Suppression of Phytophthora capsici in Chile Pepper Using Brassica juncea and Hordeum vulgare Cover Crop Residues and Trichoderma harzianum as a Biocontrol Agent.

Phytophthora blight, caused by Phytophthora capsici, is a serious disease of many vegetable crops worldwide. In New Mexico, U.S.A., the disease affects chile pepper (Capsicum annuum L.), a major crop in the state. There is no single tool that effectively controls the disease. Continuous research is needed in identifying combination of tools that can reduce the impact of Phytophthora blight. We explored the potential of combining cover crops and biocontrol agents to reduce soilborne diseases. This study aimed to evaluate the effects of Indian mustard (Brassica juncea L.) cover crop on the antagonistic ability of Trichoderma harzianum against P. capsici in vitro and to quantify the impacts of combining soil amendment with residues from B. juncea and barley (Hordeum vulgare L.) cover crops and plastic covering on infection of chile pepper seedlings by P. capsici under greenhouse conditions. Volatiles from macerated tissue of B. juncea significantly reduced P. capsici and T. harzianum growth in the absence of soil by 89.0 and 79.0%, respectively. When incorporated in soils, volatiles from macerated tissue of B. juncea significantly reduced P. capsici and T. harzianum by 33.4 and 7.8%, respectively. T. harzianum was more resilient to B. juncea biofumigation than P. capsici. Significant reduction in disease incidence was observed with B. juncea-fumigated soil, while no disease suppression was observed with soil incorporation of H. vulgare residues. Covering soil with plastic was necessary for increasing the efficacy of B. juncea biofumigation.

Competition between cheatgrass and bluebunch wheatgrass is altered by temperature, resource availability, and atmospheric CO2 concentration.

Global change drivers (elevated atmospheric CO2, rising surface temperatures, and changes in resource availability) have significant consequences for global plant communities. In the northern sagebrush steppe of North America, the invasive annual grass Bromus tectorum (cheatgrass) is expected to benefit from projected warmer and drier conditions, as well as increased CO2 and nutrient availability. In growth chambers, we addressed this expectation using two replacement series experiments designed to test competition between B. tectorum and the native perennial bunchgrass Pseudoroegneria spicata. In the first experiment, we tested the effects of elevated temperature, decreased water and increased nutrient availability, on competition between the two species. In the second, we tested the effects of elevated atmospheric CO2 and decreased water availability on the competitive dynamic. In both experiments, under all conditions, P. spicata suppressed B. tectorum, though, in experiment one, warmer and drier conditions and elevated nutrient availability increased B. tectorum's competitiveness. In experiment two, when grown in monoculture, both species responded positively to elevated CO2. However, when grown in competition, elevated CO2 increased P. spicata's suppressive effect, and the combination of dry soil conditions and elevated CO2 enhanced this effect. Our findings demonstrate that B. tectorum competitiveness with P. spicata responds differently to global change drivers; thus, future conditions are unlikely to facilitate B. tectorum invasion into established P. spicata communities of the northern sagebrush steppe. However, disturbance (e.g., fire) to these communities, and the associated increase in soil nutrients, elevates the risk of B. tectorum invasion.

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.

A warmer and drier climate in the northern sagebrush biome does not promote cheatgrass invasion or change its response to fire.

Dryland shrub communities have been degraded by a range of disturbances and now face additional stress from global climate change. The spring/summer growing season of the North American sagebrush biome is projected to become warmer and drier, which is expected to facilitate the expansion of the invasive annual grass Bromus tectorum (cheatgrass) and alter its response to fire in the northern extent of the biome. We tested these predictions with a factorial experiment with two levels of burning (spring burn and none) and three climate treatments (warming, warming + drying, and control) that was repeated over 3 years in a Montana sagebrush steppe. We expected the climate treatments to make B. tectorum more competitive with the native perennial grass community, especially Pseudoroegneria spicata, and alter its response to fire. Experimental warming and warming + drying reduced B. tectorum cover, biomass, and fecundity, but there was no response to fire except for fecundity, which increased; the native grass community was the most significant factor that affected B. tectorum metrics. The experimental climate treatments also negatively affected P. spicata, total native grass cover, and community biodiversity, while fire negatively affected total native grass cover, particularly when climate conditions were warmer and drier. Our short-term results indicate that without sufficient antecedent moisture and a significant disruption to the native perennial grass community, a change in climate to a warmer and drier spring/summer growing season in the northern sagebrush biome will not facilitate B. tectorum invasion or alter its response to fire.

Impact of Cropping Systems, Soil Inoculum, and Plant Species Identity on Soil Bacterial Community Structure.

Farming practices affect the soil microbial community, which in turn impacts crop growth and crop-weed interactions. This study assessed the modification of soil bacterial community structure by organic or conventional cropping systems, weed species identity [Amaranthus retroflexus L. (redroot pigweed) or Avena fatua L. (wild oat)], and living or sterilized inoculum. Soil from eight paired USDA-certified organic and conventional farms in north-central Montana was used as living or autoclave-sterilized inoculant into steam-pasteurized potting soil, planted with Am. retroflexus or Av. fatua and grown for two consecutive 8-week periods to condition soil nutrients and biota. Subsequently, the V3-V4 regions of the microbial 16S rRNA gene were sequenced by Illumina MiSeq. Treatments clustered significantly, with living or sterilized inoculum being the strongest delineating factor, followed by organic or conventional cropping system, then individual farm. Living inoculum-treated soil had greater species richness and was more diverse than sterile inoculum-treated soil (observed OTUs, Chao, inverse Simpson, Shannon, P < 0.001) and had more discriminant taxa delineating groups (linear discriminant analysis). Living inoculum soil contained more Chloroflexi and Acidobacteria, while the sterile inoculum soil had more Bacteroidetes, Firmicutes, Gemmatimonadetes, and Verrucomicrobia. Organically farmed inoculum-treated soil had greater species richness, more diversity (observed OTUs, Chao, Shannon, P < 0.05), and more discriminant taxa than conventionally farmed inoculum-treated soil. Cyanobacteria were higher in pots growing Am. retroflexus, regardless of inoculum type, for three of the four organic farms. Results highlight the potential of cropping systems and species identity to modify soil bacterial communities, subsequently modifying plant growth and crop-weed competition.

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.

Impact of biotic and abiotic stresses on the competitive ability of multiple herbicide resistant wild oat (Avena fatua).

Ecological theory predicts that fitness costs of herbicide resistance should lead to the reduced relative abundance of resistant populations upon the cessation of herbicide use. This greenhouse research investigated the potential fitness costs of two multiple herbicide resistant (MHR) wild oat (Avena fatua) populations, an economically important weed that affects cereal and pulse crop production in the Northern Great Plains of North America. We compared the competitive ability of two MHR and two herbicide susceptible (HS) A. fatua populations along a gradient of biotic and abiotic stresses The biotic stress was imposed by three levels of wheat (Triticum aestivum) competition (0, 4, and 8 individuals pot(-1)) and an abiotic stress by three nitrogen (N) fertilization rates (0, 50 and 100 kg N ha(-1)). Data were analyzed with linear mixed-effects models and results showed that the biomass of all A. fatua populations decreased with increasing T. aestivum competition at all N rates. Similarly, A. fatua relative growth rate (RGR) decreased with increasing T. aestivum competition at the medium and high N rates but there was no response with 0 N. There were no differences between the levels of biomass or RGR of HS and MHR populations in response to T. aestivum competition. Overall, the results indicate that MHR does not confer growth-related fitness costs in these A. fatua populations, and that their relative abundance will not be diminished with respect to HS populations in the absence of herbicide treatment.