RICE SHEATH BLIGHT-PATHOGEN BIOLOGY AND DIVERSITY

Abstract

Rhizoctonia solani Kühn was originally reported causing sheath blight of rice in Japan at the beginning of this century (Miyake, 1910). However, rice sheath blight has developed into a major disease only during the past two decades. Nowadays, the disease is second only to, and often rivals, rice blast (Pyricularia oryzae) in terms of economic importance in many major rice-growing countries. The emergence of R. solani as a major rice pathogen has been attributed to the intensification of the rice-cropping system characterised by the development of new, short-statured, high-tillering, high-yielding varieties and an increase in nitrogen fertilisation inducing a favourable micro-climate for the pathogen. The rice sheath blight pathogen has been associated with isolates of one particular intraspecific group (ISG) of Rhizoctonia solani, anastomosis group (AG) 1 IA (Yamaguchi et al., 1984; Vijayan and Nair, 1985; Borthakur and Addy, 1988; Jones and Belmar, 1989), but diversity within this ISG appears to be much greater than was previously assumed (Liu and Sinclair, 1993). Detailed knowledge of pathogen characteristics at this taxonomic level is still lacking and has hampered understanding of the patho-systems.