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Zion National Park

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Many velvet ash (Fraxinus velutina) in Zion Canyon have declined in vigor, and some are dying. This species is aesthetically and ecologically important in the canyon, because it is one of only three species that commonly grow to large size on the canyon floor. None of the three species is reproducing abundantly, because suitable seedbeds are scarce and young plants are severely browsed by deer. A disease known as ash yellows, which is caused by unnamed mycoplasmalike organisms (wall-less, obligately parasitic prokaryotic organisms, commonly called MLOs) was found in Zion Canyon in 1988, and is suspected to cause or contribute to the decline of velvet ash there. Until this project began, nothing was known about the ecology or epidemiology of ash yellows in the West.