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First Report of Angular Leaf Spot Caused by Phaeoisariopsis griseola on Phaseolus coccineus in Argentina

Angular leaf spot (ALS), caused by Phaeoisariopsis griseola (Sacc.) Ferraris, is one of the most destructive and widespread problems of common bean (Phaseolus vulgaris L.) in Tucumán and other northwestern provinces of Argentina (4). Symptoms similar to those of ALS were observed during April 2005 on most plants of runner bean (P. coccineus L.) in an 80-ha field in Tafí del Valle, Tucumán (2,000 m above sea level). Leaf lesions were brown to gray, irregular to angular to circular, and 0.5 to 1 cm in diameter. Lesions on pods were oval to circular with reddish brown centers surrounded by darker brown borders. Conidia in vivo were curved cylindrical to obclavate with one to five septa and measured 25 to 60 × 3.5 to 7 μm. The conidiophores were 100 to 250 μm high and clustered together to form synnemata measuring 20 to 50 μm in diameter. The pathogen was isolated by placing conidia from diseased leaves onto potato dextrose agar (PDA) at pH 6. Colonies measuring 2 to 3 mm in diameter composed of dense, dark olive mycelium developed after incubation in the dark at 24 ± 2°C for 3 to 4 days. Pathogenicity of the isolate was tested with conidia obtained from the second subculture of 14-day-old colonies on PDA. Conidial suspensions of 2 × 10⁴ conidia per ml were sprayed onto the upper and lower surfaces of the first trifoliolate leaves of six runner bean plants, 18 days after planting. Inoculated and control plants (sprayed with distilled water) were placed in a growth chamber with a 12-h photoperiod at 24 ± 2°C and 95 to 100% relative humidity and 48 h later moved to the greenhouse. Disease symptoms were evaluated 18 days after inoculation. While control plants were healthy, all inoculated plants showed symptoms similar to those observed in the field. The fungus that was consistently reisolated from lesions in the inoculated plants was identified as Phaeoisariopsis griseola on the basis of fungal morphology (1), symptoms produced on leaves (3), and random amplified polymorphic DNA data with primer 5′-CAATCGCCGT-3′ (2). Runner bean is a new crop in Tafí del Valle, which is a geographically isolated area. In a period of only 2 years, the area cultivated with beans increased approximately five-fold. Because of this, the presence of a pathogen like Phaeoisariopsis griseola, which causes considerable reduction in yield in most common bean-producing areas of Argentina, is of concern. To our knowledge, this is the first report of ALS occurring on P. coccineus in Argentina. This report may prompt the inclusion of regular testing of seeds for ALS in P. coccineus-production areas. A voucher culture has been deposited in the LPSC (Culture collection of the La Plata Spegazzini Institute) No. 844.

References: (1) M. B. Ellis. Dematiaceous Hyphomycetes. CMI, Kew, Surrey, UK, 1971. (2) P. Guzmán et al. Plant Dis. 83:37, 1999. (3) A. W. Saettler. Pages 15--16 in: Compendium of Bean Diseases. R. Hall, ed, The American Phytopathological Society, St. Paul, 1991. (4) S. A. Stenglein et al. Pages 209--243 in: Advances in Applied Microbiology, Vol. 52. A. I. Laskin et al., eds, Academic Press, San Diego, 2003.