A STUDY ON THE EPIDEMIOLOGY OF STRIPE RUST, PUCCINIA STRIIFORMIS WEST., IN CALIFORNIA

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Abstract
Stripe rust of wheat, Puccinia striiformis f. sp. tritici, was found to oversummer in the Sierra Nevada at altitudes of 6000 ft or above on wild grasses belonging to Elymus spp., Hordeum spp., and Sitanion spp. The similarity of infection types of stripe rust isolates from various locations and hosts on a differential set of wheat cultivars suggests that only one race of P. striiformis f. sp. tritici occurs in California.Stripe rust on Poa spp. should be considered as P. striiformis f. sp. poae f. sp. nov. because of its entirely different host range and temperature requirements.A mean temperature of 22.3 °C or mean maximum temperature of 32.4 °C measured over a 10-day period is lethal to P. striiformis f. sp. tritici, thus accounting for the absence of this fungus during the summer in all regions of California but the Sierra Nevada and the coastal area. In late autumn, recurrence of the rust in the wheat-growing areas is initiated when east winds carry uredospores from the Sierra Nevada into the central part of the Sacramento – San Joaquin Valley, infecting volunteer wheat plants and early sown wheat. In late winter, a shortening of the latent period because of rising daily temperatures results in a rapid development of stripe rust at these infection sites and consequently in secondary spread of the rust to previously uninfected areas.