3.6.11
MICRO-FUNGUS CLOSELY ASSOCIATED WITH THE LESIONS OF RED STRIPE DISEASE OF RICE

S WAKIMOTO1, PV KIM2, TTT THUY2, K TSUNO3, MK KARDIN4, RH HARTINI4, S SURANG5, P SUNETRA6, N NILPANIT6, H NEGISHIl, and K SUYAMA1

lTokyo University of Agriculture, Tokyo, Japan; 2Can Tho University, Can Tho, Vietnam; 3Miyazaki University, Miyazaki, Japan; 4Central Research Institute for Food crops, Bogor, Indonesia; 5Kasetsart University, Bangkok, Thailand; 6Ministry of Agriculture, Bangkok, Thailand

Background and objectives
A serious disease showing reddish or yellowish blight on the leaves is widely distributed in many tropical Asian countries. The disease was firstly reported as bacterial red stripe disease of rice in 1988 by Mogi and others [1,2]. However since we could not re-confirm the pathogenicity of his bacterium, experiments to find out the pathogen of the disease were carried out.

Materials and methods
To confirm the transmissibility of the disease, the healthy rice plants were kept in contact with diseased plants in the same pot, covered with Mylar's insect cage and sprayed with distilled water to keep high humidity. The lesions collected from paddy field and greenhouse were cut by scissors, directly stained with the mixture of lactophenol cotton blue on the slide glass and inspected under microscope. The samples were also fixed with 2% osmic acid and observed under scanning electron microscope. The efficacy of benomyl on the disease control was tested by spraying the chemical during transmission test. For isolating the micro-fungus, many trials were carried out by ordinary methods using various kinds of media.

Results and conclusions
The disease was readily transmitted through air to the healthy rice plants during two weeks. By microscopic observation of the lesion, extremely thin mycelia often with spores were constantly observed [3]. The thickness of the mycelia is less than 1.0 um (usually 0.2 - 0.3 mm), and the size of spore is 16.2 x 3.2 mm in average [5]. All spores are septated into four cells. Some other unknown fungal structures connected with the mycelia were also observed on/in the host tissues of some rice varieties. Under the scanning microscope, spherical or elliptical unknown structures (diam. 10 - 50mm) connected with the mycelia were observed. The mycelia tightly adhered and elongated on both upper and under surfaces of the lesion. The disease transmission was markedly prevented by spraying benomyl [4]. From these results, it is surmised that the pathogen is a kind of fungus. However, up to now, all trials for isolating pathogenic fungus were not successful.

References
1. Mogi S, 1988. Abstract of Papers 5th ICPP, Kyoto, p.388.
2. Mogi S, 1992. Papers presented at the IRRI Conference. p.10.
3. Kim PV et al., 1995. Papers presented at Scientific Conference of Faculty of Agronomy, Can Tho University. pp. 152-162 (in Vietnamese with English summary).
4. Kim PV et al., 1997. Ann. Phytopathol. Soc. Jpn. 3, 221(Abst).
5. Wakimoto S et al., 1997. Ann. Phytopathol. Soc. Jpn. 63, 221(Abst.).