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Nursery Prevention of X-disease Phytoplasma and Little Cherry Virus Recommendations

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Clean plant material is essential to limit the incidence of X-disease Phytoplasma and Little cherry Virus-1 and -2 in Washington and Oregon orchards.

Assess Risk

In developing a management strategy, nurseries need to consider:

  • What is the incidence of these pathogens in my area?
  • How close are potentially infected orchards to my operation?
  • What control measures are producers in my area using?
Preventative measures

Nurseries must consider adopting preventative measures. These include:

Sourcing propagative material
  • The Clean Plant Center Northwest
    • The clean plant center can supply small quantities of scion and rootstock buds or other propagative units to begin nursery propagation.
    • Propagate clean scion material on tested rootstocks, or rootstocks grown in protected environments.
  • Nursery-Owned Mother Blocks
    • Mother blocks should be scouted throughout the growing season, and randomly tested.
    • Propagate clean scion material on tested rootstocks, or rootstocks grown in protected environments.
  • Commercial Orchards
    • Sourcing material from commercial orchards for nursery propagation is not recommended, the risk is too great.
    • If material must be sourced from commercial orchards that are exposed to pathogen and vector pressure, each source tree must be tested i) during the active growing season before collecting budwood, and ii) in the following growing season for confirmation.
    • Trees produced from commercial orchard sourced material should be randomly tested prior to sale.
Vector management
  • Monitor for leafhopper vectors of X-disease. Place yellow sticky cards at 3 ft height[1] 2 per ½ acre and check weekly or monitor using sweep nets.
  • Consider growing mother tree and nursery trees inside netted barriers to protect from leafhopper and mealy bug vectors. Barriers should be 10 to 20 ft tall[2] with screen that is less than 1 mm at its narrowest point[3].
  • Employ spray programs which cover potential vectors and vector activity periods. See <treefruit.wsu.edu/crop-protection/disease-management/western-x> and <treefruit.wsu.edu/crop-protection/disease-management/little-cherry-disease> for details.
  • Consider vector deterrents, e.g. Extenday, kaolin clay.
Broadleaf weed management
  • Broadleaf herbicide use recommended.
  • Some broadleaf weeds are a host for X-disease phytoplasma (e.g. puncture vine, tumble mustard, and flixweed). In preliminary trials leafhoppers seem to prefer areas with broadleaf weeds.
Confirmatory measures

Active measures must be taken to ensure the continued pathogen-free status of nursery material and ensure customer confidence:

Scout & Sample
  • Scout for symptoms on trees of fruit-bearing age.
  • Randomly test both mother and production blocks for the presence of these pathogens. PCR analysis preferred.
  • Sampling rate: To be 95% sure that less than 5% of trees are infected you would have to test 56 samples for every 1000 trees. Bulk sampling of 3-5 samples may be possible (under testing).
  • Sample timing: Sample nursery trees in from July to late August (1-2 months after harvest).
  • Sample material: Sample 1-year old wood that has hardened. Four five-inch sections with leaves and woody stems. Sample side branches instead of destructive sampling of the main stem.
Tracking
  • These pathogens have long latency periods. Follow up with customers on the status of material sold.
Sentinel Trees
  • Include peach/ nectarine plants in your block which will show foliar symptoms and dieback (e.g. Honeyhaven is particularly sensitive.)
Restorative measures
  • Symptomatic or confirmed positive trees must be removed. Spray trees with insecticide before removal to limit spread from infected trees. When removing trees remove tree roots to the greatest degree possible to limit root material which may remain and root graft to newly planted tree. No trees should be planted in the same area as removed positive/symptomatic trees for at least two years.
  • Neighboring trees must be tested around confirmed positives and random testing of the block increased.
  • Trace testing, or at least follow-up scouting/customer contact should be conducted on progeny trees produced from confirmed positive trees up to one year prior.
Contacts

Tianna DuPont, WSU Extension, (509) 293-8758 tianna.dupont@wsu.edu

Scott Harper, WSU Department of Plant Pathology, 509-786-9230 scott.harper@wsu.edu

Tobin Northfield, WSU Department of Entomology, tnorthfield@wsu.edu

Bernardita Sallato, WSU Extension, (509) 4398542 b.sallato@wsu.edu

References

Cameron, A. R., & Baldock, F. C. (1998). A new probability formula for surveys to substantiate freedom from disease. Preventive veterinary medicine, 34(1), 1-17.

Cameron (1999): Survey Toolbox for Livestock Diseases – A practical manual and software package for active surveillance of livestock diseases in developing countries. Australian Centre for International Agricultural Research, Canberra, Australia.

Epitools https://epitools.fp7-risksur.eu/tools/index?toolId=46

Hancock, H., & Holler, S. (1993). Sample Size Requirements for Laboratory Sampling. Population Medicine News, 6(11), 3.

Sparks, B. Tree Source Goes All In(doors) on Citrus Production. The Greenhouse Grower. October, 2020. https://www.greenhousegrower.com/production/this-months-cover-story-treesource-goes-all-indoors-on-citrus-production/#Tinsel/151254/11

Rouse, B., Kesinger, M., Jameson, N. Citrus Nursery Issues, Protective Structures, Budwood and Tree Availability. Citrus Industry. Oct, 2007.

Citrus Nursery Stock Certification Program. Florida Administrative Code. 5B-62

Footnotes

[1] Current is evaluating trap height at 2, 4, and 6 ft heights. New information may improve this recommendation.

[2] This recommendation is under review.

[3] Mesh size recommendations are being researched and may be revised. Leafhoppers are just over 1mm wide. 60 gram insect netting has openings of (1.95 mm x 0.95 mm). SWD recommendations are for 80 gram netting (mesh size 1.0 x 0.6 mm). Barrier over the tops of fields can be larger size as leafhoppers do not have burrowing behavior.

Washington State University