Apple Viruses

Apple Chlorotic Leaf Spot Virus

Apple chlorotic leaf spot virus (ACLSV) is one of the most widely distributed viruses of fruit trees and is found in many pome and stone fruit trees. ACLSV does not cause readily observable symptoms in most commercial cultivars in production and is considered a latent virus. No natural vector of this virus is known. Chance root grafting in apple orchards is one method of tree-to-tree transmission of the virus.

Disease management

Virus certification programs are the only effective means of control since ACLSV is transmitted only through grafting.

Apple Mosaic Virus

Apple mosaic virus (ApMV) is found wherever apple trees are grown. In the spring, pale yellow to cream-colored areas develop on expanding leaves of infected apple trees. These areas may appear as small spots, flecks, irregular blotches, vein-net patterns, line patterns, or bands along major veins. As the season advances, they become light chrome yellow or white and then soon become necrotic. Severely affected leaves drop prematurely. Reduction in yield are slight; no symptoms develop on most fruit cultivars. There is no known vector of this virus. The perpetuation of the virus in orchards is primarily due to using infected stock material and through chance root grafting.

Disease management

Rogueing infected trees and using certified disease-free planting stock are effective control measures.

Apple Stem Grooving Virus

Apple stem grooving virus (ASGV) in conjunction with apple stem pitting virus and apple chlorotic leaf spot virus comprise the three common viruses in apple that are considered latent and do not induce acute symptoms. No natural vector of this virus is known.

Disease management

Virus certification programs are the only effective means of control since ASGV is transmitted only through grafting.

Apple Stem Pitting Virus

Apple stem pitting virus (ASPV) is latent in most common commercial apple cultivars. There are no obvious symptoms associated with infection on most scion/rootstock combinations, but there is still potential for reduced production, as reported for 'Golden Delicious' apples when infected with the three latent viruses: ASGV, ASPV, ACLSV. No natural vector of this virus is known.

Disease management

Virus certification programs are the only effective means of control since ASGV is transmitted only through grafting.

Tomato Ringspot Virus

Tomato Ringspot virus causes apple union necrosis. 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.

Symptoms

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.

Disease management

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 act as reservoir hosts and can play a major role in spreading ToRSV.