3.6.8
ACTUAL STATE OF SURVIVAL OF RICE SCLEROTIAL DISEASE FUNGI, RHIZOCTONIA AND SCLEROTIUM SPP. IN AND AROUND PADDY FIELDS

K INAGAKI, M SHIMIZU and A UYEDA

Faculty of Agriculture, Meijo University, Tenpaku, Nagoya 468-0073, Japan

Background and objectives
Rhizoctonia and Sclerotium spp. cause sclerotial diseases on the mainly late stage rice plants in paddy fields. These diseases are characterised by formation of sheath blight-like lesions on rice leaf sheaths. Sclerotial disease fungi, which are generally omnivorous [1,2], survive in the form of sclerotia in fields. In the present study, these fungi were examined for the actual state of their survival in fields and, furthermore, weeds growing in and around fields were examined for the possibility as infection sources.

Materials and methods
Forty plots within a field were sampled in the pre-transplanting season (early March) to examine survival of 5 fungi. These fungi were isolated from plant residues in soil samples (200g) and from two stubbles collected from each plot. The number of fungal isolates (IS) obtained and that of field plots (PL) where the respective pathogen was detected were counted. Two test fields (8 ha) surveyed for 3 years were severely attacked by S. fumigatum and moderately by the other fungi. In the rice growing season, 26 weeds which belong to 8 families and 21 genera were collected in and around fields for fungal isolation.

Results and conclusions
PL and IS from both soil and stubble were most abundant in S. hydrophilum and S. fumigatum among 5 fungi. In R. oryzae and S. oryzae-sativae, PL and IS from soil were very few compared with those from stubble. AS to weed-test, S. fumigatum was isolated from 5 weeds belonging to Gramineae, R. oryzae from 1 (Gramineae), R. solani AG-2-2 (IHB) from 1 (Gramineae) and S. oryzae-sativae from 1 (Cyperaceae). S. hydrophilum was not isolated from any kind of weeds. These results indicate that rice sclerotial disease fungi differ considerably in their surviving places within a paddy field and S. hydrophilum survives and distributes widely in paddy fields irrespective of comparatively lesser outbreak of disease caused by this pathogen. Further, it is possible to assume that some Gramineae plants widely growing in and around fields are available for infection sources of sclerotial diseases, especially one by S.fumigatum .

References
1. Nakata K, Kawamura E, 1939. Mat.Improv.Agr.Forest Japan 139, 176.
2. Inagaki K, Okuda K, Makino M, 1978. Sci. Rept. Fac. Agr. Meijo Univ. 14, 1-6.