Leaf Spot Disease of Broad Bean Caused by Alternaria tenuissima in Japan.

In April 1999, a leaf spot of broad bean (Vicia faba L.) was observed in commercial fields in Shimane prefecture of Western Japan. Lesions were concentric and brown in color. Older leaves were particularly affected. In later stages of the disease, plants defoliated as leaves blighted from margin to the center. Isolation was made from infected leaf tissue. The isolated fungus produced conidia on V8 medium (2) either in dark or under continuous irradiation of near ultraviolet radiation (NUV) from BLB fluorescent lamps. Conidial chains were unbranched or rarely formed a few lateral branches with a few conidia. The conidia of the fungus grown under continuous NUV were dark and smoothly tapered into the apical beak, and each conidium had a conspicuously thickened primary septum with a constriction of the conidial wall and often a darker median transverse septum. The conidia measured 21.2 to 45.5 μm (mean = 32.9 μm) × 7.3 to 17.7 μm (mean = 11.4 μm ) on V8 medium. Conidia produced on leaves and stem collected from field were similar in size and appearance. The fungus was identified as Alternaria tenuissima based on its cultural and morphological characteristics (2). An isolate was also sent to CABI Bioscience Identification Services (Egham, UK), which also identified the fungus as A. tenuissima. A conidial suspension (10 spores/ml) was prepared and used to inoculate detached leaves and intact plants of broad bean. Intact plants were inoculated by spaying with spore suspension and covered with polyethylene bags for maintaining high humidity. Detached leaves in moist petri dishes were inoculated with drops of spore suspension. Symptom developed on both detached and intact leaves 3 to 4 days after inoculation. Reisolating the pathogen from infected leaves completed Koch's postulates. In June 2000, the leaf spot was observed in all 15 fields surveyed in other areas of Shimane prefecture. In some fields, plants were defoliated and stems and pods were also infected. Isolates of A. tenuissima also were obtained from those fields. This pathogen has been isolated from other hosts in Japan (1). This is the first report of A. tenuissima on broad bean in Japan. References: (1) Anonymous. 2000. Common Names of Plant Diseases in Japan. The Phytopathological Society of Japan, Tokyo. (2) E. G. Simmon. Mycotaxon 37:79-119, 1990.