Disease Note.

First Report of a Phoma sp. on Common Ragweed in North America. S. C. Briere, Department, McGill University, Ste-Anne-de-Bellevue, PQ, Canada, H9X 3V9. A. K. Watson, T. C. Paulitz, and S. G. Hallett, Plant Science Department, McGill University, Ste-Anne-de-Bellevue, PQ, Canada, H9X 3V9. Plant Dis. 79:968. Accepted for publication 21 July 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0968B.

A Phoma sp. was isolated from diseased common ragweed (Ambrosia artemisiifolia L.) on the island of Montreal in Quebec, Canada, in the fall of 1993. The disease was present on several plants that were collected in an urban area at two roadside sites approximately 1 km apart. Abundant pycnidia, 75 μm in diameter, were observed on necrotic areas of infected leaves. Hyaline, nonseptate conidia measuring 2.5 x 5.5 urn were exuded in cirri from ostiolate pycnidia. Three-week-old ragweed seedlings were sprayed with a conidial suspension (1 x 106 conidia/ml) of pure cultures of the Phoma sp., placed in a dew chamber for 24 h, and grown on a growth bench at 23°C. Inoculated plants developed small, dark, necrotic lesions on leaves after 2 weeks and pycnidia developed on the senescent leaves after 3 weeks. Inoculated plants frequently exhibited systemic infections in leaf petioles and stems, which often led to the dieback of the petioles and stems. In many cases the growing point of plants and/or the emerging male flower were colonized and little or no pollen was produced. The fungus is being considered as a potential bioherbicide candidate for the control of common ragweed. A closely related Phyllosticta sp. (P. ambrosiae J. J. Davis) was previously described on ragweed, but our isolates are distinct from that species. No species of Phoma have been described previously on ragweed in the U.S. or Canada. Identification is currently being carried out to determine if this fungus is a previously undescribed Phoma species.