Abstract

CO 29, a high yielding short duration blast resistint rice variety has replaced the existing susceptible varieties in many parts of the Tamil Nadu. But during winter season 1966, CO 29 variety was found to be severely affected by the blast disease resulting in almost complete loss of crop in many parts of Coimbatore district. During 1967 winter season also, the blast incidence was noticed on CO 29 in many other districts in the Tamil Nadu. Such breakdown in resistance may be due to the change in environment as in the case of 'Polygenic resistance' or due to the development of new races of the pathogen as in the case of Single gene resistance' (Walker, 1959). Suryanarayanan (1958) reported that even the resistant varieties when subjected to low night temperature (20°C), showed characteristic blast lesions on inocula- tion with Piricularia oryzae Cav. Padmanabhan (1965) also has reported the occurence of races of P. oryzae in India. The present investigation was undertaken to elucidate the factors responsible for the breakdown of resistance to blast in CO 29 rice variety.

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