Assessing the effect of resistant-susceptible associations and determining thresholds for associations in suppressing leaf and neck blast of rice

Blast has become a major problem in the upland rice of Indonesia, as the improved resistant varieties with high yield performances released in the 90's became susceptible only a few years after their release. The present study investigated the efficiency of association of susceptible and resistant varieties in reducing leaf and neck blast and increasing yield. Two sets of experiments were conducted in fields with associations through using different proportions of resistant variety, and two types of susceptible variety (moderately and very susceptible) in the association. The efficiency of the association was measured by comparing disease intensity and yield in the variety when grown in association and when grown as a pure stand. ANOVAs and logistic regressions indicated that resistant-susceptible variety association was more efficient in reducing neck blast than leaf blast. Associations were more efficient (1) when the proportion of resistant variety was increased, (2) when a moderately susceptible variety was used instead of a very susceptible variety, and (3) when disease level in the pure stand was higher. Associations did not penalize the performances of the resistant variety, neither in terms of disease intensity, nor yield. Logistic regressions allowed to estimate the probability of a given association to reduce disease (or to increase yield) above a preset threshold, according to the proportion of resistant variety and to the type of susceptible variety.