Bacterial wilt of cucurbits: impact of plant age on symptom progression and pathogen movement and locating genes associated with host preference and pathogenesis in E. tracheiphila

Abstract

Bacterial wilt of cucurbits is a devastating disease of cucurbit crops in the midAtlantic 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 withinplant 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