Sensitivity of apple (Malus domestica) indicator cultivars to russet ring disease, and the results of graft-transmission trials of other fruit-affecting disorders of apple

In an evaluation in New Zealand of four Canadian apple (Malus domestica Borkh.) cultivars used as indicators for the russet ring and leaf pucker group of diseases, 'Jubilee' and 'Yellow Newtown' proved to be the most sensitive to the diseases present in New Zealand, with 'Blaxstayman' being almost as sensitive. A leaf chlorosis, and fruit russet and distortion condition of 'Fuji' apple was shown to be graft-transmissible and probably caused by a virus. The conditions causing leaf malformation and a streaked fruit russet of 'Braeburn', and a bumpy russet of 'Royal Gala' appear to be caused by genetic mutations. Of three red-skinned sports of 'Braeburn', bud-inoculated with type B russet ring, only 'Braestar' produced fruit symptoms, several seasons after bud inoculation. A test of some selected apple rootstocks from commercial nurseries indicated that they were free from type B russet ring.