Responses of six Eurasian Ulmus cultivars to a North American elm yellows phytoplasma

Elms (genus Ulmus) of six clonal cultivars representing Eurasian species an d hybrids were grafted when 2 to 3 years old with bark patches from U. rubr a infected with an elm yellows phytoplasma or were left untreated as contro ls. The cultivars were U. glabra x minor 'Pioneer', U. minor x parvifolia ' Frontier', U. parvifolia 'Pathfinder', U. wilsoniana 'Prospector', and the complex hybrids 'Homestead' and 'Patriot'. Trees were evaluated for infecti on and symptoms 1 or 2 years after inoculation. Infection was detected via the 4',6-diamidino-2-phenylindole(.)2HCl (DAPI) fluorescence test in 26 of 86 grafted trees representing five cultivars. Infection of selected trees w as confirmed by polymerase chain reaction (PCR) amplification of a fragment of phytoplasmal rDNA, and the phytoplasma was identified by restriction fr agment length polymorphism (RFLP) analysis of the amplified DNA using restr iction enzymes AluI, RsaI, and TaqI. Elm yellows phytoplasma was also ident ified by nested PCR and RFLP analysis in two of seven inoculated healthy-ap pearing, DAPI-negative trees and one noninoculated control tree. All RFLP p rofiles were identical to that of reference strain EY1. Phytoplasma-associa ted symptoms, observed in five cultivars, included suppressed growth, progr essive size reduction of apical shoots and leaves, chlorosis, foliar redden ing, witches'-brooms, and dieback. Phytoplasma was not detected in cv. Home stead. Possible resistance of this cultivar to elm yellows phytoplasma was indicated by localized phloem necrosis in stems below inoculum patches.