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Disease Management

Introduction

A disease can be described as any change from the normal condition of plants, which detract from their appearance, or make them less useful or valuable. Here we will cover biotic diseases caused by bacteria, fungi and viruses, specifically affecting fruit trees or their fruit.

Biotic diseases are caused by infections by a living pathogen that can reproduce and infect other plant parts of the tree, and spread to other trees in the orchard. Diseases affect the trees’ health, productivity, fruit quality, and the ability to market and export fruit. The orchards of North Central Washington have less fungal and bacterial disease pressure than many areas of the world due to the low levels of precipitation in the spring and summer, and abundance of sunshine. Fruit trees need to be monitored for signs and symptoms of disease throughout the year. Symptoms of disease may include: branches with wilted, discolored, or dying/dead leaves; tree limbs with visible cankers; and rotting fruit in the tree or on the ground. Many fruit tree diseases originate in the orchard, and appear after harvest in cold storage. Diseases that occur or develop symptoms in cold storage are discussed in the Postharvest section of this site. Some of the more common diseases that occur in the orchard are listed below. Washington State University scientists are working to find new control methods for tree fruit diseases, as well as breeding new disease resistant varieties. See the Washington Tree Fruit Research Commission (WTFRC) reports listed below for more information on current disease research, as well as the resource links for information on tree fruit disease management.

The USDA Specialty Crop Research Initiative (SCRI) RosBREED 2 project  involving 35 scientists from 14 Universities, are currently working on the project “Combining Disease resistance with horticultural quality in new Rosaceous cultivars,” using DNA-based tools to find new cultivars. Disease threats that researchers are addressing include: scab, blue mold, and fire blight in apple; fire blight in pear; powdery mildew in sweet cherry; and bacterial spot and brown rot in peach, among others. For more about this project see the Good Fruit Grower article $10 million RosBREED grant, and the Growing Produce article RosBREED 2 Program Focuses on Disease Resistance and Quality.

Types of Tree Fruit Diseases

Bacterial

Bacteria are microscopic, single celled organisms that reproduce by simple division, rapidly growing the population during ideal weather and host conditions. The presence of water is necessary for the spread of most bacterial infections, as the bacterial cells are motile (can swim in water). Plant bacteria require an opening in plant tissue in order to infect the plant. Wounds, pruning cuts, environmental stress damage, as well as natural openings in the plant can act as entry points for pathogens. Once inside the plant, bacteria can spread throughout the plant. Examples of bacterial fruit tree diseases are fire blight, and bacterial spots and cankers.

Common Tree Fruit Bacterial Diseases
Fire Blight
Fireblight symptoms on pear
Fireblight symptoms on pear
  • Causative agent: Erwinia amylovora
  • Crops affected: Apple & Pear
  • Disease description: Bacteria overwinter in tree cankers from infections that occurred during the previous year.  Bacteria will ooze from cankers a few weeks before bloom, and can be carried by rain or insects to blooms, other parts of the tree and adjacent trees. Warm, wet weather during bloom time promotes the disease.
  • Management: Its important to monitor for cankers and remove them all and destroy debris during the dormant season to keep the disease from spreading. Fire blight has been controlled with antibiotics, but these chemicals are now in the process of being phased out. Research is being conducted on alternative control methods.
  • Resources:
Pseudomonas Bacterial Canker
  • Causative agent: Pseudomonas syringae
  • Crops affected: Cherry
  • Disease description: Bacterial canker is favored by cool, wet weather. The bacteria overwinter in tree cankers, buds and other host tissues. Dark cankered areas on trunks and branches may develop and expand in early spring. The infected tissues may produce gum, although gumming can also be caused by other factors. The cankers often girdle twigs and branches, causing dieback above the lesion. Leaves on girdled twigs often yellow and fall by late summer.
    Infected buds may be killed or leaf infections may occur as the new growth emerges resulting in collapse of leaves. Infection can be spread by wind, rain, insects, pruning tools, or by planting or grafting with infected stock. The disease may spread throughout the entire tree (systemic infection) with or without visible symptoms.
  • Management:
    • The primary focus should be on cultural controls first. Avoid injury to the tree and tissues.
    • Burn (cauterizing) or cut out cankers on branches or trunks. Cauterizing should be done in the spring prior to bloom. Check cauterized areas for continued bacterial activity 15-20 days later.
    • Control weeds, which may be a source of bacteria.
    • Prune out and destroy infected tissues during dry weather. Make cuts well below visible canker and sterilize tools frequently. Do not remove cankers at the same time as regular pruning.
    • Remove severely infected trees.
    • If this is supplemented with a chemical control, make one application in October during the fall prior to fall rains. Then make a second application in early January. Products containing copper may have limited efficacy due to resistance.
  • Resources:

Fungal

Fungi are organisms that require a food source (plants) because they cannot make their own food. Most fungi reproduce by spores. Fungal pathogens enter plant hosts in different ways. Some are capable of penetrating plant tissue, and others require openings like stomata in leaves, lenticels in fruit, flowers, or wounds.
Fungi can overwinter in infected buds, shoots, and cankers of host plants; and on dried leaves, limbs and other debris on the orchard floor. In the spring, fungi will spread by spores with rain and wind, and infect new tissues including leaves and new shoots on trees. Examples of fungal fruit tree diseases are apple scab, powdery mildew, and brown rot. Some postharvest fungal decays may originate in the orchard, but fruit show no symptoms until undergoing cold storage. Examples of such fungal decays include: Bull’s eye rot, Sphaeropsis rot, Phacidiopycnis rot. Additional fungal infections can occur at harvest through damage on the fruit skin caused by stem punctures, limb rubs and bruising.

Common Tree Fruit Fungal Diseases

Common Tree Fruit Fungal Diseases

Apple Scab/Pear Scab
  • Causative agent: Venturia inaequalis (on apples) and V. pirina (on pears)
  • Crops affected: Apple and Pear
  • Disease description: The fungus overwinters on dead apple leaves on the ground. Rainfall in the spring moves spores to infect developing leaves and fruit. Black, sooty lesions form on infected leaves and flowers in the early spring. Fruit symptoms: Fruit can be infected, resulting in scars and fruit distortion. Symptoms can range from a pinpoint spot to stellar lesions, yellow flecks, or necrotic lesions.
  • Management: Scab is controlled in the orchard with effective spray treatments. Follow state or extension recommendations. WSU-DAS does have a Scab prediction model with management recommendations.
  • Resources:
Bull’s Eye Rot/Perennial Canker
  • Causative agent: major species: Neofabraea perennans, N. alba and Cryptosporiopsis kienholzii (the asexual state of a Neofabraea spp.) (major export quarantine pest)
  • Crops affected: Apple & Pears
  • Disease description: Neofabraea causes perennial canker on apple trees. On pear trees, it may survive on the dead bark. Perennial canker is a problem east of the Cascades where winters are colder and summers tend to be dry and hot. The cankers are composed of series of concentric rings because growth of the canker is renewed annually. Cankers can become large enough to kill scaffold limbs. This renewal of growth is caused by the woolly apple aphid, which makes small galls at the canker margin; these galls rupture during cold winters. The perennial canker fungus produces conidia in the canker, and these conidia readily infect the ruptured galls at the canker margin, thus initiating another annual cycle of canker extension. Conidia are most abundant in the orchard during cool, wet conditions in fall and winter.
  • Fruit Symptoms: Perennial canker can infect fruit and produce bull’s eye rot. A bull’s eye rot lesion is circular, flat to slightly sunken and appears light brown to dark brown with a lighter brown to tan center. Decayed tissue is firm. Cream-colored spore masses in the aged decayed area may appear. Bull’s eye rot commonly originates from infection at lenticels on the fruit skin, but stem-end Bull’s eye rot is also commonly seen on Golden Delicious and Gala apples, particularly on the fruit from orchards with over-tree evaporative cooling or irrigation. Calyx-end Bull’s eye rot has also been observed on Golden Delicious fruit.
  • Management: Water spreads the fungal inoculum and creates conditions conducive for fruit infection. Therefore, it is recommended that overhead irrigation be avoided and that over-tree cooling be limited in duration to only the amount needed for sunburn prevention. Preharvest fungicides such as Topsin M, Pristine or Ziram applied near harvest as a ground application reduce Bull’s-Eye Rot on fruit after harvest. Good coverage is important to the effectiveness of preharvest fungicide spray. A postharvest fungicide drench with Penbotec (pyrimethanil), Mertect (thiabendazole), or Scholar Max (fludioxonil+thiabendazole) is effective for control of Bull’s- Eye Rot on apple fruit.
  • Resources:
Powdery Mildew
  • Causative agent: Podosphaera clandestine (cherry) and P. leucotricha (apple and pear)
  • Crops affected: Apples, Pears (Anjou and Comice particularly susceptible) and  Cherry
  • Disease description: Powdery mildew may be found on blossoms, leaves, twigs and fruit. New growth is particularly susceptible, since the fungus overwinters in buds. The entire terminal may become covered with powdery mildew.  Leaves typically develop a characteristic gray-white powdery growth, often on the underside. Infected young leaves may be curled and distorted. Powdery mildew on young foliage reduces photosynthetic efficiency. Infected foliage is brittle and may be killed. Dark brown fungal fruiting bodies may be seen by midsummer, when the white fungal mats turn brown. Fruit is most susceptible during the period around petal fall. Affected fruits typically show a net-like pattern of russeting in the infected areas. When fruit are infected, the surface may become russetted or discolored, and sometimes dwarfed.
  • Management:
    • Select resistant varieties
    • Prune out and destroy severely infected shoots as they appear. It is important to remove the early-spring
      infected shoots.
    • For chemical management see the WSU Crop Protection Guide for the appropriate control program.
    • Powdery mildew and scab are usually treated with the same fungicide and timing.
  • Resources:
Speck Rot
  • Causative agent: Phacidiopycnis washingtonensis (major export quarantine pest)
  • Crops affected: Apple and ‘Manchurian’ crabapple
  • Disease description: Speck rot is a postharvest disease affecting apples, but it originates in the orchard. The inoculum comes from dead or diseased plant tissues of the ‘Manchurian’ crabapple pollinizers in affected orchards. The fungus produces small black dots (fruiting bodies = pycnidia) on ‘Manchurian’ twigs, tree branches and crabapple fruit. Fruiting bodies contain millions of infective spores that can be spread by rain, irrigation or over-tree cooling to nearby apple trees and fruit. Although apple fruit infection occurs in the orchard, fruit rot symptoms develop during storage or at the market.
    The cankers and twig dieback caused by P. washingtonensis are not common on apple trees in commercial apple orchards in Washington State, but the ‘Manchurian’ crabapple pollinizer trees are highly susceptible. Detailed pruning of this pollinizer is strongly recommended to significantly reduce the infective potential in commercial orchards.
    Fruit symptoms: Speck rot can appear as either a stem-end rot or calyx-end rot, or both. Affected tissue is spongy to firm, which is not differentiable from gray mold and not readily separable from the healthy tissue. The color of the decayed areas varies from light brown to dark brown or occassionally black. Speck rot is so named because of the brown to black specks with white to light tan centers that may appear around the lenticels, especially on red apple cultivars.
  • Management:
    • Detailed pruning of the pollinizer ‘Manchurian’ crabapple is strongly recommended to significantly reduce the infective potential in commercial orchards. Removal of twigs with dieback and cankers will help reduce inoculum of the fungus in the orchard.
    • Since water spreads the inoculum and creates a favorable environment for the disease, it is recommended that overhead irrigation be avoided and that over-tree cooling be limited in duration to only the amount needed for sunburn prevention.
    • Preharvest fungicides such as Ziram, Topsin M or Pristine applied near harvest as a ground application reduce speck rot caused by P. washingtonensis. Good coverage is important to the effectiveness of preharvest fungicide sprays.
    • Postharvest control: A postharvest fungicide drench with Penbotec (pyrimethanil), or Scholar (fludioxonil) is highly effective in controlling this disease on apple fruit. A postharvest drench with Mertect (thiabendazole) is also effective. These three postharvest fungicide treatments are more effective than the preharvest fungicide sprays.
  • Resources:
Sphaeropsis Rot
  • Causative agent: Sphaeropsis pyriputrescens (major export quarantine pest)
  • Crops affected: Apples and Pears
  • Disease description: This is a postharvest decay with its origin in the orchard. First discovered in D’Anjou pears, but was later determined to cause worse problems in apples. Sphaeropsis rot has occurred on Red Delicious, Golden Delicious, Fuji, and Granny Smith. The source of inoculum is diseased or dead plant tissue in affected orchards. Cankers caused by S. pyriputrescens are not common on apple trees in commercial apple orchards, but the ‘Manchurian’ crabapple used as a pollinizer is highly susceptible. On ‘Manchurian’ it causes twig dieback and cankers. The fungus also infects the crabapple fruit. The fungus can produce small black dots (fruiting bodies) to form on infected ‘Manchurian’ twigs and tree branches. Fruiting bodies contain millions of spores that serve as inoculum for fruit infection. Rain, irrigation water or over-tree cooling can spread the spores to nearby apple trees and fruit. Although apple fruit infection occurs in the orchard, fruit rot symptoms develop during storage or at the market.
  • Management:
    • Detailed pruning of the pollinizer ‘Manchurian’ crabapple is strongly recommended to significantly reduce the infective potential in commercial orchards. Removal of twigs with dieback and cankers will help reduce inoculum of the fungus in the orchard.
    • Since water spreads the inoculum and creates a favorable environment for the disease, it is recommended that overhead irrigation be avoided and that over-tree cooling be limited in duration to only the amount needed for sunburn prevention.
    • Preharvest fungicides such as Ziram, Topsin M or Pristine applied near harvest as a ground application reduce Sphaeropsis rot caused by S. Pyriputrescens. Good coverage is important to the effectiveness of preharvest fungicide sprays.
    • Postharvest control: A postharvest fungicide drench with Penbotec (pyrimethanil), or Scholar (fludioxonil) is highly effective in controlling this disease on apple fruit. A postharvest drench with Mertect (thiabendazole) is also effective. These three postharvest fungicide treatments are more effective than the preharvest fungicide sprays.
  • Resources:
Verticillium Rot
  • Causative agent: Verticillium dahliae, a fungus introduced to Central Washington and built to high levels in the soil during the production of potatoes or mint.
  • Crops affected: Cherry and other stone fruit trees
  • Disease description: Young trees are attacked through the roots, penetrating into the xylem. With each years growth, the fungus progressively plugs more xylem. The tree may become overwhelmed in its first few years and will start collapsing one or two scaffolds a year. The young tree may stay slightly ahead of the disease progression until its first heavy crop. At this point slower tree growth results in faster progression of the fungus into the xylem leading to full tree wilt.
  • Management: This is a very persistent soil fungus. The best management includes:
    • …keeping the affected orchard growing well. Good fertility, weed control and irrigation may help trees stay ahead of the disease progression. New wood production is critical. Some weeds promote a build up of the fungus, but grass does not. So good weed control is important.
    • Recently dead or dying wood should be removed promptly to prevent a build-up of the shot-hole beetle in those affected areas. Beetles feeding in affected area can spread the fungus to unaffected areas.
  • Resources:

Viral

Viruses are submicroscopic parasitic particles that depend on their host for reproduction. Viruses enter living cells, which do not recognize the virus as an invader. The host cells use valuable energy to help the viruses replicate. They spread from host to host in different ways. Vectors can be other organisms (e.g. insects) that pick up the virus, and pass it along to healthy plants (example: mealy bugs and little cherry virus 2). Some viruses are spread in plant pollen; and others by the seeds of flowering plants infected with a virus. Viruses can also be transmitted by vegetative plant propagation (e.g., budding or grafting), through natural root grafting, and by nematodes.

One of the most important things to do to avoid virus diseases is to select virus-tested and certified virus-free planting material from a reputable commercial nursery. See the Clean Plant Center of the Northwest page for information on viruses, virus-free materials, and testing services.  Visit the National Clean Plant Network–Fruit Trees site here. For information on how virus diseases are spread, virus testing, a list of potential viruses, symptoms, and possible transmission patterns, see the eXtension Apple Tree Viruses page.

For information about testing for virus see: Virus Testing on this website.

Common Tree Fruit Viral Diseases
Viruses Affecting Apple

Generally, apple viruses are transmitted through grafting. Tomato ringspot virus is an exception, and it causes apple union necrosis and decline.  When trees are healthy, and they have virus-free rootstock, the tree will usually remain healthy. Infected trees in the orchard can spread viruses slowly to adjacent trees through natural root grafts (reference). Here is a list of the most common viruses affecting apple. Note that some of these may also affect other crops and will be noted in the descriptions. A list of major viral diseases affecting pear are listed after the apple section.

Apple Mosaic Virus

  • Causative agent: Apple Mosaic Virus
  • Vector: Spreads by vegetative propagation, it is not thought to spread by insects or pollen. It may spread by root grafting.
  • Crops affected: Apples, Pear, Plum, Peach
  • Disease description: Pale or yellowish banding areas along the leaf veins occur on the leaves of many cultivars, in the spring. The chlorotic spots may become necrotic later in season from sun and heat, and leaves may fall early. All apple varieties are susceptible, but ‘Golden Delicious’, ‘Jonathan’ and ‘Granny Smith’ are very susceptible. Related to Prunus necrotic ring spot virus (PNRSV). Fruit may be small or deformed. Loss of crop can occur, depending on cultivar. Pears are symptomless.
  • Management: Transmitted through budding and grafting with infected trees. Use certified virus tested and virus-free planting material to propagate trees. Affected trees and roots must be removed and destroyed.
  • Resources:
    • Apple Mosaic Virus, Pacific Northwest Plant Disease Management Handbook, webpage. (Accessed: 1/17/17).
    • Apple Mosaic Virus, E. V. Podleckis, et. al., eXtension webpage, 2011.  (Accessed: 1/17/17).
    • Apple Mosaic, University of California IPM webpage. (Accessed: 1/17/17).

Apple Union Necrosis and Decline (AUND)

  • Causative agent: Tomato ringspot virus (TmRSV),
  • Vector: spread by Dagger nematodes, Xiphinema americanum (sensu lato).
  • Crops affected: Apple, Peach, Nectarine, Prune
  • Disease description: Grafted trees with fruiting varieties resistant to TmRSV and rootstocks tolerant to this virus have a problem with this disease (e.g., ‘Delicious’/MM.106). A tolerant rootstock can harbor the disease but not suffer from it. When infected trees are at bearing age, budbreak may be delayed, leaves might be small and sparse, and pale green. Shoot growth is reduced. Separation of graft union can occur in severe infections. Tree death may occur. Under bark near union there is a distinct necrotic line. The graft is spongy and orange colored. With strong winds, it is possible for the top of the tree to break at the graft union. The virus is present in broadleaf weeds in orchards (e.g. dandelions) and spread to trees through nematodes. Hosts to this virus include fruit and ornamental trees, vegetables and weeds. AUND is caused by incompatible graft unions involving a resistant scion grafted onto a susceptible but tolerant rootstock.
  • Management: Use virus-free and nematode resistant rootstock. Grow trees in sites with no history of the disease and preferably that were preplant fumigated to control nematodes. Use broadleaf herbicides to control weeds.
  • Resources:

Latent Viruses in Apple

Latent viruses live in their host plant without causing symptoms. When a virus-infected scion is grafted onto susceptible rootstock, the virus is transmitted. (Three common latent viruses in apple include: Apple chlorotic leaf spot virus (ACLSV), Apple stem pitting virus (ASPV), and Apple stem grooving virus (ASGV). They can occur individually or together, and can cause diseases in other fruit crops. They are not as much a problem in commercial apple production.

Apple Chlorotic Leaf Spot (ACLSV)

  • Causative agent: Apple chlorotic leaf spot virus (ACLSV)
  • Vector: none known; graft transmissible.
  • Crops affected:  Apple, Crabapple, Pear.
  • Disease description: Many crabapple cultivars are affected with apple chlorotic leaf spot. Some new cold tolerant and fire blight resistant rootstocks are hypersensitive to one or more latent viruses.  This virus causes pear ring pattern mosaic, and has been found in all pome and stone fruit species.  Leaf symptoms include: chlorotic spots, distortion and stunting.
  • Management: Use only virus-free certified planting material.
  • Resources:

Apple Stem Pitting Virus (ASPV)

  • Causative Agent: Apple stem pitting virus (ASPV)
  • Vector: none known; graft transmissible.
  • Crops affected: Apple, Crabapple, Pear.
  • Disease description: When a susceptible rootstock (e.g., new cold tolerant and fire blight resistant rootstocks) has an infected scion grafted to it, hypersensitive reactions can occur including a downward curving of leaves and pitting and weakening of the rootstock.
  • Management: Use only virus-free certified planting material.
  • Resources:

Apple Stem Grooving Virus (ASGV)

  • Causative Agent: Apple stem grooving virus (ASGV)
  • Vector: none known; graft transmissible.
  • Crops affected: Apple, Crabapple.
  • Disease description: Rootstocks of M. sylvestris ‘Virginia Crab’ can develop Apple Decline and a brown necrotic line at the graft when infected; complete breakage of the tree can occur at the graft union. Scion tissue above the graft union may swell. Some new cold tolerant and fire blight resistant rootstocks are hypersensitive to one or more latent viruses.
  • Management: Use only virus-free certified planting material.
  • Resources:

Apple Green Crinkle Disease

  • Causative agent: Trees with green crinkle disease are infected with Apple Stem Pitting Virus (ASPV), Apple Stem Grooving Virus (ASGV), and Apple Chlorotic Leaf Spot Virus (ACLSV), but it is not known which are responsible for the disease.
  • Vector: unknown
  • Crops affected:  Apple.
  • Disease description:  Fruit develop depressions that become more severe as the fruit matures. Distorted fruit may appear on one or two limbs. There are no leaf symptoms.
  • Management: Use only virus-tested planting material that is free of all known viruses.
  • Resource:

Flat Apple Virus

  • Causative agent:  Cherry Rasp Leaf Virus (CRLV).
  • Vector: Dagger nematodes, Xiphinema americanum; Can also spread through contaminated budwood during propagation.
  • Crops affected:  Apple, Cherry
  • Disease description:  Apple fruits become flattened from both the stem and calyx sides of fruit. Leaf rolling up from the midrib may occur in infected varieties (e.g., ‘Red Delicious’).  Disease is spread from tree to tree by nematodes in the soil. This virus can move between cherry and apple plantings. Apple trees planted on sites that previously had cherry trees affected with CRLV will become infected. Many orchard weeds serve as a host. Flat apple symptoms are more severe on ‘Red Delicious’ and related cultivars.
  • Management: The use of virus-tested, virus-free planting material; Remove infected trees as soon as symptoms appear. Fumigation for replant sites may delay disease development; control the nematode vector.
  • Resource:
    • Flat Apple Disease, Pacific Northwest Plant Disease Management Handbook, webpage.  (Accessed: 1/17/17).

Russet Ring Virus

This virus occurs in ‘Golden Delicious’ and ‘Newtown’ apples. Symptoms are rings of russeted areas on the skin of the fruit. Develops more in cool growing seasons. Can only control by using virus-free nursery stock. See the Market Diseases of Apples, Pears and Quinces, C. Pierson et. al. and the Pacific Northwest Plant Disease Management Handbook for more information about this virus disease.

Star Crack Virus

The ‘Golden Delicious’ and ‘Gravenstein’ varieties in the Pacific Northwest are susceptible. Affected fruit are distorted, and cracks may develop anywhere. Cork-like tissue forms on surface of fruit on healed cracks.  Must use virus free bud and nursery stock to control this disease.  See the Market Diseases of Apples, Pears and Quinces, C. Pierson et. al. and the Pacific Northwest Plant Disease Management Handbook for more information about this virus disease.

Viruses Affecting Pear

Pear Stony Pit Virus

  • Causative agent:  unknown
  • Vector: None known
  • Crops affected:  Pears. The Bosc pear variety is highly susceptible to this virus.  It can also be found in d’Anjou and Forelle.  Bartlett is a symptomless carrier.
  • Disease description:  The pits appear to be cone-shaped. Numerous pits can develop in the fruit, giving it a distorted appearance. Cells at the base of the pits develop hard, stone-like masses. Transmitted by budding, grafting, and root cuttings. Associated with Apple Stem Pitting Virus (ASPV).
  • Management: Use virus-free planting material, remove infected trees
  • Resources:

Pear Stem Pitting Virus

  • Causative Agent: Apple Stem Pitting Virus (ASPV)
  • Vector: graft transmissible
  • Crops affected: Pears
  • Disease description: The Apple stem pitting virus causes infection in some pear cultivars, even though symptoms are not seen. There are longitudinal grooves on the xylem under the bark near the lower stem when tree is infected. Ridges form on the inner bark also.
  • Management: Use virus-free planting material
  • Resource:

Pear Red Mottle Disease

  • Causative Agent: Apple Stem Pitting Virus (ASPV; or very closely related)
  • Vector: none known, graft transmissible
  • Crops affected: Pears
  • Disease description: Symptoms are easy to see in late summer and fall. Dark red spots near finer leaf veins appear.
  • Management: Use scions and rootstocks that are virus-free
  • Resource:

Pear Vein Yellows Virus

  • Causative Agent: Apple Stem Pitting Virus (ASPV; or very closely related)
  • Vector: not known
  • Crops affected: Pears
  • Disease description: Smaller secondary leaf veins are yellow in color.  Along veins there may also be flecking, red mottling and necrotic spots; it has been reported that symptoms are more pronounced when the weather is cool.  Trees that are infected may have reduced vigor compared to healthy trees.
  • Management: Use scions and rootstocks tested and virus-free.
  • Resource:

Pear Necrotic Spot

  • Causative Agent: Apple Stem Pitting Virus (ASPV; strain of the same filamentous virus)
  • Vector:  not known, graft transmissible
  • Crops affected: Pears
  • Disease description: Circular chlorotic spots 2-3 mm in diameter on leaves in early summer. Spots will turn deep red or black. Trees may shed leaves prematurely and have a decreased yield. Fruit show no symptoms for most cultivars.
  • Management: Use only virus-free certified planting material
  • Resource:

Apple Stem Grooving Virus (ASGV) and Apple Chlorotic Leaf Spot Virus (ACLSV)

These are both major viruses affecting pear. The summary descriptions are listed above under Latent Apple Viruses. Additional information can be found in this research article:

Effect of thermotherapy on elimination of apple stem grooving virus and apple chlorotic leaf spot virus for In-vitro-cultured pear shoot tips, L. Wang, et. al., HortScience, 41(3):729-732. 2006.

Viruses Affecting Cherry

Little Cherry Disease

Western X Disease

  • Causative Agent: Candidatus Phytoplasma pruni
  • Vector: Leafhoppers (Colladonus geminatus); also transmitted by budding or grafting
  • Crops affected: Cherry
  • Disease description: Fruit are small and pointed, and pale in color. Fruit has a bitter taste. Affected fruit is often in one area of the tree, while the rest of the tree has normal fruit.  Trees on Mazzard rootstock can show symptoms for years and not die; trees on Mahaleb rootstock are quite susceptible, and usually die within a few years or less. Between the scion and rootstock, a dark line may occur under the bark with pits and grooves.
  • Management: Spray trees to kill vectors, then remove trees. If many trees are infected, removing them may not be economically advisable, as healthy looking fruit can still be produced. Manage nearby ornamental hosts including choke cherry and bitter cherry. Treat orchard for leafhopper, and weeds that harbor leafhoppers.
  • Resources

Cherry Leaf Roll Virus: (Cherry Decline)

  • Causative Agent: Cherry leaf roll nepovirus
  • Vector: spread by use of infected budwood, and root grafting; may be transmitted by pollen also.
  • Crops affected: Cherry
  • Disease description: Flowering is delayed. Leaf margins may roll upward, as if wilted. Leaves in some cultivars may turn a purple-red color early in the season, and light green spots may appear in others. Fruit is small, and ripens late.  Shoot dieback can occur.  Tree decline occurs more quickly if trees are simultaneously infected with Prune Dwarf Virus (PDV) and Prunus Necrotic Ringspot Virus (PNRSV).
  • Management: Remove infected trees.  Apply herbicide to the freshly cut stump of removed trees.  Use only virus tested and virus free planting material.
  • Resources:

Cherry Mottle Leaf Virus

  • Causative Agent: Cherry Mottle Leaf Virus
  • Vector: Microscopic scale mite (Eriophyes inaequalis); spread by budding or grafting.
  • Crops affected: Cherry, especially Bing and Napoleon (Royal Ann); Van, Rainier, and Lambert are symptomless carriers.
  • Disease description: Trees on the edge of orchard are usually infected first.  Leaves on affected trees may be distorted, puckered, with irregular mottling that is light green to yellow in color.  Fruit set of infected trees is reduced and fruits may lack flavor besides ripening late and being small.  Terminal shoot growth is stunted.
  • Management: Use virus free planting material. Remove infected trees to prevent spread of the virus. Destroy wild cherry trees near commercial orchards.
  • Resources

Cherry Twisted Leaf

  • Causative Agent: Cherry twisted leaf virus
  • Vector: Spread by budding and grafting; no known vectors
  • Crops affected: Cherry.  Bing and Rainier are susceptible cultivars.
  • Disease description: Leaves twist downward and to the side, and remain small. Tissue sections on the underside of midrib and veins on leaf die.  Older trees of medium vigor tend to show greater symptom severity.
  • Management: Use virus free planting material. Remove infected trees when symptoms appear. Choke cherry stands near commercial orchards should be removed.
  • Resources:

 Cherry Rasp Leaf Virus

  • Causative Agent: Cherry rasp leaf virus
  • Vector: spread by dagger nematodes (Xiphenema americanum); grafting; dandelion and red raspberry are host plants for the virus.
  • Crops affected: Cherry
  • Disease description: The spread of disease in a tree or orchard is usually slow, and causes flat apple on apple trees. Lumps or growths form on the underside of leaves between veins. Leaves may fold and distort, and surface texture becomes rough.  Fruit production is reduced in infected trees.  Young trees and branches that are severely affected will die.
  • Management: Use virus free planting material. Remove infected trees. Pre-plant fumigation for nematodes.
  • Resources:

Necrotic Rusty Mottle

  • Causative Agent:  Necrotic rusty mottle virus
  • Vector: Spreads through infected bud wood; vector is unknown
  • Crops affected: Cherry, all cultivars are susceptible
  • Disease description:  More severe symptoms may occur in cool springs.  Spots and areas of dead tissue appear on leaves around 3-6 weeks after bloom. Severely decayed leaves will drop. Buds at the ends of shoots do not open or swell and will die. Bark of infected trees may have shallow areas of decay with associated wound gumming.  Infected trees may die in as few as two years.
  • Management: Remove infected trees; Use virus-free planting material. Some pollinizing cultivars are symptomless hosts.
  • Resources:

 Prunus Necrotic Ringspot

  • Causative Agent: Prunus necrotic ringspot virus (PNRSV)
  • Vector:  Spreads by infected pollen; transmitted by budding and grafting
  • Crops affected:  Most Prunus species including sweet cherry, sour cherry, peach, almond, rose, etc.
  • Disease description:  Leaf and bloom development may be delayed in the spring for infected trees.  Leaf symptoms usually consist of a few rings or chlorotic areas that can develop into necrotic spots and a shot-hole or tattered appearance later.  The disease is associated with reduced fruit growth, yield, and delayed fruit maturity.  Large areas of bark on the tree may die and produce wound gumming.  Susceptibility of infected trees to winter injury is increased.
  • Management:  Use virus free planting material.
  • Resource:

Cherry Rugose Mosaic

  • Causative Agent: Prunus necrotic ringspot virus
  • Vector: Grafting, seeds, and pollen from infected trees spread this disease.
  • Crops affected: Cherry
  • Disease description: Turned up leaf tips and delayed fruit maturity (up to 7-10 days) occur with this virus. Leaf symptoms include brown necrotic spots which drop out, giving leaves a shot-holed appearance.
  • Management: Remove infected leaves before bloom. Use virus-free planting material and scion wood.
  • Resources:

 Rusty Mottle

  • Causative Agent: Rusty mottle virus
  • Vector: Spread by infected budwood.  No known vector.
  • Crops affected: Cherry
  • Disease description: Older leaves turn a mottled yellow, symptoms appear first in lower center of tree. Two-three weeks before harvest rusty yellow leaves drop, removing 30-70% of tree’s leaves.  Infected trees will have reduced vigor and some main limbs may die back.
  • Management: Remove diseased trees when found. Plant virus-free trees.
  • Resources

 Prune Dwarf Virus

  • Causative Agent: Prune dwarf virus
  • Vector: Spread by budding, grafting, seeds and pollen
  • Crops affected: Sweet cherry and Sour cherry
  • Disease description: Sweet cherry leaves may be mottled, show yellow rings, have shot-holes, or have no symptoms.  They may be longer and narrower than usual.  In trees 25 years old or older, there may be more blind wood in infected trees.
  • Management: Use virus-free trees; Rogue out and replant young, infected trees
  • Resources
Viruses Affecting Stone Fruit

Peach Mosaic Virus

  • Causative Agent: Peach Mosaic Virus
  • Vector: unknown
  • Crops affected: peach
  • Disease description: Leaves develop chlorotic patterns in the early season. Spots become necrotic and fall out leaving a shot-hole appearance. Fruit will show symptoms of a rough and bumpy surface. Fruit size is reduced.
  • Management: Use virus tested, virus-free planting material
  • Resource:

Prune Dwarf Virus

  • Causative Agent: Prune dwarf virus (PDV)
  • Vector:  Spread by infected pollen; Transmitted by budding and grafting and by seed
  • Crops affected: Sweet Cherry, Sour Cherry, Italian prune
  • Disease description: Leaves on sweet cherry trees are longer and narrower with the virus. Symptoms may only show on one branch or section of an infected tree.  Leaves may show chlorosis, necrosis, leaf distortion and stunting.
  • Management:  Use virus tested and virus free bud wood and rootstocks.
  • Resource:
    • Prune dwarf virus, Pacific Northwest Plant Disease Management Handbook, webpage. (Accessed: 1/17/17).

Prunus Stem Pitting (also called prune brownline and constriction disease)

  • Causative Agent: Tomato ringspot virus
  • Vector:  Dagger nematodes (Xiphinema spp.) transmit the disease from weeds to trees, seeds (e.g. dandelion) can spread the virus over long distances. Spread through grafting and budding of infected plant tissue.
  • Crops affected: All stone fruit species, and it is found in broadleaf weeds such as dandelion
  • Disease description: Leaves cup upward along middle, turn shades of yellow to red, and fall from tree early. Symptoms result from girdling of the trunk.  Tissue at the graft union may die and tree breakage can occur under high winds.  Trees that are infected may produce many small fruit, which will ripen early and drop.  Affected trees may die within two years of leaf symptom expression.
  • Management: Use virus tested, certified virus free trees. If nematodes are found, fumigation can be considered. Control broadleaf weeds.
  • Resources:

Plum Pox Virus (Sharka)

  • Causative Agent: Plum pox polyvirus (PPV)
  • Vector: transmitted by aphids; can be spread through infected bud and nursery stock.  Virus can spread by root grafting.
  • Crops affected: All Prunus species including apricot, plum, peach, latent in cherry trees.
  • Disease description: Serious virus of stone fruit with potential to devastate production. On plum, pale green spots and rings or lines appear on the leaves; Rings and spots can occur on the fruit, as well as sunken lesions. Peach leaf symptoms appear as chlorotic bands along veins, and twisting. Rings and spots appear on peach fruit. Various common weeds are hosts (e.g. white clover).  Fruit from infected trees lack flavor, may mature early and often drop prematurely.   Tree roguing should include roots.
  • Management: Use propagative material that has been tested and is known to be virus free.
  • Resources:
Additional Virus Resources

WSU Resources

 

Virus Research Reports

 

Extension Resources

 

Trade Articles About Viral Diseases

 

Technical Articles About Select Viral Diseases

 

Additional Tree Fruit Diseases Caused by 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).

The Disease Process

Plant diseases can only develop when three things occur at the same time creating the “disease triangle.” The disease causing pathogen, the right environment, and an entry way into a host plant are the three elements that all need to be present at the same time for disease to occur. Minimizing the influence of any of the three components can reduce the severity of the disease. Some of the bacterial and fungal diseases that afflict fruit trees in our area start to grow and spread in the spring when rain showers and temperatures increase, with just the right windows of time for incubation, spreading bacterial cells and fungal spores to sensitive new leaves and flowers (examples: fire blight, scab). Insect pests, and environmental stresses like wind, hail, sun, or excessive rain can provide pathogens an opportunity to infect the tree through openings in tree bark, leaf and fruit tissues.

General Control Tactics

Selection of Varieties & Rootstocks

Select disease-resistant fruit tree varieties/rootstocks for your location and growing conditions. Also, buy certified virus-free trees from reputable nurseries. See the Clean Plant Center of the Northwest page for information on viruses, virus-free materials and testing services.

Monitoring

Plant protection involves treating healthy plants before they become diseased, and disease prevention is essential to preserving fruit quality. Monitor trees for signs of disease throughout the year. Integrated pest management (IPM), is a multi-tactic approach to managing, among other pests, plant diseases. Disease management techniques include cultural, biological, and chemical methods, and allowing low, non-damaging levels of a problem. Chemical or biological methods can be used to control insects that cause diseases (examples: sooty molds caused by secretions from aphids and pear psylla). Cultural sanitation methods include pruning out infected twigs and limbs, removing and destroying the debris (examples:, burning or composting) so it won’t re-inoculate the trees.

DSS Tools

Use decision support systems and tools like the WSU-DAS Cougar Blight model for fire blight to know when the disease is very likely to occur during bloom, if fire blight is already in your orchard. Additional information about fire blight can be found on the Chelan-Douglas County Cougar Blight webpage.

Chemical Protection

Chemical protection is one of the main control methods for bacterial and fungal diseases. Treat severe infestations during the growing season. Spray treatments are applied during the dormant season to inhibit disease colonies. Orchard sanitation, pruning and quickly destroying debris from diseased tree tissue is important to prevent the spread of the pathogens throughout the tree and to other trees in the orchard. It is recommended to remove and destroy the debris from fire blight pruning. Federal and state plant quarantine regulations require the control of tree fruit diseases for fruit destined for export to foreign countries (e.g. Manchurian crab apple fungal diseases and exports to China) See the 2016 WSU Crop protection guide for tree fruits in Washington here.
Always read labels, and follow the state or extension recommendations.

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