Micrometeorological determination of release rate of Venturia inaequalis ascospores from a ground-level source during rain

Abstract

Apple scab, caused by the fungus Venturia inaequalis, is an important disease of apples throughout the world. Disease epidemics are initiated in spring time by ascospores, which are discharged from infected leaves on the ground during periods of rain. The probability of new infections for such an aerially dispersed plant disease depends on the aerial concentration of spores around the plant. A major difficulty in predicting concentration lies in determining the rate of release, Q, of spores from a source. The rate of release of V. inaequalis ascospores was determined using two methods. The first method involves the measurement of vertical profiles of ascospore concentration and wind speed above a source in the field, which were used to obtain profiles of horizontal spore flux. The vertical integration of the horizontal fluxes was used in conjunction with a mathematical model for conservation of spore numbers to estimate Q during natural release events. The second method employed a spore tower in the laboratory to measure the ascospore release potential from source leaves collected twice weekly from the field sources. The two independent methods yielded values for the cumulative total seasonal release of ascospores which agreed to within a factor of two for a source in a grass field and to within a factor of three for a source in an orchard of dwarf apple trees. The spore tower method offers a relatively simple yet quantitative means for estimating the release of ascospores from field sources and should aid in modeling disease spread and in making management decisions.