Diseases

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  • Alternaria blotch

    The disease primarily affects the foliage, causing circular, necrotic lesions with a light brown interior (A) that later become surrounded by a darker purplish halo (B). Defoliation can occur by late summer on susceptible varieties that are heavily infected (C). The pathogen can also attack green, woody tissue, but it rarely attacks the fruit.

  • Alternaria fruit rot

    The disease appears as velvety dark green to black, circular, sunken lesions on mature fruit; the infected tissue is firm and brown (A, B). Disease is typically associated with over-ripe or damaged fruit, or fruit held in storage (where serious losses can occur). The pathogen has also been reported to cause superficial red spotting on the surface of apricots and peaches. The spots eventually turn tan to brown, becoming necrotic, but typically retaining a red halo.

  • American hawthorn rust

    Attacks only the leaves (not the fruit) of apple and pear; affects the apple varieties McIntosh and Cortland in particular (A).

  • Anthracnose

    Lesions start as small, circular, tan to brown spots on mature or nearly mature fruit. Lesions expand rapidly, with a tendency to form concentric rings that may or may not be sunken. Lesions are firm to the touch but typically develop orange to pink, slimy spore masses in their centers. Individual lesions may reach a diameter of 4–5 cm, but may coalesce to form more extensive areas of infection (A).

  • Apple anthracnose

    Branch lesions first appear as small, circular spots that are purple or red when wet. As lesions enlarge, they become elliptical, sunken and turn orange to brown. A distinct margin develops between healthy and diseased tissue, which eventually causes the bark to crack around the infected area (A). The infected bark tissue over the canker separates into small pieces and curls upwards from the lesion. On older cankers, the bark sloughs off leaving only the last fibers behind. These fibers run lengthwise across the lesion and their appearance as such has often been referred to as "fiddle strings". Anthracnose cankers typically do not enlarge during their first year of growth.

  • Apple latent viruses

    Latent viruses are viruses that survive in their host without causing symptoms. These viruses are transmitted when a virus-infected scion is grafted onto a susceptible rootstock. There are number of latent viruses that affect apple, but these three are the most common. They may occur together, in pairs, or singly. Affected trees begin to show a general decline 1–2 years after grafting. Familiar symptoms associated with the specific viruses are: ACLSV, chlorotic leaf spots, leaf distortion, or leaf stunting (A); ASGV, chlorotic leaf spots, stem grooving and pitting, union necrosis, and swelling above the graft union (B); ASPV, pitting and grooving, epinasty and decline (C).

  • Apple mosaic virus

    Young leaves develop pale to bright cream-colored spots, blotches, bandings or patterns as they expand in the spring (A). These turn brown and become necrotic as they age and premature defoliation may occur when infection is severe. Symptom expression is highly variable among varieties.

  • Apple scab

    On leaves, young lesions are velvety brown to olive green with indistinct margins, and will often not be readily noticeable until after petal fall in commercial orchards (A). The number of lesions can vary from few to several hundred per leaf, depending on the season and varietal susceptibility. Older leaf lesions are typically raised, with a corresponding cupping on the underside of the leaf, and dark green to gray to brown in color, with distinct margins (B, C). Leaves that are heavily infected tend to curl, shrivel, and fall from the tree. On the fruit, young lesions appear similar to those on leaves (D). Although the entire surface of the fruit is susceptible to infection, lesions often cluster around the calyx end of the fruit. As lesions get older, they become brown and corky and take on a "scabby" appearance (E).

  • Apple union necrosis and decline

    AUND is due to an incompatibility at the graft union where a resistant scion is grafted onto a susceptible, but tolerant rootstock, most commonly MM.106. Symptoms appear approximately 4–6 years after planting, usually once the tree is capable of bearing a full fruit load. Affected trees show a general decline beginning with delayed budbreak. The canopy tends to be sparse, bearing small, pale green leaves, and premature defoliation is possible. A distinct black, sunken line at the union is apparent underneath the bark (A). The graft union may be weakened to the point where the scion and rootstock separate partially or completely or crack under stressful conditions (B).

  • Armillaria root rot

    The bark at the crown and roots sloughs off easily, exposing the dense white growth of the fungus (A). The growth extends in a fan-like pattern underneath the bark. Black shoestring-like strands (rhizomorphs) may be obvious on the surface of the bark (B). In the fall, yellow/brown mushrooms may appear at the base of the tree, especially if killed trees are left in place (C). Trees in affected orchards will often die in a circular pattern from one or more foci in the orchard.

  • Bacterial blossom blast of pear

    The most common symptoms are wilting followed by browning or blackening of blossoms, often spreading through the entire blossom truss and killing the fruiting spur (A, B). If infection is restricted to the calyx-cup and the blossom is not killed, this may lead to the development of black lesions on developing fruit; many of these fruit drop. Infection of the leaves leads to the development of small lesions and "shot holes"; entire leaves may be killed.

  • Bacterial canker (blossom blast)

    Leaf scars, stomata, and areas of injury are the principal sites of infection. The most conspicuous symptoms are limb and trunk cankers (A, B), blossom blast (C), "dead bud", and leaf spotting; these symptoms may or may not occur together. Cankers can girdle and kill entire limbs, reducing the tree's fruiting capacity. Infection of the trunk, particularly on young trees, often results in tree death. Crotches are particularly susceptible to infection, which often leads to extensive gumming (D). Infections of the blossoms cause blossom blast and loss of fruiting spurs (E). Infections of dormant flowering and vegetative buds result in a condition called "dead bud" in which buds fail to break dormancy in spring. On leaves, lesions are tan to brown and initially surrounded by a yellow halo. Lesions may be small or they may coalesce to form large areas of infection. They are eventually walled off and the center of the lesion drops out to give the leaf a shot-holed appearance (F). On fruit, lesions tend to be circular, brown, and sunken (G).

  • Bacterial spot

    On leaves, lesions are small, tan to brown in color, eventually becoming necrotic, and usually surrounded by a yellow halo. There are often numerous lesions on a leaf and they tend to be restricted to areas between veins, which gives them an angular appearance. Lesions are eventually walled off and the center of the lesion drops out, giving the leaf a tattered or shot hole appearance (A). Severe foliar infections results in the yellowing of leaves and premature leaf drop. Bacteria from leaf infections move to the current year's twig growth, leading to canker formation. On fruit, the bacteria cause dark brown lesions or blemishes. Lesions often become sunken and the skin of the fruit cracks, causing deep pits that leave the fruit unmarketable (B, C).

  • Bitter pit and cork spot

    Small, green to purplish to light brown, slightly sunken lesions appear on the surface of mature fruit (A). Individual lesions on the fruit surface are dry and do not extend deep into the fruit (B); however, cutting into the fruit can reveal numerous internal lesions. Bitter pit usually develops in storage and is most severe at the calyx end. A similar calcium-related disorder that occurs only on d'Anjou pears is named cork spot.

  • Bitter rot

    Bitter rot appears on young fruit as small, circular brown lesions. Lesions expand rapidly and radially under wet and warm conditions. As they age, they turn darker brown and become sunken (A). When several lesions occur on a fruit they tend to coalesce and no longer appear circular. The spores of the fungus are creamy, white to pink, and tend to form in concentric circles within the lesion (B). The rotted flesh is often watery and appears V-shaped in cross section (C). The fruit eventually dries, mummifies, and may fall to the ground or remain hanging from the tree throughout the duration of the winter.

  • Black knot

    Black knot usually develops over two seasons. The disease first appears in late summer or autumn as an olive-green swelling on new shoots (A). Disease develops rapidly the following summer, forming a characteristic dark, course-textured warty knot (B, C). Knots vary from 2.5 cm to nearly 30 cm in length and may or may not encircle the branch. The vascular tissue becomes restricted in infected branches, ultimately leading to the death of the branch.

  • Black pox of apple (blister canker of pear)

    On apple, conical, smooth, shiny black swellings are evident on current season's growth. As lesions age, they become ovoid with raised borders (A). On leaves, lesions begin as small, circular green spots surrounded by a red halo. Lesions expand to 1.5–6 mm in diameter and the center of the lesions turn brown. On apple fruit, lesions are approximately 3–9 mm in diameter and appear shiny black, round and sunken (B). On pear branches, fully developed lesions are generally circular, and the interior of the lesion cracks, exposing darkened wood beneath the bark (C).

  • Black rot (Blossom end rot, Frogeye leaf spot)

    Fruit infections that occur early in the season appear at the calyx end and typically develop into blossom end rot that may not appear until the fruit begin to mature (A). Seed cavity infections are associated with infections of the carpel, particularly in varieties with Delicious parentage, often leading to premature fruit drop within 1 month after petal fall. Late fruit infections occur through cracks in the cuticle, wounds and possibly lenticels. On mature fruit, lesions enlarge rapidly (slower in the northeast), typically producing a series of concentric bands that alternate in color from brown to black (B). The flesh beneath the rot remains firm and leathery. Infected fruit color early and ripen in advance of healthy fruit, up to 3–6 weeks earlier in the southeast. Small black fungal bodies form on rotted fruit surfaces. Eventually, infected fruit dry down to mummies that remain attached to the tree, serving as inoculum sources in the spring. Frogeye leaf spot: Lesions start as small purple specks on the upper surface of leaves 1–3 weeks after petal fall and enlarge to 3–6 mm in diameter. Mature lesions are circular, tan to brown with distinct purple margins (C). Heavily infected leaves become chlorotic and drop. Cankers appear primarily in trees where the older wood has been extensively invaded by basidiomycete wood-rotting fungi (D, E), the presence of which is denoted by darkened wood visible in the center of a cross-section of an infected limb (F). Healthy apple limbs show no darkened wood in cross-section.

  • Blister spot

    Lesions begin as small, darkened, water-soaked areas, generally around lenticels and typically on the lower half of the apple. Small raised blisters form shortly thereafter, becoming purplish black as they expand (A). The infections are shallow, not extending more than 1–4 mm into the fruit flesh. The epidermal layer covering the blister dies and will often flake off the surface (B). The lesions are generally circular and rarely become larger than 4–5 mm in diameter. Leaves of Mutsu may also experience a mid-vein necrosis (C).

  • Blue mold

    Blue mold enters the fruit through wounds, stem-end invasion, or as a core rot (A). Infection is first visible as a soft and sunken, yellow to pale-brown circular lesion on the surface of the fruit. Lesions expand rapidly and can quickly macerate the fruit (B). A diagnostic symptom of this rot is a strong earthy or musty odor and unpleasant taste. If fruit are stored under wet or humid conditions, the fungus produces numerous blue-green tufts of spores on the surface of the fruit (C); sporulation typically does not occur under CA conditions (compare with gray mold).

  • Brooks fruit spot

    Appears as irregular, slightly sunken dark green lesions on immature fruit. On red-skinned varieties, fully developed lesions are dark red to purple and are generally no larger than 10 mm in diameter (A). On lighter-skinned varieties, lesions turn dark green. Brooks fruit spot does not cause significant browning of the flesh beneath the lesion (B).

  • Brown rot

    Infected flowers turn brown, wither, and either become fixed to twigs as a gummy mass or drop like unpollinated flowers (A). Apricot is the most susceptible to blossom blight, followed by prune, sweet cherry, peach, sour cherry, and then plum. If infected blossoms do not drop off, the fungus may grow through the flower stem and into the twig below. Twigs develop elliptical to fusoid cankers with profuse gumming at the margin between diseased and healthy tissue (B). Leaves on infected shoots turn brown and wither, but remain attached. In some instances, twigs are girdled and killed. On the fruit, infection first appears as soft brown spots. These rapidly expand and become covered with powdery masses of tan spores (C). Infections spread rapidly from fruit to fruit when they are touching one another. Rotted fruits typically shrink into a wrinkled "mummy" as they dry on the tree (D). Both immature and mature fruit infected with brown rot tend to remain on the tree and form mummies (E). Mummies that fall to the ground may produce a spore-bearing mushroom-like structure called an apothecia the following spring (F); its importance in the disease cycle is minimal in the Northeast and mid-Atlantic region.

  • Calyx end rot

    Symptoms begin at the calyx end of the fruit, causing a reddish discoloration at the site of infection. The rot is at first soft, but eventually dries out, turning tan to brown with a red border. Calyx end rot is caused by the "white mold" fungus. Infected fruit have a tendency to drop prematurely.

  • Cedar apple rust

    On leaves, the disease appears on the upper surface as small, faint, yellow spots (A) shortly after the appearance of active cedar galls (B) found on the alternate host for this fungus, the red cedar (Juniperus virginiana L.). Lesions eventually turn dark yellow to yellow-orange and develop a reddish border (C). Tiny pustules form in the center of these lesions, eventually turning black. Blisters form on the underside of lesions by midsummer and produce small, tubular projections (D). As lesions age, they split and the walls curve back, forming a cup with masses of powdery orange to brown spores. On the fruit, lesions usually appear on the calyx end, similar to those on leaves, but only a bit larger, and are usually slightly raised. A dark-green border forms around the yellow to orange colored lesion as they age (E, F).

  • Cherry leaf spot

    Lesions begin as small, circular red to purplish spots on the upper leaf surface (A). Spots enlarge as they grow older, typically coalescing and turning brown (B). Lesion centers may eventually drop out to give the leaf a "shot-hole" appearance, particularly on plum. The most striking symptom of cherry leaf spot, especially on sour cherry, is the yellowing of older leaves prior to their falling from the tree (C). When infection is severe, the entire tree may be defoliated by midsummer. Spores are produced on the underside of leaf lesions in acervuli and appear as a white to pinkish mass in the center of the lesion (D).

  • Constriction disease of Stanley plum (Brown line)

    Brown line disease is due to an incompatibility at the graft union when Stanley plum and some other European or hybrid plum varieties are grafted onto Myrobalan (Prunus cerasifera) rootstock. Asian plums are not affected. Infected trees show a general decline and bear small, pale green leaves. The scion grows quicker than the rootstock, giving the rootstock a constricted appearance just below the graft union (A). A distinct brown, sunken line at the union is apparent underneath the bark (B).

  • Crown gall

    Infected trees are often stunted and produce small, chlorotic leaves. Spherical to elongated swellings (galls) along the roots or on the trunk just above the soil line is the primary symptom (A). Young galls are smooth and soft, and the bark tissue is often pale relative to the surrounding healthy tissue, but galls darken as they age. The galls may completely surround the root or crown or may appear as a growth off to one side (B). Galls start small and can grow to a typical 0.6 to 10 cm in diameter.

  • Dry eye rot (blossom end rot)

    Symptoms begin at the calyx end of the fruit, causing a reddish discoloration at the site of infection. The rot is at first soft, but eventually dries out, turning tan to brown with a red border. Dry eye rot is caused by the "gray mold" fungus. Infected fruit have a tendency to drop prematurely. If harvested though, infected fruit will develop gray mold in storage.

  • Fabraea leaf spot

    Lesions on leaves and petioles start as small, circular purple to black pinpoint spots (A, B). They enlarge quickly to a diameter of about 10 mm, develop a dark brown to black interior, and may coalesce to form larger areas of infection. On fruit, lesions have a similar appearance to those on leaves but tend to be larger and cause the fruit to crack (C). Heavily infected leaves and fruit drop prematurely.

  • Fire blight

    Blossom blight occurs in the spring. Infected blossoms first exhibit a water soaking, followed by wilting and their eventually turning brown on apple and nearly black on pear (A). Individual flowers or the entire cluster may be affected. Typically, infected blossoms do not fall and bacteria progress into the tender shoot growth. In the shoot, the bacteria travel along the midvein of the leaves and they soon wilt, shrivel, and turn brownish black, killing the entire shoot. Bacterial ooze may be observed under warm and humid conditions (B). Flowers will cling to the infected stem and often remain attached throughout the season. Infected fruit appear black and shriveled and usually remain attached to the tree. Shoot blight develops in late spring or early summer on actively growing terminal shoots, including suckers and water sprouts. Infected shoots first have an oily appearance and turn a dark green. On apple, the infected shoot becomes light to dark brown, in contrast to pear, in which the shoot becomes black. Blighted shoots often form a characteristic "shepherd's crook" at their tip and the dead leaves remain clinging to the affected twigs (C). The disease can progress to whole limbs and, when infection is severe, the whole tree appears to be scorched by fire, hence the name fire blight. Rootstock blight is the result of infections that travel from infected blossoms or shoots into the rootstock, although direct infection of the rootstock through sucker growth is possible. Infected rootstocks are discolored and there is usually a sharp demarcation between infected rootstock tissue and scion tissue when the bark is peeled back (D). Under favorable conditions, the bacteria may ooze from the rootstock, leaving streaks down the rootstock.

  • Flyspeck

    Sooty blotch and flyspeck are found together on the same fruit and affect only the epidermal layer of the fruit (A). Flyspeck colonies appear as distinct groupings of shiny, black fungal bodies on the surface of the fruit (B). The number of colonies or "specks" range from a few to over 50 per grouping.

  • Fusicoccum canker (constriction canker)

    On new shoots, small, reddish brown to dark, oval cankers centered on infected buds or leaf scars, or at the base of current season's twigs are found in early spring (A, B). As lesions enlarge, they develop a necrotic center surrounded by a purplish halo and eventually girdle and kill the branch above the lesion, giving it a blighted appearance. Lesions may be dotted with very tiny black fruiting bodies (pycnidia). Tendrils of conidia are exuded from pycnidia under favorable conditions and can be seen with a hand lens (C). On leaves, large, irregularly shaped brown spots are formed.

  • Gray mold

    Lesions usually start at the calyx or stem end of the fruit or at wound sites as small water-soaked areas. As lesions age, they enlarge, turning from grayish-brown to light brown, and eventually to a darker brown. White or grayish-white mycelium form on the surface of the rot under humid conditions (A); however, little sporulation occurs at cold-storage temperatures. Apples coming out of CA storage appear firm and tan, and when completely decayed, look like baked apples (B) (compare with blue mold).

  • Green fruit rot

    Both fungi attack the blossoms but rarely invade the twig (A). Blighting of the blossoms followed by gray spore masses is typical of infection from B. cinerea. S. sclerotiorum infects senescing floral parts; the white mycelium of the fungus is sometimes observed on blighted blossoms. Fruit infection occurs when developing fruit are in direct contact with blighted blossoms. Botrytis-infected fruit develop a typical gray sporulation (B). Sclerotinia-infected fruit develop a dense white fungal growth that often leaves the fruit deformed (C).

  • Green ring mottle virus

    The virus produces symptoms on sour cherry, primarily the variety Montmorency. Apricot, peach, and sweet cherry are symptomless hosts. Yellow mottling with irregularly shaped green islands or rings appear on the leaves of infected trees (A). A less common symptom is yellowing of the lateral veins, usually accompanied by a tip distortion (B). Fruit are misshapen with corky-brown, discolored pits, streaks or rings in the epidermis that extend into the flesh of the fruit. Infected fruit are bitter.

  • Moldy core and core rot

    Moldy core is associated with several different fungi. Infection is initiated at the calyx end and the fungi proceed to grow inward into the carpel tissue or locules and cause a core rot. External rot symptoms are rare. Fruits ripen early and decay is only obvious when fruit are cut in half (A, B). Core rot appears similar but occurs after harvest when the fruit are dipped into contaminated water in the packinghouse. A wet rot develops once the fruit are in storage.

  • Mucor Rot

    Infected tissue appears light brown, soft, and watery [Mucor piriformis on decaying pear-TM.tif]. The infection usually develops at wound sites, at the calyx end, or at the stem end of the fruit. Complete decay occurs rapidly under packinghouse conditions and in about 2 months in cold storage [Mucor piriformis on Fuji apples-TM.tif]. Fully rotted apples release large amounts of liquid containing the infective propagules of the fungus, which may spread the disease, but this is relatively uncommon.

  • Mycosphaerella leaf spot

    Lesions are primarily circular, 3–5 mm in diameter, and have a grayish white interior, with a distinct purple margin; small, black pycnidia develop in the interior of older lesions (A). The fungus occasionally attacks the fruit, producing small, dark lesions.

  • Necrotic leaf blotch

    Medium to large, irregular necrotic lesions occur on the foliage of mature leaves during mid- to late summer. The remaining green tissue generally turns yellow shortly after the appearance of symptoms (A, B). The onset of symptoms is often sudden and occurs in 2 to 4 waves during the summer. Defoliation rapidly follows the onset of symptoms and may affect fruit quality if it is extensive.

  • Nectria canker

    Cankers are often associated with nodes, often appearing as elliptical sunken areas (A). Sometimes callus production stops fungal invasion and cankers die by season's end. Other times, the fungus, walled off by callus formation during the current growing season, re-invades callus tissue when active growth resumes the following season, giving older cankers a zonate or target-like appearance (B). Enlarging cankers girdle infected twigs and branches, killing tissue above cankers. During damp weather, gelatinous white to cream-colored sporodochia produce spore masses that ooze from cankers; bright red to orange fruiting bodies (perithecia) may appear on older cankers (similar to Nectria twig blight).

  • Nectria twig blight

    Typically, small cankers can be found girdling the base of cluster buds that bore fruit the previous year. This leads to the wilting and dying of leaves and twigs of current season's growth (A). Bright pink to orange fruiting bodies appear at the nodes (B) or on developed cankers or pruning stubs during wet weather in early summer (C).

  • Peach leaf curl

    The pathogen infects young undeveloped tissue of leaves and fruit. Infection is most severe when cool conditions prevent rapid development of the foliage. Infected leaves curl and blister, leaving them severely deformed (A). Blisters may become discolored, ranging from light green to purplish (B). Severely infected leaves eventually shrivel and fall to the ground. Infected fruit either drop prematurely or remain on the tree and develop blisters or wart-like deformities on their surfaces (C).

  • Peach scab

    On fruit, lesions begin as small, greenish circular spots that gradually enlarge and darken as spore production begins (A). These spots appear when fruit are half grown and are most common on the stem end of the fruit, but can occur over the whole surface. Secondary infections may occur on twigs (B), and late-season variety fruit.

  • Peach tree short life (PTSL)

    Trees in their third to sixth year show a sudden wilt and collapse of new blossoms and death of branches, with tree death following within weeks of initial symptoms (A). The bark of affected trees appears reddish and water-soaked and gum exuding from these tissues often has a "sour sap" odor. Cracking of the outer bark is common and extends into the xylem. The cambial tissue is discolored, but the discoloration does not extend below the soil line (B). Few secondary or feeder roots are evident, but the primary roots generally appear healthy.

  • Pear scab

    Lesions on leaves begin as pinpoint spots, enlarging and becoming velvety brown to olive green with indistinct margins (A). Older lesions may remain singular or coalesce with other leaf lesions; they eventually stop expanding and develop distinct margins. On fruit, young lesions appear similar to those on leaves and can be found starting 1 month after fruit set (B). Although the entire surface of the fruit is susceptible to infection, lesions often cluster around the calyx end of the fruit. Infected fruit tend to be misshapen and eventually have dark brown to black spots or patches where fruit are infected (C). New lesions on developing shoots appear similar to leaf and fruit lesions. Eventually, the lesions become corky in appearance, where they will overwinter and produce conidia the following season (D).

  • Pear stony pit

    Dark-green spots appear on developing fruit about two to three weeks after petal fall. The tissue around the affected areas continues to grow, forming deep pits. Older pits develop necrotic centers. Heavily infected fruit are often severely deformed, gritty, and difficult to slice through (A). Doing so, however, reveals pitting throughout.

  • Pear vein yellows

    Faint, yellow vein banding, particularly of the secondary veins on current year’s growth, is the most common symptom. Red mottling or the development of necrotic spots of various intensities may also accompany vein-banding symptoms (A); the specific symptoms are dependent upon the particular strain of the virus.

  • Perennial canker of apple and pear

    Branch lesions are elliptical, sunken, and orange, purple, or brown in color. A raised layer of callus tissue forms around the infected tissue to isolate the diseased tissue. This occurs year after year as the fungus continues to invade healthy tissue, resulting in a series of concentric callus rings (A). Acervuli are produced in the most recently colonized tissue and appear as small, raised black bodies. The woolly apple aphid (Eriosoma lanigerum) can be found invading these cankers in regions where they both exist.

  • Perennial canker of stone fruit

    Small twig infections are usually found around winter-killed buds, leaf scars, and picking and pruning injuries. They appear as sunken discolored areas with alternating zonation lines and may ooze amber gum unless the twig is killed. On infected branches, the leaves often turn yellow, wilt and die (A). Dead twigs, branches, and canker margins become covered with pinhead-sized black pimples (pycnidia) that break through the bark (B). Pycnidia exude flesh- to orange-colored tendrils of spores under wet conditions. Main trunk and branch infections usually start at pruning wounds or winter-killed tissue. Cankers are elliptical and exude excessive amounts of amber-colored gum. Cankers often develop a series of concentric callus rings, reflecting the yearly alternation of callus formation and tissue invasion (C).

  • Phony peach disease

    The canopy of infected trees is flattened and compacted due to shortening of the internodes; the foliage tends to be a darker green (A). Infected trees may also flower and set fruit earlier, bear smaller fruit, and may suffer a substantial reduction in yield.

  • Phytophthora root, crown, and collar rot

    Crown and collar rot are often and mistakenly used interchangeably. Collar rot refers to infection that affects the bark tissue of the scion portion of the tree at or just below the soil line, whereas crown rot affects the bark tissue of the rootstock portion of the tree. Infected trees often have a normal bloom, but developing fruits tend to be small, the leaves wilt and drop, and the tree shows a general decline and eventually dies (A). Symptoms on apple usually develop over several seasons, becoming progressively worse. Apple, cherry, peach, and apricot trees are more susceptible to infection than pear and plum trees. Phytophthora-infected tissue often shows a characteristic reddish-brown discoloration of the inner bark several inches below the soil line with a characteristic, clear-cut margin of diseased from healthy tissue (B, C).

  • Plum pockets

    Infections occur soon after blossoms open and are first evident on fruits when they reach 6–12 mm in diameter. Symptoms first appear as white to off-white spots or blisters that enlarge rapidly to cover the entire fruit. Infected fruit are distorted, spongy, and abnormally large (A). The tissues of the seed cavity wither and die, forming a pocket within the fruit (B). As the fruit dry, they turn velvety gray as a result of spore production on their surfaces, eventually turning brown, withering and falling from the tree. Diseased leaves are thickened and curled, similar to peach leaf curl. Symptoms on leaves may or may not coincide with fruit symptoms.

  • Plum pox virus

    There is considerable variation in symptoms, depending on which species of stone fruit is affected, variety, age, and general nutritional status of the tree. On leaves, symptoms may include vein yellowing, banding, or the formation of light green to yellow rings (A). Peach and apricot fruit may develop light yellow rings on their skin or pits, become misshapen, or develop necrotic lesions (B, C, D). Plums are the most severely infected stone fruit. Fruit develop distinct dark rings or spots on their skin, the flesh shows a red discoloration, and fruit may drop prematurely (E). Affected fruit tend to be tasteless due to lowered sugar content, and the flesh may be dry.

  • Powdery mildew of apple and pear

    The fungus overwinters in leaf buds and sometimes flower buds. Mycelium develops rapidly on unfolding leaves and appears as white, felt-like patches or as a solid mat on the upper or undersurface of the leaf (A). Infections on the underside of the leaf may cause chlorotic patches or spots to occur on the upper side of the leaf. Infected leaves tend to crinkle, curl, or roll upwards along the edges, giving them a narrow appearance. The blossoms, petals, sepals, receptacles, and peduncles may become infected and covered with the fungus. Blossom infections are less common but are important because infected blossoms will either fail to set fruit (B) or produce small, stunted or russetted fruit (C).

  • Powdery mildew of apricot, nectarine, peach and plum

    Infection appears as white circular lesions of patches of powdery growth on either side of the leaf, or on the terminal ends of new shoot growth. Severely infected leaves curl upward or blister, may be stunted, but eventually drop as infection progresses (A). Leaves on new shoots may be narrow, strap-like, and distorted. Except for plum, young foliage is affected by S. pannosa; older foliage is affected by P. clandestine. Infections of the fruit begin with appearance of white, circular spots (B). On young fruit, the infection may progress and cover the entire fruit, causing them to become deformed. On older fruit, the lesions eventually cause the surrounding tissue to become necrotic and scabby; nectarines remain green (C).

  • Powdery mildew of cherry

    The fungus attacks young leaves and shoots and tends to cause more damage on sour cherry than sweet cherry. Infections appear as white circular lesions or patches of powdery growth on either side of the leaf or on the terminal ends of shoots (A). Severely infected leaves curl upward or blister but eventually drop as infection progresses. Towards the end of the season, small, black fungal bodies (cleistothecia) are visible within powdery mildew colonies (B). Infected fruit are deformed when infected young, or develop circular, slightly sunken lesions when infection occurs on mature fruit.

  • Prunus necrotic ringspot

    Individual branches or the entire tree shows delayed budbreak or foliation, stunted wavy leaves, and shortened blossom pedicels in spring. Leaves develop chlorotic spots, lines, or rings as they emerge (A). In severe cases, chlorotic areas become necrotic and fall out, leaving the leaves "shot-holed" or tattered (B). Fruit maturity may be delayed, and fruit may be marked (C). In other cases, trees may develop symptoms as described one year and then remain symptomless in subsequent years ("shock" symptoms).

  • Prunus stem pitting

    Affected trees appear weak and show a general decline (A). Leaves may have upward cupping, turning prematurely yellow or reddish purple, droop, and then prematurely drop. The bark is abnormally thick and spongy and the wood underneath has a severely pitted, indented texture (B). Symptoms are most severe in the wood just above and below the soil line.

  • Quince rust

    Attacks only the fruit (not the leaves) of apple and pear. Symptoms begin as a purplish lesion, usually appearing on the calyx end of the fruit. As the disease progresses, the entire calyx end becomes blistered and deformed. Tube-like structures eventually form and produce powdery, bright orange spores (A).

  • Replant disorders

    In general, trees suffering from replant disease show slow and uneven growth within the first three years of planting (A). Both specific (e.g., apple after apple) and non-specific (e.g., stone fruit after vegetables) replant disorders are known. The disorder is characterized by reduced shoot growth, severe stunting, rosetted leaves, and reduced fruit production. The root systems of affected trees are fibrous, poorly developed and are often in a state of decay.

  • Rhizopus rot

    Although the rot is predominantly a postharvest problem, symptoms may also develop in the field. Rotted fruit appears similar to brown rot, but Rhizopus-affected fruit appears slightly darker, the skin may slip away from decaying flesh underneath, or the fruit may be very leaky. Visible fungal mycelium may be white and fluffy, appearing like whiskers when the fungus sporulates. Infected fruit lying on the orchard floor or in packaged containers will often be engulfed by the fungus, appearing fluffy, with the fungus appearing to turning from gray to black as the fruiting bodies of the fungus develop at the tips of the "whiskers" (A, B).

  • Root-lesion nematode

    Root-lesion nematodes (A) are microscopic, migratory endoparasites that feed on the root systems of many crops. Affected trees appear stunted, may exhibit chlorosis or yellowing of the leaves, and have poor yields; young trees may be killed. Newly infected roots typically show a reddish brown, elongated lesion in the vicinity of invasion. Severely infected root systems lack the fine-texture, fibrous roots or may have tufts of necrotic roots that resemble a witches' broom. Disease is common on light-textured soils.

  • Rusty spot

    Lesions begin as small, circular, tan to orange blemishes approximately 3–5 mm in diameter. The discoloration is due to discoloring of the fuzz on the fruit (A). As lesions enlarge, the interior, reddened fuzz is shed. At this point, lesions appear green and smooth, surrounded by a tan to orange halo. Lesions may continue to expand and may coalesce with other lesions to form larger smooth patches of infection. Eventually, lesions stop expanding, harden, and turn orange-brown.

  • Silver leaf

    Silvering of the foliage is the characteristic symptom (A). At first, silvering may be associated with only one or two major branches, but eventually the entire tree becomes silvery in appearance. When infection is severe the leaves may curl upward. On apple, symptoms are usually evident shortly after petal fall. The symptoms progress over several seasons and trees slowly decline before dying. In some instances, affected limbs may recover and produce no symptoms in following years. The heartwood of affected trees is typically stained brown. In autumn, the fruiting bodies of the fungus (basidiocarps) appear on the surface of dead or severely infected limbs or the trunk (B).

  • Sooty blotch

    Sooty blotch and flyspeck are found together on the same fruit and affect only the epidermal layer of the fruit (A). Sooty blotch appears as various shades of olive-green on the surface of the fruit. Colonies range in shape from nearly circular colonies with distinct margins to large, amorphous colonies with diffuse margins (B). The variation in colony appearance can be attributed to the interaction among the different fungi causing the disease and environmental conditions, specifically temperature and relative humidity.

  • Sour cherry yellows

    Young leaves develop chlorotic yellow rings or mottle; shot hole may occur in severe cases or as lesions age. These symptoms rarely recur after the first year of infection. In subsequent years, leaves develop a distinct irregular green to yellow mottling and interveinal chlorosis, then drop 3 to 4 weeks after petal fall (A). Successive waves of mottling and dropping occur in response to day/night temperature fluctuations. Older trees show a willowy type of growth or bare wood from a reduction of fruiting spurs (B). Fruit are sparse but large. Similar symptoms occur on sweet cherry. Infected plum develops narrow, strap-like leaves that are thicker than normal.

  • Southern blight

    Trees attacked by the fungus show a general decline (A). In the early phase of disease, a dense mat or web of white mycelium is evident at the base of the tree (B). The mycelium eventually disappears and leaves behind masses of hardened, fungal bodies called sclerotia (C). The sclerotia are globular, vary in size from 0.5–2.0 mm in diameter, and are at first white in color and turn tan to reddish or dark brown as they age.

  • Verticillium wilt

    Leaves wilted or browned on one or several branches, often remaining attached (flagging); the rest of the tree appears healthy (A). Young trees are often killed by infection. Older trees, except for sweet cherry, can recover from infection, often leafing out in the following year, only to be affected again. Affected trees often have dark streaks in their sapwood of 2–3-year or older wood (B, C). Symptoms become more severe with water stress in midsummer.

  • White rot

    Fruit lesions become visible 4–6 weeks before harvest, and appear as small, circular, slightly sunken tan to brown spots, sometimes surrounded by a red halo on yellow-skinned fruit. On red-pigmented fruit, the halo appears dark purple to black. Expanding lesions develop in a cylindrical fashion to the fruit core, unlike bitter rot lesions, which tend to be V-shaped. Rotted fruit appear tan to light brown (A) and scattered clumps of black fruiting structures develop on the surface of fruit in the late stages of white rot. The fruit may become soft and watery in warm weather. Fruit rot developing under cooler conditions is firmer and has a deeper tan color, similar to black rot caused by B. obtuse. White rot cankers begin as small, sunken, reddish brown lesions in spring and enlarge throughout the season, often girdling the infected limb. Pimple-like fruiting bodies develop on canker surfaces 4–8 weeks after infection (B). Shoot dieback may appear above the canker, particularly when the branch is girdled; the appearance of yellow foliage in late May to early June on infected limbs is one of the more striking symptoms of the disease. In the northeast, white rot infection may remain superficial, surviving in the outer bark without damaging the phloem or cambium until drought stress allows the fungus to penetrate and kill these tissues.

  • X-Disease

    This disease is caused by a mycoplasma and infects many varieties of stone fruits. On cherry, infected trees tend to develop a dieback and a generally unthrifty appearance (A). Infected trees decline, but the rate of decline is dependent on the rootstock. Cherry trees on resistant rootstock, such as Mahaleb, decline rapidly because of a hypersensitive response at the graft union. On susceptible rootstock, such as Mazzard or Colt, decline occurs over several years. Cherry fruit on these trees tend to be small, flattened, pointed, and pale-colored and are confined to a few branches, but mixed with some normal fruits (B). On peach, leaves curl inward after several months. Water-soaked spots turn red, become necrotic and drop out, giving leaves a tattered appearance (C). Localized areas or the entire canopy defoliates, leaving foliage only at the tips. The entire tree may show symptoms 2 to 3 years after the initial infection (D).

The MSU IPM Program maintains this site as an access point to pest management information at MSU. The IPM Program is administered within the Department of Entomology, fueled by research from the AgBioResearch, delivered to citizens through MSU Extension, and proud to be a part of Project GREEEN.