEAF

Foci of bean rust were established in a bean field by inoculating single trifoliolate leaves with urediniospores of Uromyces appendiculatus. The initial spread of disease from these foci was quantified by counting the number of pustules on entire plants within 3 m of the inoculated plant. A model was derived for spore deposition close to a source based on the assumption that spores move in approximately straight line trajectories within a few meters of a source. This model leads to an equation of the form P1 = A(1 + s2/L2)-1, in which P1 is the number of pustules per leaf on individual plants, A and L are parameters of the model, and s is distance from the source. The observed primary spread of bean rust was described well by the model, with the parameter L determined from the size of the plants. The scaling parameter A is related to the number of spores released at the source and is the only adjustable parameter in the model.