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Avoiding the Introduction of Exotic Pests Through Proper Site Preparation John M. Halbrendt Fruit Research & Extension Center Biglerville, PA [email protected] PPV Quarantine Area, Quarantine Removed/Replanting Allowed: 1/22/07

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Avoiding the Introduction of Exotic Pests Through Proper Site Preparation

John M. Halbrendt

Fruit Research & Extension Center

Biglerville, PA

[email protected]

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Could the introduction of Plum Pox Virus (PPV; Sharka) into Pennsylvania have been avoided?

If PPV eradication is successful, can we prevent re-introduction?

Are there other exotic diseases we need to be concerned about?

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Lessons Learned from Pennsylvania have been avoided?

Plum Pox Virus

Peach ‘Encore’

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• Prior to 1999 PPV was not known in North America Pennsylvania have been avoided?

• Virus can be vectored by aphids, or move long distances in infected propagation material

• Entry into US/PA - unknown

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PPV Symptoms - Apricot Pennsylvania have been avoided?

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PPV Symptoms - Plum Pennsylvania have been avoided?


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PPV - Leaf Symptoms Pennsylvania have been avoided?

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PPV - Blossom Break Pennsylvania have been avoided?

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Do other exotic diseases pose a serious threat to Pennsylvania?

  • > 40 virus-like agents that infect Prunus

  • Some recorded in U.S.

  • Several new outbreaks, in peach and in sweet cherry

  • Several causing severe

  • economic damage

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Examples of other Endemic and “Exotic” Virus Diseases infecting Prunus of concern to Pennsylvania growers

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Prune Dwarf Virus- infecting pollen / seed borne transmission

PDV Infected


• Common and


• Management: 

Virus-free certification

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Prunus Necrotic Ringspot (PNRSV) infecting

Pollen borne virus

Fruit yields may be decreased by 40

to 50 percent, depending on conditions

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Cherry Leaf Roll Virus (CLRV) infecting

• Infection by Cherry leaf roll virus alone is difficult to recognize; symptoms are vague, and difficult to distinguish from other stress

• Infection by CLRV and another virus such as PDV or PNRSV can produce very dramatic symptoms

• Vector unknown

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Diseased infecting


Cherry leaf roll virus & Prunus necrotic ringspot virus

SYMPTOMS: Weak terminal growth; delayed fruit development; poor leaf color and size

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Cherry Rasp Leaf Virus infecting (CRLV)

• Distribution - Western States, e.g., Colorado

• Transmitted by grafting and by dagger nematodes

• Alternate Hosts - weeds such as dandelion

• Fruit production is reduced and young trees may die

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Cherry – Rasp Leaf Symptoms infecting

  • Prominent projections (enations) are found on undersides of leaves

  • Deformed leaves; many are extremely narrow, folded, and distorted

  • Not all parts of a tree will show symptoms at the same time

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Flat Apple infecting

Disease caused


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Peach Rosette infecting Mosaic Virus

• Distribution - MI and NY

• Vector - dagger nematodes

(also found on grape)

(Photo by Don Ramsdell)

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Peach infecting Mosaic

Pale green areas vary from tiny flecks to large blotches

These spots become necrotic and fall out causing


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Cherry Mottle Leaf Virus infecting

  • Mottle-leaf

  • pattern -principle symptom

  • Most severe on

  • Bing,

  • Royal Ann,

  • and Lambert

  • Mottling - irregular and chlorotic, causing a leaf distortion

  • early in the season; leaves pucker increasing as the

  • season advances

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Cherry Twisted Leaf Virus Symptoms infecting

• Abrupt kink in the midrib or the petiole, which causes the leaf to appear twisted

• Leaves develop asymmetrically or stay small; spurs appear bunchy due to shortened internodes

• Once the tree is infected, the

disease spreads rapidly to all branches

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4 healthy cherries among infected fruit infecting

Photo part of OSU Extension Plant Pathology slide collection, 1949

Apricot Ring Pox on Bing

Healthy and Little Cherry-infected Fruit

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Cherry infecting Rosette

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Prunus Stem Pitting infecting

Causal agent: Tomato ringspot virus (ToRSV)

Vector: Dagger Nematode

Distribution: Common and widespread

Alternate Hosts: At least 23 species of broadleaf weeds

Management: Virus-free trees; control nematode vector and broadleaf weeds (i.e., reservoirs of ToRSV)

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Apple Union Necrosis and Decline infecting (AUND)

  • Caused by ToRSV - vectored by dagger nematode

  • Common and widespread

  • Scion/stock interface reveals a poorly formed union

  • Susceptibility to this problem depends on the scion/rootstock combination

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Dapple Apple Pennsylvania growers

  • Dappling more intense as fruit mature, surface somewhat flattened

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Apple Mosaic Virus Pennsylvania growers

  • Leaves develop pale yellow to bright cream-colored areas as they expand during early spring

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Pear Stony Pit Pennsylvania growers

  • The causal agent of pear stony pit has not yet been isolated

  • Transmitted by grafting

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Elimination of Virus from Plants Pennsylvania growers

There is no practical way to eliminate a virus once it is in a field setting - the only solution is to destroy the host

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The Key to Virus Control: Pennsylvania growers


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ONLY PLANT CLEAN TREES Pennsylvania growers

  • Buy certified virus-free trees whenever possible

  • Buy from a reputable nursery

  • Ask questions! Are the trees virus-tested? Tested for which viruses?

  • All trees ~ pollinizers, replants, trial varieties ~ are important

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DON’T INTRODUCE PROBLEMS Pennsylvania growers

  • Follow production guide recommendations for nematodes, weeds…

  • Don’t bring trees of unknown virus content near your trees

  • Don’t bring fruit of unknown virus content near your trees

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Bring in experts to consider possible causes….

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Symptoms of Virus Infection Pennsylvania growers

  • Foliar discoloration or deformity

  • Change in growth habit

  • Change in maturity timeline

  • Stem pitting or union necrosis

  • Fruit deformation

  • Lack of vigor; decline

  • Inability to cope with other stressors

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START WITH A CLEAN SITE Pennsylvania growers

  • Follow university site preparation guidelines

  • Test for nematodes

  • Consider previous crops

  • Check out the

  • neighborhood

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Cultural Practices - Minimize Alternate Hosts Pennsylvania growers

Weeds / Alternate Hosts for Dagger Nematode

and Tomato Ringspot Virus

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To Prevent Spread!

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Why is it important to remove old roots? Pennsylvania growers

  • Root tissue may harbor pathogens

  • Roots may send up suckers

  • Roots interfere with soil

  • fumigation

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GOOD TREE REMOVAL Pennsylvania growers


• Kill / remove as much root as possible

• Prevent sucker re-growth

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Evaluate the site for potential replant problems to determine if corrective measures are needed

• Nematode assay

• Soil fertility (include pH and organic matter)

• Consideration of the cropping history

(risk factors) and crop to be planted

• Advice from consultant / specialist

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REPLANT PROBLEMS determine if corrective measures are needed



Replant Disease



Loss of OM








pH - toxicity


Complex Disease

Nema + Virus

Nema + Fungi

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Replant problems are determine if corrective measures are needed

caused by changes in soil

conditions resulting from

years of monoculture

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Remediation of replant sites is an attempt re-establish optimum soil conditions for healthy tree growth

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Soil Fumigation optimum soil conditions for healthy tree growth

  • Soil temperature40o-80o F

  • Soil moisture

  • Moist from 2” to 12” below surface

  • Determined by “feel”

  • Varies with texture

  • Soil tilth

  • Free of clods and plant residue

  • Seedbed-ready soil condition

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Soil fumigant injection optimum soil conditions for healthy tree growth

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Broadcast Application optimum soil conditions for healthy tree growth

Essential to seal soil surface

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Benefit of Soil Fumigation on Apple Replants (WA) optimum soil conditions for healthy tree growth

Five year old Fuji on M.26 root, 100 ft. away and adjoining the planting shown on the left

Five year old Fuji apple on M.26 root planted on non-fumigated old orchard site

Causal agent - unknown

Tim Smith- WSU Extension

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Benefits of Rotation Crops optimum soil conditions for healthy tree growth

• Suppress weeds

• Reduce nitrate leaching

• Control erosion

• Improve soil structure

• Some suppress nematodes

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Cover Crop Trial Plots optimum soil conditions for healthy tree growth

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Peach seedling growth following optimum soil conditions for healthy tree growth

cover crop treatments


Cover Crop


Cover Crop

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Not all rotation crops provide the same benefits - e.g., corn is a good source of organic matter while soybean fixes nitrogen

Crops that suppress one type of nematode may increase another

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Effects of Organic Matter nematodes

Improved soil physical properties

eg. structure and texture

Increased fertility

Air flow

Water retention

Soil porosity

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Decomposition of Rapeseed little organic matter

releases toxic compounds





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Biofumigation with Rapeseed little organic matter

Must Chop and Till for Biofumigation to Occur

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Summary: Control is difficult! little organic matter

Disease Prevention is best!!

  • Buy certified virus-free plants

  • Start with a clean site

  • Maintain good cultural practices - sanitation

  • Control virus vectors

  • Control alternate hosts - reservoir plants

  • Correct for any potential replant problems

    • e.g., fertility, pH, loss of organic matter, compaction