Past and present status of virus and phytoplasma diseases in apple rootstocks in New Zealand

Abstract

The high incidence of virus and phytoplasma diseases in older apple (Malus domestica Borkh.) trees in New Zealand can be traced in part to infection in the ‘Northern Spy’ clonal rootstock which was used extensively in the early part of this century to combat woolly apple aphid infestation, and in the ‘Mailing’ series of clonal rootstocks which were imported in 1930 for controlling the size of apple trees. The “latent virus infections"—stem pitting, stem grooving, chlorotic leaf spot, and scaly bark were commonly present in these rootstocks, as was apple mosaic virus. The phytoplasma disease rubbery wood occurred only in ‘ Mailing 12 ‘. Presence of apple mosaic virus and rubbery wood phytoplasma in the rootstocks may explain the unusually high incidence of these two diseases in New Zealand apple trees in comparison to other apple growing countries. Most of the apple rootstocks imported since 1936, which include the now widely used ‘Merton 793’ and ‘Malling-Merton 106’, were free from infection when imported. In tests for virus and phytoplasma diseases in rootstocks grown in 12 commercial nurseries from 1987 to 1995, most were free from infection. Stem pitting, stem grooving, chlorotic leaf spot, and scaly bark viruses were found in a few of the samples tested, but none were infected with either apple mosaic virus or rubbery wood phytoplasma. © 1996 Taylor & Francis Group, LLC.