Common Pome Fruit Diseases

The major pome fruit diseases are listed alphabetically using commonly accepted disease names. Each disease name is followed by the Latin binomial for the pathogen(s), a letter to indicate whether the pathogen is a bacterium (B) or a fungus (F), and a brief description of disease symptoms and control strategies.

Apple replant disease is caused by an undefined complex of soil organisms. The disease causes stunting or reduced growth of young apple trees planted into old orchard sites. Various studies have implicated nematodes, actinomycetes, fungal root pathogens such as Pythium, herbicide residues from previous plantings, low soil pH, and drought stress of newly planted trees. Soil fumigation prior to planting is beneficial in some sites but not in others. Effects of the disease can be avoided at many sites by using good preplant site preparation coupled with irrigation of young trees during the year of planting.

Apple scab (F: Venturia inaequalis) is the most common and economically important disease of apples. Lesions appear on leaves and fruit nine to 17 days after infection (Figure D1.1), with the length of the incubation period depending on temperature. Young lesions appear as circular, velvety, olive-brown spots that gradually turn black-brown with age. Severely affected trees defoliate by midsummer and may fail to form fruit buds for the next year's crop. Fruit that become infected early in the season are misshapen and may crack. Infections on fruit pedicels may cause fruit to drop during June.

Primary inoculum consists of ascospores that are released from overwintering leaves on the orchard floor during rains. Ascospore release begins at or soon after trees reach budbreak in spring. It peaks when trees reach the pink- or full-bloom stage and terminates near petal fall or within two weeks thereafter. The "Mill's Table" lists the minimum wetting period required for infections to occur at various

FIGURE D1.1. Apple scab lesions on a leaf (left) and on fruit (right)

temperatures and is widely used to monitor potential infection periods. Infections initiated by ascospores produce abundant conidia that are disseminated to new leaves and fruit by splashing and windblown rain. Most infections on fruit occur as a result of secondary spread from primary lesions established on prebloom foliage. Infected fruit that have no symptoms at harvest may develop pinpoint scab (small black lesions 1 to 3 millimeters in diameter) during cold storage.

Scab is usually controlled by applying fungicides to prevent primary infections. Scab-resistant cultivars are available but are not widely accepted or easily marketed. Removing leaf litter before spring rains or using cultivation, urea sprays, or saprophytic fungi to speed leaf degradation can reduce the amount of primary inoculum but rarely provide complete control of scab.

Bitter rot (F: Colletotrichum species) is a common summer fruit rot in hot humid climates, but it is rare in cooler growing regions and in arid climates. Infections are most common on the side of fruit facing the sun. They start as sunken tan lesions that gradually enlarge, sometimes developing masses of slimy pink spores in the centers of lesions. Decayed tissue extends in a narrow "V" pattern toward the seed cavity. The disease can spread rapidly during hot, wet weather and is controlled with fungicides. Captan is often used to control bitter rot during summer.

Black rot (F: Botryosphaeria obtusa) is a summer fruit rot characterized by firm dark lesions, sometimes with a bull's-eye pattern, and sometimes with black pycnidia evident in the center of larger lesions. Infections on fruit can occur anytime from before bloom until harvest. Killed fruitlets that remain attached to the tree after fruit thinning are frequently colonized and provide inoculum both for later summer infections and for infections of fruit and foliage the next spring. Dead branches, including branches killed by fire blight, are rapidly colonized and will produce both ascospores and conidia if left in the tree over winter. The pathogen also causes frogeye leaf spot and limb cankers. Benzimidazoles, mancozeb, metiram, and strobilurin fungicides provide good control.

Blue mold (F: Penicillium expansum) is the most economically important postharvest decay of apples and pears. Decayed fruit flesh is soft and watery, separates easily from healthy tissue, and has a musty, earthy odor. Spores are disseminated by air or in water flumes. The fungus rapidly invades wounds in fruit and can also invade through fruit stems during long-term controlled atmosphere storage. Blue mold is controlled by using sanitation measures to limit exposure to inoculum and by using postharvest fungicide treatments.

Bull's-eye rot (F: Pezicula malicorticis) is a late-summer fruit decay that is common in northwestern United States and is occasionally found in more humid climates. Decay lesions are brown, often with a lighter brown or tan center. Affected tissue is slightly sunken, firm, and does not easily separate from healthy adjacent tissue. Inoculum comes from cankers in apple trees or from fungal colonies in the dead outer bark of pears. The disease is controlled with fungicides applied during late summer.

Cankers (various causes) are diseased areas on tree limbs or trunks where pathogens have killed the tree bark. Cankers can be caused by bacteria (e.g., fire blight cankers) or fungi. Common canker-causing fungi include Botryosphaeria obtusa (black rot canker), Nectria galligena (European apple tree canker), Pezicula malicorticis (an-thracnose canker), and Neofabraea perennans (perennial canker). Many canker-causing fungi are weak pathogens that invade through wounds or stressed tissue. For example, black rot cankers on apples in northeastern United States occur primarily on limbs where xylem-inhabiting basidiomycetes such as Trametes versicolor or Schizo-phyllum commune have extensively colonized the internal woody cylinder within the limbs.

Cedar apple rust (F: Gymnosporangium juniperi-virginianae) causes yellow or orange lesions on leaves and fruit of susceptible apple cultivars in regions where the alternate host, eastern red cedar (Juniperus virginiana), is endemic. Basidiospores produced on galls in cedar trees are released during spring and early summer rains. Most infections on apple leaves and fruit occur between tight cluster and first cover, but leaf infections may continue for up to six weeks after petal fall. There is no secondary disease cycle on apples. Aeciospores produced on apple can only infect cedars. Fungicides are used to control this disease, but disease pressure can be reduced by removing cedars within several hundred feet of orchard perimeters.

Fabraea leaf and fruit spot (F: Fabraea maculata) is common where susceptible pear cultivars (e.g., 'Bosc') are grown in warm, humid climates. The disease appears during summer as small, 1 to 3 millimeter red or purple spots on leaves and fruit. The spots turn brown with age and produce an abundance of splash-dispersed conidia. The disease spreads extremely rapidly and can cause complete defoliation and crop loss. In northeastern United States, fungicides are needed from petal fall through mid-July to prevent primary infections. Later sprays are needed when primary infections are not completely controlled.

Fire blight (B: Erwinia amylovora) is the most destructive disease of apples and pears because it spreads rapidly and can kill entire trees. Bacteria released from cankers are spread to blossoms by insects and splashing rain. Blossom infections result in collapse of the blossom cluster and invasion of the subtending branch. On twigs killed by fire blight, the leaves collapse and turn blackish brown but do not abscise. Shoot tips on killed twigs frequently bend over to form a diagnostic "shepherd's crook." Bacterial ooze is usually visible on recently invaded leaves, twigs, branches, and fruit. Secondary shoot infections occur when inoculum contacts and invades succulent tissue in growing shoot tips. Rootstock blight occurs when the pathogen kills susceptible apple rootstocks even though the scion portion of the tree may be only mildly affected by fire blight. Under some conditions, the pathogen can move internally from blossom and twig infections to rootstocks without causing apparent damage to intervening tissue in limbs and trunks. Trauma blight occurs when bacteria invade tissue that is damaged by hail or frost. Apple and pear cultivars show wide variations in their susceptibility to fire blight.

Fire blight is controlled by pruning out infected tissue to reduce inoculum. A delayed dormant copper spray can help to suppress inoculum levels in orchards that were infected the previous year. The most important control measure involves well-timed applications of antibiotics (streptomycin, or terramycin where bacteria are resistant to streptomycin) during bloom.

Flyspeck (F: Schizothyrium pomi) appears during summer as a skin blemish on apples grown in warm, wet climates. Fungal colonies are usually 1 to 3 centimeters in diameter and consist of a few to more than 50 shiny black spots reminiscent of deposits left by a large fly (Figure D1.2). Most infections originate with ascospores or conidia blown into orchards from the numerous wild hosts found in hedgerows and wood lots. Infected fruit must be exposed to more than 250 hours of accumulated wetting before symptoms become visible. Fly-

FIGURE D1.2. Flyspeck (FS), sooty blotch (SB), and white rot (WR) on apple fruit at harvest

speck is controlled by application of fungicides beginning at petal fall.

Frogeye leaf spot: See black rot.

Gray mold (F: Botrytis cinerea) is the most important postharvest disease on pears and the second most important on apples. The decay is pale brown, soft, and watery on ripe apples and pears, but it can cause a firm decay of apples during controlled atmosphere storage. Conidia can infect through stems on pears or through wounds on pears and apples. Calyx infections that occur in the field may remain quiescent until fruit are moved to storage. Botrytis cinerea spreads rapidly from one fruit to other contacting fruit. As a result, infection originating in a single fruit can cause large losses during long-term storage.

Leaf spot (various causes) is a generic term often used for nondescript brown spots 1 to 3 millimeters in diameter. The spots can be caused by various fungi or by abiotic factors such as spray injury. Two common leaf spot diseases, frogeye leaf spot on apple and Fabraea leaf spot on pear, were described earlier under their own names. Cedar apple rust can cause extensive rust-induced leaf spotting in apple leaves where development of rust lesions is arrested by host resistance after rust basidiospores have already germinated and killed leaf cells. This rust-induced leaf spotting is common in 'Empire' and 'Cortland' trees that are not protected with fungicides during rust infection periods. Many weak pathogens, including species of Alternaria, Phomopsis, and Botryosphaeria, will invade leaftis-sue that has been injured by rust or by other abiotic factors, but these pathogens have only limited capabilities for invading healthy leaf tissue. An exception is the strain of Alternaria mali that causes Alternaria leaf spot in southeastern United States and in Asia.

Pear blast (B: Pseudomonas syringae) affects pear flowers during cool, wet weather and can result in significant reductions in fruit set. Symptoms include blackening of the calyx ends of fruit and, occasionally, collapse of entire spurs. However, the subtending wood beneath killed spurs remains healthy. Losses to this disease are often blamed on light frost or poor pollination. Streptomycin sprays applied prior to infection periods have been shown to reduce losses.

Pear scab (F: Venturiapirina) on pears parallels apple scab on apples. The life cycle and control measures for pear scab are very similar to those described for apple scab.

Phytophthora crown and root rot (F: Phytophthora cactorum and other Phytophthora species) is the most important soilborne disease of tree fruit. The pathogens release flagellated zoospores that can infect tree roots and crowns in water-saturated soils. The fungus kills the bark on the crown and larger roots. Aboveground symptoms become apparent after trees are girdled by the fungal infection. Infected tissue is usually apparent several inches below the soil line where infected bark and inner phloem tissue have a soft texture and rusty red-brown color when cut. The disease is best managed by planting trees on well-drained sites, tiling poorly drained sites prior to planting, planting trees on raised berms, and avoiding susceptible rootstocks such as MM.106 and M.26 on sites with questionable internal soil drainage. The fungicide mefenoxam can be used to protect susceptible trees.

Powdery mildew (F: Podosphaera leucotricha) is second only to apple scab as the most important fungal disease of apples. The fungus overwinters in infected buds and grows to cover the new green tissue when these buds begin growing in spring. Infected leaves develop a white powdery coating of spores and mycelium. The conidia from these primary infections can initiate secondary infections on new leaves. Fruit infected at the pink-bud stage develop a netlike russeting, but mildew is primarily a foliar disease.

Powdery mildew is the only aboveground fungal disease of tree fruit that thrives in dry climates because it does not require free water

(rains or dews) for infection. Conidia can germinate on leaf surfaces when relative humidity is as low as 70 percent. This disease is controlled with fungicides. Some apple cultivars are relatively resistant to infection. Highly susceptible cultivars will require more fungicide protection than less susceptible cultivars.

Quince rust (F: Gymnosporangium clavipes) is a disease that affects apple fruit but not foliage. Infected fruit are often misshapen with deeply sunken lesions. The life cycle of quince rust is similar to that of cedar apple rust, but quince rust produces perennial cankers in cedars instead of galls.

Sooty blotch (F: a complex of Peltaster fruticola, Leptodontium elatius, and Geastrumia polystigmatis) causes superficial gray, black, or cloudy areas on apple fruit (Figure D1.2). The disease cycle and controls are similar to those described for flyspeck.

Virus diseases are less important in pome fruit than in stone fruit. Many old apple cultivars and rootstocks contained one or more latent viruses such as apple mosaic virus, apple stem pitting virus, apple stem grooving virus, or apple chlorotic leaf spot virus. These viruses caused no visible symptoms on most cultivars and were considered benign even though some of them reduced productivity of some cultivars. Other viruses or viruslike diseases such as stony pit in pears cause fruit deformities that make fruit from infected trees unmarketable. Tomato ringspot virus causes a tree decline in certain cultivar-rootstock combinations (e.g., 'Delicious' on MM.106), but it does not affect most apple rootstocks.

The best defense against pome fruit viruses is to establish orchards using trees that are certified to be free of known viruses. Almost all of the pome fruit viruses are disseminated primarily via virus-infected propagation material. Tomato ringspot virus is the exception. It is vectored by several species of Xiphenema nematodes.

White rot (F: Botryosphaeria dothidea) is a summer fruit rot that occurs primarily in warm, humid growing regions. Lesions usually become visible during late summer as small, brown to tan spots, often with a red halo around the margins. As the decayed area expands (Figure D1.2), it extends in a cylindrical pattern toward the core of the fruit. (This contrasts with the "V"-shaped pattern characteristic of bitter rot.) Under warm conditions, rotten fruit have a soft, watery, "applesauce-in-a-bag" composition, but under cooler conditions white rot is not easily distinguished from black rot. The disease is con trolled with fungicides (benzimidazoles, mancozeb, metiram, strob-ilurins) and by pruning out dead wood that can harbor this fungus.

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