Endria inimicaSay acquired the North American type of wheat striate mosaic virus during periods of 15 minutes or longer on diseased plants held at five constant temperatures ranging from 10 to 33 deg;C. When infective insects were given inoculation access periods varying from 1 to 4 days at different temperatures, the percentage of test plants infected increased with temperature from 12.5 at 10deg; to 81.4 at 33 deg;C. After an acquisition access period of 2 days at 24 deg;C, insects kept at 8 or 10 deg;C did not transmit virus, but the percentage of others that transmitted at successively higher temperatures increased from 3.3 at 16 deg;C to 73.3 at 33 deg;C. The preinfective period was more than 29 days for insects kept at 16 deg;C and only 5 days for some kept at 27, 30, and 33 deg;C. The average preinfective period was 11 days at 20 deg;C, but decreased to 6.4 days as temperature increased to 33 deg;C. The percentage of test plants that became infected increased from 0.1 at 16 deg;C to 44.3, at 33 deg;C. Stewart and Ramsey wheat seedlings exposed to infectiveE. inimicafor 2 days did not develop symptoms during a subsequent 60 day period at 10 deg;C. After the same plants were placed in a greenhouse at 20ndash;25 deg;C, 26 and 27, respectively, developed symptoms. The incubation period for symptoms in plants ranged from 17 to more than 62 days at 16 deg;C. It decreased as temperature increased but varied from 6 to 25 days at 30 deg;C. Forty-two and 48 of Stewart and Ramsey wheat plants respectively, developed symptoms at 16 deg;C, and increased to almost 100 for both varieties at 30 and 33 deg;C. The above results indicate that high temperatures during early summer are prerequisite for severe epidemics of wheat striate mosaic in spring wheat.
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