Biophysical scaling and the passive dispersal of fungus spores: relationship to integrated pest management strategies

Successful integrated pest management strategies depend on an accurate evaluation of 'immigrant' inoculum coming into a managed area. Dispersal of plant pathogenic fungus spores is comprised of a series of inter-connected events. Starting with spore production, dispersal depends, in turn, on spore release or removal from a substrate, spore escape from the canopy space, transport and dilution of a spore cloud by turbulent wind, loss of inoculum viability during transport, spore removal from the atmosphere by precipitation, spore deposition on host tissue, and the infection efficiency of deposited spores on susceptible host tissue. All of these components change in time and space, challenging the aerobiologist to identify critical biophysical processes that control disease spread. Some of these processes will be illustrated for two plant diseases: apple scab caused by the pathogen Venturia inaequalis and tobacco blue mold caused by the pathogen Peronospora tabacina. Both these pathogens are dispersed by airborne spores. Ascospores of the apple scab pathogen become airborne primarily during rain events, while P. tabacina sporangia become airborne primarily during dry, convective conditions. Initial success of dispersal depends on, wind gusts which allow escape of spores from a crop canopy or the boundary layer of air just above the ground. Microclimatic conditions at the time (e.g. day or night, rain or no rain) and the location of spore release (upper or lower canopy) can have a significant effect on subsequent dispersal of pathogens. The rate of advance of a disease frontal boundary during long-distance disease spread can be as much as six times faster than the local rate of disease spread. This apparent anomaly will be discussed using tobacco blue mold as an example. The rate of movement of the disease front in a spatially heterogeneous distribution of hosts, is determined largely by the relative rate of local disease development, the length scale of the dispersal function, distance between regions of host plants, and the area of those host regions. Factors such as ground transportation of diseased transplants and changes in the pathogen's sensitivity to fungicides can, at times, override the biophysical constraints on long-distance spore dispersal. (C) 1999 Elsevier Science B.V. All rights reserved.