Biotic and abiotic stress responses of interspecific hybrid cherry rootstocks

Interspecific hybrid cherry rootstocks have been under horticultural test in the United States for about 10 years. Several of the more promising rootstocks (in terms of improved precocity and/or vigor control) are being examined at Washington State University for responses to critical biotic (ilarviruses) and abiotic (flower bud cold hardiness) stresses in the Pacific Northwest, where the majority of North American fresh market sweet cherries (Prunus avium) are grown. Among the best rootstocks are hybrid crosses between P. avium or P. cerasus and P. canescens or P. fruticosa. The latter three species are known to express sensitive reactions, in some cases, to infection with Prunus necrotic ring spot (PNRSV) and/or prune dwarf(PDV) ilarviruses. Furthermore, sweet cherries on some of these hybrid rootstocks have been shown to bloom earlier than on standard P. avium (Mazzard) rootstocks, raising the possibility that flower bud cold hardiness may be influenced by rootstock. Controlled freezing of 'Bing' budsticks from several of the hybrid rootstocks from Giessen, Germany, revealed apparent differences (1 to 2 degrees C) in bud hardiness in late winter, probably indicating earlier or more rapid deacclimation during ecodormancy. The magnitude of these differences varied by sample date, with Mazzard and GI 148/1 having the greatest flower bud hardiness and GI 148/2 having the least. Responses to PDV and PNRSV inoculation varied by plant age (non-bearing vs, mature). Responses could be categorized from tolerant (no apparent symptoms) to severe and lethal. All rootstocks with P. fruticosa as a parent tended to exhibit the maximum severity of response. Rootstocks with P. cerasus or P. canescens had variable responses, including differences between sibling germplasm.