SPREAD OF PLANT DISEASE ON A CONTINENTAL SCALE: ROLE OF AERIAL DISPERSAL OF PATHOGENS

Successful transmission of plant diseases over long distances through the atmosphere depends on the reproductive rate of the pathogens, on the carrying capacity of the source locality, on atmospheric turbulence, stability, and wind speed, and on the survival of spores during exposure to inhospitable temperature and humidity and to UVB radiation from the sun. These interacting factors were incorporated into a model to estimate the rate and extent of seasonal incursions of disease from southern into northern areas of the United States. The model indicates a practical limit for long-distance dispersal (LDD) of a plant pathogen, which depends strongly on its fecundity and on its ability to survive in the atmosphere. Two classic plant diseases, stem rust of wheat and tobacco blue mold, are used to illustrate the model. Both diseases appear to spread northward on average at about the same rate as the seasonal advance of the "green wave" of available susceptible host tissue. The near concordance of the disease wave and the green wave underscores two important points. First, it suggests that disease spread over long distances may be limited more often by pathogen establishment than by LDD. The green wave also reduces the stochastic variability and speed of disease spread by presenting a barrier to potential long-distance, low-probability dispersal events. Second, it helps to focus attention on alternative pathways for disease spread and on possible unappreciated niches for overseasoning, both of which can have important implications for disease control strategies.