Bacterial blight (Xanthomonas arboricola pv. corylina) (Xac) of hazelnut (Corylus avellana L.) was described first in Oregon in 1915 and is now recognized as a damaging disease of young hazelnut trees worldwide. Stressed hazelnut trees in conditions such as planting on marginal sites, and trees between 1 and 4-years-old are most susceptible to bacterial blight. The Willamette Valley of Oregon is where 99% of the U.S. hazelnut crop is grown. Bacterial blight was the most impactful disease in the Pacific Northwest (PNW) until the devastating fungal canker disease eastern filbert blight (EFB) made its way to the hazelnut growing region in the mid-1970s. However, since the release of a series of cultivars with genetic EFB resistance starting in 2005, there has been a surge in new hazelnut plantings. Widespread planting of new hazelnut cultivars has created a resurgence in interest in bacterial blight throughout the PNW growing region. No research has been conducted on the disease in Oregon since the 1970s. In this thesis, a comprehensive review of the literature was conducted on bacterial blight research over the history of the Oregon hazelnut industry, and the more recent history of the disease in Europe. Experimental research began with a characterization study on Xac utilizing morphological, biochemical, and molecular techniques on field-collected bacterial populations. In other experiments, hazelnut cultivars were assessed for their response to
inoculation of Xac in vitro with hazelnut explants cultured in micropropagation media, as well as in young potted hazelnut trees grown in a greenhouse and inoculated under orchard conditions. The bacteria recovered from symptomatic hazelnut tissue in commercial orchards were positively identified as Xac, and the phylogenetic diversity of the pathogen present in European countries was confirmed to also be present in Oregon. The disease progression in vitro on explants and on potted tree inoculations was documented and found consistent with symptomology and disease incidence reports from the field. Tissue culture was demonstrated to be an effective research environment to evaluate the infection efficiency of Xac on hazelnut cultivars. It was found from inoculations of both explants and potted trees that each hazelnut cultivar was susceptible to bacterial blight infection from artificial inoculations, though some cultivars were more severely affected. The bacterial blight susceptibility of each of the EFB resistant hazelnut cultivars evaluated indicates that management of the disease must rely on maintaining healthy cultural practices, sanitation of pruning tools, and timely copper sprays to minimize the spread, until bacterial blight resistance can be discovered and bred into hazelnut cultivars. This research on characterization of Xac and its application to assess hazelnut cultivars to bacterial blight has laid a foundation for further applied research on the disease. Genomic characterization of Xac will be the next step to increase the understanding of the molecular interactions of Xac with the host plant. Populations of Xac isolates are available for use in further applied research to investigate cultural bacterial bight management strategies.