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 seaso ns are required to identify stable disease resistance in rice (Oryza s ativa L.). CIAT's Rice Breeding Program aims to develop lines with sta ble resistance; therefore, the knowledge of how long a germplasm shoul d be assessed before being used in crosses is important, For this rese arch, 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 (Hash ioka Sr Yokogi) W. Gams syn, Rhynchosporium oryzae (Hashioka & Yokogi) ], and gain 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 consi dered as the most reliable period, in which an ideal set of 13 cultiva rs was identified based on their disease resistance stability (M, b, a nd Sb values), To identify the minimum number of seasons required to a ssess stability of resistance, simulations were made with these 13 cul tivars and the 11 seasons and the coincidence in selection and the num ber of misclassified entries were calculated, The M and b correlation values and the number of misclassified entries suggested that seven co nsecutive growing seasons (two per 12-mo. period) are necessary to ide ntify germplasm with stable reaction to leaf and panicle blast, leaf s cald, and gain discoloration.