No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

The determination of the geographical extent of English boxwood decline in Virginia, description of the symptoms of decline, determination of organisms associated with declining plants, investigation of host-parasite relationships and determination of the environmental aspects of the decline were investigated under laboratory, greenhouse, and field conditions. English boxwood decline is increasing in northern Virginia and the Shenandoah Valley and recent reports indicate an increase in the Richmond area. Phytophthora was not associated with the present decline but Paecilomyces buxi and Fusarium oxysporum were consistently isolated from healthy and declining plants. Paecilomyces buxi was isolated, in most cases, at the point where lateral roots emerge from main roots while F. oxysporum was isolated mainly from root tips. Phoma sp. was isolated in the spring months but was not recovered at any other time. Foliar infection did not have a significant role in the decline syndrome as the cause was determined to be a root-rot. In a root-dip study, P. buxi produced greater damage to the root system than the uninoculated treatments although there were significant differences among isolates of P. buxi. Results from field inoculation studies were not significant as uninoculated plants developed symptoms as severe or more severe than several treatments. Growth of P. buxi, F. oxysporum and Phoma on excised and attached roots demonstrated both saprophytic and parasitic capabilities of these three organisms. Results of soil moisture studies were inconclusive as decline occurred in uninoculated checks but disease severity was higher and significantly different as soil moisture decreased. Rainfall data for the period 1960-1970 at Lincoln and Lexington, Virginia, show a moisture deficit further indicating that soil moisture may have been important in the present decline. Temperature was the best correlated environmental factor studied in relation to disease development. Disease severity indices were higher at the highest temperature tested (30° C) and generally decreased with decreasing temperature. Plant parasitic nematodes, particularly Pratylenchus (meadow) and Helicotylenchus (spiral) were recovered from healthy and declining English boxwood but present field populations did not appear to be the cause of the present decline. Field populations remain static while populations on greenhouse grown plants steadily increase indicating that some factor was responsible for preventing increase of the field populations. The results of this study indicate that several fungi possess pathogenic capabilities on English boxwood. Greenhouse and field studies gave some insight to the nature of this present decline but were by no means conclusive. A complex of factors possibly involving environmental stresses appear to be associated with this disease and further research is needed to isolate and discover the role of each in this decline. / Ph. D.

A system with hysteresis is one that exhibits path dependent but rate independent memory. Hysteresis can be observed physically through the magnetization of a ferromagnetic material. In order to mathematically describe systems with hysteresis, we use the Preisach model. A discussion of the Preisach model is given as well as a method for computing the hysteretic transformation of an input variable. The focus of this paper is hysteresis in economics, namely, unemployment. We consider essential time series techniques for analyzing time series data, i.e. unit root testing for stationarity. However, we point out problems in modeling hysteresis with these techniques and argue that unit root tests cannot capture the selective memory of a system with hysteresis. For that, hysteresis in economic time series data is modeled using the Preisach model. We test the explanatory power of the previous unemployment rate on the current unemployment rate using both a hysteretic and non-hysteretic model. We find that the non-hysteretic model is better at explaining current unemployment rates, which suggests hysteresis is not present in the United States unemployment rate. / Master of Science

The inheritance of resistance to black root rot in tobacco, caused by the fungus Thielaviopsis basicola (Bek. and Br.) Ferr., was studied with a view of developing higher resistance by combining genes from various sources. The studies were conducted under greenhouse conditions during the winter and summer seasons of 1951-52. The five parental lines, the F₁ hybrids between them and five of the F₂ populations were included in the winter study when the temperature averaged between 65° and 70° F. The five parental varieties, all of F₁, F₂, and the backcross populations were included in the summer study when the temperature could not be controlled in the greenhouse. The evaluation of resistance was done on the basis of visible damage to the roots of the seedling plants. / M.S.

This thesis presents an analytical approach using Root-Loci method for designing optimum passive series RC snubbers for continuous-current synchronous buck switch mode power supply (SMPS). Synchronous buck SMPS is the most popular power converter topology found in modern consumer electronics. It offers relatively good efficiency to target the high-current and low-voltage requirements while it is also relatively inexpensive to implement. Passive series RC snubbers are simple, efficient and cost-effective open-loop equalizer circuit for synchronous buck SMPS. Its purpose is to control and to balance between the rate of rise and the overshoots of transient switching waveform in order to optimize efficiency and reliability Existing methods of RC snubber design are solely based on second-order approximation. It is investigated in this research that this approximation is highly inaccurate in SMPS applications because higher order equivalent models are required for the load path of the SMPS. The results using the RC snubbers obtained from existing method are shown to be unsatisfactory without correlation to the calculations and simulations based on second-order approximation. Optimum RC values obtained using Root-Loci approach presented in this thesis are shown to correlate to both Spice simulation and lab measurements.