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SUGAR CRAB, AN EFFECTIVE INDICATOR FOR DAPPLE APPLE.
In 1958, the causal agent was shown to be graft transmissible.
Although the disease has been reported from other commercial apple cultivars, it appears to be of major concers only in the cultivars ‘Cortland’, ‘McIntosh’ and ‘Macoun’, which are grown primarily in the New England fruit-producing states.
The disease appears not to be restricted to North America; recent reports from Japan and the United Kingdom indicate that the same or a very similar disease occurs there also.
The causal agent of dapple apple has not been isolated or identified; detection has required the use of woody indexing procedures employing the cultivar ‘Cortland’ as the indicator host.
In a search for other sensitive indicator hosts for dapple apple, the crabapple cultivar ‘Sugar Crab’ has proven extremely effective.
The clone of ‘Sugar Crab’ used in our studies is infected with aplle chlorotic leafspot virus; however, this latent contaminant does not appear to interfere with symptom expression of dapple apple. ‘Sugar Crab’ is easily propagated and precocious.
Fruit on trees infected with dapple apple develop deep surface cracks which suberize and become dark or black in color.
Few, if any, normal fruit are produced on an infected tree, and the tree continues to produce severely affected fruit in succeedding years.
No other symptoms in ‘Sugar Crab’ have been associated with dapple apple infection.
The dapple apple agent seems to be refractory to common thermotherapy treatment.
The same or very similar fruit symptoms are produced by ‘Sugar Crab’ trees inoculated with buds from source trees with symptoms of dapple apple or scar skin.
This would suggest that the two diseases have common causal agents.