Biological Limitations of Protomyces gravidus as a Mycoherbicide for Giant Ragweed, Ambrosia trifida

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Abstract
Stem gall disease ragweed, Ambrosia trifida, was found widely distributed at endemic levels in lowland areas in eight northern Arkanas counties. Resting spores of the pathogen Protomyces gravidus germinated after 5 months of weathering in infected tissue in the field. Cultures of yeastlike ascospores were obtained on nutrient agar and increased in liquid shake culture to concentration of 5.9 .times. 105 spores/ml in 5 days 16-24 C. Inundative inoculation of giant ragweed seedlings produced galls on 100% of the plants when incubated in a dark dew chamber for 48 hr at 20 C. Plants were killed by the disease systemically infected. Common ragweed, A. artemisiifolia, and cocklebur, Xanthium strumarium, were the only other hosts identified after inoculaitng 27 species in eight plant families. The major limittion to use of P. gravidus as a mycoherbicide is its low infectivity rate and lack of virulence in environmental conditions comparable to environment where it incites endemic disease.