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First Report of Exserohilum rostratum Associated with Rice Seed in Venezuela

Rice (Oryza sativa L.) is an important cereal crop in Venezuela and is planted on approximately 200,000 ha. Fungal diseases caused by the Helminthosporium complex of fungi are considered a major constraint to rice production. In 2005, a blotter method was used to identify fungi associated with rice seed. Seeds (150) of cv. INIA-017 were placed on a plate with moistened filter paper and incubated at 26 ± 2°C with a 12-h light/12-h dark cycle for 5 days until fungal sporulation. Single spores were transferred to 2% water agar (WA). Germinated spores were then transferred to potato dextrose agar (PDA), and resultant colonies were preserved until used. To induce sporulation, 4-mm-diameter discs were cut from 72-h-old cultures, transferred to WA, and incubated at 26 ± 2°C for 48 h (modified technique of Alcorn [1]). Discolored seed yielded isolates of Bipolaris oryzae (Berda de Haan) Shoemaker and Exserohilum rostratum (Drechs.) Leonard and Suggs (2). Pathogenicity tests were performed on 60-day-old rice plants of cv. Blue Bonnet 50. Four plants per pot were used for each isolate. Before inoculation, plants were placed in humidity chambers for 48 h. Spores were harvested from discs into a beaker containing 50 mL of a dilute gelatin solution (Gelatin Difco; 5 g/100 mL of sterile distilled water [SDW]). Spore suspensions were filtered through cheesecloth and adjusted to 7 × 10⁴ spores per ml. The inoculated plants were placed in humidity chambers for 72 h. Control plants were sprayed with SDW. Inoculated plants were removed from humidity chambers and placed on a greenhouse bench. Plants inoculated with B. oryzae were symptomatic 5 days after inoculation; resultant lesions were red-brown with chlorotic borders. Lesions subsequently turned gray with reddish borders and were rhombic in shape. Plants inoculated with E. rotratum were symptomatic 7 days after inoculation with elliptical lesions that were orange-brown along venation. These lesions turned gray with reddish borders. Both fungi were reisolated from symptomatic leaf tissue. To our knowledge, this is the first report of E. rostratum from rice in Venezuela.

References: (1) J. L. Alcorn. Mycotaxon. 17:1, 1983. (2) A. Sivanesam. Mycol. Pap. 158, 1987.