Zachariah Miller

Impacts of Crop Variety and Time of Inoculation on the Susceptibility and Tolerance of Winter Wheat to Wheat streak mosaic virus

Plant genotype, age, size, and environmental factors can modify susceptibility and tolerance to disease. Understanding the individual and combined impacts of these factors is needed to define improved disease management strategies. In the case of Wheat streak mosaic virus (WSMV) in winter wheat, yield losses and plant susceptibility have been found to be greatest when the crop is exposed to the virus in the fall in the central and southern Great Plains. However, the seasonal dynamics of disease risk may be different in the northern Great Plains, a region characterized by a relatively cooler fall conditions, because temperature is known to modify plant-virus interactions. In a 2-year field study conducted in south-central Montana, we compared the impact of fall and spring WSMV inoculations on the susceptibility, tolerance, yield, and grain quality of 10 winter wheat varieties. Contrary to previous studies, resistance and yields were lower in the spring than in the fall inoculation. In all, 5 to 7% of fall-inoculated wheat plants were infected with WSMV and yields were often similar to uninoculated controls. Spring inoculation resulted in 45 to 57% infection and yields that were 15 to 32% lower than controls. Although all varieties were similarly susceptible to WSMV, variations in tolerance (i.e., yield losses following exposure to the virus) were observed. These results support observations that disease risk and impacts differ across the Great Plains. Possible mechanisms include variation in climate and in the genetic composition of winter wheat and WSMV across the region.

Effects of Soil Nitrogen and Atmospheric Carbon Dioxide on Wheat streak mosaic virus and Its Vector (Aceria tosichella Kiefer)

Management of vector-borne plant viruses requires understanding how abiotic (e.g., resource availability) and biotic (e.g., virus-vector interactions) factors affect disease via effects on epidemiological parameters that drive disease spread. We conducted two complementary experiments using Wheat streak mosaic virus (WSMV): (i) a field study to determine the effects of nitrogen (N) fertilization on winter wheat (Triticum aestivum L.) susceptibility to WSMV infection and (ii) a growth chamber study to evaluate the effects of N and carbon dioxide (CO2) enrichment on population growth rates of the wheat curl mite (WCM), the vector of WSMV, and whether the effects of nutrient addition on WCM reproduction were modified by WSMV infection. The relationship between N fertilization and plant susceptibility to WSMV infection was nonlinear, with infection rates increasing rapidly as soil nitrate increased from 0 to 20 ppm and more gradually at higher nitrate concentrations. In the growth chamber study, N fertilization increased WCM population growth rates when the vectors transmitted WSMV but had the opposite effect on nonviruliferous mites. CO2 enrichment had no observable effects on WCM populations. These results suggest that, whereas the spread of WSMV is facilitated by N addition, increases in atmospheric CO2 may not directly alter WCM populations and WSMV spread.

Relative Canopy Height Influences Wild Oat (Avena fatua) Seed Viability, Dormancy, and Germination

Abstract The environment in which a plant grows (maternal environment) can affect seed viability, germination, and dormancy. We assessed the effects of maternal environment on wild oat seed viability, germination, dormancy, and pathogen infection by collecting and analyzing wild oat seed from above and below a barley canopy at three field sites in Montana. The viability of wild oat seed collected below a crop canopy was consistently less than it was for seed from the overstory but varied among sites and years. Reductions in viability because of relative canopy position ranged from 10% to 30%. Effects of position relative to crop canopy on weed seed germination/dormancy rates varied by site and suggest that the direction and magnitude of the effects of maternal environment on dormancy depend on environmental conditions. These effects may be driven by crop competition or by changes in seed pathogen pressure or both. Seven species each of fungi and bacteria were isolated from wild oat seeds. The only fungi causing reductions in seed viability (15%) was isolated from understory seeds, and several bacteria from both overstory and understory sources reduced seed germination. Results suggest that, in addition to the known weed-suppressive effects of using taller or earlier emerging varieties of crops, such crops can reduce weed spread through effects on weed seed demography because weeds growing beneath the crop canopy produce a reduced amount of viable seed that is less likely to germinate in the following year. Nomenclature: Wild oat, Avena fatua L. AVEFA, barley, Hordeum vulgare L.

Wheat and Barley Susceptibility and Tolerance to Multiple Isolates of Wheat streak mosaic virus

One of the greatest virus disease threats to wheat production in the Great Plains of the USA is Wheat streak mosaic virus (WSMV). Breeding programs have developed wheat varieties that are resistant or tolerant to WSMV infection, but these characteristics are climate dependent, and may also vary by WSMV isolate. We tested 10 spring and nine winter wheat (Triticum aestivum) varieties and two barley (Hordeum vulgare) varieties for resistance and tolerance to one WSMV isolate over four years. In spring wheat and barley, there were year by cultivar interactions in terms of resistance and tolerance. However, in winter wheat, yield losses due to WSMV were relatively consistent across years and varieties. Additionally, we tested the impacts of three WSMV isolates individually and in a mixture on twelve, two, and twelve varieties of spring wheat, barley, and winter wheat, respectively. Resistance and tolerance varied by isolate and cultivar, but there were no isolate by cultivar interactions. For spring wheat and barley, yield impacts were greater for two of the three single isolates than for the isolate mixture, whereas in winter wheat, the isolate mixture caused greater yield losses than the individual isolates. Overall, the results indicate that resistance and tolerance phenotypes were influenced by environmental conditions and by WSMV isolate or combination of isolates, suggesting that cultivar screening should be conducted over multiple years and with multiple virus isolates.

Temperature and alternative hosts influence Aceria tosichella infestation and Wheat streak mosaic virus infection

Wheat streak mosaic, caused by Wheat streak mosaic virus (WSMV; family Potyviridae), is the most important and common viral disease of wheat (Triticum aestivum L.) in the Great Plains of North America. WSMV is transmitted by the wheat curl mite (WCM; Aceria tosichella). We evaluated how mean daily temperatures, cumulative growing degree-days, day of the year, and surrounding alternative host identity affected WCM infestation and WSMV infection of wheat from late summer through early autumn in Montana, United States. Cumulative growing degree-days, warm mean daily temperatures (i.e., >10°C), and surrounding alternative hosts interacted to alter risk of WCM infestation and WSMV infection. Wheat surrounded by Bromus tectorum L. and preharvest volunteer wheat had WCM infestation and WSMV infection rates of 88% in years when the mean daily temperature was 15°C in October, compared with 23% when surrounded by bare ground, and <1% when the temperature was 0°C regardless of surrounding alternative host. Mean daily temperatures in the cereal-growing regions of Montana during autumn are marginally conducive to WCM population growth and movement. As the region continues to warm, the period of WCM movement will become longer, potentially increasing the frequency of WSMV outbreaks.