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 soilborne wheat mosaic virus by analysis of disease incidence data. Plant Disease 90(8):1039-1044. Interpretive Summary: Soilborne wheat mosaic virus (SBWMV) infects wheat, rye, triticale, and barley, causing disease and yield loss on those crops sown in the autumn. SBWMV was first detected in New York State in 1998 and has since been spreading in the Finger Lakes region, where autumn-sown small grains are important in crop rotations with vegetables and other field crops. SBWMV is presumably 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. In New York, symptom expression begins in late April with typical chlorotic symptoms appearing in patches of plants where machinery first entered the field, reflecting recent introduction. Symptoms on individual leaves are a genotype-specific mosaic. Symptoms fail to develop on emerging leaves when the average temperature exceeds 20C, but can re-initiate on new growth as flag leaves emerge given conducive weather conditions. Wet autumns and prolonged cool periods in spring are conducive for disease development. A controlled environment study using infested field soil suggested that optimal SBWMV transmission requires at least 24 hours of matric potentials greater than –20kPa (wetter than field capacity) any time after seedling emergence but before soil temperatures drop below 7C. Suboptimal soil temperatures or moistures in the autumn result in less efficient transmission and therefore reduced disease incidence the following spring. Furthermore, cool weather in the spring promotes the development and maintenance of symptoms and disease severity. Currently, the only feasible control tactic for SBWMV, once inoculum has become established in a field, is the planting of resistant cultivars. Resistance to SBWMV appears to be controlled by 1-3 genes. Resistance is most often expressed as a reduction in the incidence of symptomatic plants, but not necessarily as reduced titer or severity in infected plants. Since cultivar resistance against the putative vector has not been identified, SBWMV is likely to be transmitted to the cortical root tissue of most host genotypes. Therefore, the resistance observed involves the inability of SBWMV to move from infected roots to foliar tissue and result in symptoms. Resistance evaluation using mechanical inoculation of leaves does not translate to field resistance, as resistance appears to be directed against root-to-shoot movement and not systemic movement in the shoots. We assessed one hundred fourteen regionally-adapted small grains genotypes for resistance to SBWMV over four growing seasons. No genotype assessed in multiple years was immune to infection. However, sixty-eight of the regionally-adapted genotypes tested repeatedly expressed strong resistance to SBWMV, providing growers a choice of cultivars for reducing losses to SBWMV.
Technical Abstract: Soilborne wheat mosaic virus (SBWMV) was detected in New York in 1998 for the first time and has been associated with yield loss where identified. We assessed one hundred fourteen regionally-adapted small grains genotypes for resistance to SBWMV over four growing seasons. Resistance to SBWMV reduces the percentage of plants that develop detectable viral titer and symptoms. Logistic regression was used to analyze disease incidence data and was compared to a general linear model for categorizing relative resistance to SBWMV. Logistic regression facilitated assessment of the effects of small sample size, low disease incidence, and non-uniform disease distribution. By increasing sample size from 20 to 30 stems per replicate, the number of resistance categories was increased through improved resolution of intermediate resistance classes. In environments with low disease incidence, the number of genotypes categorized as susceptible decreased while intermediate genotypes appeared to be resistant in the analysis. Inclusion of disease distribution data as covariates in a spatially-balanced experiment did not increase the power of the logistic analysis. No genotype assessed in multiple years was immune to infection. However, sixty-eight of the regionally-adapted genotypes tested repeatedly expressed strong resistance to SBWMV, providing growers a choice of cultivars for reducing losses to SBWMV.