Bacterial canker (B: Pseudomonas syringae pv. syringae) affects all stone fruit but can be especially severe on sweet cherries and apricots where it causes cankers and kills spurs. The bacteria overwinter in cankers, buds, and sometimes in symptomless host tissue. Severe infections of cherry leaves and fruit often occur in association with light frosts. Symptoms consist of irregular spots on leaves and sunken lesions on fruit. Cankers on twigs and branches are often initiated in autumn. Copper sprays applied at leaf fall have been used to control the canker phase in sweet cherries, but some strains of the pathogen are resistant to copper.
Bacterial spot (B: Xanthomonas arboricola pv. pruni, formerly Xanthomonas campestris pv.pruni) causes spots on leaves, fruit, and twigs of peaches, nectarines, Japanese plums, and apricots. In the United States, it occurs east of the Rocky Mountains and is especially severe where trees are grown on light, sandy soils in humid environments. The bacteria invade leaf scars in autumn and overwinter in buds, small cankers, or twig surfaces. Bacteria spread to new leaves and fruit during rains beginning at late bloom. Spots on leaves originate as angular purple lesions 1 to 3 millimeters in diameter and are usually concentrated along the leaf midrib or toward the tips of leaves. Spots on fruit may become visible about three to five weeks after petal fall. The disease is best controlled by selecting resistant cultivars. Susceptible cultivars must be protected with copper sprays in fall and/or spring to reduce inoculum levels and terramycin or copper at petal fall and early cover sprays to protect fruit and leaves.
Black knot (F: Apiosporina morbosa) appears as black swellings or knots on twigs and branches of plums and cherries in eastern United States. Ascospores are released from the knots during spring rains beginning when trees are at white bud and continuing for several weeks after petal fall. The ascospores infect nodes on new shoots. New knots usually become apparent about one year after infection and produce ascospores the second year. Severely affected trees become unproductive. Black knot is controlled by removing in fected wild Prunus from hedgerows prior to planting, by pruning out infected knots as they appear, and by protecting trees with fungicides during the period of ascospore release.
Brown rot (F: Monilinia fructicola, M. laxa, M. fructigena) is the most widespread and economically important fungal disease of stone fruit. It causes a blossom blight, twig blight, canker, ripe fruit rot, and postharvest fruit rot. Monilinia fructicola is the primary pathogen in eastern North America, whereas M. laxa predominates in Europe. Both species are found in California and South America. The brown rot fungi overwinter in cankers, mummified fruit in trees, and fallen fruit on the orchard floor. The latter may produce apothecia and asco-spores in spring, but ascospores are usually less important in the disease cycle than conidia that are produced on cankers or mummies in trees. Blossoms and fruit become infected during warm rains. Green fruit are more resistant to infection than blossoms or ripening fruit. However, green fruit may develop quiescent infections that become active as the fruit ripen in the field or after harvest. Brown rot is controlled by pruning out cankers and fruit mummies during winter and by using fungicides to protect blossoms, ripening fruit, and fruit after harvest.
Cherry leaf spot (F: Blumeriella jaapii) affects both sweet and sour cherries in eastern North America and Europe. Ascospores are produced in apothecia on overwintering leaf litter and are released during rains starting during late bloom and continuing for about six weeks thereafter. Infections appear as small red to purple spots on the upper leaf surface that gradually enlarge to 3 millimeters and turn brown. On the underside of leaves, lesions appear pink or cream-colored during wet weather due to production of conidia. Secondary infections continue to occur through summer and can cause early defoliation, thereby leaving affected trees susceptible to winter damage. The disease is controlled by applying fungicides to prevent primary infection.
Leucostoma canker or Cytospora canker (F: Leucostoma per-soonii) affects peaches, nectarines, and sweet cherries and is especially severe in regions where trees may be damaged by cold winter temperatures. Cankers on older wood are usually elliptical, blackened, and gummy. Conidia are produced in cankers throughout the year. Infections occur only through wounds, dead tissue, or injuries (including sunburn and cold injury). The disease can be avoided by keeping new plantings away from old diseased plantings and by using good horticultural practices to minimize tree stress and injuries. Fungicides are not effective.
Peach scab (F: Cladosporium carpophilum) is important in warm humid production regions. It affects primarily peaches and nectarines, but can also occur on plums and apricots. The disease overwinters in infected buds and twigs. Conidia are released during periods of high humidity beginning about two weeks after shuck split and can infect fruit, leaves, and green twigs. Fruit infections first appear as small pinpoint, olive-green spots that gradually enlarge to 2 to 3 millimeters in diameter. The disease is controlled by applying fungicides for several weeks after shuck split.
Peach leaf curl (F: Taphrina deformans) causes red blotching, thickening, and puckering of peach and nectarine leaves. It occasionally affects fruit. The fungus overwinters as a yeastlike saprophyte on twigs and in buds, then invades developing leaf tissue during cool, wet weather in early spring. It can be controlled with copper sprays or other fungicides applied at leaf fall in autumn or before budbreak in spring.
Phytophthora crown and root rot (F: Phytophthora species): See comments under Common Pome Fruit Diseases.
Powdery mildew (F: Sphaerotheca pannosa, Podosphaera clandestina, P. tridactyla) can affect all stone fruit and is most severe where stone fruit are grown in arid climates. Infected leaves and shoots develop a powdery white coating of mycelia. Sphaerotheca pannosa overwinters in buds on peaches and roses, with the latter sometimes acting as an inoculum source for orchard infections. Podo-sphaera clandestina overwinters as cleistothecia trapped in the bark of cherry trees or on the orchard floor. Ascospores are released from cleistothecia during spring rains, but conidia from primary infections can spread and infect during periods of high relative humidity in the absence of rain. The greatest losses occur on sweet cherries, when young fruit become infected and deformed. The disease is controlled with fungicides applied to prevent early season infections.
Virus or viruslike diseases cause extensive losses in stone fruit. The most common viruses are prunus necrotic ringspot virus (PNRSV) and prune dwarf virus (PDV). The numerous strains of PNRSV cause a variety of symptoms, the most common being a necrotic leaf spot that appears the first year a tree is infected but rarely thereafter. In fected trees may show no obvious symptoms, but productivity is often reduced. Both PNRSV and PDV can be transmitted by seed and by pollen. Pollen transmission allows the disease to spread rapidly from tree to tree in the field. PDV causes sour cherry yellows, a disease characterized by leaf yellowing and abscission in midsummer and presence of long, barren shoots caused by a lack of fruiting spurs. PNRSV and PDV are controlled by using virus-certified planting stock and by keeping new plantings away from old infected orchards.
Tomato ringspot virus (TmRSV) causes constriction disease of plums and prunus stem pitting in peaches, nectarines, and cherries. The disease is most common in temperate growing regions of the eastern United States. The virus is seed transmitted in dandelion and presumably in other weed hosts. It is also transmitted by dagger nem-atodes (Xiphinema species). European plums propagated on Myroba-lan rootstocks develop a constriction below the graft union and a brown line or pitting in wood at the graft union. Affected trees decline rapidly four to seven years after trees are planted. On peaches, nectarines, and cherries, TmRSV causes deep pitting in the woody cylinder of the rootstock, and affected trees decline. Control measures include preplant soil treatments to reduce or eliminate populations of vector nematodes and regular use of broadleaf herbicides to keep alternate hosts from becoming established in the orchard.
Plum pox, or sharka, is caused by plum pox virus (PPV) and is common throughout Europe. PPV is vectored by aphids. Depending on the species and cultivar of stone fruit, PPV may cause deformed fruit, loss of productivity, tree decline, or no visible symptoms at all. PPV was recently introduced in Pennsylvania and Canada but is being controlled by eradication of infected trees.
X-disease is caused by a phytoplasma and affects peaches, nectarines, and sweet cherries. Phytoplasmas lack cell walls, live in plant phloem, and are vectored by leafhoppers. Leafhoppers acquire the phytoplasma from infected sweet cherry trees or from wild hosts, primarily chokecherry (Prunus virginianae) or naturalized sweet cherry seedlings. Leaves on affected peach and nectarine limbs develop red, water-soaked lesions and abscise prematurely, leaving a tuft of young leaves at the ends of denuded shoots. The disease usually kills peach, nectarine, and cherry trees on P. mahaleb rootstock within two to four years. Cherry trees on mazzard rootstock (P. avium) survive many years and can be detected only by uneven fruit ripening and production of small fruit with delayed maturity. The disease can be controlled only by eliminating infected hosts within 500 feet of new plantings.
A wide array of fungal, bacterial, and viral pathogens can attack fruit, leaves, wood, and roots of temperate zone fruit trees. Viruses cause the largest number of diseases, but most viral diseases are rare in commercial orchards. Fungal diseases are common and would cause total crop loss in many years and locations if they were not controlled with fungicides. Bacterial diseases, although few in number, are difficult to control and cause extensive losses in some years. Researchers continue to devise cost-effective IPM strategies to manage the common diseases of tree fruit.
Related Topics: INSECTS AND MITES; NEMATODES; PLANT-PEST RELATIONSHIPS AND THE ORCHARD ECOSYSTEM; SUSTAINABLE ORCHARDING
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