Disease Note.

First Report of Protomyces gravidus Causing Stem Galls on Giant Ragweed in Louisiana. G. E. Holcomb, Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. Plant Dis. 79:860. Accepted for publication 23 June 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0860D.

Galls up to 75 mm long and 45 mm in diameter were observed frequently on lower stems of giant ragweed, Ambrosia trifida L, growing on banks of drainage ditches in East Baton Rouge Parish in Louisiana. Free-hand sections of galls revealed the presence of thick-walled, ascogenous cells belonging to the fungal genus Protomyces. Ascogenous cells were smooth-walled, did not germinate when placed in water, and measured 32 to 64 ? 35 to 81 ?m. Based on these characteristics and the recognized host specialization among Protomyces species, the ragweed pathogen was identified as P. gravidus J. J. Davis (1). Pathogenicity tests were not attempted with the fungus because ascogenous cells of Protomyces species are known to have a dormancy period of up to 7 months (1). Protomyces gravidus was reported previously on Ambrosia and Bidens species in New York and Wisconsin, but has not been reported from the southern United States (1,2). Protomyces species cause leaf, petiole, and stem galls on plants restricted to the families Asteraceae and Umbelliferae.