Biological Limitations of Protomyces gravidus as a Mycoherbicide for Giant Ragweed, Ambrosia trifida. R. D. Cartwright, Former Research Assistant, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. G. E. Templeton, University Professor, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. Plant Dis. 72:580-582. Accepted for publication 12 January 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0580.

Stem gall disease of giant ragweed, Ambrosia trifida, was found widely distributed at endemic levels in lowland areas in eight northern Arkansas 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 concentrations of 5.9 × 10⁸ spores/ml in 5 days at 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 when systemically infected. Common ragweed, A. artemisiifolia, and cocklebur, Xanthium strumarium, were the only other hosts identified after inoculating 27 species in eight plant families. The major limitation to use of P. gravidus as a mycoherbicide is its low infectivity rate and lack of virulence in environmental conditions comparable to environments where it incites endemic disease.