Bacterial wilt of cucurbits is a devastating disease of cucurbit crops in the mid-Atlantic region of the USA. Caused by Erwinia tracheiphila, it is transmitted by striped (Acalymma vittatum (F.)) and spotted (Diabrotica undecimpunctata howardi (Barber)) cucumber beetles. The management of bacterial wilt relies primarily on the control of cucumber beetles with insecticides. However, alternative management strategies are needed due to the high cost of insecticides and their negative impact on the environment. Understanding bacterial wilt etiology and the ecology of E. tracheiphila may provide insights to help control bacterial wilt of cucurbits.

The first objective of my thesis research was to evaluate the impact of host age on the rate of symptom development and the extent of bacterial movement in the xylem of muskmelon. Wilting occurred more rapidly on muskmelon seedlings that were 2 or 4 weeks old when inoculated than those that were 6 or 8 weeks old. Culturing of a rifampicin-resistant strain of E. tracheiphila from muskmelon stem segments after inoculation revealed that vascular spread of E. tracheiphila was more extensive below than above the inoculation point. These findings provide experimental evidence that host age impacts the rate of symptom development in cucurbit bacterial wilt and that within-plant movement of the xylem pathogen E. tracheiphila occurs primarily in the downward direction.

The second objective of my thesis was to locate the genetic basis of host preference and pathogenicity in E. tracheiphila. Previous studies at Iowa State University suggested that specificity of E. tracheiphila strains was correlated with host genus (Cucumis and Cucurbita). Differences between isolates from Cucumis and Cucurbita hosts were further evaluated in the present work through physiological comparison focused on growth and a functional genomic comparison focused on two putative effector genes, eop1 and dspE, that may be a host-limiting factor and pathogenicity effector in E. amylovora, respectively. Quantification of growth rates of 65 E. tracheiphila strains showed that Cucumis strains grew faster, on average, than Cucurbita strains, which supports a possible division of the Cucumis and Cucurbita strains into distinct subspecies of E. tracheiphila. Alignments of the sequences of eop1 and two important regions in a functional domain of dspE from 9 strains of each of the putative subspecies indicated complete sequence identity for both genes within each subspecies group, but multiple nucleotide polymorphisms in each gene across the subspecies groups, consistent with the existence of two putative subspecies. To begin to evaluate the roles of these genes in pathogenesis and host preference, two mutagenesis systems were examined. A low efficiency of transformation hindered the success of these mutagenesis systems, suggesting that E. tracheiphila is not readily induced to competency and that future studies should employ conjugation as the primary means of DNA introduction. This finding highlights the need to employ conjugal vectors in the development of molecular tools to genetically manipulate E. tracheiphila.