GRC, International Rice Research Institute, PO Box 933, Manila, Philippines

Background and objectives
Utilizing rice cultivars with durable blast resistance is the most preferred option for blast management in many blast-prone rice growing areas. By definition, durable resistance is recognized only after prolonged cultivation in a large disease-prone area. Various evaluation schemes have been proposed to identify potentially durable resistance. The multilocation test is commonly utilized as an initial evaluation step to assess spatial resistance. Spatial resistance is the effectiveness of genetic resistance against diverse blast populations present in many test sites. This is based on the assumption that the broad spectrum of spatial resistance might be highly associated with durable resistance [1]. Temporal resistance refers to the stability of genetic resistance, either complete or partial, over a prolonged period of close association between rice cultivar and blast pathogen in a given area or test site. Stability of complete and/or partial resistance in a specific pathosystem is an important trait of all durably resistant rice cultivars. An earlier study has demonstrated that the durably resistant rice cultivars have generally exhibited a broad spectrum of spatial resistance and highly stable temporal resistance in their respective areas of cultivation. However, the relation between the spatial and temporal resistance in conventional evaluation schemes was not examined in depth. The present analysis was done to determine the relationship among durable resistance of known rice cultivars, and the spatial and temporal resistance in current evaluation schemes for blast resistance.

Results and conclusions
Eight durably resistant rice cultivars showed a broad spectrum of spatial resistance in the International Rice Blast Nursery (IRBN) and highly stable temporal resistance in farmers' fields and/or sequential evaluation plots. These include Oryzica Lianos 5 from Colombia, IR64 from the Philippines, and Samgangbyeo from Korea. Sequential evaluation was made by continuously monitoring disease severity in a series of staggered plantings of the same test cultivars inoculated with all available compatible isolates [2]. This technique can determine the temporal stability of partial resistance over a prolonged period. Three non-durable rice cultivars-CICA 4, IR50 and Yushin exhibited narrow spatial resistance and unstable temporal resistance. Though exhibiting a broad spectrum of spatial resistance in IRBN, five rice cultivars -- Tetep, Carreon, CICA 8, CICA 9, and C101A51 -- did not show stable temporal resistance in respective areas of cultivation or in sequential evaluations. Todorokiwase, a japonica rice, showed an intermediate degree of spatial resistance, but exhibited stable temporal resistance in farmers' fields in Japan. This analysis indicates that the evaluation of spatial resistance alone is not sufficient to identify potentially durable resistance. The spatial resistance is not related with the temporal resistance. Accordingly, the assessment of spatial resistance using multilocation evaluation should be followed by the assessment of temporal resistance. No simple method to determine the temporal resistance is available yet. Yearly testing of blast resistance for a short period can not reliably assess temporal resistance, because it does not allow a prolonged interaction between rice cultivars and potentially compatible pathogen populations. The sequential evaluation technique appears suitable for determining the stability of partial resistance over time.

1. Ahn SW, Ou SH, 1982. Phytopathology 72, 282-284.
2. Kim CH, Ahn SW, 1991. Korean J. Breeding 23, 103-110.