Study Of Etiology Of A New Rice Disease, Sheath Black Spot

A new rice disease, characterized by elliptical black spots with obscure border on sheaths, was found in Huayuan county of Hunan province in July2011. The black spots are similar to stem rot but different by lack of black spot on stem. It should be a new disease. The isolation, identification and biological characteristics of the pathogen were studied systematically. And several fungicides were screened in laboratory for better control.1. Identification of the causal agent of rice sheath black spotAn elliptical black spot, with obscure border, about10×5mm in size, was observed on inspected rice. The leaf blades of infected sheaths became yellow and withered. Typical Curvularia spp. was observed under microscope. Conidium straight or slightly curved, which showed4-septate conidia with3dark central cells and subhyaline terminals, the measured average size of conidia was30.4×12.5μm. No sexual stage has been found. Isolation Strain(HNHY001) appeared as dark green velvet with branchy black stroma scattered on PDA plate after5days cultivation. After artificial inoculation both in vitro and in vivo, HNHY001caused black spots on rice sheaths,there have yellow halo around it.We can get the pathogenic which is the same as the original separation strains after isolation the black spot.No disease spot on control group. PCR amplification of HNHY001was carried out by utilizing universal rDNA-ITS primer pair ITS4/ITS5. After BLAST, the sequence (GenBank Accession No. JQ360963) of HNHY001rDNA-ITS was found to be the most homologous with those of Cochliobolus geniculatus and its mitosporatic4-septate conidial Curvularia spp. Considering the fact that we have not observed the sexual stage of HNHY001, during confrontation cultures with other4-septate conidial Curvularia isolates, we therefore regarded the isolate HNHY001as Curvularia fallax, one of C. geniculatus mitosporatic Curvularia spp. based on the asexual stage characteristics, i.e., slightly straight4-septate conidium and branchy stromata. Study results were published in the Plant Disease2012, Volume96(8):12242.Study on the Biological Characteristics of4-septate Conidial Curvularia spp.27Curvularia isolates were obtained from the150rice seed sets in Hunan province. Among those, three isolates were found to contain4-septate conidia. Then, researched and comparatived the biological characteristics of these three isolates with HNHY001.The result show us the differences of them at the conidium morpHology、size、sporulation quantity and the adaptation to temperature、pH、illumination. The research showed that apart from HNHY001,there were two isolates could produce branchy stroma.For mycelial growth,the range of temperature was28-30℃.For conidial production, the range of temperature was25-35℃.The optimum temperature of germination rate of conidia was30℃.4-septate conidia adapt to the wide range of pH.The optimum pH for mycelium growth and conidial production was between pH7to8.The lethal temperature of conidial was55℃.Light conditions have a certain effect on the mycelium growth and conidial production.The dark environment conducive to the growth of mycelium,but in the light from the darkness alternating environment more conducive to the conidial production.For mycelium growth, the optimum carbon source was maltose and dextrose.The optimum nitrogen source was L arginine and L-proline.3.Fungicides Toxicity Determination in LaboratoryThe results showed that different fungicides had the significant difference for the inhibiting effect on Curvularia falla,40%flusilazole and45%prochloraz had strong toxicity on Curvularia fallax, with the EC50of0.0624and0.0829mg-L-1respectively. Followed by5%hexaconazole and10%with0.1363and0.2445mg-L-1EC50values respectively. Chlorothalonil、thiodiazole-copper、azoxystrobin and carbendazim had no obvious inhibitory action on Curvularia fallax.It is an important innovation of this study that the disease of rice sheath black spot is actually caused by Curvularia fallax, one of C. geniculalus mitosporatic Curvularia spp.