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Abstract
Nonsported ‘Rhode Island Greening’ consistently produced its characteristic smooth and symmetrical fruits following ionizing irradiation or vegetative propagation, except for a low incidence of russet-fruited scions in one irradiation experiment. Three russet-fruited sports consistently produced russet fruits after either irradiation or propagation, as in the original limb, but more than half of the scions of another russet sport bore nonrusset fruits following either irradiation or propagation. Two sports with furrowed fruits, when irradiated, pruned severely, disbudded or propagated, produced smooth, symmetrical fruits on 0 - 100% of the scions or branches, with an incidence of 0 - 11% russet-fruited scions in one irradiation experiment. Three sports with russet-furrowed fruits, when irradiated, disbudded or propagated, produced a graduated series of fruits ranging from russet-furrowed to furrowed to smooth and symmetrical, with an incidence of 0 - 8% russet-fruited scions in one irradiation experiment. Another sport with russet-furrowed fruits was stable in limited observations. There was no evidence that variation in fruit type was related to virus infection. The sports are hypothesized to be periclinal chimeras with a mutation for russet in layer I of the apical meristem (unstable sports) or in layers I and II (stable sports), or with a mutation for furrowing in layer III (sports with furrowed fruits) or in layers II and III (sports with russet-furrowed fruits). The appearance of russet fruits in irradiation experiments with nonrusset clones was considered to be the result of new mutation.
Abstract
Of 31 preliminary selections from the Geneva scab-resistance apple breeding program inoculated with apple chlorotic leaf spot virus (CLSV), 18 clones exhibited sensitivity during the first 2 growing seasons. Six advanced selections, all derivatives of the virus-sensitive Malus floribunda 821, were inoculated with CLSV or with a combination of other graft-transmissible agents that occur commonly as latent infections in apple. None of the 6 exhibited visible symptoms associated with virus content during 3 growing seasons; growth of 1 selection was slightly depressed. In a 3rd test, 1 of 7 advanced selections derived from M. floribunda exhibited depressed growth in an orchard trial on virus-infected Mailing (M) 9 rootstocks, compared to growth on virus-free M 9.