First Report of Root Rot on Asparagus Caused by Phytophthora megasperma in Canada

In August 2002, Phytophthora megasperma Drechs. was isolated from wilted plants of Asparagus officinalis L. cv. Guelph Millenium displaying spear and crown rot. Six affected plants were sampled in a commercial asparagus field located in the Saguenay-Lac-Saint-Jean Region (300 km northeast of Montreal, Quebec, Canada). The fungus was isolated from asparagus fern stalks, crown tissue, and spears after a rainy period and identified using morphological and cultural characteristics (2). In pinkish 4-week-old cultures, unbranched stalks bore abundant sporangia, which were ovoid to obpyriform in shape, 15 to 45 m long, and 10 to 30 m in diameter. Characteristic circular oospores >30 m in diameter were produced on V8 juice agar at 25°C in darkness after 1 month. Pathogenicity was tested on asparagus cvs. Guelph Millenium and Jersey Knight. A mycelium suspension (3 ml at 10⁶ CFU/ml) prepared from 1-week-old shaken potato dextrose (PD) broth was sprayed on 30 1-week-old seedlings grown in petri plates filled with sterilized, moist, sandy soil and held at 20°C (day/night). Controls received sterile PD broth. Within 3 weeks of incubation in the dark, inoculated seedlings exhibited necrotic symptoms similar to those observed initially, while controls remained healthy. The pathogen was isolated from 75% of the 'Guelph Millenium' and 98% of the 'Jersey Knight' symptomatic seedlings, but not isolated from the control seedlings. In North America, disease caused by P. megasperma resulting in yield loss has been reported in California and New York (1,3). In Canada, the etiology of asparagus diseases is not well characterized. To our knowledge, this is the first report of P. megasperma on asparagus plants in Canada. References: (1) P. A. Ark and J. T. Barrett. Phytopathology 28:754, 1938. (2) D. C. Erwin et al. Phytophthora: Its Biology, Taxonomy, Ecology, and Pathology. The American Phytopathological Society, St Paul, MN, 1983. (3) T-L. Kuan and D. C. Erwin. Phytopathology 70:333, 1980.