First Report of Verticillium dahliae Causing Dieback of Highbush Blueberry (Vaccinium corymbosum) in Oregon and Washington

In the spring of 2011, 2013, 2015, and 2016, cultivar Aurora, Blue Ribbon, Bluetta, and Cargo blueberry plants (Vaccinium corymbosum L.) from Oregon and Washington were sent to the Oregon State University Plant Clinic for diagnosis. Symptoms included foliar chlorosis, reddening, blackening of either part or the entire stem followed by stem dieback, vascular discoloration in the lower stems, and occasional wilt. The diseased plants were scattered in the fields, with incidence ranging from less than 5 to 40% of acreage. Isolations were made from surface-disinfected debarked stems and crown tissues to water agar and potato dextrose agar. Verticillium dahliae Kleb. was recovered from stems and crowns of all four cultivars, from which single-spore isolates were derived. Morphology of the conidiophores, conidia, and microsclerotia fit the description for V. dahliae (Klebahn 1913). Two of the isolates (OSU herbarium #OSC# 156080 and #OSC# 156081) were first identified by multiplex PCR assay (Inderbitzin et al. 2013) and subsequently sequenced. Partial DNA sequencing of the internal transcribed spacer (ITS) regions and TEF-1 alpha from both isolates were deposited in GenBank as accession numbers KY039311 and KY039312 (ITS) and KY039313 and KY039314 (TEF-1), and they showed 100% identity to the epitype strain of V. dahliae (ITS accession no. HQ206718, 461/461 and 466/466 bp for the two isolates, respectively; TEF-1 alpha accession no. HQ414624, 579/579 bp for both). Pathogenicity tests were performed by wounding and inoculating three lower stems each of three 2-year-old Bluetta blueberry plants with agar plugs of a 7-day-old single-spore culture of V. dahliae (#OSC# 156080). Three stems of another plant were wounded and inoculated with uncolonized agar plugs. Three months after inoculation, the stems had turned black up to 30 cm above inoculation sites, and the vascular tissue was discolored. Microscopic examination revealed masses of embedded V. dahliae microsclerotia along entire stems. Control plants remained symptomless, and V. dahliae was not recovered. Using the same two V. dahliae isolates as above, 15 cm tall blueberry plants of cultivars Bluetta, Earliblue, Draper, Polaris, and Rubel were inoculated by removing the bottom 1 cm of roots and dipping them in a conidial suspension (5 × 10⁶ conidia/ml) for 10 min, followed by planting in the greenhouse at 27°C. After 10 weeks, all inoculated cultivars showed defoliation, stunting, leaf reddening, vascular discoloration, and blackened stems. V. dahliae morphologically similar to our isolates was consistently recovered from stems of all inoculated cultivars, at least 2.5 cm above the soil line. Control plants remained healthy and did not yield V. dahliae. Verticillium-infected field-grown blueberries were largely from sites previously planted with peppermint or potato, crops susceptible to V. dahliae. Verticillium wilt has previously been reported on lowbush blueberry (V. angustifolium) in the United States and Canada (Brisson et al. 1976), on Vaccinium myrtillus (listed on PlantWise database), and on blueberry (Vaccinium sp.) in Chile (Cisternas and France 2009). None of these reports were confirmed by DNA evidence. To our knowledge, this is the first report of V. dahliae causing disease of highbush blueberry in the United States. Our results strongly suggest that fields with a history of V. dahliae infestation should not be planted with blueberry.