Date of Award

1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Diplodia gossypina was only isolated from the basal portion of sweet potato sprouts growing from mother roots which were bedded adjacent to diseased roots, bedded in infested soil or inoculated with conidial suspension before bedding. It was recovered 1-2 cm above and 2-4 cm below the point of inoculation 2 months after inoculation of wounded established vines. It was not isolated from the proximal or distal ends of roots harvested from field plots in which vine cuttings or established vines were artificially inoculated or when vine cuttings were planted in artificially infested soil. Roots harvested from infested soil showed higher disease incidence but those harvested from inoculated vine cuttings or established vines did not differ from the noninoculated control. Freshly harvested roots inoculated with artificially infested soil at cut ends developed higher disease incidence than roots inoculated with conidial suspensions, mycelial plugs, or infected tissue. Diplodia gossypina survived in field soil for at least one year. Its population was reduced by high soil moisture. Conidia collected from crates used 8 months earlier germinated well. Thus, the pathogen may survive in soil or on crates and infect roots through wounds made at harvest or in postharvest handling. The optimum temperature range for D. gossypina decaying slices of root was 20-28(DEGREES)C. Decay was much less at 32(DEGREES)C, and was negligible at 12, 16 and 36(DEGREES)C. Freshly harvested roots inoculated with 3000 viable conidia/site did not show differences in disease incidence following washing, storing at 7(DEGREES)C or in anaerobic conditions for 8 days before or after inoculation. Freshly harvested roots inoculated with artificially infested soil at cut ends had lower disease incidence when properly cured and stored than when cured and stored at 30-32(DEGREES)C. Roots stored for 5 or 8 months were more susceptible to infection than those freshly harvested, and developed higher disease incidence when incubated in curing conditions than when incubated at room temperature after inoculation. Most sweet potato cultivars and breeding lines had uniformly low disease incidence when inoculated just after harvest, but showed differences in susceptibility when inoculated after 8 months storage.

Pages

76

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