Preliminary recommendations. By Tianna DuPont, WSU Extension, Scott Harper, WSU Plant Pathology, Tobin Northfield, WSU Entomology. Last updated March 20, 2024.
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 are the control measures used by producers in my area?
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
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- Monitor for leafhopper vectors of X-disease. Place yellow sticky cards at 2 to 4 ft height 2 per ½ acre, 2 to 4 trees from the edge of the block and check weekly or monitor using sweep nets. Consider where leafhoppers will be. Avoid bare areas such as roadways. Leafhoppers will often be moving in from weedy irrigated areas.
- 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 [1] with screen that is less than 1 mm at its narrowest point [2].
- Consider vector deterrents, e.g. Extenday, kaolin clay. Recent research has shown that kaolin clay (Surround) applications can reduce leafhopper vector populations in high pressure blocks by 48% percent and reduce leafhopper movement into the canopy (Marshall et al 2024). Kaolin not only deters leafhoppers from orchards it also repels leafhoppers from trees.
- 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.
Broadleaf weed management
- Planting grass in drive rows can reduce leafhopper numbers (50% compared to weedy drive rows, Northfield 2023).
- Broadleaf herbicide use recommended (in drive row as well as tree row).
- Many broadleaf weeds are a host for X-disease phytoplasma and prefered by leafhoppers (e.g. dandelion, mallow, lambsquarter, chickweed, knotweed and plantain).
- Leafhoppers seem to prefer areas with broadleaf weeds and irrigation.
Confirmatory measures
Active measures must be taken to ensure the continued pathogen-free status of nursery material and ensure customer confidence:
Scout & Sample
- See X-disease Phytoplasma Sampling Recommendations for Nurseries
- Mother trees
- Sample scion trees every 36 months for viruses and virus-like organisms that do not present foliar symptoms.
- Sample timing: Target sampling in mid-to-late June for mother trees.
- Sample material: Collect four five-inch cuttings of 1/4-1/2″ (pencil diameter) diameter limb from previous season or older wood which include flower buds, fruit stems.
- Send to labs with current proficiency test results. Labs Testing for Little Cherry Virus and X-disease Phytoplasma
- Production blocks
- 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 (Cameron 1998, 1999, Epitools). Additional research is underway to bring sampling rate down to an economically feasible level.
- Sample timing: Sample nursery trees from July to late August (1-2 months after harvest). Sampling the young dormant trees after digging them out of the ground, or once they are in cold storage and easily accessible, is possible.
- Sample material: One 3-4″ cutting from the tip of the tree including and buds + one 3-4″ cutting from the first order lateral roots (at least ¼” thick), or from the taproot/root core should it be present (Figure 2). Diagnostic laboratories should process these two tissues together.
Tracking
- These pathogens have long latency periods. Follow up with customers on the status of material sold.
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) 713-5346, 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
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.
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] This recommendation is under review.
[2] 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.
