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First Report of Angular Leaf Spot Caused by Phaeoisariopsis griseola on Bean in Ontario, Canada

During the summer of 2000, brown, angular-shaped leaf spots, frequently surrounded by a chlorotic halo, were observed in commercial fields of green bean (Phaseolus vulgaris L.) in Ontario, Canada. Brown synematta (30 to 60 μm × 160 to 330 μm) and brown conidia (2 to 5 septate, usually curved, 5 to 7.3 μm × 35 to 66 μm) from the underside of diseased leaves were plated onto V8 agar. Within 3 to 4 days, dark olive green colonies formed, and after 10 to 14 days, white mycelial growth occurred on the upper surface of colonies, and colonies appeared gray. The causal agent was tentatively identified as Phaeoisariopsis griseola (Sacc.) Ferraris, the cause of angular leaf spot of bean, and this identification was later confirmed by the Centraalbureau voor Schimmelcultures, The Netherlands. New colonies of the fungus were started by streaking conidia across plates of V8 agar and new conidia were produced within 36 h. Conidial suspensions of 1 × 10⁵ conidia/ml were sprayed onto leaves of green bean varieties Goldrush, Strike, Bronco, and Gold Mine. Plants were placed in a mist chamber at 20 ± 2°C for 11 days and then kept at high humidity for four more days. Lesions were observed 8 to 10 days after treatment and synematta developed 12 to 14 days after treatment. Disease symptoms and synematta were observed on all bean varieties tested. P. griseola was reisolated from inoculated plants, fulfilling Koch's postulates. To our knowledge, this is the first report of angular leaf spot occurring on P. vulgaris in Ontario.