Thesis (Ph. D.)--University of Wisconsin--Madison, 1967. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

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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 disease of boxwood, so called "boxwood blight", began causing serious damage to boxwood plants after·the drouth of 1930. Four fungi were isolated from the affected parts of diseased plants. Macrophoma candollei waa isolated from the leaves, a species of Fusar1a from diseased stems and twigs, a species of Coniothyrium from the cankers on the stems, and a species of Verticillium from the infected roots. Inoculation experiments were performed on healthy young boxwood plants that were growing in the greenhouse. Pure cultures of Fusaria, Coniothyrim, and Verticillium were used as inoculum. In general the results were negative. The Fusarium culture was the only one that produced any infection and that infect1on waa very slight. Observations made of infected plants in the greenhouse at V.P.I. and at a nursery at Marion, Virginia, and. also a nursery at South Boston, Virginia, indicate that probably the fungi associated with boxwood blight, infect only thoae plants that have been weakened by some physiological condition such as lack of moisture, winter injury, malnutrition, insect injury, or improper transplanting. / M.S.

Phytophthora parasitica is an important root rot pathogen of American and English boxwood, Buxus sempervirens and B. sempervirens var. suffruticosa, respectively, shrubs used extensively in the landscape in Virginia and North Carolina. Studies were undertaken to screen microorganisms for antagonism to Phytophthora in vitro; test two polymers, carboxymethyl cellulose (CMC) and methylcellulose (MC) for their suitability as carriers, storage media, and carbon substrates; and finally to determine the efficacy of selected antagonist pairs in controlling Phytophthora root rot on containerized American boxwood. On the basis of in vitro inhibition tests, 14 organisms were selected. These included six bacteria, five fungi and three actinomycetes. Populations of conidia-forming fungal isolates increased or remained fairly stable when stored in 0.25% suspensions of CMC and MC, indicating that spores may be a more suitable propagule than mycelial fragments when storing fungal isolates for more than 1 week. No significant or consistent changes in population viability occurred among the actinomycete and bacterial isolates. Carboxymethyl cellulose was a more readily utilizable carbon source than MC as indicated by the smaller specific growth rates of microorganisms in MC. Methylcellulose was not utilized by actinomycete and non-spore-forming fungal isolates. Six of seven antagonist pairs were efficacious in controlling root rot in the greenhouse when suspended in CMC. In general, application of antagonist pairs to boxwoods stimulated growth relative to control plants. The polymers may provide the biocontrol organisms with a protective environment and/or food base for establishment of stable population levels in the infection court, providing protection of the root system when challenged with the pathogen. / M.S.