TEMPERATURE IN RELATION TO THE TRANSMISSION AND PATHOGENICITY OF WHEAT STRIATE MOSAIC VIRUS

Endria inimica Say 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 °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 10° to 81.4% at 33 °C. After an acquisition access period of 2 days at 24 °C, insects kept at 8 or 10 °C did not transmit virus, but the percentage of others that transmitted at successively higher temperatures increased from 3.3% at 16 °C to 73.3% at 33 °C. The preinfective period was more than 29 days for insects kept at 16 °C and only 5 days for some kept at 27, 30, and 33 °C. The average preinfective period was 11 days at 20 °C, but decreased to 6.4 days as temperature increased to 33 °C. The percentage of test plants that became infected increased from 0.1% at 16 °C to 44.3%, at 33 °C. Stewart and Ramsey wheat seedlings exposed to infective E. inimica for 2 days did not develop symptoms during a subsequent 60 day period at 10 °C. After the same plants were placed in a greenhouse at 20–25 °C, 26% and 27%, respectively, developed symptoms. The incubation period for symptoms in plants ranged from 17 to more than 62 days at 16 °C. It decreased as temperature increased but varied from 6 to 25 days at 30 °C. Forty-two and 48% of Stewart and Ramsey wheat plants respectively, developed symptoms at 16 °C, and increased to almost 100% for both varieties at 30 and 33 °C. The above results indicate that high temperatures during early summer are prerequisite for severe epidemics of wheat striate mosaic in spring wheat.