Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Plant Pathology and Microbiology


Studies were conducted in the growth chamber, greenhouse and field to characterize the response between Xanthomonas phaseoli var. sojensis (Xps) and resistant and susceptible soybeans. The first trifoliate leaves of resistant and susceptible soybean plants were inoculated with Xps. At all inoculum concentrations, Xps populations increased at similar rates in both resistant and susceptible leaves. In susceptible leaves (cv. Clark), the ratio between initial pathogen population and the number of pustules 10 days after inoculation was 1.3:1. In resistant leaves (cv. Clark63) this ratio was approximately 7.7:1. Pustules on resistant plants were approximately 25% smaller than those on susceptible plants;Dispersal of a rifampin-resistant mutant of Xps (XpsRa) was studied under field conditions. The pathogen was dispersed at the same rate (0.1 m per day) in all directions in plots of resistant and susceptible soybeans. Epiphytic populations on symptomless leaves of resistant and susceptible cultivars were not significantly different (P = 0.05) during the first 3 weeks after inoculation. At 35 days after inoculation, when the greatest populations were detected, epiphytic XpsRa populations were 20- to 40-fold higher on the susceptible plants. All resistant plants remained symptomless throughout the season;In field studies to determine the potential of seed transmission of Xps, epiphytic populations of XpsRa developed on both resistant and susceptible seedlings from either external or internal seedborne inoculum. On susceptible plants that developed a large populations of XpsRa, 90% of the pustules contained XpsRa. Also, XpsRa was present as a contaminant on seed harvested from these hills;The pustule pathogen was able to overwinter in Iowa in plant debris on the soil surface or buried at a depth of 15 cm. The pathogen remained virulent during the overwintering period.



Digital Repository @ Iowa State University,

Copyright Owner

Donald Ernest Groth



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74 pages

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Botany Commons