EVALUATION OF RESISTANCE TO WHEAT SPINDLE STREAK MOSAIC VIRUS BY ANALYSIS OF DISEASE INCIDENCE DATA

Authors: Cadle-Davidson, Lance; SORRELLS, MARK - CORNELL UNIVERSITY; Gray, Stewart; BERGSTROM, GARY - CORNELL UNIVERSITY

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2006
Publication Date: 8/1/2006
Citation: Cadle Davidson, L.E., Sorrells, M.E., Gray, S.M., Bergstrom, G.C. 2006. Evaluation of resistance to wheat spindle streak mosaic virus by analysis of disease incidence data. Plant Disease 90(8):1045-1050.

Interpretive Summary: Wheat spindle streak mosaic virus (WSSMV) causes significant disease resulting in measurable yield loss in autumn-sown wheat in most years in the Northeastern United States. WSSMV is putatively vectored by the protozoan plasmodiophorid Polymyxa graminis, an obligate parasite of wheat roots that can survive in dry soil without a host for at least ten years while retaining transmissibility of the viruses it may harbor. Transmission occurs shortly after autumn planting, but because of slow systemic viral movement, symptoms are typically not expressed until the following spring. In New York, symptom expression begins in early May (stem elongation stages) with typical symptoms appearing as elongated spindle-shaped chlorotic streaks, each surrounding a dark green island. Symptoms fail to develop on emerging leaves when the average temperature exceeds 15C, but can re-initiate on growth as late as the flag leaf given conducive weather conditions. Chemical and cultural control tactics for managing WSSMV have not been identified. Due to the longevity of WSSMV and P. graminis in soil even in the absence of a host, the only practical control tactic once WSSMV becomes established in a field is the planting of resistant genotypes. Resistance to WSSMV is highly heritable and controlled by a few dominant genes. In a population of recombinant inbred lines derived from a cross between resistant ‘Geneva’ and susceptible ‘Augusta,’ one major gene for resistance to WSSMV resided on chromosome arm 2DL. Cultivar resistance against the putative vector has not been identified. Presumably, the vector is able to transmit WSSMV to the cortical root tissue of all host genotypes. Resistance to WSSMV has been characterized as a reduction in the incidence of symptomatic plants but does not necessarily correspond with a reduction in virus titer or symptom severity in infected plants. Using serological methods, WSSMV has been detected in roots of the resistant wheat cultivar ‘Geneva’ not expressing foliar symptoms, suggesting that resistance blocks neither transmission nor cell-to-cell movement in roots. In a study looking at the incidence of Wheat spindle streak mosaic (WSSM) on individual stems in the field, a small proportion of individual ‘Geneva’ plants became systemically infected with WSSMV and produced foliar symptoms. Highly resistant ‘KS92WGRC22’ had a low incidence of symptoms on individual plants, and on those plants that were symptomatic, individual stems were asymptomatic. Together, these observations suggest that one mechanism of resistance to WSSMV is to restrict the movement of virus into foliar portions of the plant and that this restriction can be overcome in individual plants, resulting in foliar symptoms. We assessed 113 genotypes of autumn-sown wheat, rye, triticale, and barley for resistance to WSSMV in a three-year trial in a field that had been used continuously for WSSMV evaluation for over twenty years. None of the genotypes were immune to infection. Sixty-seven of the regionally-adapted genotypes repeatedly expressed strong resistance to WSSMV, thus providing growers with a choice of cultivars for reducing losses to WSSMV.

Technical Abstract: Once Wheat spindle streak mosaic virus (WSSMV) becomes established in a field, the only available control tactic is the planting of resistant genotypes. In this study we assessed 113 genotypes of autumn-sown wheat, rye, triticale, and barley for resistance to WSSMV in a three-year trial in a field that had been used continuously for WSSMV evaluation for over twenty years. Because resistance to WSSMV reduces the percentage of plants that develop detectable virus titer and symptoms, we collected and analyzed disease incidence data. A statistical approach was devised using logistic regression to compare objectively the relative susceptibility of genotypes without losing power to severity or ranks classification. None of the genotypes were immune to infection. Sixty-seven of the regionally-adapted genotypes repeatedly expressed strong resistance to WSSMV, thus providing growers with a choice of cultivars for reducing losses to WSSMV. Because of a significant interaction between genotypes and environment (year), genotypes should be assessed for incidence of symptomatic plants in multiple years, particularly when differentiating intermediate resistance responses from other classes.