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Western X Phytoplasma

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Written by: Scott Harper, WSU Plant Pathology; Teah Smith, Zirkle Fruit Company; Rob Curtiss, Washington Tree Fruit Research Commission. December 2018. 

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Organism: Phytoplasma

Host Range: Most Prunus species, ex. cherries, peaches, nectarines, almonds, plums, and choke cherry. Infects weeds: ex. puncture vine, tumble mustard, and flix weed.

Vector: Leafhoppers. Known vectors: (click for full-size image)

Distribution: 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 around Hood River.

Symptoms: On sweet cherries, infection reduces fruit size and quality. The disease stages are only clearly visible approximately two weeks prior to harvest. Molecular methods are more accurate for detection when symptoms are not visible, such as in the early stages of infection:

  1. 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.
  2. Middle infection (Years 2-3), systemically infected tree, small fruit observed on multiple or all limbs, and poor color development is pronounced.
  3. Terminal infection (4+ years), cultivar dependent, but characterized by reduced fruit yield, and dieback of limbs.

Little cherry virus-2 (LCV-2) or Western X? LCV-2 reduces fruit sugar content, affects the taste of fruit, and may reduce fruit size with tip taking on a pointed, triangular appearance.

Sample collection: Good sampling technique is essential to detecting Western X using molecular methods. To sample, collect plant material from several points around the tree (divide the tree into four sections and sample each). If sampling before harvest collect both leaves and cherry fruit stems. If sampling post-harvest collect only leaves. Tissue samples should be stored in clean plastic bags and kept refrigerated. For more information visit the CPCNW website: http://cpcnw.wsu.edu/fruit-tree-sampling/.

Control: 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:

  1. Identify and Remove Infected Trees: Primary control measures rely on identification and removal of infected trees. Infected trees spread the pathogen to neighboring trees by insect vectors or via root-grafting from tree to tree. To determine if root-grafting has occurred, treat stumps of removed trees with herbicide and look for herbicide injury on neighboring trees.
  2. Spray Vectors: Leafhoppers may be present as early as February and as late as November, so insecticides must be applied throughout the season, including post-harvest. Most leafhoppers feed on multiple host species so suppression of their populations is critical.
  3. 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.

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 Western X 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.

Contact: If you have any questions about Western X phytoplasma or little cherry disease, please contact:

Dr. Scott Harper
Department of Plant Pathology
Washington State University
509-786-9230 or scott.harper@wsu.edu


Washington State University