Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Christopher A. Clark


During three seasons, mother storage roots of Beauregard sweetpotato were inoculated with Erwinia chrysanthemi in March, and the pathogen was recovered from them the day they were bedded, 7 days later, and at the first and second pullings of slips as well as from underground stems at harvest in September and October of the first and second transplantings, respectively. Little stem rot occurred in the fields during the three seasons. Thirteen strains of E. chrysanthemi, 8 isolated originally from sweetpotato and 5 from other hosts, and one strain of E. carotovora subsp. carotovora were inoculated on sweetpotato and 12 different hosts. Strains from hosts other than sweetpotato caused little or no disease on sweetpotato. Of 95 carbon sources tested, sweetpotato strains used the same 25, were unable to grow on 57, and varied on 13 substrates. Jewel and Beauregard sweetpotato stem cuttings were inoculated with E. chrysanthemi and/or Fusarium solani before planting in the greenhouse and in the field. In the greenhouse, stem rot lesion length was greater when the two pathogens were inoculated together than the sum of stem lesion length produced by both pathogens inoculated separately. The same effect was observed on Beauregard in the field in 1989 and 1990. When storage roots were punctured with a flamed needle and incubated for 5 days submerged in distilled water in plastic bags, 85 to 100 % rotted in 2 tests. In the same tests, 15% and none of storage roots rotted when surface disinfested with NaOCl. Two distinguishable pectolytic spore-forming, anaerobic bacteria resembling Clostridium in morphology were found in the lesions, regardless of treatment. Inoculation of storage roots with pure cultures of these bacteria and incubation under anaerobic conditions induced rapid maceration and production of gas, typical of "souring", a disorder previously regarded as solely physiological in origin.