Evaluation location: Oregon, United States
Abstract
Blueberry shock virus (BlShV) is a serious problem in blueberry production in the Pacific Northwest (PNW) region of North America. Infection occurs during bloom and the virus moves into other parts of the plant in an uneven but steady manner and may take several years to become fully systemic in mature bushes. In the year after infection, emerging flower and leaf tissues die rapidly at full bloom followed by regrowth. Once symptoms have been exhibited in all parts of the plant, the plant remains asymptomatic in subsequent years. Infection in young plants of some cultivars can stunt the plant for the remainder of its life, however, more typically production is lost for one year. While there appears to be no immunity as tested by grafting, it has been recognized that the rate of infection varies among genotypes. Plants in the HCRU breeding program and at the NCGR, have been tested regularly by ELISA for BlShV, in some cases for nearly 20 years, and germplasm that is particularly slow to become infected has been identified. At Oregon State University, North Willamette Research and Extension Center (NWREC), ‘Legacy’, ‘Toro’, ‘Bluecrop’, ‘Baby Blues’, ‘Darrow’ and all rabbiteye cultivars have tested negative for BlShV for over 10 years while growing among many known positive plants. On the other extreme, ‘Berkeley’, ‘Bluegold’, ‘Brigitta Blue’, ‘Nui’ and ‘Spartan’ tested positive in the year following their 1st bloom. At the NCGR, ‘Bladen’, ‘Harding’, ‘Lateblue’, ‘Legacy’, ‘Razz’, US 612, US 693, US 845 and US 847 all tested negative after 20 years in the field. ‘Toro’ and ‘Bluecrop’, which had tested negative after 20+ years at NWREC, were positive at the NCGR, as were many rabbiteye cultivars. Crosses have been made among genotypes that are slow to test positive for BlShV to try to develop this trait in cultivars.
Methods
Leaf samples were taken from all plots in the USDA-ARS, HCRU trials (Corvallis, OR and at Oregon State University - North Willamette Research and Extension Center [NWREC], Aurora, OR) in spring of all years beginning the first year the plants had flowers (typically two years after planting). For plots that tested BlShV+, samples from individual plants were taken and reanalyzed to determine whether all plants in the plot were BlShV+. Samples were similarly taken in multiple years at the USDA-ARS, NCGR (Corvallis, OR), however, the results from the 2015 are the only ones examined. All fields contain many genotypes that have tested BlShV+ for many years providing plenty of inoculum for infecting newly established plants. The leaf samples were homogenized in blueberry buffer and the standard procedures for testing for BlShV by enzyme-linked immunosorbent assay (ELISA) testing were followed as described by Bristow and Martin (1999).