Survival ofPuccinia reconditaandP. graminisUrediniospores Exposed to Temperatures from Subfreezing to 35 C

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Viable urediniospores and dormant mycelial infections are the principal inoculum sources that contribute to the establishment and development of destructive wheat rust epidemics in the central Great Plains. Viability of urediniospores of Puccinia recondita and P. graminis was measured by exposure in environmental chambers set at constant temperatures ranging from −6 to 35 C. Exposure of P. recondita and P. graminis urediniospores for up to 120 h to constant temperatures of 5-35 C did not significantly (P ≤ 0.05) affect spore viability among isolates. However, P. recondita and P. graminis urediniospore viability as measured by germination observed after exposure to constant temperatures of -6, −4, −2, 0, 5, 10, 20, 25, 30, or 35 C for 24, 48, 72, 96, or 120 h indicated significant differences in viability among temperature treatment means (P ≤ 0.05). Urediniospores remained viable up to 864 h at constant temperatures between 10 and 30 C and up to 504 h at constant temperatures of 5 and 35 C. Viability of urediniospores exposed at temperatures ranging from −2 to 0 C decreased rapidly after the first 4 h of exposure. At below-freezing temperatures, viability of spores declined rapidly after 1-2 h of exposure. Viability of urediniospores exposed to freezing or below-freezing temperatures was not restored by heat shocking at temperatures typically encountered in the field during an epidemic. Infectivity of P. recondita and P. graminis urediniospores exposed at 5-35 C for 120 h was not significantly different from infectivity of unexposed urediniospores; an average of 58% of the viable urediniospores produced appressoria