Partial resistance in rice to blast and how to select for it

A detailed study of three components of partial resistance (PR) to leaf blast in tropical lowland rice genotypes was made. Among the components relative infection efficiency (RIE), measured as the number of sporulating lesions that developed, lesion size, and latent period, the (RIE), appeared to be the most important component. Genotypes with a higher level of PR developed less lesions which usually had smaller sporulating areas compared to the susceptible check. No difference of any importance between genotypes was found for latent period. Genetic analysis showed that the (RIE), is controlled by several minor genes. Small differential interactions between host-cultivars and virulent pathogen genotypes were detected, indicating that RIE-reducing genes in the host operate on a gene-for-gene basis with genes in the pathogen. Only young leaf tissue is susceptible to the blast pathogen. PR rapidly increases with aging of the leaf, and the higher the level of PR of a genotype, the faster the leaf tissue become resistant. A higher level of resistance was mainly expressed as a more rapid reduction of the number of sporulating lesions.

Genotypes developing fewer lesions per plant in the greenhouse demonstrated a higher level of PR in field studies. The total number of sporulating lesions per plant and the number of leaves (from the top) in which these lesions developed were closely associated in pure, as well as in segregating lines. Therefore, counting the number of leaves with sporulating lesions instead of counting all sporulating lesions per plant will suffice when selection for improved PR to leaf blast is applied. Since PR to blast is controlled by several genes, selection for improved PR should preferably occur in early generations of a breeding program. The present study shows that such selection is feasible. The (RIE), was significantly reduced by selection in F ₂ populations. However, due to the high error of individual plant assessment, selection among lines was much more effective. Selection of the plants with the fewest lesions from the most resistant F ₃ lines of crosses is recommended to accumulate genes for PR to blast.

A higher PR to leaf blast was usually, but not always, linked to a higher PR to neck blast. As in leaves, PR to neck blast rapidly increased with aging. This was mainly expressed as a rapid increase of resistance to growth of the pathogen after infection. The lesion length was closely correlated to yield loss that occurred. Because PR to neck blast can be affected considerably even by small differences in development stage of the panicle, meaningful comparison of PR to neck blast between genotypes requires infection at an identical stage of panicle development. Inoculation of neck nodes at the time of flowering is recommended for screening PR to neck blast. Promising entries are those in which typical neck blast symptoms are observed after exposure to a virulent isolate, but in which relatively few necks become infected, and in which the yield loss in panicles that develop neck blast remains relatively small.