Symptoms and Signs
Diseased trees produce pebbly, leathery-skinned, pale fruit that is most evident at harvest. On Mahaleb rootstocks, trees rarely have fruit symptoms but suddenly wilt and collapse above the graft union. X-disease is caused by a phytoplasma organism that is found in phloem cells of infected trees. Trees are typically infected in summer and fall (July to October) and show symptoms the following year.
Comments on the Disease
X-disease, also called cherry buckskin, is a major cause of tree decline and serious losses of sweet cherry trees in some areas of California, including the northern San Joaquin Valley (not south of Madera), Sierra foothills, and North Coast.
The disease is spread by leafhoppers, which acquire the disease organism when feeding on diseased cherries or other plants that host the disease organism.
Effective management of this disease, involves:
- Annual surveys of the orchard to promptly identify and mark infected trees for removal
- Postharvest treatment of the orchard for leafhopper vectors (see sections on CHERRY LEAFHOPPER and MOUNTAIN LEAFHOPPER for more information)
- Removal of infected trees following postharvest treatment for leafhoppers
- Management (i.e. removal or sprays to control leafhoppers) of nearby ornamental hosts of the cherry leafhopper (boxwood, ceanothus, crabapple, hawthorn, lilac, myrtle, privet, pyracantha, viburnum).
- Removal of abandoned or volunteer cherries, almonds, and Japanese plum trees
- Control of weed hosts that harbor the pathogen. These include:
- In addition, curly dock, Rumex crispus, is a favored host of the mountain leafhopper and encourages the presence and breeding of this pest in the orchard so should be targeted for control as well.
- Avoiding the use of clover cover crops in the orchard, as they can harbor the pathogen
In areas where X-disease is prevalent, survey the orchards looking for fruit symptoms just before harvest. Only a single limb may have symptoms in newly infected trees. Collect fruit with symptoms and send them to a lab for confirmation. Mark any infected trees for removal after applying a postharvest treatment for leafhoppers. Be sure to treat stumps so that they do not resprout. Infected trees may live for many years with minimal foliar symptoms (but clear fruit symptoms) and serve as infection reservoirs for the rest of the orchard. It is critical to detect infected trees just as fruit becomes mature and symptoms are obvious.
Orchards on Mahaleb rootstock rarely show fruit symptoms but instead exhibit yellowing foliage and a rapid decline a few months after infection. These symptoms may look similar to the rapid decline caused by root rot or gopher damage. Survey orchards on Mahaleb rootstock periodically during the season (i.e. once in spring, summer, and fall) for signs of tree decline. Check the wood under the bark at the graft union for the characteristic appearance of zippering in the wood, which indicates X-disease infection. Mark and remove infected trees after a postharvest treatment for leafhoppers.