Barnyardgrass, Echinochloa crus-galli (L) Beauv., is ranked as the world's third worst weed (2). Punctiform, purplish brown spots were found on leaves and sheaths of 7- to 8-week-old barnyardgrass in a rice crop cultivated in La Plata, Argentina (34° 54′S, 58° 30′W). Individual lesions ranged from 1 to 2 mm diameter. Isolates from the lesions were identified as Exserohilum monoceras (Drechsler) Leonard & Suggs (1) (confirmed by A. Watson, Univ. McGill, Canada). Cultures on potato dextrose agar formed dark green colonies. Conidia were 75 to 135 × 15 to 19 μm, 5- to 7-distoseptate, straight or slightly curved, fusoid, tapering gradually towards the base, pale- to mid-olivaceous brown, with a small protruding plenum-type hilum. Barnyardgrass at the 3-leaf stage was inoculated in a greenhouse with a suspension of 105 conidia per ml of water to confirm the pathogenicity of E. monoceras. Plants were bagged and kept in a humidity chamber for 48 h, at approximately 16°C. After 10 days lesions developed that were similar to those found on infected plants in the field. E. monoceras was reisolated from these lesions. This pathogen is being studied as a biocontrol agent for barnyardgrass in tropical areas (2).
References: (1) J. L. Alcorn. Mycotaxon 7:411, 1978. (2) W. Zhang and A. Watson. Can. J. Bot.75:685, 1997.