A Search for Aecial Hosts of Melampsora medusae Among Some Conifers Grown in the Eastern United States

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Abstract
Epidemics of cottonwood leaf rust, caused by Melampsora medusae, occur annually in the central and southern United States at great distances from tamarack (Larix laricina), the recognized native aecial host in eastern North America. Eight conifer species (Larix decidua, Pinus echinata, P. rigida, P. strobus, P. sylvestris, P. taeda, P. virginiana, and Tsuga canadensis) grown in the eastern United States were planted around enclosures containing telia-laden, overwintered cottonwood leaves to determine if they could provide primary inoculum for southern epidemics by serving as aecial hosts. Aecia were observed annually on L. decidua during a 5-yr test period, but not at all on the other conifers tested. Previous reports on the occurrence of overwintered urediospores in Texas and of different physiological races of M. medusae in the north and south suggest that primary inoculum may be produced locally in areas far removed from tamarack. Species of larch planted in the central and southern states can provide some of this inoculum.