In July 2011, we observed a new rice disease, black sheath spot, on tillering rice plants in Huayuan County, Hunan Province, China. Field surveys indicated that this disease covered a ~1,000-ha paddy field, mainly on Y series 2-line hybrid rice cultivars, especially Y Liangyou 7 and Y Liangyou 302, in hills of which 10 to 70% infection was observed (average of 45%), causing estimated damages up to $1.2 million (US). The diseased rice plants were cultivated with standard practices. Weather, flooding, herbicide damage, and fertilizer application did not appear to account for the symptoms. Typical symptoms of the disease included elliptical spots, about 10 × 5 mm, dark brown to black and with a diffuse yellow-brown margin. The leaf blades with diseased sheaths became yellow and blighted. After 7 days of growth on PDA, groups of cylindrical and branched stromata scattered over dark green colony. Conidia averaged 30.4 × l2.5 μm and contained five cells with three dark central cells. The morphology of spores was consistent with that of C. fallax (1). We inoculated 8-cm healthy rice sheath segments with culture disk of an isolate HNAH001. Sheath segments were divided into four groups: I) disks on wounded sheath surfaces; II) disks on non-wounded sheath surfaces; III) disks between the leaf sheath and stem, and IV) a sterilized water control. The segments were maintained in moist, covered plates in a 25°C incubator after inoculation. After another 24 h, circular brown lesions, less than 10 mm long, appeared on all inoculated segments in treatments I,II, and III. Treatment I induced the most serious symptoms. No lesions developed on control segments. For further verification of pathogenicity, we sprayed a spore suspension of HNHY001 on healthy rice plants at the boot stage. Black spots reappeared on the sheaths after 5 days. No lesions appeared on the stems or the leaf blades of inoculated rice plants. We recovered HNHY001 from the spots on inoculated plants and completed all steps of Koch's postulates. For molecular identification of the fungus, DNA was extracted from mycelia and used as a template for PCR with a primer pair of ITS 5 and ITS 4 targeting the rDNA-ITS. The sequence of the PCR product (Accession No. JQ360963) had 100% identity with rDNA-ITS of Cochliobolus geniculatus (teleomorphic state of C. geniculata) and C. affinis in GenBank after a BLAST search and clustered with them after a phylogenetic analysis. There was no sequence of C. fallax on the BLAST list because ITS sequence of C. fallax had not yet been submitted to any nucleotide databank. Hosokawa et al. concluded that C. fallax and C. affinis are synonyms for C. geniculata (2), which is supported by our results from BLAST and phylogenetic analysis. In view of its relative straight conidia and branching stromata, we suggest that the causal agent of black sheath spot of rice be C. fallax. Although Curvularia spp. were reported as pathogens responsible for black kernel of rice, there is no report of sheath spot of rice caused by Curvularia (3). To our knowledge, this is the first report of a rice sheath disease caused by C. fallax. It seems likely the disease exists in areas beyond Huayuan County. Further field inspection and molecular identification are necessary.
References: (1) K. B. Boedijn. Bull. Jard. Bot. Buitenzorg, ser. 3 13:129, 1933. (2) M. Hosokawa et al. Mycoscience 44:227, 2003. (3) S. H. Ou. Page 317 in: Rice Diseases, 2nd Edition, Commonwealth Mycological Institute, Kew, UK, 1985.