Immigrant Amplification and Population Dynamics of Fungi on Leaf Islands

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Microorganisms that grow within or upon leaves alternate between a life cycle phase associated with the leaf, a transport phase in the atmosphere, and growth on another leaf of the same or different plant species. This project seeks to explain, both by mathematical models and experimental tests, how a resident population of microbes can be elevated synergistically by the influx of relatively small numbers of immigrant microbe cells to the leaf. The models will be tested by examination of natural and experimentally enhanced levels of a yeast- like fungus, Aureobasidium pullulans, on apple leaf surfaces. The field tests will be facilitated by use of molecular marker methods, which allow the background population of A. pullulans to be discriminated from the introduced immigrant population.This project has two significant scientific implications. First, it may help explain how populations of species survive and compete in nature in isolated patches when the number of immigrants is small relative to the size of the extant population. Second, microbes inhabiting leaves cause important plant diseases. These pathogens share many of the features of A. pullulans. Therefore, this system should give greater quantitative insight into the biology and epidemiology of microbial plant diseases.