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Air dispersal was found to be an important means of disseminating conidia of Monilinia fructicola. In the Murrumbidgee Irrigation Areas of New South Wales only one major dispersal period occurred in each of the four years of this study. This period commenced about one month before peach harvest and continued for about two months after harvest. Greatest numbers of conidia were trapped in seasons when rain fell about harvest time. Conidial concentrations were related to the number of fruit infections present. Diurnal periodicity in spore concentration was noted, the maximum concentration of air-borne spores usually occurring in early afternoon. There was wide variation in the number of conidia present in air at different sites within orchards at a given time. Conidia could be detected in air up to 300 feet from a source orchard, but not beyond this in the one test where this point was examined. Rainfall, humidity, windspeed, temperature, and dew records were compared with concurrent spore trappings, and possible relationships noted. The practical significance of the results is discussed in relation to chemical control and the culture of varieties of differing maturity.

In a typical season brown rot development in canning peaches on the Murrumbidgee Irrigation Areas was found to follow a basic pattern. There is little or no blossom blight or post-blossom infection. Infection of fruit is first seen in January and increases until harvest. Infections of fruit during the preharvest period are almost all initiated in injuries. Damage caused by larvae of the oriental fruit moth is the most important form of fruit injury. Infections of injured immature fruits may occur in dry weather and cannot be prevented by the application of fungicides. These injured rotting fruits produce inoculum which can start epiphytotics if rain falls when the fruit is ripe. Blighted blossoms are usually unimportant as inoculum sources at this stage, but they are important links in the infection chain since they can provide inoculum for the first infections of injured fruits. Dried fruit beetles were found to be vectors during the preharvest build-up of the disease, and there was some evidence that they may carry inoculum to blossoms in spring. Quiescent and latent fruit infections initiated in spring were noted, but they are thought to have little overall significance in the aetiology of the disease. Some infections occurring during the ripening period may exhibit latency. The implications of these findings for disease control and their relevance to brown rot aetiology in other, more humid fruit-growing areas are discussed.

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