Source: H.J. Larsen, Bugwood.org
Ornamental crabapples and other Malus species appear unaffected, as do most apple cultivars on seedling rootstocks. Tomato ringspot virus (ToRSV) has been isolated from clonally propagated, size-controlling rootstocks and Malling Merton 106 (MM.106) is the most frequently, naturally infected clone.
Apple trees infected with ToRSV normally begin to exhibit symptoms when they reach bearing age. Foliation is delayed on infected trees, the leaves are small and sparse, and their color is a dull, pale green. Terminal shoot growth is reduced, the stem internodes are short, and infected trees flower heavily and set large crops of small, highly colored fruit. Partial or complete separation of the graft union is common on severely affected trees. Removal of the bark above and below the graft union reveals abnormally thick, spongy, orange-colored bark and a distinct necrotic line at the scion-rootstock union.
The severity of apple union necrosis is influenced by the cultivar-rootstock combination. Red Delicious on MM.106 rootstocks is the most severely affected combination and may exhibit severe graft union necrosis followed by decline and death. The symptoms are generally less severe on other cultivars.
Although dagger nematodes are the primary vectors of ToRSV, other factors are important in the spread of the virus in woody fruit crops. Because apples are propagated by grafting, it is important to purchase certified virus-free trees. A good weed management program is necessary since infected weeds can also play a major role in spreading ToRSV.