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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, lowprobability 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.

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Transmission of random noise through dense corn, a dense hemlock plantation, an open pine stand, dense hardwood brush, and over cultivated soil was measured. The relation between attenuation and frequency in these diverse cases suggested models that permit the prediction of attenuation in any configuration of vegetation and soil. The corn crop had an excess attenuation of 6 dB/100 ft for each doubling of frequency between 500 and 4000 Hz. On the other hand, the stems of the hemlock, pine, and brush all reduced noise by only about 5 dB/100 ft at 4000 Hz. Bare ground attenuates frequencies of 200–1000 Hz, and the frequency of maximum attenuation depends on the soil permeability to air. Thus, tilling the soil reduced the frequency of peak attenuation from 700 to 350 Hz and increased maximum attenuation at 52 m from the source by nearly 80%. Furthermore, earlier conflicting reports of noise attenuation by vegetation appear reconciled if ground attenuation is taken into account. Scattering and ground attenuation are the principal factors in sound attenuation by vegetation. Both factors attenuate relatively less sound as distance from the sound source increases. Hence measurements far from the source can underestimate the effect of a narrow band of vegetation or soil.

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