RESPONSE TO SELECTION FOR HIGH AND LOW PARTIAL RESISTANCE TO LEAF BLAST IN F2 POPULATIONS OF 3 RICE CROSSES

Divergent selection for higher and lower partial resistance to leaf bl ast was applied in F-2 populations of crosses between the rice cultiva rs IR36, IR64 and CO39 after exposure to a virulent isolate. IR36 and IR64 are partially resistant while CO39 is highly susceptible. As sele ction criteria the number of sporulating lesions in leaves of the main culm or the lesion density in the topmost leaf relative to other F-2 plants with the same stage of development were used. A highly signific ant and meaningful response to selection was obtained in most cases, b ut the heritability was low. Realized heritabilities varied from 0.14 to 0.25 depending on the cross and were similar for both selection cri teria. Selection for improved partial resistance to leaf blast is poss ible as early as the F-2. The efficiency of selection is probably much higher if replicated tests could be made, and better results are ther efore expected if selection among F-3 lines is carried out. The result s indicated that the relatively low infection efficiency in IR36 and I R64 is oligo- or polygenically controlled.