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.

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The minimum and maximum temperatures for germination of uredospores of Puccinia striiformis, P. recondite, P. coronata and P.gramnis isolates from south‐central Chile were 0°C and 26°C, 0°C and 32°C, 8°C and 30°C, and 4°C and 34°C, respectively, whereas the shortest latent period was 8.5 days for P. struformis at 20°C, 5 days for P. recondite at 26°C, 5.5 days for P. coronata at temperatures, reaching the minimum and maximum threshold temperatures at 0.006°C and 23°C for P. strüformis, 0.31°C for P. recondita, 2.87°C and 30°C for P. coronata, and 1.72°C and 32°C for P. graminis. rspectively. The cardinal temperatures give no explanation as to the observed sequential appearance of these rusts during the growing season. Other phenomena like the systemic mycelial growth of P.striiformis might be involved here. At temperatures between 19°C and 22°C, the average daily increase of the area of sporulation of P.striiformis in wheat leaves varied between 9.05 and 22.48 mm2/infection site. This variation was due to substrate (plant) history and environmental factors.

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To study the over-summering of wheat stripe rust (Puccinia striiformis f.sp. tritici) in the California Central Valley (CCV), temperature records from various locations in the CCV during the period 1950-2009 were examined for the occurrence of lethal maximum temperatures for the uredinia and uredinio-mycelium of this fungus. The lethal upper threshold temperature for the uredinial stage of P.s. tritici, estimated to be 40.5 ºC on the basis of data published elsewhere, and the sum, accumulated during ten consecutive days, of the respective lethal temperature quotients (ALTQ10), accounting for the partial lethal effect of the daily ambient temperatures between 30 and 40.5 ºC on the uredinial stage of P.s. tritici, were used as yardsticks for thermal lethality. The results indicate that, in these 60 yr, the uredinia and the uredinio-mycelium of P.s. tritici could not possibly have over-summered at any of the locations studied. The Sierra Nevada Mountains, together with the Tulelake Basin and the coastal zone of the Pacific Ocean are the only two areas in California with appropriate environmental conditions for the summer-survival of the uredinial stage of stripe rust. Therefore, it is presumed that the inoculum for the initial infections of P.s. tritici in wheat fields in the CCV during the following growing season originates in either one or both of these areas, although, a potential third source of inoculum for the initial infections of stripe rust in the CCV could also be involved. Namely, the possible presence of telia with viable teliospores of P.s. tritici in autumn on straw of the threshed wheat fields or on volunteer wheat plants in the CCV, in conjunction with the accidental concurrence of nearby stripe rust susceptible barberry (Berberis spp.), could lead to the development of alternative, endogenous sources of inoculum in the CCV.