Effect of Field Resistance on Leaf Blast Development in Mixtures of Susceptible and Resistant Rice Cultivars

Abstract

Effect of field resistance on the development of leaf blast in mixtures of susceptible and resistant rice cultivars was studied in three field experiments (one in the upland nursery and two in the paddy fields) by using two susceptible rice cultivars with different (low and intermediate) levels of field resistance to the disease. In the upland nursery experiment where infected leaves with blast fungus were strewed uniformly in the plots and in the paddy experiment in 1982 where the disease occurred naturally, the disease severity on respective susceptible cultivars decreased with the reduction in the proportion of the susceptible cultivars in the plots, and the decrease for the cultivar with a low level of field resistance (LFC) was greater than that for the cultivar with an intermediate level of the resistance (MFC). Similar results were obtained in the paddy experiment in 1981 where an inoculum source was placed at the center of each plot. Since leaf blast severity in each experiment was smaller for MFC than for LFC and the disease gradient on the susceptible cultivar from the inoculum source was steeper for MFC than for LFC in the paddy experiment in 1981, it was assumed that less spread of the disease between plants on MFC reduced the effectiveness of the mixtures for the control of the disease.