What is apple union necrosis and decline, and how do I identify and manage the disease?

Apple union necrosis and decline is caused by tomato ringspot virus, which also causes disease in other fruit trees, small fruits, and many herbaceous plants. Symptoms appear as infected trees reach bearing age. Bud break is often delayed in the spring, and leaves are small and sparse with a dull, pale green color. Terminal shoot growth is reduced, with shortened internodes. Infected trees flower heavily and set large numbers of small, highly colored fruit. Leaf discoloration and leaf drop occur prematurely in infected trees. Affected trees often produce large numbers of sprouts from the rootstock. Swelling may occur above the graft union. Partial to complete separation of the graft union is common on severely affected trees; sometimes the top breaks off at the union in strong winds. Decline and death are possible, although infection is not always lethal. Removal of the bark above and below the graft union reveals it to be abnormally thick, spongy, and orange-colored, and there is a distinct necrotic line at the scion/rootstock union. Trees cannot be cured of virus after they are purchased and established. To avoid introducing the virus into new plantings, purchase only certified virus-free trees from a reputable nursery. For more detailed information, see this article on Apple Union Necrosis and Decline.

Answer provided by Alan R. Biggs, West Virginia University.