D Aylor

Connecticut Agricultural Experiment Station, New Haven, Connecticut, USA

Background and objectives
Some plant pathogens rarely, if ever, survive the winter season in northern latitudes of the mid-Atlantic and north-eastern USA. These pathogens are introduced anew into these growing regions from inoculum sources in the south. Tobacco blue mould (caused by Peronospora tabacina Adam) is a persistent problem in the south-eastern USA and occurs in most years. The disease appears in northern growing regions on an infrequent basis. Blue mould appeared in Connecticut (4175' N, 7266' W) in significant amounts in only four out of the last 40 years (1979, 1980, and 1996, 1997); each time it caused serious economic losses. The long interval (approximately 15 years) between outbreaks underlines the conclusion that long-range transport of this disease to Connecticut is an unlikely event. The purpose of this study is to identify the biophysical and atmospheric parameters that govern long-distance transport of inoculum and use these to estimate limits on the rate of northward spread of disease from inoculum sources in the south.

There are two ways that the fungus migrates. The first is by spores being carried on the wind and the second is by the movement by people of infected transplants. The probability that long-distance aerial transport of spores will lead to disease spread depends on the combined effect of several factors [1]. These include (1) the number of spores released at the source, (2) favourable 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 favourable for infection at the target.

Materials and methods
The four most recent pandemics of tobacco blue mould in the eastern USA were examined. The dates of disease onset in growing areas extending from about 29 to 42  N were obtained from first confirmed reports of disease. The northward spread of disease was represented by trajectories in distance-time-space. The slopes of these trajectories 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 apparent infection rate, length of the latent and infectious periods, size of the crop areas, distance between crop areas, wind speed and turbulent diffusivity, time scales for removal of airborne spores by rain and loss of spore viability owing to exposure to UVB. 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 [2].

Results and conclusions
The average rate, R, of blue mould spread from south to north was about 15, 13, 11, and 19 km/day in 1979, 1980, 1996, and 1997, respectively. The model mimicked the observed rates of disease spread using realistic values of the model parameters. A slight concave downward curvature of the observed disease front trajectories could be explained, in part, by the decrease in host plant acreage with more northerly latitudes. The values of R were not too different from the average rate of northward movement of the mean monthly 12.7C isotherm (green wave of planting) and in at least one year (1980) appeared to be constrained by it. The effect on R of complicating factors, such as ground transportation of diseased transplants and changes in the fungus response to temperature and its sensitivity to fungicides, will be elucidated.

1. Aylor DE, 1986. Agricultural and Forest Meteorology 38, 263-88.
2. Aylor DE, Sutton TB, 1992. Phytopathology 82, 532-40.