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Avoiding the Introduction of Exotic Pests Through Proper Site Preparation John M. Halbrendt Fruit Research & Extension Center Biglerville, PA jmh23@psu.edu PPV Quarantine Area, Quarantine Removed/Replanting Allowed: 1/22/07

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Avoiding the Introduction of Exotic Pests Through Proper Site Preparation

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Avoiding the Introduction of Exotic Pests Through Proper Site Preparation

John M. Halbrendt

Fruit Research & Extension Center

Biglerville, PA


PPV Quarantine Area, Quarantine Removed/Replanting Allowed: 1/22/07

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?

Lessons Learned from

Plum Pox Virus

Peach ‘Encore’

• Prior to 1999 PPV was not known in North America

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

• Entry into US/PA - unknown

PPV Symptoms - Apricot

PPV Symptoms - Plum


PPV - Leaf Symptoms

PPV - Blossom Break

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

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

Prune Dwarf Virus- pollen / seed borne transmission

PDV Infected


• Common and


• Management: 

Virus-free certification

Prunus Necrotic Ringspot (PNRSV)

Pollen borne virus

Fruit yields may be decreased by 40

to 50 percent, depending on conditions

Cherry Leaf Roll Virus (CLRV)

• 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



Cherry leaf roll virus & Prunus necrotic ringspot virus

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

Cherry Rasp Leaf Virus (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

Cherry – Rasp Leaf Symptoms

  • 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

Flat Apple

Disease caused


Peach Rosette Mosaic Virus

• Distribution - MI and NY

• Vector - dagger nematodes

(also found on grape)

(Photo by Don Ramsdell)


Pale green areas vary from tiny flecks to large blotches

These spots become necrotic and fall out causing


Cherry Mottle Leaf Virus

  • 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

Cherry Twisted Leaf Virus Symptoms

• 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

4 healthy cherries among infected fruit

Photo part of OSU Extension Plant Pathology slide collection, 1949

Apricot Ring Pox on Bing

Healthy and Little Cherry-infected Fruit


Prunus Stem Pitting

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)

Peach Stem Pitting - Autumn

Apple Union Necrosis and Decline(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

AUND - Gingergold / M26

Examples of other tree fruit virus diseases of concern to Pennsylvania growers

Dapple Apple

  • Dappling more intense as fruit mature, surface somewhat flattened

Apple Mosaic Virus

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

Pear Stony Pit

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

  • Transmitted by grafting

Apple Decline - Cause Unknown; Appears to be Virus

Elimination of Virus from Plants

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

The Key to Virus Control:



  • 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


  • 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


Bring in experts to consider possible causes….

Symptoms of Virus Infection

  • 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


  • Follow university site preparation guidelines

  • Test for nematodes

  • Consider previous crops

  • Check out the

  • neighborhood

Cultural Practices - Minimize Alternate Hosts

Weeds / Alternate Hosts for Dagger Nematode

and Tomato Ringspot Virus


To Prevent Spread!

Why is it important to remove old roots?

  • Root tissue may harbor pathogens

  • Roots may send up suckers

  • Roots interfere with soil

  • fumigation



• Kill / remove as much root as possible

• Prevent sucker re-growth

Sub-soil to remove old roots and loosen soil

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




Replant Disease



Loss of OM








pH - toxicity


Complex Disease

Nema + Virus

Nema + Fungi

Replant problems are

caused by changes in soil

conditions resulting from

years of monoculture

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

Soil Fumigation

  • 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

Soil fumigant injection

Broadcast Application

Essential to seal soil surface

Benefit of Soil Fumigation on Apple Replants (WA)

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

Benefits of Rotation Crops

• Suppress weeds

• Reduce nitrate leaching

• Control erosion

• Improve soil structure

• Some suppress nematodes

Cover Crop Trial Plots

Peach seedling growth following

cover crop treatments


Cover Crop


Cover Crop

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

Corn - increases organic matter but is a good host for many nematodes

Effects of Organic Matter

Improved soil physical properties

eg. structure and texture

Increased fertility

Air flow

Water retention

Soil porosity

Rapeseed green manure controls certain nematodes but adds little organic matter

Decomposition of Rapeseed

releases toxic compounds





Biofumigation with Rapeseed

Must Chop and Till for Biofumigation to Occur

Summary: Control is difficult!

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

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