Ecology and Epidemiology of Wheat streak mosaic virus, Triticum mosaic virus, and their mite vector in wheat and grassland fields
Price, Jacob A.
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The Great Plains region serves as one of the most important areas for wheat production in the United States. This region, including Montana through Texas, produces approximately 59% of the U.S. wheat production. In the more southern and southwestern regions, winter wheat typically planted as a dual purpose crop for both cattle grazing and grain production. This system allows producers to earn extra income for grazing, however early fall planting also exposes the wheat to a variety of pests and pathogens. Some of the most important viral pathogens affecting wheat in this area are the mite-vectored viruses Wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV), and Wheat mosaic virus (WMoV). Within this virus group, WSMV and TriMV are most commonly found during co-infection, which increases disease severity. These viruses are transmitted by the wheat curl mite, (WCM) Aceria tosichella, which relies on wind for passive movement from host to host. The WCM and viruses survive between wheat seasons on reservoir hosts such as volunteer wheat and some perennial and annual grasses. Management of these pathogens are mainly aimed at cultural control through destruction of alternative hosts and delayed planting. Many of these methods are outdated and have not been evaluated in the Texas High Plains. Therefore studies were conducted to examine the ecology and epidemiology of WSMV and TriMV by evaluating alternative host sources, time of vector movement during the year, and pathogen distribution within the host and vector. Native and introduced grasses, commonly found in CRP and rangeland plantings, were evaluated for potential to serve as alternative host reservoirs for WCM and WSMV and TriMV. Three fields classified as a CRP, rangeland, and an underdeveloped grassland field were evaluated for WCM population dynamics. Higher eriophyid mite population numbers, including the WCM, were detected in the underdeveloped field when compared to the CRP or rangeland fields. The underdeveloped field contained a large number of other grasses not found in CRP and rangeland plantings. Therefore to further examine CRP grasses for their ability to serve as alternative hosts, CRP grasses were infested by the WCM and inoculated with WSMV and TriMV. Grasses commonly planted in CRP fields did not serve as hosts for either the WCM or WSMV and TriMV. However, other grasses including western wheatgrass and rescuegrass were found to be hosts for the WCM but not WSMV or TriMV. To identify areas that pose a potential source for wheat virus disease, surveys were conducted on CRP, rangeland fields, and roadside areas adjacent or next to wheat production fields. All grass species found within each field and roadside site were identified as either hosts or non-host species for both the WCM and viral pathogens. CRP and rangeland fields contained low numbers of host grasses, except in the case of witchgrass, which was detected in 33% of CRP fields. However, roadside areas contained a large number of warm season hosts for both the WCM and WSMV and TriMV. Samples of witchgrasss, prairie grass, and barnyardgrass, collected from a single roadside location adjacent to volunteer wheat infected by WSMV and TriMV, tested positive for virus infection.. Roadside areas containing these and other warm season grasses were determined to be a more important sources of wheat viral disease. Studies were also conducted to evaluate the effectiveness of late planting as a tactic to reduce mite movement and pathogen spread from volunteer wheat. Movement of WCMs from the volunteer wheat during the winter months was detected even after delayed planting in October. Incidence of WSMV also was detected as early as December. These early infections resulted in reductions in grain yield that increased in proximity to the source point. Therefore, even delayed planting in the presence of volunteer wheat does not protect wheat against winter infection. Further studies examined co-infection of both WSMV and TriMV within host plants and the WCM along disease gradients. Incidence of co-infection with both WSMV and TriMV were found to be higher at the edge of the field, closest to the area of initial pathogen/vector spread. Co-infections decreased with distance into the field with single infections by WSMV continuing to a greater distance. Single infections of only TriMV were found in low incidence in both fields. WCMs also displayed differences in pathogen distribution. Field 1 contained a large percentage of mites carrying both WSMV and TriMV near the edge of the field. Field 2 contained lower numbers of co-viruliferous mites and no mites carrying TriMV only. Mites collected from co-infected wheat tillers were also examined for differences in WSMV and TriMV titer levels. Plants co-infected with both WSMV and TriMV contained higher levels of TriMV titer in both fields. However, mites collected from these tillers contained higher levels of WSMV than TriMV demonstrating preferential uptake of WSMV over TriMV. These findings help to illuminate pathogen distributions detected within fields containing co-infections of both WSMV and TriMV and higher transmission efficiencies of WSMV. Multiple aspects of WSMV and TriMV ecology and epidemiology were explored to evaluate management tactics involving CRP and alternative hosts, delayed planting to reduce infection incidence, and disease spread during co-infection by WSMV and TriMV. Grasses commonly used in CRP and rangeland fields do not serve as significant sources of WSMV and TriMV infection. However, roadside areas contain a variety of host grasses and could serve as a parietal risk for wheat disease. WCM populations are capable of movement even after delayed planting therefore strict volunteer control is need. Vector populations have an effect on disease distribution within fields containing co-infections of WSMV and TriMV due to preferential uptake of WSMV over TriMV. These factors need further exploration in the case of synergistic interactions between WSMV and TriMV.