Written by: Scott Harper, WSU Plant Pathology; Teah Smith, Zirkle Fruit Company; Rob Curtiss, Washington Tree Fruit Research Commission. February 2019. Vector management reviewed by Tobin Northfield July 2019. Updated by Tianna DuPont, WSU Extension November 2019. Updated January 2020. Updated April 2020. Updated June 2020.
X-disease phytoplasma, Little cherry virus 2 (LChV2) and Little cherry virus 1 (LChV1), cause small cherry symptoms often described as ‘Little Cherry’, ‘X-disease’ or Western X. Diseased trees produce cherries of small size and poor color and flavor making the fruit unmarketable. X-disease is at epidemic levels in the Columbia River basin, with high incidence in Yakima, Benton, and Franklin counties, and present in Oregon, The Dalles area.
X-disease (Western X) is not a new problem. It was first identified in cherry trees of WA State in 1946. In a 1947 survey, about 1% of cherry trees were found to be infected, and it has remained present ever since, fluctuating in frequency.
On sweet cherries infection reduces fruit size and quality. The disease symptoms are only clearly visible starting the week prior to harvest. In contrast to the effects of Little cherry virus 2 where fruit often has little flavor, fruit from X-disease infected
trees generally have a bitter taste. In addition to fruit symptoms you will see reduced growth and extension of infected limbs, sometimes leading to crowding of leaves into dense clusters (called a rosette) when trees have been infected for multiple years. Molecular methods are more accurate for detection when symptoms are not visible, such as in the early stages of infection.
Symptoms in cherry
Small and misshapen fruit.
Poor color development.
Fruit lacking in flavor/ bitter.
Symptoms can be confused with unripe fruit until close to harvest.
Symptoms in peaches, plums, and nectarines
Yellowed curled leaves
Leaf shot hole
Small deformed fruit.
Leaf yellowing symptoms on infected peaches and nectarines begin to appear about 2 months prior to harvest, and get progressively worse, with shot holes appearing as the season progresses.
Early infection (Year 1), small fruit may be restricted to one branch, or cluster, fruit color may develop normally, or individual pale to white fruit may be observed.
Middle infection (Years 2-3), systemically infected tree, small fruit observed on multiple or all limbs, and poor color development is pronounced.
Terminal infection (4+ years), cultivar dependent, but characterized by reduced fruit yield, and dieback of limbs.
X-disease phytoplasma is not a virus, but instead is a type of wall-less bacteria known as a phytoplasma. The X-disease phytoplasma lives and replicates in the in the vascular phloem of infected trees, interfering with tree growth and development.
X-disease is present across North America, throughout Washington State, and at epidemic levels in the Columbia River basin, with high incidence from Yakima, Benton, and Franklin counties, and present in Oregon in The Dalles area.
X-disease: Most Prunus species, ex. cherries, peaches, nectarines, almonds, plums, and chokecherry. Infects weeds: ex. puncture vine, tumble mustard, and flixweed.
Grafting: X-disease phytoplasma is readily transmitted by all types of grafting; other means of spread are not known. Vector: Leafhoppers. Known vectors: Colladonus montanus, Fiebriella florii, Scaphytopius acutus, Paraphlepsius irroratus, Colladonus reductus, Colladonus geminatus.
Life Cycle of Organism
The X-disease phytoplasma replicates in the phloem tissue of the tree. It is believed that the phytoplasma either ceases to replicate or dies in the aerial parts of the tree as the branches go dormant during the winter months, but active, living phytoplasma cells overwinter in the roots. In the spring, the aerial portions of the tree become re-infected as the phytoplasma moves up the phloem of the tree, usually following the same general route as in the previous year. As a result, you may see symptoms in one limb for a year or more, but symptoms will eventually appear in additional limbs. Removing a symptomatic limb does not eliminate the phytoplasma since it is already in the root system before symptoms appear.
Sampling and Testing for X-disease
Material to sample: Submit four five-inch cuttings from the diseased limb(s) including leaves, and FRUIT STEMS.
Where to sample: Trees with symptoms: Sample from symptomatic limbs. Trees with no symptoms: Sample from each leader. *Samples only needed in non-confirmed blocks/ adjoining trees. See flow chart.
When to sample: The week before harvest to mid-August.
Sample condition: Keep tissue moist and cool (e.g. package with a cold pack). Old or dried tissue is more likely to have false negatives.
There is no cure and an infected tree will remain infected for the rest of its life. There are no commercial products that have been proven in scientific studies to have an effect on the phytoplasma. Management requires a combination of these three strategies:
Pathogen-Free Planting Sources: Replacement trees must be obtained from pathogen-free planting stock. Nursery trees can be free of symptoms and still be infected. Manage your risks – if in doubt, have the material tested before you buy or plant.
Identify and Remove Infected Trees: Primary control measures rely on identification and removal of infected trees. Remove infected trees following postharvest treatment for leafhoppers. Infected trees spread the pathogen to neighboring trees by insect vectors or via root-grafting from tree to tree. Treat stumps with herbicide immediately after cutting or inject into trees before cutting trees to ensure roots are dead. Several glyphosate products are labeled, see BMPs for Tree Removal. In an early study, orchards where infected trees were removed as soon as they were observed, the disease incidence remained below 2% and decreased over time.
Monitor and Manage Vectors:Consider timing. Both leafhopper populations numbers and X-disease phytoplasma concentration in the tree are likely to be higher after harvest. When phytoplasma concentration in the tree is higher leafhoppers are more likely to acquire and transfer the pathogen. Concentrate monitoring and management efforts when risk is highest after harvest. Monitor. Monitor leafhopper populations in early and late season, including postharvest in order to manage populations not controlled by your general insect management program. Monitor using yellow sticky card traps (best) placed 3-5 feet high in the cherry canopy (VanSteenwick 1990). Rotate leafhopper products when populations are present. University trials for postharvest management of leafhoppers are not available in the Pacific Northwest. See management 2020 X-disease vector management for details.
Manage alternative hosts of the phytoplasma and of the leafhoppers: clovers, dandelions, curly dock, bitter cherry, chokecherry. Broadleaf weeds are difficult to eliminate from an orchard but by suppressing them, your orchard becomes less attractive to the leafhopper vectors.
Finally, control of this disease requires a community-wide effort, what your neighbor does or doesn’t do, affects you (and vice versa). The key to ending the current X-disease epidemic relies on reducing the amount of pathogen present in the state. This can only be done by removing infected trees because it is from those trees that the leafhoppers are acquiring and spreading the pathogen.
Van Steenwyk, R.A., Havens, D.M., Freeman, R. 1990. Evaluation of Trap Types for Two Vectors of Western X Disease: Colladonus montanus and Fieberiella florri. Journal of Economic Entomology. 83: pg 2279
Purcell, A.H., Elkinton, J. 1980. A Comparison of Sampling Methods for Leafhopper Vectors of X Disease in California Cherry Orchards. Journal of Economic Entomology 73: 854-880.
Purcell, A.H. 1987. Buckskin Disease of Cherry. California Agriculture.