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X-Disease of Peach, Nectarine, and Cherry

PLPATH-FRU-0013
Agriculture and Natural Resources
Date: 
10/16/2024
Revised by:
Melanie L. Lewis Ivey, Associate Professor, Department of Plant Pathology, The Ohio State University - Wooster, Wooster, OH
Francesca Rotondo, Program Director, C. Wayne Ellett Plant and Pest Diagnostic Clinic, Wooster, OH
Ashley Leach, Assistant Professor, Department of Entomology, The Ohio State University - Wooster, Wooster, OH

X-disease affects peach, nectarine, sweet cherry, and sour cherry trees in the Great Lakes regions of the United States and Canada. In the Northwestern United States, the disease also occurs on plums. On peach, nectarine, and plum trees the disease is referred to as Peach X-disease. On cherry trees it is called Western X-disease or cherry buckskin. The distribution of these diseases mirror that of the wild chokecherry, which serves as a major reservoir for the pathogen. The pathogen, ‘Candidatus Phytoplasma pruni,’ is a phytoplasma and is transmitted by various species of leafhoppers. X-disease symptoms differ depending on the crop type, variety, rootstock (cherry only), and environmental conditions, but are most recognizable on peach (Figure 1).A collage of four images showing symptoms of X-disease on peach and cherry. Top two images show a tree with yellow (chlorotic) leaves, and a close-up of curled leaves with brown spots. Bottom two images show close-up of leaves that are necrotic (brown), and a view of red sweet cherries hanging from branches next to a cluster of diseased white cherries.

Disease Development and Symptoms

X-disease is caused by the phytoplasma 'Candidatus Phytoplasma pruni.’ Phytoplasma are specialized bacteria that live in the phloem cells (plant sap) of plants. Wild chokecherry plants are the primary reservoir for 'Candidatus Phytoplasma pruni' although wild plum, wild cherry, and weeds such as dandelions, lambsquarter, pigweed, and clover are also hosts (Figure 2).

Leafhoppers (Figure 3) are the only known vectors of the phytoplasma that causes X-disease. In the United States, seven species of leafhoppers are known vectors of the pathogen, acquiring the pathogen by feeding on the shoots and leaves of infected host plants. The leafhoppers then feed on healthy peach, nectarine, sweet cherry, and sour cherry trees, introducing the pathogen into the trees' phloem. Although peach and nectarines are very susceptible to infections, they are poor hosts and generally not involved in the spread of the pathogen. However, sweet and sour cherry, particularly those grafted on Mahaleb and Mazzard rootstock, are suitable hosts, and are involved in the spread of disease. The rate of transmission by leafhoppers varies depending on the level of infection in the orchard and leafhopper abundance. The phytoplasma overwinter in the roots, and in the spring they move upward in the phloem, re-infecting the shoots and leaves.Graphic showing three images stacked vertically, labeled from top to bottom, chokeberry, broadleaf weeds, and infected propagation material, with arrows pointing from each image to two larger images of trees with peaches showing root grafting.

'Candidatus Phytoplasma pruni' is also transmitted by grafting, including root grafting from tree-to-tree, and can be moved around the region via infected propagation material.

Peach, nectarine, and plum symptoms

Symptoms on peach, nectarine, and plum are predominantly foliar, but fruit may also be affected. The symptoms first appear on the leaves in midsummer, about two months prior to harvest. The leaves curl inward and develop irregular, reddish-purple spots and blotches. Eventually the spots turn brown and drop out, leaving the leaves tattered with a “shothole” appearance. As the disease progresses the leaves drop prematurely beginning at the base of the shoot. Eventually, only a tuft of leaves remains at the tips of infected shoots. Initially, infected trees will have a mixture of healthy and diseased branches on the same tree and fruit set will appear normal. However, two to three years after the initial infection, most branches will show symptoms, fruit set will be reduced, and the fruit will be small and misshapen. No symptoms occur on the roots of infected trees.

Cherry Symptoms

Six images positioned side-by-side vertically, showing an overhead view of leafhoppers with long, clear wings and heads with large eyes.There are two major types of symptoms associated with X-disease on cherry. Trees on Mahaleb rootstock suddenly wilt and collapse above the graft union and rarely have fruit symptoms. In contrast, trees on Mazzard rootstock decline slowly and produce fruit that are small, misshapen, show poor color development (i.e., pink to white in color), and have a bitter flavor. Fruit symptoms can be confused with unripe fruit until harvest. Dieback of limbs on trees on Mazzard rootstock occurs in trees that have been infected for several years. Roots on trees affected by X-disease appear normal.

Management

Prevention of X-disease relies on the use of pathogen-free propagation and planting material, early detection and testing, management of alternative hosts especially chokecherries near fruit orchards, and leafhopper control.

Clean Plants

Trees certified to be pathogen-free (clean plants) should be purchased from certified nurseries. A clean plant is the product of a process designed to ensure that plants distributed to nurseries and growers are free of known pathogens. Most nurseries sell certified and non-certified trees, so it is important to confirm that the trees you are purchasing are certified.

Monitor and Manage Vectors

Leafhoppers are the only known vectors that transmit X-disease phytoplasma. These insects can be monitored using sweep nets or insect vacuums. Yellow sticky traps can also be used season-long to passively monitor for leafhopper presence. It is recommended to place one trap every two acres. Traps should be suspended in trees approximately 2–4 feet above the orchard floor. Position traps along the orchard's perimeter or in areas previously affected by X-disease. Regular monitoring of the traps every 7–10 days is necessary for early detection and a rapid response. The presence of one or more leafhoppers on a trap, initiates the application of an insecticide.

Early detection and testing

Early detection and removal of infected trees is critical to preventing the spread of the X-disease phytoplasma through the orchard and to neighboring orchards. Annual surveys of the orchard and confirmation of disease through testing must be done. Every row and both sides of every tree should be scouted. Trees with X-disease symptoms should be flagged or marked (Figure 4), and samples immediately collected and tested. Cuttings (4–5 inches) from the previous season’s symptomatic limbs (pencil size or larger)—including leaves and fruit stems—are the most suitable for reliable test results. Leaves from symptomatic tissues may test negative even if the woody tissue tests positive, therefore leaves alone are not reliable. In Ohio, samples can be submitted to the C. Wayne Ellett Plant and Pest Diagnostic Clinic (phone: 330-263-3650).

Cultural Practices

Removal of alternative hosts

Wild chokecherry plants (see Box 1) are the primary reservoir of the X-disease pathogen. These plants should be eradicated when they are near stone fruit orchards. Chokecherry plants can resprout from roots and trunks, so it is important to remove all above- and below-ground parts of the plant. Trees can be removed by bulldozing, deep plowing, burning, or pulling out individual bushes. Areas where trees were removed should be checked for chokecherry sprouts and seedlings during the growing season. Sprouts or seedlings should be pulled out or spot treated with an herbicide.

Broadleaf weeds serve as secondary reservoirs for the X-disease pathogen. Conversely, grasses are not typical hosts for ‘Candidatus Phytoplasma pruni’ and do not support leafhopper populations since leafhoppers do not feed on grasses. Planting and maintaining healthy, weed-free grass in the drive rows and around the orchard can be an effective weed-control strategy that also reduces leafhopper populations, aiding in overall X-disease management in the orchard.

Two photos side-by-side vertically. Photo on left shows leafy, bush-like vegetation with combs of white flowers. Photo on rights shows close-up of leafy branch holding clusters of small red berries.Box 1. Chokecherry (Prunus virginiana) is often confused with wild black cherry and wild pin or fire cherry. It is important to be able to distinguish this species from others because only chokecherry is important in the spread of the phytoplasma. Unlike black cherry and pin cherry trees, which can grow up to 50 feet or more, chokecherries typically grow as shrubs reaching heights up to 15 feet. Chokecherries are also often found growing in clusters. Chokecherries produce fruit along a central stem that turns black when ripe, maturing earlier than black cherry fruit. Unlike black cherry, where the calyx cup remains attached to the fruit, it does not persist on chokecherry fruit. Pin cherries, on the other hand, bear fruit in clusters like sour cherries and are quite distinct from chokecherries. Chokecherry leaves are generally wider and broader than those of black cherry or pin cherry, with more pronounced and widely spaced serrations along the leaf margins.

In addition to X-disease, chokecherry is a host of the black knot fungus (Apiosporina morbosa) and the eastern tent caterpillar, and its leaves, bark, and stone are poisonous to livestock.

 

Chemical and Biological Control

Two photos positioned side-by-side vertically. Photo on left shows tree with yellow leaves. Photo on right shows tree being pulled from ground by steam shovel.Once a tree is infected there are no effective chemicals or biologicals to control the disease, and the trees must be removed (Figure 4). However, chemicals and some biologicals can be used to eradicate chokecherries, control weeds, and/or reduce leafhopper populations.

Chokecherry eradication

Chokecherry trees are commonly found in hedges, along property lines, in open woods, in overgrown meadows, and in abandoned fields. Systemic herbicides, specifically brush killers, applied in the summer and fall are the most economical and effective way to kill chokecherry trees. Herbicides can also be used to spot-treat new sprouts and seedlings.

Weed control

Broadleaf herbicides used between the rows and around the perimeter of the orchard will reduce alternative hosts and indirectly reduce leafhopper populations by eliminating the leafhopper’s food source.  For guidance on suitable herbicides for use in the orchard, refer to the Midwest Fruit Pest Management Guide (Bulletin 506).

Leafhopper population control

A broad-spectrum, seasonal, insect-management program that includes insecticides effective against leafhoppers will reduce the spread of X-disease phytoplasma in the orchard. Later detection of leafhoppers on yellow sticky traps may necessitate additional insecticide applications. Insecticides belonging to neonicotinoid, pyrethroid, and organophosphate classes have efficacy against leafhoppers. For guidance on suitable insecticides targeting leafhoppers, refer to the Midwest Fruit Pest Management Guide (Bulletin 506).

In organic orchards, refined kaolin clay (Surround) applied every 7–10 days after petal fall can deter leafhopper feeding. Pottery grade kaolin clay should not be used as it will negatively impact plant health by reducing photosynthesis. One drawback of using kaolin clay is that it can deter beneficial mite predators, therefore oil sprays may be necessary for mite control.

Useful Resources

Check out the following resources for more information on X-Disease of peach, nectarine, and cherry:

Originally written in 2008 by Michael A. Ellis, Department of Plant Pathology, The Ohio State University.

Originally posted Oct 16, 2024.
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