DISEASES IMPACT MANAGEMENT OF USDA CLONAL VACCINIUM GENEBANK

Abstract

The USDA Agricultural Research Service maintains a diverse collection of Vaccinium genotypes at the National Clonal Germplasm Repository, a temperate fruit and nut genebank in Corvallis, Oregon, USA. Vaccinium species are hosts for two pathogens that occur in the U.S. Pacific Northwest and impact collection management. One is the fungus Phytophthora ramorum and the other is Blueberry shock virus (BlShV). P. ramorum is a devastating pathogen of certain oak species and has a very broad host range with varying symptoms including foliar blight in some Vaccinium species. Vaccinium germplasm must be inspected and certified to be free of this pathogen to protect the U.S. nursery industry and native flora. The pollen-borne BlShV has made it necessary to move the primary Vaccinium clonal collection from a field planting to a protected potted collection. More than 600 clonal accessions are maintained in greenhouses or screenhouses, protected from pollinators and other virus vectors. These clonal accessions represent more than 60 unique Vaccinium taxa from around the world and include 171 cranberry (Vaccinium macrocarpon), 46 lingonberry (Vaccinium vitis-idaea), 15 lowbush blueberry (Vaccinium angustifolium), and 182 highbush (Vaccinium corymbosum) and/or rabbiteye (Vaccinium ashei) blueberry genotypes. Migration to a protected container collection required the development of a management strategy to maintain plants in a vigorous condition that would provide high quality vegetative growth suitable for propagation. However, tree bark in container growth media is a potential source of P. ramorum dissemination. Trials were conducted to select a bark-free medium with good fertility and porosity that would be stable for an expected 8-10 year lifespan of a potted blueberry plant. A blend of volcanic pumice (50%), un-milled, coarse, sphagnum peat moss (40%), and sandy loam (10%) was selected. A 3-5 cm deep pumice top-dress (collar) was added to the surface of each pot to create a semi-sterile, dry, inorganic surface that prevents weed, moss, and fungus gnat growth. This top-dress combined with a stable, bark-free potting medium creates a growing system that greatly reduces water use, nutrient leaching, salt build-up, and moisture stress. Woody Vaccinium clones are hard-pruned in late winter to remove all flower buds and six to eight upright shoots are selected and allowed to grow. These shoots provide cuttings that are distributed by the genebank for research or propagation.