Cause These diseases are caused by an unknown graft-transmissible agent. They were assumed to be caused by phytoplasmas, but repeated attempts to confirm this association have failed. Many old rootstocks introduced from Europe were uniformly infected with the pathogens that cause these diseases.
Symptoms On many commercial cultivars, there are no acute symptoms, although fruit quality and yield may be adversely affected. On sensitive cultivars, branches fail to develop properly and remain very pliable and droop under their own weight. The affected limbs are also more prone to cold and frost damage than healthy limbs. On cultivars such as 'Gravenstein', the disease appears as a flattening of shoots and limbs on 2- to 3-year old growth. As the branches become older, the flattening becomes more pronounced and deep furrows develop. Trees exhibit reduced production, and the diseased limbs are very prone to breaking.
Both types of symptom are more severe in moderate climates and less severe in warmer environments.
Cultural control The use of certified virus-tested (and found to be free of all known viruses) propagation material has dramatically decreased the incidence of these diseases.
References Hadidi, A., Barba, M., Candresse, T. and Jelkmann, W. 2011. Virus and Virus-like Diseases of Pome and Stone Fruits. St. Paul, MN: APS Press.
van Oosten, J.J. 1983. Growth, yield and fruit quality of virus-infected and virus-free Golden Delicious apple trees, 1968-1982. Acta Horticulturae 130:213-220.