Burkholderia glumae and B. gladioli Cause Bacterial Panicle Blight in Rice in the Southern United States.

Bacterial panicle blight (BPB) is among the three most limiting rice diseases in Louisiana and the southern United States. The identity and characterization of pathogens associated with this disease was unclear. This research details studies carried out on the pathogens causing BPB on rice in Louisiana and other rice producing southern states. Bacterial strains were isolated from BPB-infected sheath, panicle, or grain samples collected from rice fields in Louisiana, Arkansas, Texas, and Mississippi. In greenhouse inoculation tests, 292 of 364 strains were pathogenic on rice seedlings or panicles. Identification of strains in the pathogen complex by growth on S-PG medium, carbon source utilization profile (Biolog), cellular fatty acid analysis, and polymerase chain reaction (PCR) methods revealed that 76 and 5% of the strains were Burkholderia glumae and B. gladioli, respectively. The other strains have not been conclusively identified. Although strains of both species produced similar symptoms on rice, B. glumae strains were generally more aggressive and caused more severe symptoms on rice than B. gladioli. Virulent strains of both species produced toxoflavin in culture. The two species had similar growth responses to temperature, and optima ranged from 38 to 40°C for B. glumae and 35 to 37°C for B. gladioli. PCR was the most sensitive and accurate method tested for identifying the bacterial pathogens to the species level. The 16S rDNA gene and 16S-23S rDNA internal transcribed spacer (ITS) region sequences of the B. glumae and B. gladioli strains from rice showed more than 99% sequence homology with published sequences. A real-time PCR system was developed to detect and quantify this pathogen from infected seed lots. Our results clearly indicate that B. glumae and B. gladioli were the major pathogens causing BPB in the southern United States.

Outbreak of False Smut of Rice in Louisiana.

False smut, caused by Ustilaginoidea virens (Cooke) Takah., has been occurring in Louisiana rice since at least 1906 (4). A color plate (no. 69) of the disease was published in the Compendium of Rice Diseases published by the American Phytopathological Society (3). The slide for this plate was taken by M. C. Rush in 1976 of rice grown at the Rice Research Station at Crowley, LA. Since that time, the disease has been sporadic and light in Louisiana. In 1997, however, incidence was high. False smut was present on many germ plasms at the Rice Research Station in Crowley and was observed on commercial cultivars in several growers' fields in southwestern Louisiana. Incidence ranged from 1 to 15% of tillers infected with at least two to three spore balls per infected panicle. The disease occurred on both long- and medium-grain cultivars. False smut of rice occurs in the field at the hard dough to mature stages of the crop. A few spikelets in a panicle transform into globose, yellowish green, velvety spore balls that are 2 to 5 cm in diameter and covered by a thin orange membrane. The membrane bursts open and releases powdery dark green spores. The chlamydospores formed in the spore balls are spherical to elliptical, warty, olivaceous, and 3 to 5 × 4 to 6 µm in dimension. Some of the spore balls develop one or more sclerotia, which are the overwintering structure, in the center. False smut has been considered a minor disease of rice that occurs sporadically in Louisiana. The recent discovery of ustilotoxin, a phytotoxin and mycotoxin, produced by this pathogen on diseased tissues suggests that the fungus may be of concern as a contaminant on rice products consumed by livestock and humans (1,2). This increases the need to monitor the incidence of this disease. References: (1) Koiso et al. Ustiloxin: A phytotoxin and a mycotoxin from false smut balls on rice panicles. Tetrahedron Lett. 33:4157, 1992. (2) Koiso et al. Ustiloxins, antimitotic cyclic peptides from false smut balls on rice panicles caused by Ustilaginoidea virens. J. Antibiot. 47:765, 1994. (3) F. N. Lee and P. S. Gunnel. 1992. Compendium of Rice Diseases. The American Phytopathological Society, St. Paul, MN. p. 28. (4) W. A. Orton. 1907. Plant diseases of 1906. Yearbook U.S. Department of Agriculture. U.S. Government Printing Office, Washington, DC, pp. 499-508.

First Report of the Occurrence of White Leaf Streak in Louisiana Rice.

White leaf streak, caused by Mycovellosiella oryzae (Deighton and Shaw) Deighton (syn. Ramularia oryzae), was found in Louisiana rice. The symptoms closely resemble those of narrow brown leaf spot caused by Cercospora janseana (Racib.) O. Const. (syn. C. oryzae (Miyake)), and it is difficult to distinguish between these two diseases. Initially both produce similar elongated light brown lesions, but later the lesions of white leaf streak become wider with a whitish center and are surrounded by a narrow light brown margin (2,3). The disease was first observed at the Rice Research Station, Crowley, LA, in 1996 on older leaves of the cultivar Lemont at maturity. Leaves containing the unusual lesion types were placed in a moist chamber and incubated at 28°C for 5 days. Abundant conidia were produced and the fungus was isolated on acidified potato dextrose agar (APDA) by single spore isolation and by plating infected tissues after surface sterilization in 40% Clorox for 10 to 15 min. The colonies grew slowly on APDA and were dark gray in color. The conidia formed in branched chains or singly. They were hyaline, cylindrical with tapering ends and a thick hilum; 0 to 3 septate, and 15 to 35 m long (1,3). Pathogenicity tests were conducted in the greenhouse on the Lemont and Cypress rice cultivars by spraying a conidial suspension (103-4 conidia per ml) onto leaf blades at boot stage. Conidia were produced by growing the fungus on PDA for 10 to 14 days. Inoculated plants were placed inside a humid chamber in a greenhouse and maintained for 4 to 5 weeks. Many elongated lesions similar to those observed in the field were produced 3 to 4 weeks after inoculation. Reisolation from these lesions yielded M. oryzae. With the same methods, 45 cultivars and lines were inoculated to determine their reactions to this disease. Most of the cultivars grown in the southern United States were moderately susceptible or susceptible to white leaf streak. Foreign cultivars tested, including BR-7, BR-11, Cica-4, Cica-6, Cica-7. Cica-8, Cica-9, Oryzica llanos, Rax clear, Teqing, and Tetep, were resistant. In 1997, the disease was found prevalent on many cultivars grown at the Rice Research Station, Crowley, LA. As symptoms of both white leaf streak and narrow brown leaf spot were sometimes observed on the same leaf; it is possible that the disease has been present, but not identified as a separate disease because of the similarity of the symptoms of the two diseases. A thorough survey is necessary to determine the extent of its occurrence and further studies are necessary to determine its yield loss potential. At present it appears to be a minor problem for Louisiana rice. White leaf streak has previously been recorded from Papua New Guinea on cultivated Oryza sativa, and from the Solomon Islands, Sabah, Nizeria, and Sierra Leone on cultivated O. glabberima Steudel and on wild perennial rice O. berthii A. Chev. (2). This is the first report of white leaf streak on cultivated rice in the United States. References: (1) F. C. Deighton. Mycol. Pap., CMI 144:1,1979. (2) F. C. Deighton and D. Shaw. Trans. Br. Mycol. Soc. 43: 515, 1960. (3) B. C. Sutton and A. K. M. Shahjahan. Nova Hedwigia 25:197, 1981.