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 °C on the basis of data published elsewhere, and the
sum, accumulated during ten consecutive days, of the respective lethal temperature quotients (ALTQ
10), accounting for the
partial lethal effect of the daily ambient temperatures between 30 and 40.5 °C 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.
