Determination of minimum number of growing seasons for assessment of disease resistance stability in rice

The literature reports that evaluations over a number of growing seasons are required to identify stable disease resistance in rice (Oryza sativa L.). CIAT's Rice Breeding Program aims to develop lines with stable resistance; therefore the knowledge of how long a germplasm should be assessed before being used in crosses is important. For this research, 69 commercial cultivars grown in Latin America and the Caribbean were evaluated during 11 growing seasons in a 'hot spot' site for the four most important diseases of rice: leaf and panicle blast (caused by Pyricularia grisea Sacc.), leaf scald [caused by Microdochium oryzae (Hashioka & Yokogi) Samuels & I.C. Hallett syn. Gerlachia oryzae (Hashioka & Yokogi) W. Gams syn. Rhynchosporium oryzae (Hashioka & Yokogi)], and grain discoloration (caused by species of Sarocladium, Bipolaris, Alternaria, Gerlachia, Fusarium, Phoma, Curvularia, Trichoconiella, and Pseudomonas). Cultivar mean disease severity (M), its response to increase in disease pressure (b), and the standard deviations from the regression (Sb) were calculated. Eleven evaluation seasons were considered as the most reliable period, in which an ideal set of 13 cultivars was identified based on their disease resistance stability (M, b, and Sb values). To identify the minimum number of seasons required to assess stability of resistance, simulations were made with these 13 cultivars and the 11 seasons and the coincidence in selection and the number of misclassified entries were calculated. The M and b correlation values and the number of misclassified entries suggested that seven consecutive growing seasons (two per 12-mo. period) are necessary to identify germplasm with stable reaction to leaf and panicle blast, leaf scald, and grain discoloration. (RA) eng