Epidemic Development and Virulence in 1995–1998 of Puccinia coronata, a Potential Biocontrol Agent of Wild Oats on San Clemente Island

The biology of islands differs from that of large land masses in having less complex ecosystems. Introduced exotic weeds are often a major threat to fragile island ecosystems because of their expansion into habitats previously occupied by endemic species. San Clemente Island, 120 km off the California coastline, is an example of this process in which numerous exotic weed species have been introduced and some are endangering the native flora. Crown rust of oats caused by Puccinia coronata f.sp. avenae was investigated as a potential biocontrol agent against two wild oat species, Avena barbata and Avena fatua, introduced on San Clemente Island. Epidemiology and virulence of this rust were studied. The island was surveyed from 1995 to 1998 for occurrence of P. coronata on wild oats. Wild oats were found sprouting in the northern part of the island shortly after autumn rainfall and subsequently covered the main grasslands of the island. The rust also appeared first on the northern part of the island and progressively spread south. However, disease severities in the south were considerably lower than those in the north. Diverse virulence types, although related to Californian and Mexican forms, were detected among the isolates. The potential use of P. coronata as an augmentative biocontrol agent for wild oat species on San Clemente Island is discussed.