Some plant pathogens rarely, if ever, survive the winter season in northern latitudes of the mid-Atlantic and Northeastern United States. These pathogens are introduced anew into these growing regions from inoculum sources in the South. Tobacco blue mold (caused by Peronospora tabacina Adam) is an example. It occurs in the Southeastern U.S. in most years, but appears in northern growing regions on an infrequent basis. Blue mold appeared in Connecticut USA in significant amounts in only four out of the last 40 years (1979, 1980, and 1996, 1997). The long intervals between outbreaks indicate that long-range aerial transport of blue mold to Connecticut is an unlikely event. The probability that long-distance aerial transport of spores will lead to disease spread depends on the combined effect of several biometeorological factors. These include 1) the number of spores released at the source, 2) favorable wind direction, 3) airborne travel time, 4) dilution of the spore cloud by turbulence, 5) loss of viable inoculum from the air column either by mortality or by physical removal by wet and dry deposition processes, and 6) susceptible host and conditions favorable for infection at the target.
The four most recent pandemics of tobacco blue mold in the eastern U.S. were examined. The dates of disease onset in growing areas extending from about 29 to 42 degrees N latitude were obtained from first confirmed reports of disease. The northward spread of disease was represented by trajectories in distance-time space, and the slopes of these trajectories were taken to represent the rate of progression of the disease front. Progress of the pandemics was described by a combined infection threshold-finite leap model. The model expresses the rate of advance of the disease front in terms of: disease infection rate, length of the pathogen's latent period, size of the crop areas, distance between crop areas, wind speed and turbulent diffusivity, and time scales for removal of airborne spores by rain and loss of spore viability due to exposure to ultraviolet rays in sunlight. The dispersal function used contains a self-similar term due to turbulent atmospheric dispersion and an exponential term due to scrubbing by rain and spore mortality. The average rate of blue mold spread from south to north was about 15, 13, 11, and 19 km per day in 1979, 1980, 1996, and 1997, respectively. The model mimicked the observed rates of disease spread using realistic values of the model parameters. The effect on rate of disease spread of complicating factors such as ground transportation of diseased transplants and changes in the fungus response to temperature and its sensitivity to fungicides will also be discussed.