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Incidence of Colletotrichum coccodes in Certified Potato Seed Tubers Planted in Washington State

Incidence of Colletotrichum coccodes in lots of certified seed tubers planted in Washington state, originating from nine western and midwestern states in the United State and two provinces in Canada, ranged from 0 to 90% in 1994 and 0 to 53% in 1995. In 1994, significant interactions between state/province and cultivar, and between seed grower and cultivar, were evident. In 1995, incidence of C. coccodes in seed lots did not vary significantly among states and cultivars. C. coccodes was not isolated from nuclear seed tubers and incidence of infected tubers was higher with higher seed generations. The fungus was isolated from the tuber periderm and outer medulla tissues and isolation frequency was greater from tuber stem ends than from either bud ends or lateral sections. Significantly greater stem infections developed in plants grown from seed tubers in which C. coccodes had been detected than in plants grown from seed tubers from which C. coccodes had not been isolated. This study confirms that C. coccodes is distributed among potato-production areas within seed tubers, and that seed tuber infection increases the incidence of early-season plant infection.