No description available.

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.

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.

No description available.

Bibliography: leaves 149-173.

Bibliography: leaves 184-209. This thesis describes the occurence of natural, biologically based suppression of Rhizoctonia barepatch in a direct drilled system at Avon, South Australia. The supressive characteristics are transferable, removed by biocidal treatments, and active against increasing doses of R. solani AG-8, Gaeumannomyces graminis var. tritici and Fusarium graminearum. Disease severity and the viable population of Rhizoctonia are reduced in suppressive soil but the causal agent is still present. The microbial populations in suppressive and non-suppressive soil appear to differ both in their functioning and composition. The control strategy is developed through manipulation of the existing soil biota with farming practices.

No description available.

Bibliography: leaves 149-173. / xiv, 173 leaves : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Plant Science, 1994

Bibliography: leaves 184-209. / xx, 219 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / This thesis describes the occurence of natural, biologically based suppression of Rhizoctonia barepatch in a direct drilled system at Avon, South Australia. The supressive characteristics are transferable, removed by biocidal treatments, and active against increasing doses of R. solani AG-8, Gaeumannomyces graminis var. tritici and Fusarium graminearum. Disease severity and the viable population of Rhizoctonia are reduced in suppressive soil but the causal agent is still present. The microbial populations in suppressive and non-suppressive soil appear to differ both in their functioning and composition. The control strategy is developed through manipulation of the existing soil biota with farming practices. / Thesis (Ph.D.)--University of Adelaide, Dept. of Soil Science, 1996

Diss. (sammanfattning) Alnarp : Sveriges lantbruksuniv. / Härtill 5 uppsatser.