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Phytoplasmas and Viroids

Other Diseases Caused by Phytoplasmas and Viroids

Apple Rubbery Wood

  • Causative Agent: unconfirmed graft-transmissible agent, perhaps phytoplasma.
  • Vector: None known
  • Crops Affected: Apple
  • Disease Description: Abnormal flexibility of stems and branches. Reduced growth, vigor, and yield.  On ‘Gravenstein’, 2-3 year old limbs and shoots will have a flattened appearance.  Diseased limbs are more brittle.
  • Management:  Use virus tested, certified virus free trees
  • Resource:

Apple Scar Skin Viroid and Dapple Apple

  • Causative agent:  Apple scar skin viroid
  • Vector:  unknown; it can spread through seeds, infected tools, and possibly spread by root grafts
  • Crops affected:  Apple, Pear (a symptomless carrier)
  • Disease description: Many cultivars of apple and pear do not exhibit disease symptoms. Affected fruit have small circular spots near the calyx, which increase in size as the fruit matures. In severe cases, the circular patches become brown and necrotic.  Fruit from infected trees are typically smaller than those on healthy, uninfected trees.
  • Management: Use virus tested and virus-free certified planting material. Disinfest contaminated pruning tools.
  • Resource:

Pear Blister Canker Viroid

  • Causative agent:  pear blister canker viroid
  • Vector:  unknown
  • Crops affected:  Pear
  • Disease description:  In the spring, small blisters erupt on the bark of one and two year old shoots.  Cracks associated with these blisters can lead to premature tree death.  Older trees that survive are generally smaller and may have fewer flower buds develop than uninfected healthy trees.
  • Management:  Use virus tested and virus-free certified planting material.
  • Resources:
    • Pear blister canker, Pacific Northwest Plant Disease Management Handbook, webpage. (Accessed: 1/17/17)

Pear Decline

  • Causative Agent: Phytoplasma organism (Candidatus Phytoplasma pyri)
  • Vector: Pear psylla, or by grafting infected material to healthy trees
  • Crops affected: Pears
  • Disease description:  The roots of infected trees serve as a pathogen reservoir.  Trees may experience a quick or slow decline that may be rootstock and stress related.  Trees may wilt and die in a few weeks; or lose vigor over several years where the leaves may roll and turn red, and foliage becomes sparse.  Leaves may drop prematurely.
  • Management: Use resistant or tolerant rootstocks. Maintain good tree vigor with proper management. Remove infected trees and roots from orchards. Control pear psylla in trees.
  • Resources:
    • Pear Decline, Pacific Northwest Plant Disease Management Handbook, webpage. (Accessed: 1/17/17).
    • Pear Decline, University of California- Davis IPM webpage.  (Accessed: 1/17/17).
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