In April 2013, upright, straw-colored panicles were observed in rice (Oryza sativa L.) fields with center pivot sprinkler irrigation at Ukulima farm in Limpopo Province, South Africa. Affected panicles contained florets that initially exhibited discoloration ranging from light green to light brown on the basal portion of the glumes and later developed a reddish-brown margin separating this area from the rest of the glume that became straw-colored. The stems of infected florets remained green. Affected grains were partially filled or aborted, causing panicles to remain upright. These symptoms appeared on approximately 25% of the rice plants of the varieties Presidio and WAB56-104. A gram-negative, rod-shaped bacterium was isolated consistently from symptomatic panicles. These strains produced circular, smooth-margined, convex colonies on King's B medium (KB) with a yellowish-green, diffusible non-fluorescent pigment. Four strains, two from each of the two varieties, were selected for physiological, molecular, and pathogenicity assays. These strains grew on nutrient-broth yeast (NBY) agar at 40°C and in NBY amended with 3% NaCl. They utilized acetate, adonitol, and citrate, and were positive for gelatin hydrolysis, but negative for oxidase and arginine dihydrolase. These morphological and physiological data indicate that the characteristics of these strains were similar to those of Burkholderia glumae. The strains were further verified using B. glumae-specific PCR method (2). In a greenhouse, 12 panicles each of Presidio and WAB56-104 were spray-inoculated at the flowering stage with a bacterial suspension (108 CFU/ml) of each strain generated from cultures grown on KB at 30°C for 24 h. Twelve panicles of each variety treated in the same way with sterile water served as controls. After inoculation, treated panicles were covered with transparent polyethylene bags and maintained in the greenhouse with temperatures ranging from 24 to 35°C. After 2 days, the polyethylene bags were removed. Symptoms similar to those observed in the field developed on all the panicles inoculated with each strain by 10 days of inoculation. No symptoms were observed on the control panicles. Bacteria with the same morphological and cultural characteristics of B. glumae were consistently re-isolated from diseased panicles. All eight randomly selected, re-isolated strains were confirmed to be B. glumae based on the B. glumae-specific PCR and physiological assays described above. This pathogenicity experiment was repeated once with similar results. This is the first report of bacterial panicle blight of rice caused by B. glumae in South Africa. Overhead sprinkler irrigation appeared to promote the development and spread of the disease. Since the bacterium is seedborne (3) and there are no acceptable levels of resistance available in commercial varieties, bacterial panicle blight poses a threat to rice production. This bacterium was first described in Japan as the causing agent of grain rot and seedling rot on rice grown under flood irrigation conditions (1). Bacterial panicle blight caused by B. glumae has since been reported in China, Korea, Latin America, the Philippines, and the United States (2).
References: (1) K. Goto and K. Ohata. Ann. Phytopathol. Soc. Jpn. 21:46, 1956. (2) R. Nandakumar et al. Plant Dis. 93:896, 2009. (3) R. J. Sayler et al. Plant Dis. 90:603, 2006.
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