Apple union necrosis and decline
Tomato Ringspot Virus (TmRSV)
AUND is due to an incompatibility at the graft union where a resistant scion is grafted onto a susceptible, but tolerant rootstock, most commonly MM.106. Symptoms appear approximately 4–6 years after planting, usually once the tree is capable of bearing a full fruit load. Affected trees show a general decline beginning with delayed budbreak. The canopy tends to be sparse, bearing small, pale green leaves, and premature defoliation is possible. A distinct black, sunken line at the union is apparent underneath the bark (A). The graft union may be weakened to the point where the scion and rootstock separate partially or completely or crack under stressful conditions (B).
Common to all fruit-growing regions in eastern North America.
Any disease or disorder affecting the root system or rootstock/scion union can produce similar above-ground symptoms. AUND can be distinguished from similar diseases by cutting away the bark and observing the darkening at the graft union.
TmRSV can survive in a number of weed hosts, is seed-transmitted in dandelion, and is vectored by the dagger nematodes Xiphinema americanum Cobb and X. rivesi Dalmasso. Thus, good weed control and pre-planting site preparation can help to reduce the incidence of this disease. Selecting resistant rootstock or, in some instances, using a resistant interstem will reduce the incidence of disease. However, the ranking of rootstocks has been difficult in some cases due to inconsistencies among reports. Nonetheless, rootsotcks considered tolerant to AUND (i.e., those in which the development of symptoms is possible) include MM.106 (particularly on Delicious), M.26, and P.2. Those considered resistant include M.4, M.7, and Ottawa 3. Several other rootstocks, including M.9, MM.111, G.16, G.30, and B.9, have given mixed results.
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