Union Necrosis and Decline
Affected trees begin to exhibit symptoms when they reach bearing age. Leaves are small and sparse and their color is a dull pale green. Other symptoms include a reduction in terminal shoot growth and shortened stem internodes. Infected trees flower heavily and set large crops of small, highly colored fruit. On severely infected trees, partial to complete separation of the graft union is common. Removal of bark above and below the graft union reveals abnormally thick, spongy bark and a distinct necrotic line at the graft union. Symptoms on apple are similar to those on peach (see Prunus stem pitting).
Symptoms at the graft union may differ depending on the cultivar. The severity of apple union necrosis is influenced by the cultivar and rootstock combination. There has been some reported resistance of cultivars and rootstocks to this disease. Check your production guides for specific details.
Disease Cycle and Management
Apple union necrosis and decline are caused by the tomato ringspot virus, which is vectored by the dagger nematode. The disease cycle and disease management tactics are the same as that for Prunus stem pitting.