Host plant resistance to fire blight (Erwinia amylovora) in the Rosaceae subfamily Maloideae

by Bell, Andrew Claude.

Bell, Andrew Claude. Host Plant Resistance to Fire Blight (Erwinia amylovora) in the Rosaceae subfamily Maloideae (Under the direction of Dr. Thomas G. Ranney.) Fire blight, caused by the bacterium Erwinia amylovora (Burrill) Winslow et al., is a devastating disease of apple (Malus Mill. spp.), pear (Pyrus L. spp.), and other taxa in the Rosaceae subfamily Maloideae. Currently, the antibiotic, streptomycin, is the only effective chemical to control this disease among susceptible taxa; however, development of streptomycin-resistant strains of E. amylovora threatens the future use of this antibiotic. Moreover, environmental and health concerns associated with the use of antibiotics may ultimately result in a widespread ban that already exists in some countries. Significant variation in resistance to fire blight among some host plants has been reported. Selecting and developing new cultivars with enhanced fire blight resistance is an environmentally safe, cost effective, and practical means of control for this disease. Therefore, three studies were conducted to document resistance to fire blight and investigate the role of secondary metabolites in disease resistance. In the first study, artificial inoculations, using a virulent strain of E. amylovora were conducted to determine levels of resistance to fire blight among taxa of flowering pear and quince (Chaenomeles Lindl. spp.). Significant variation in resistance to fire blight was observed among many widely cultivated taxa of flowering pear and quince and highly resistant taxa were identified. Pyrus calleryana ‘Chanticleer’ was significantly more resistant than ‘Bradford’. Prior to this study, only limited information was available regarding fire blight resistance among taxa of flowering quince. In a second study, resistance to fire blight among 49 taxa of flowering crabapples was evaluated based on observations of natural infection and results from artificial inoculations using three virulent strains of E. amylovora. Considerable variation in resistance was observed with cultivars ranging from highly susceptible to highly resistant. Artificial inoculations provided a consistent method for evaluating disease resistance. Malus ‘Adams’ and ‘David’ were among the most resistant taxa to natural infection and to artificial inoculations. Several phenolic compounds found in Malus spp. exhibited strong antibiotic activity in vitro against E. amylovora. Levels of endogenous phenolic compounds were analyzed by high-performance liquid chromatography (HPLC). Three unidentified constitutive compounds were correlated with resistance to fire blight among cultivars of flowering crabapples. In the final study, three cultivars of flowering crabapple, ‘Adams’, ‘Canary’, ‘Schmidtcutleaf’, representing a range of resistance to fire blight were artificially inoculated to evaluate pathogen-induced changes in phenolic compounds over time. Seven major phenolic components were analyzed by HPLC. There were significant cultivar, inoculation, and time effects and various interactions among factors depending on the specific compound. Component A showed an inducible response and levels were higher in the resistant and moderately resistant cultivars following inoculation. Levels of component X were significantly lower in the highly susceptible cultivar, decreased in the moderately resistant cultivar following inoculation, but were highest and remained unaffected following inoculation in the resistant cultivar. This research provides information on methods for screening and evaluating for resistance, identification of resistant taxa, and insights into the role of endogenous phenolic constituents in host plant resistance to fire blight. These results will aid in further evaluation, selection, and improvement of cultivars with superior disease resistance.