Symptoms and Signs
Mission trees with yellow bud mosaic appear open and stunted. Leaves may be crinkled and distorted. Occasionally, chlorotic spots may develop that eventually become necrotic and are abscised, leaving holes or resulting in a tattered appearance. Leaves may also be stunted and form small rosettes or short tufts. They stand out in sharp contrast to normal leaves on the same branch or in other parts of the tree. In severe cases, the leaves may develop a mottled appearance.
Leaves with symptoms develop sporadically throughout the tree canopy. Affected branches lack lateral growth and have little terminal growth from one year to the next. Frequently, one or more normal shoots are produced on a branch that is otherwise completely diseased. Fruit set on diseased trees is reduced. Hulls are wrinkled or rough, and fruit appear larger than healthy fruit because the hulls are abnormally thick. Trees with yellow bud mosaic will live for many years but do not produce well; yield reduction is directly proportional to the severity of the symptoms.
Comments on the Disease
Yellow bud mosaic was first described in California in 1936 on peaches and almonds in Solano and Yolo counties. It can affect most stone fruits. On almond, yellow bud mosaic seriously damages the cultivar Mission. Many other plants provide natural reservoirs of Tomato ringspot virus, which is transmitted from plant to plant by the dagger nematode Xiphinema americanum.
The yellow bud mosaic strain of Tomato ringspot virus can infect most rootstocks, except Marianna 2624 plum. Besides Mission, symptoms occasionally may be seen on water sprouts in the lower scaffold limbs of cultivars Nonpareil and NePlus Ultra, and a form of the disease has occurred in the Patterson area of Stanislaus County that caused Nonpareil trees to grow in a spiral form, similar to the characteristic spiraling of the IXL cultivar.
Tomato ringspot virus is indigenous to California and is widely distributed in the coastal areas and the Sacramento Valley; it also occurs in a few scattered areas of the San Joaquin Valley. Tomato ringspot virus is spread by budding and grafting and by dagger nematodes, Xiphinema spp., in the orchard soil. The virus is seedborne in dandelion, and infects a number of other broadleaf weeds (e.g., bristly oxtongue, little mallow, chickweed, lambsquarters, mullein, plantain, spurge, white clover) as well as apricot, apples, caneberries, peach, and grapevines. The nematode vector acquires the virus by feeding on the roots of infected hosts. Xiphinema juveniles remain infective until they molt. Adults remain infective for 3 to 8 months.
Susceptible rootstocks become infected with Tomato ringspot virus when infected dagger nematodes feed on their roots. Gradually, over the course of several years, the virus spreads to the upper shoot terminals. If nematodes carrying the virus feed only on one root, the disease will initially develop only on one side of the tree and then spreads throughout the tree. If the nematodes are distributed throughout the soil and feed on all the roots, the disease will progress more rapidly and uniformly.
Tomato ringspot virus is spread to new locations when floods or cultural operations move infective nematodes. Trees in areas adjacent to east-west streams flowing from the coastal mountains or Sierra foothills have a higher incidence of this disease. The disease also can spread when wind-disseminated seeds of virus-infected plants such as dandelion germinate and grow in an orchard with dagger nematodes. Within an orchard, the disease spreads slowly to adjacent trees, gradually enlarging the original area of infection. Frequently, symptoms first develop in border trees and subsequently spread to adjacent trees. Although it is possible for infection to spread from tree to tree through root grafts, this is less common. The disease spreads mainly by dagger nematodes. Mission trees planted in infested soil will exhibit symptoms in 2 years and will never be productive. When older Mission trees become infected as a result of natural spread of nematodes in the soil, they generally become uneconomical producers 3 to 5 years after the symptoms first appear.
Management
Once yellow bud mosaic symptoms appear in an orchard, it is extremely difficult to eliminate the virus. Deep soil fumigation at rates high enough to kill the nematode is expensive and often not practical, especially if other factors may also be affecting the growth of the tree.
If you have an area that contains Tomato ringspot virus, try to limit its spread to other parts of the orchard. Do not perform any cultural operations, including cross cultivation and flood irrigation that may move soil from this area to other locations. If the affected area involves only a small part of the orchard, remove infected trees and trees in at least two rows beyond. If several areas of diseased trees are present, you may need to remove the entire block. Before removing the trees, treat them with an herbicide or girdle them to aid in killing the roots. Remove as many roots (potential virus reservoirs) as possible. For best results, fallow the ground for 2 years and then fumigate the soil to kill the nematodes.
Do not plant Mission trees on peach or almond rootstock in infested areas. Marianna 2624 plum rootstock is resistant to the virus. Replant with cultivars that are compatible with Marianna 2624, or with non-hosts of the virus, such as walnut, pear, and plum or prune on Marianna 2624 rootstock.