Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/7/2002
Publication Date: 12/15/2002
Citation: CARROLL, J.E., BERGSTROM, G.C., GRAY, S.M. ASSESSING THE RESISTANCE OF WINTER WHEAT TO WHEAT SPINDLE STREAK MOSAIC BYMOVIRUS. CANADIAN JOURNAL OF PLANT PATHOLOGY. 2002. Interpretive Summary: Wheat spindle streak mosaic virus is widespread throughout North American wheat growing regions and can cause a significant yield reduction. Resistance to the virus or its soil-borne protozoan vector will be the best control strategy, yet resistant wheat cultivars have been difficult to identify. Resistance is generally assessed on the basis of disease symptom expression, however, symptoms are highly influenced by environmental conditions and plants are not uniformly infected. A more complete understanding of resistance mechanisms is required to improve the identification of resistant germplasm. We conducted greenhouse and laboratory evaluations of wheat cultivars that had been identified as resistant in field trials. When virus was introduced directly into the leaves of a resistant plant it was able to cause an infection, whereas root inoculation yielded, in most cases, no infection of the aerial portions of the plant. In cultivars where virus infection of the roots was also limited, these plants were not a good source of virus for further infections. If virus did multiply in the roots then the plant was a potential source of inoculum regardless of whether the virus infected the leaves. Evaluation of resistance should not rely solely on symptom expression in leaves, but a test to determine root infection is required.
Technical Abstract: Management of wheat spindle streak mosaic bymovirus (WSSMV) relies on resistant cultivars. We compared mechanical inoculation with virus-infected plant sap to previous results from natural infection in field trials. The six cultivars tested ('Century', 'Wichita', 'TAM 200', 'Geneva', 'Harus', and 'Augusta') became infected via mechanical inoculation, ruling out resistance to virus replication in leaves as the basis for field resistance in the cultivars considered resistant in field trials. Augusta (susceptible), Geneva (resistant), and germplasm K92WGRC22 (highly resistant) were compared in field plots for numbers of symptomatic vs. asymptomatic tillers. Individual plants having both infected and uninfected tillers were found only in the two resistant wheats. Resistance in Geneva and K92WGRC22 could be due to uneven movement of virus from roots into tillers or to reduced virus replication in roots. If resistance reduces virus replication in roots, cropping with such a cultivar would contribute less to the inoculum potential of soil than susceptible cultivars would. Soil transmission of virus occurred in field soil gathered from the root zone of symptomatic Augusta, but not in field soil from the root zone of asymptomatic KS92WGRC22. These results suggest that infected plants, in a single crop cycle, contribute significantly to the soil reservoir of WSSMV.