The etiology of root rot and decline of English boxwood (Buxus sempervirens cv. surfruticosa L.)
Montgomery, George Bryan
MetadataShow full item record
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.
- Doctoral Dissertations