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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:
Washington State University