Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Plant Pathology and Microbiology

First Advisor

Edward J. Braun

Second Advisor

Denis C. McGee


The process of seed transmission was elucidated for Clavibacter michiganense ssp. nebraskense, causal agent of Goss's wilt of corn. In the field corn plant leaves were inoculated with a rifampin-tolerant strain of the bacterium. Seed infection averaged from 1% to 30.7% and C.m. nebraskense populations in seed samples ranged from 0 to 1.7 x 10[superscript]7 cfu. Seed infection levels tended to increase with more inoculations and later sampling. Infected seed was detected in non-inoculated control plots that became naturally infected by pathogen spread from inoculated plots. Seed-borne inoculum was not reduced by drying seed at 35 C from harvest moistures of 38% and 25% to 10% or 12%, respectively. Pathogen transmission from infected seeds to seedlings could not be demonstrated either in pasteurized soil in the greenhouse or in the field. Transmission occurred under greenhouse conditions in from 0.1% to 0.4% of seedlings growing from seed inoculated by vacuum infiltration of the pathogen. Characteristic symptoms developed and the pathogen was re-isolated;The pathogen's ability to overwinter in infested leaf debris was established. Pathogen populations in infested debris on the soil surface were monitored between October and April of 1983/84 and 1985/86. They declined by 94% in 1983/84 and 17% in 1985/86. In the laboratory declines in pathogen populations of 70% and 0% occurred over this time;These findings suggest that although seed infection may readily occur, the inefficiency of transmission through seed indicates that seeds are an unimportant inoculum source where the disease already occurs. In these cases infested residues are likely to be of more importance. In areas where the pathogen has not been recorded seed should be considered as a potential inoculum source;The pathogen could be detected by plating seeds on culture media. Two media were used: Corynebacterium nebraskense selective medium with reduced lithium chloride; and, for rifampin tolerant isolates, nutrient broth yeast extract agar + 0.05 g rifampin + 0.04 g cycloheximide/litre. A method could be developed to determine infection levels in seed lots. This could be useful in reducing the potential for pathogen spread by seed-borne inoculum.



Digital Repository @ Iowa State University,

Copyright Owner

Julie Anne Biddle



Proquest ID


File Format


File Size

92 pages