Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

<I>Bean pod mottle virus </I>(BPMV, Genus <I>Comovirus</I>, Family: <I>Comoviridae</I>)is an important virus in soybean (<I>Glycine max</I> (L.) Merrill), causing quality and yield loss due to seed coat mottling and seed weight reduction. Although BPMV has been known in Virginia since 1958 and has always been regarded as causing negligible losses, its impact is changing as BPMV incidence has increased in many soybean growing areas of Virginia and the USA in general. From 1997 to 2001, a total of five BPMV isolates (V-W1, V-W2, V-S98-1, V-S98-15 and V-S01-10) were collected in Virginia and characterized. In this study, the effects of these isolates were studied, alone or with Soybean mosaic virus (SMV, Genus Potyvirus, Family Potyviridae) strain SMV G1, and isolates S98-51 and S98-52, on selected soybean cultivars. Individual isolates of BPMV showed variable symptom severity, and resulted in yield loss of between 40.4 to 58.1%, while SMV caused 23.7% in the most severe interactions. Up to 100% yield loss was realized from double inoculations of selected BPMV and SMV isolates, BPMV V-S98-1 + SMV S98-52 and BPMV S98-15 + SMV S98-52 on Hutcheson and Hutcheson Roundup Ready&#174; (BC5) soybeans, respectively. Time of inoculation, a critical factor in the impact of many virus diseases, affected seed coat mottling in four cultivars and seed weight in two cultivars, in tests with four BPMV isolates and three stages of soybean development. All BPMV isolates inoculated to plants at vegetative stage V1-V3 severely increased seed coat mottling and reduced seed weight than those inoculated at V4-V6 and reproductive stage R1-R3. Seedlings grown from non-mottled seeds germinated more uniformly had fewer thin-stemmed seedlings and grew faster than those grown from mottled seeds. Inoculation of various cultivars and breeding lines showed that there was no correlation between the severity of virus-induced foliar symptoms, relative accumulation of SMV, and extent of seed coat mottling. Thus, by avoiding the presence of BPMV at an early growth stage through proper timing of planting to avoid vectors, proper cultural practices like weed control, use of SMV free seeds, and chemical control, it is possible to greatly improve seed quality and reduce yield losses in soybean. / Ph. D.

Annual grassy weeds are problematic in intensively managed turfgrass systems due to a lack of selective and affordable control options. Four projects were conducted from 2020-2023 to evaluate novel techniques for Annual bluegrass (Poa annua L.), goosegrass (Eleusine indica L. Gaertn.), and smooth crabgrass (Digitaria ischaemum Schreb.) control on golf course putting greens or putting green surrounds. Hybrid bermudagrass Cynodon transvaalensis Burtt. Davy. x dactylon L. Pers.) tolerated cumyluron regardless of application timing, endothall when applied during full dormancy, and methiozolin when applied during mid-transition. Methiozolin half-life in the upper 2-cm of 12 hybrid bermudagrass putting greens was approximately 14 days and was prolonged in similar studies by seven orders of magnitude when herbicide was applied to bare ground compared to adjacent Kentucky bluegrass (Poa pratensis L.) turf. In a study conducted in Alabama, California, Florida, and Virginia, methiozolin at labeled use rates applied biweekly controlled smooth crabgrass >80% in creeping bentgrass (Agrostis stolonifera L.) and hybrid bermudagrass turf. Although similar programs also controlled goosegrass, acceptable control required more applications than are allowed on the product label. Targeted application devices (TAD), such as spot sprayers and dabbers that are used for individual plant treatment of escaped weeds, were tested for uniformity of fluid delivery. Fluid output of dabbing devices was highly variable and dependent on reservoir fill level, reservoir air seal, human user, and contact time, but largely independent of peak force exerted during the dabbing event. These studies suggest that new products are available to improve annual grassy weed control in turfgrass systems, but proper application timing and device calibration is important to achieve best results. / Doctor of Philosophy / Annual grasses are difficult to control in "high-end" golf turf because few herbicides can be safely used near greens and key weeds have become resistant to the most common products. Several new products were tested for safety on hybrid bermudagrass greens. Methiozolin (PoaCure) was safe for use after post-dormancy greenup, endothall was safe when used while turf was still dormant, and cumyluron was safe regardless of application timing. The duration of preemergence weed control with methiozolin is reduced as temperatures increase and in turfgrass compared to bare ground systems. Half of the methiozolin product will dissipate in 14 days or less when applied to hybrid bermudagrass putting greens or Kentucky bluegrass lawns in spring. Methiozolin controlled smooth crabgrass for the entire season in several Southeastern states, but goosegrass control was slightly below acceptable levels when the product was used within annual dose restrictions. Applicators, such as dabbers and spot sprayers, that are used to treat individual plants improve turf safety and reduce chemical cost, but these devices had not been previously tested for uniformity of fluid output. Studies found that these devices can vary in output by several orders of magnitude depending on the type of devices used, the person using the device, and duration of contact with the turf as the user presses a dabbing device over a weed. Within-device errors were equally problematic and governed by the amount of downward pressure exerted by the fluid contained in the device reservoir. For every 10% of fluid capacity added, fluid dispense rate increases approximately 33%. When the air seal of the fluid fill cap is broken, fluid output approximately doubles compared to when this seal is maintained because loss of vacuum increases downward force of the fluid column. These studies suggest that new products are available to improve annual grassy weed control in ornamental turf, but proper application timing and device calibration is important to achieve best results.

The objectives of this research were to examine some representative surface waters in the Yazoo River Basin (YRB) during the growing season for a few selected herbicides; to compare the type and abundance of pesticides in the atmosphere at a agricultural and an urban site; to determine the load of nitrogen and phosphorus being discharged from the YRB to the Mississippi River and ultimately the Gulf of Mexico; to determine how the adsorptive properties of a common Delta soil differ under no-tillage (NT) versus conventional tillage (CT) for two commonly used herbicides; and to validate the Soil and Water Assessment Tool (SWAT) model for herbicide runoff in the YRB. Herbicides, in low concentrations (generally less than 10 parts per billion) were frequently detected in 3 streams in the YRB throughout the growing season. Pesticides were detected in air and rain samples from both urban and agricultural areas. The concentrations in the agricultural area generally were an order of magnitude higher and types of pesticides detected were different: more insecticides in the urban area and more herbicides in the agricultural area. The annual load of nitrogen being contributed to the Mississippi River from the YRB was less than what might be expected based on discharge, and the load of phosphorus was slightly higher than what might be expected. The amount of atrazine and fluometuron adsorbed was similar for a soil under CT and NT, but much more herbicide was adsorbed by the NT soil. At the scale of the Bogue Phalia Basin (too large for specific information to be available and too small for averaging to eliminate the need for site specific data) there are considerable uncertainties associated with input data and these, together with the simplifying assumptions within the model, mean that SWAT should not be used to predict the exact date, time, and concentration of a pesticide in a stream. However, the model does offer the potential to assess the likelihood of contamination of surface waters by a given compound in a given situation and as such could provide a useful tool for planning, management and regulatory purposes.

Water level declines in the Mississippi River Valley Alluvial Aquifer (MRVAA) are attributed largely to withdrawals for rice (Oryza sativa L.) irrigation. This study was performed to determine if alternative irrigation strategies for rice could reduce withdrawal from the MRVAA without having an adverse effect on grain yield, grain quality, control of barnyardgrass, and profitability. Research was conducted in Stoneville, MS and 19 on-farm locations across the Delta region of Mississippi from 2014 through 2017 to determine the irrigation threshold for alternate wetting and drying (AWD) rice irrigation, the effect of AWD management on barnyardgrass control, and effects of irrigation water management practice, i.e., conventional flood via cascade (CONV), multiple side inlet (MSI), and MSI coupled with AWD, on aquifer withdrawal, rough rice grain yield, irrigation water use efficiency (IWUE), and net returns above irrigation costs. An AWD threshold of -20 cm below the soil surface had no adverse effect on grain yield or grain quality, reduced irrigation applied by 50%, and improved IWUE by 45% compared to a continuous flood (CF). Control of barnyardgrass in AWD was either maintained or improved compared to CF for both Clearfield and conventional rice production systems. At the production scale, up to 39% less water was applied to AWD compared to CONV and MSI. Rice grain yield for AWD was not different from either CONV or MSI, despite substantial reductions in water use. Relative to standard irrigation strategies, AWD maintained or increased net returns up to $238 ha-1 for pumping depths from 5.5 m to 122 m and diesel prices from $0.42 L-1 to $0.98 L-1. Irrigation water use efficiency was up to 59% greater for AWD relative to conventional systems due to the positive effects of the former on water use while maintaining yield. These data demonstrate that AWD can reduce withdrawal from the MRVAA while maintaining or improving yield and net returns relative to irrigation strategies currently employed across the midsouthern USA rice belt.

No description available.

No description available.

No description available.

No description available.

No description available.