Septoria Leaf Spot Lesion Density on Trap Plants Exposed at Varying Distances from Infected Tomatoes

Abstract

Twelve trap plant experiments were conducted over 6 years to examine the spatial dispersal of conidia of Septoria lycopersici (causal agent of Septoria leaf spot) during and immediately following rain events. The leaf area of each trap plant was determined before exposure. Trap plants were placed out in the field for 1 to 5 days at perpendicular distances of 0.5 to 106 m from rows (29 to 67 m long) of inoculated tomato plants (Lycopersicon esculentum). Trap plants were retrieved and then incubated for 10 to 14 days in a greenhouse. Septoria leaf spot lesions were then counted, and disease severity was expressed as the number of lesions per unit leaf area (lesions/m 2 ). Disease severity was high (10 3 to 10 6 lesions/m 2 ) within a meter of the source and low (10 to 10 3 lesions/m 2 ), but detectable at distances of up to 106 m from the line source. The logarithm of the resulting disease severity on trap plants was found to be well correlated (r 2 = 0.59 to 0.98, df = 10 to 30) with the logarithm of distance from the line source, suggesting a dispersal function described by an inverse power law of distance. The observed transport at long distance suggests that at least some conidia are carried in very small rain droplets or secondary splash droplets. This long-range dispersal of spores may have a major impact on the epidemiology of this disease.