Sweet cherry rootstock/virus interactions

Sweet cherry trees in the U.S. are grown primarily on Mazzard (Prunus avium) or Mahaleb (P. mahaleb) seedlings, or clonally-propagated 'Colt' (P. avium x P. pseudocerasus) rootstocks. These rootstocks are generally tolerant of infection by the pollen-borne ilarviruses, prune dwarf virus (PDV) and Prunus necrotic ringspot virus (PNRSV), which are found in many mature orchards in the Pacific Northwest. Most PDV and PNRSV strains elicit few, if any, obviously detrimental symptoms in trees on these rootstocks. With the advent of new precocious and/or dwarfing rootstocks that are either interspecific Prunus hybrids or open-pollinated selections of P. cerasus, the performance of infected sweet cherry trees on these rootstocks needs to be examined. Uphoff et al. (1988) demonstrated that direct inoculation of rootstocks by PDV or PNRSV resulted in a variety of symptoms among the genotypes in the rootstock breeding population at Giessen, Germany. These ranged from apparent tolerance to rootstock mortality. In early summer 1995, we bark graft-inoculated PDV, PNRSV, or PDV+PNRSV into canopy branches on young (Mahaleb, GI 148/1, GI 148/2, GI 148/8, GI 172/9, and GI 195/1) or mature (18 rootstock genotypes in the 1987 NC-140 rootstock trial) trees of 'Bing' sweet cherry. Within 10 weeks of inoculation, gum exudation at the graft union was apparent in young trees on GI 172/9 and GI 195/1, and in mature trees on GI 154/7, GI 172/7, and GI 173/9. Many of these trees subsequently died within two years. With the exception of the young trees on GI 195/1, the common parent species of these hypersensitive rootstocks was P. fruticosa. Milder symptomologies, generally involving foliar responses and inhibition of lateral shoot elongation, have been observed on several of the genotypes derived from P. canescens and P. cerasus, although some siblings (including GI 148/1, GI 148/2, and GI 195/2) appear to be tolerant. The interactions between all specific rootstocks and viruses evaluated to date are discussed.