Assessment of Apparent Infection Rate and Disease Intensity of Bipolaris oryzae in Different Cultivars of Rice in Central India

Background and Objective: Brown spot of rice is an important disease causing considerable yield losses in Central India. Screening of improved and local cultivars of rice was conducted in Rewa, Madhya Pradesh, in India for brown spot resistance and improvement in yield characters under irrigated conditions. Materials and Methods: Twelve rice cultivars were screened for the mechanism of resistance against brown spots. Among these six were local (Indrajal, Gurmatia, Dehula, Aajaan, Lochai and Newari) and six commercial including Pusa Basmati-1, Vandana, Govinda, Jaya, Kalinga and IR-64. Standard farming practices were applied for optimum crop growth and observation of different mechanisms of the host-pathogen and disease developments in different periodic phases. Brown spot symptoms starting from the date of first appearance approaching maturity were recorded during both years. The assessments were done by estimating the apparent infection rate using standard evaluation systems (0-9 scale) and disease index (DI) of the cultivars was also reported. Results: The apparent infection rate increases at weekly intervals till the successful invasion of the pathogen. The cultivars show a remarkable variation in the rate of infection, however, the disease intensity was higher in local races compared to improved varieties. High susceptibility was reported in local races ‘Gurmatia’ and ‘Dehula’ and the hybrid ‘Pusa Basmati-1' was found moderately susceptible, whereas other varieties also show disease incidence though the degree of susceptibility was comparatively low. Improved mega variety IR-64 expressed minimum DI and maximum yield compared to other tested cultivars. Conclusion: Low rainfall and temperature were more conducive for brown spot occurrences in the region. Therefore, the selection of landraces and quality seeds is a decisive factor in minimizing brown spot infection and restricting disease epidemics in the region.