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First Report of Bipolaris sorghicola on Johnsongrass in Argentina

Target leaf spot was found on johnsongrass (Sorghum halepense (L.) Pers.) in La Plata, Argentina (34°54′S, 58°30′W). Up to 80% disease severity was observed in adult plants. In most diseased plants, spots were well defined, delimited by leaf veins, and elongated. Individual lesions ranged in size from small spots (2 to 3 mm) to large lesions (10 to 15 mm). Lesions sometimes coalesced to produce extensive areas of necrosis. Isolates identified as Bipolaris sorghicola (Lefebre & Scherwin) Alcorn (1) grew slowly on potato dextrose agar medium, producing a compact dark brown mycelium. Conidia were 40 to 80 μm long, slightly curved and golden brown, with three to seven pseudosepta. Although germination of conidia was mainly bipolar, lateral germination sometimes was observed. Twenty monoconidial isolates were inoculated at 10⁵ conidia per cm³ on johnsongrass at the 4-leaf stage to confirm the pathogenicity of B. sorghicola. After inoculation, plants were placed in a high-humidity chamber for 24 h; thereafter, they were kept at 25°C day and 18°C night temperatures. Typical lesions developed after 10 days. The pathogen was reisolated from lesions after 15 days. Johnsongrass is one of the most noxious weeds in the world. Due to its potential as a biocontrol agent, further studies are needed to determine the effect of B. sorghicola on johnsongrass.

Reference: (1) J. L. Alcorn. Mycotaxon 27:1, 1983.