DYNAMICS OF WHEAT SPINDLE STREAK MOSAIC BYMOVIRUS IN WINTER-WHEAT

The dynamics of wheat spindle streak mosaic bymovirus in winter wheat were studied during two crop cycles in a field site with a history of high virus incidence. Individual plants of two susceptible cultivars w ere sampled from autumn to spring and the presence of virus antigen in roots and leaves was determined by ELISA. Virus incidence was higher in cv. Frankenmuth than in cv. Augusta. During year one, incidence of viral antigen in roots remained very low for four months after sowing, and did not reach maximum levels until the following spring. During y ear two, incidence of viral antigen in roots rose to maximum levels in autumn, only three months after sowing. These results strongly sugges ted that root infection occurred in spring as well as in autumn. In bo th cultivars and in both years, we detected the virus in roots one mon th prior to its detection in leaves, suggesting that virus moves slowl y from roots into leaves. Maximum incidence of virus in leaves occurre d in spring of both years, coinciding with the period of symptom devel opment. Typical symptoms (yellow streaks, spindles, and mosaic) were o bserved in year two, whereas only mild mosaic was observed in year one . Virus antigen was detected in nonsymptomatic leaves from two months after sowing through crop senescence. Because antigen could be detecte d in roots throughout the crop cycle, and zoosporangia and cystosori o f the fungal vector could be detected one and two months, respectively , after sowing, it is possible that wheat spindle streak mosaic bymovi rus is acquired and/or spread by the vector during the majority of the crop cycle.