Tree Fruit: Integrated Pest Management (IPM)

Key Diseases Affecting Apples

Apple Scab

• Disease causing organism: Venturia inaequalis (Cooke) Wint.
• Dissemination: Ascospores are disseminated by wind during rainy periods in the spring, and conidia are disseminated by wind and rain once infection is established on the tree.
• Symptoms: Infections are most obvious on the leaves and fruit. When the infection first occurs on leaves, it is visible as a lighter shade of green when compared to the rest of the leaf surface. The lesion soon becomes covered with fungal growth that radiates from the site of infection. The lesion increases in size and becomes olive-colored as infection progresses, taking on a velvet appearance. The inner portion of the lesion may become gray - brown as the fungus and the leaf tissue in that area die. Under severe conditions, the whole leaf can take on a velvet appearance, a phenomenon known as sheet scab. This condition can lead to premature defoliation. Lesions which develop on the fruit are very similar to those that develop on the leaves, but tend to be smaller, develop more slowly, and become corky with age. Foliar lesions which are formed in the autumn (late season scab) may appear on either surface of the leaf as a small, round, tan or black spot that closely resembles other fungal lesions which can effect apple leaves. Early infection of the fruit often results in large lesions which deform the fruit, and may cause it to drop off. Later in the season, symptoms of fruit infection may not appear until after the fruit has been in storage.
• Disease cycle: The fungus primarily overwinters on infected leaves on the orchard floor. Ascospores are released in the spring at about budbreak, and disseminated by wind during rainy periods. Moisture is required in order for the spores to germinate. The time it takes for infection to occur is a function of the number of hours of leaf wetness and the temperature. Several secondary cycles of infection, arising from spores produced in primary lesions, may occur during the growing season.
• Management:
• Key times for management: During the primary stage of disease development, when ascospores are released from leaves on the orchard floor, begin the degree day model in order to determine ascospore maturity. Use the Revised Mills table in order to determine when environmental conditions are such that infection is likely to occur. The ascospore maturity model and the Revised Mills table are available in the New England Apple Pest Management Guide. Orchards should be scouted on a weekly basis during the primary stage. Approximately seventeen days after predicted depletion of ascospores a final monitoring should be conducted for primary scab lesions. Management of scab for the remainder of the season should be based on the incidence of primary scab lesions. Late season determination of inoculum levels in the orchard may be done after harvest. See New England Apple Pest Management Guide for details.
• Management Options: Scab is controlled primarily with fungicides. Prediction methods, such as the Revised Mills table based on temperature and leaf wetness, may be used in order to predict when infection is likely to occur. This aids in determining whether the trees are protected by sprays that have been applied. Methods of leaf sampling have been developed in order to determine the level of scab present in the orchard and the level of inoculum which will be present in the orchard in the spring. Systems have also been developed for determining when fungicide application must begin in orchards with low levels of inoculum. See New England Apple Pest Management Guide for details.
• Cultural: When establishing a new orchard, plant cultivars which are resistant to the disease. Applications of 5% urea to leaf litter may be applied in the fall in order to hasten leaf decomposition and therefore reduce primary inoculum. Flail mowing of the leaf litter on the orchard floor also hastens leaf decomposition. Prune trees to open the canopy to light, air, and spray penetration.
• Chemical: see New England Apple Pest Management Guide

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Fire Blight

• Disease causing organism: Erwinia amylovora (Burrill) Winslow
• Dissemination: Disseminated by bees and other pollinating insects and by rain
• Symptoms: Affected parts appear to be scorched by fire. A watery ooze may be exuded from infected plant parts. The disease may kill entire trees. Fruits which are infected early remain small and appear shriveled, dark, and ‘water soaked’. They will remain attached to the cluster. Fruits affected later are less shriveled and discolored. Infection of the vegetative shoots often results in the bending of the shoot tip to resemble a shepherd’s crook.
• Disease cycle: The bacteria overwinters in bark tissues along the edges of cankers caused by infection in previous years. The bacteria multiplies in the spring, the cankers exude a characteristic ooze, and the bacteria are disseminated by rain and insects to vulnerable tissues - especially open blossoms, tender vegetative shoot tips, and young leaves. The bacteria penetrates the tree at natural openings or wounds. Secondary infection arises from ooze from fresh infections.
• Monitoring and Management:
• Key times for management: Bloom. The need for chemical control depends on a combination of orchard risk factors and weather conditions. Infection is most likely to occur after 200 degree hours (base 65° F) have accumulated since the first blossom has opened in the orchard. Predictive models of likely infection periods (such as the Washington fire blight risk model and the Maryblyt model) can help growers to determine when such sprays may be needed. See the New England Apple Pest Management Guide for details. Inspect orchards weekly in the summer for fresh infection. Remove diseased parts during the growing season.
• Management Options: Control of fire blight is aimed at reducing the level of inoculum in the orchard, reducing the susceptibility of the trees through horticultural practices, and preventing infection at critical times through the use of bactericides.
• Cultural Practices: Reduce primary inoculum by removing infected plant material when winter pruning. Inspect orchard weekly during the growing season and remove infected plant material. When removing infected plant material, cut infected branches at least 12 - 18 inches below the lowest evidence of disease. See the ‘ugly stub’ method of pruning out diseased tissue in the New England Apple Pest Management Guide. When removing infected plant material during the growing season, prune only on sunny, hot days when rain is not predicted. Control insect vectors in the orchard. When planting new orchards, avoid susceptible cultivars. Plant on well-drained soil. Maintain proper orchard nutrition in order to discourage excessive tree vigor.
• Chemical: see New England Apple Pest Management Guide

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Powdery Mildew

• Disease causing organism: Podosphaera leucotricha (Ell. and Ev.) Salmon
• Dissemination: In the spring, spores are produced by the powdery mildew fungus which has overwintered in the buds, and are carried by winds to vulnerable (young) tissues.
• Symptoms: Evidence of powdery mildew infection may appear on the leaves, buds, shoots, blossoms, and fruit. Symptoms of infections on the leaves most often are seen as whitish, felt-like patches of fungal mycelium and spores. These symptoms most commonly appear first on the lower surface of the leaves. Lesions may also appear on the upper surface as chlorotic spots, or cover the entire leaf with powdery, white spores and mycelium. Curling and crinkling of the leaves can occur as a result of infections along the leaf margin. Leaves effected severely by the disease may fall off prematurely. Branch terminals which are severely infected may be stunted. Buds which are infected with powdery mildew can become more susceptible to winter injury. Fruit infections on certain cultivars result in a netlike russeting.
• Disease Cycle: The fungus overwinters in the dormant buds that had been infected in the preceding growing season. Buds which have been infected are more susceptible to winter damage. Below temperatures of -12° F the survival rate of the overwintering buds is generally less than 5%. The overwintering fungus within the bud produces spores that initiate the primary infections of the disease cycle in the spring. Unlike the apple scab fungus, the spores of powdery mildew do not require a film of water in order to germinate. The resulting infections of the young leaves and blossoms then provide inoculum for secondary cycles of the disease.
• Management:
• Key times for management: From tight cluster until terminal growth stops, particularly the period after petal fall when vegetative growth is rapid. Begin to look for signs of infection when leaves emerge from the bud.
• Management Options: Depending on the susceptibility of the cultivar and the impact of cold weather temperatures on overwintering inoculum survival, chemical control may be needed.
• Cultural Controls: Plant cultivars which are less susceptible to the disease. Reduce humidity in the tree canopy by pruning to increase air, light, and spray penetration.
• Chemical: see New England Apple Pest Management Guide

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Cedar Apple Rust

• Disease causing agent: Gymnosporangium juniperi-virginianae Schwein.
• Dissemination: Spores produced on eastern red cedar are discharged following rain, and disseminated by wind currents to apple hosts. Spores produced on apple may also be carried by wind to cedars.
• Symptoms: Infections first appear on apple as bright yellow-orange lesions on the upper surface of leaves, petioles, and young fruit. Lesions may be bordered by a red band or a chlorotic halo. Lesions which occur on fruit are superficial and extend not more than 1/16 inch into the flesh. They occur most often on the calyx end of the fruit. On cedar the fungus produces brown to reddish brown leaf galls.
• Disease Cycle: Unlike some other diseases, cedar apple rust requires two hosts in order to complete its lifecycle. On eastern red cedar, the fungus causes galls to form. Spring rains cause the red cedar galls to exude horn-like structures called telia, which become swollen and jelly-like, and which dry back to dark brown threads when the rains cease. Swelling and drying of the telia occurs repeatedly throughout the spring. The telia produce teliopores, which germinate during rain to produce basidiospores. The basidiospores may be carried by the wind for more than 1 mile to vulnerable apple tissue. The spores which land on young apple tissue then germinate if there is a film of water upon the leaves for a sufficient period of time. One to two weeks after infection the lesions on the upper sides of the leaves (or fruit) produce watery orange drops, then produce small orange-brown dots (pycnia) containing pycniospores. Two weeks after the formation of the pycniospores, aecia bearing aeciospores appear on the lower surface of the leaves (or on fruit). These are released during dry weather during July and August. These are then disseminated by the wind and those that land on eastern red cedar infect them, and form mature galls after two years of infection.
• Management:
• Key times for management: Spores are released from cedar during rainy periods from the last week in April until mid-June, with the peak release from pink until full bloom. Lesions are first visible on the upper sides of the leaves in spring to early summer.
• Management Options: The grower should be most concerned about cedar apple rust if the cultivars grown in the orchard are susceptible to the disease, if there are eastern red cedars in the vicinity, and if there are numerous rainy periods during the spring.
• Cultural Management Options: Sources of infection may be reduced by cutting down nearby eastern red cedar, but it is difficult to entirely eliminate sources of infection due to the distance spores are able to be carried by the wind. Some apple cultivars are resistant to cedar apple rust, and may be grown without fungicide sprays to control it.
• Chemical: Certain scab fungicides may also control cedar apple rust. See New England Apple Pest Management Guide

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Black Rot

• Disease causing organism: Botryospaeria obtusa (Schwein)Shoemaker
• Dissemination: Spores are disseminated by wind and rain
• Symptoms: Signs of leaf infection usually appear 1-3 weeks after petal fall and become visible as small purple flecks that enlarge to form circular lesions. The margins of the lesions remain purple while the centers become tan to brown (this disease is often called "frogeye leafspot" due to these symptoms). Symptoms on the fruit first appear as red flecks that will turn purple and be bordered by a red ring. As the lesions enlarge, they form a series of rings alternating from black to brown. The lesions of the fruit will remain firm. Also, symptoms include cankers on limbs that begin as reddish brown areas which are slightly sunken. Cankers on limbs may remain small or enlarge to become several feet long.
• Disease cycle: The fungus overwinters in cankers, dead bark, twigs, and in mummified apples on the tree. Spores are released during rainy periods. Infection occurs through stomata, cracks, and wounds.
• Management:
• Key times for management: A critical time for management of leaf infections is between tight cluster and pink. Fruit infection can begin as sepal infection early in the season and new infection can occur throughout the growing season.
• Management Options: A major decrease in incidence of this disease can be attained though cultural means. Control is usually attained through a combination of cultural and chemical methods.
• Cultural Controls: Piles of prunings should be removed from the orchard and burned, or they may be left on the orchard floor and chopped with a flail mower. Remove mummified apples, dead wood, and shoots infected with blackrot in order to remove sources of inoculum.
• Chemical: Some fungicides used for scab control are also effective against black rot. See New England Apple Pest Management Guide

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Sooty blotch

• Disease causing organisms:a complex of fungi which includes Leptodontium elatius (G. Mangenot) De Hoog, Peltaster fructicola Johnson, and Geastrumia polystigmatus Batista
• Symptoms: Colonies of the fungi on the fruit resemble sooty or cloudy olive green blotches on the surface of the fruit. The fungus grows superficially on the surface of the fruit and may be easily wiped away.
• Dissemination: Sooty blotch spores are released during rainy periods and are disseminated by wind.
• Disease Cycle: The fungi overwinter on infested apple twigs and on numerous forest trees. During rains in the spring and early summer the spores are dispersed, with secondary spread occurring throughout the season. Fruit infection can occur as early as 2-3 weeks after petal fall. Infections are most likely to occur during periods of frequent rain and high humidity. Symptoms of infection may not be apparent until some time (typically 20 to 25 days) after the initial establishment of infection.
• Management:
• Key times for management: The key time for management begins 1-2 weeks after petal fall and continues until 2-3 weeks before harvest. Temperatures between 60 and 70° F, with a relative humidity greater than 96% are optimal conditions for infection.
• Management Options: Sooty blotch is often controlled by fungicides applied for apple scab. Cultural practices aid in the control of the disease.
• Cultural practices: Remove reservoir hosts from the orchard and from the perimeter of the orchard. Sooty blotch thrives in the humid conditions present in poorly pruned trees and in damp, slow-drying areas of the orchard. Winter and summer prune in order to increase air, light, and spray penetration. Thinning fruit to separate fruit clusters facilitates the drying of the fruit after rainy periods.
• Chemical: see New England Apple Pest Management Guide

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Flyspeck

• Disease causing organism: Zygophiala jamaicensis E. Mason
• Dissemination: Spores which produce primary infection are disseminated by wind during rainy periods from overwintering sites. Spores which produce secondary infections may be wind or water borne.
• Symptoms: Flyspeck appears on the fruit in well-defined groups of shiny, black dots which grow on the surface of the fruit. Areas of these dots are usually less than an inch in diameter. Fungi grow superficially on the fruit and may be wiped from the surface easily.
• Disease cycle: The fungus overwinters on infested apple twigs and on reservoir hosts such as brambles. Spores are then released during rainy periods just before bloom to 1-2 months after bloom, and are carried by wind to the host in order to create the primary infection. The spores then germinate and create superficial fungal growth. Colonies become visible and spores are produced in approximately three weeks. Secondary infection develops on the twigs and on the apple fruit. The spores which create secondary infection are also airborne and are released after sunrise.
• Management:
• Key times for management: A key time for apple infection occurs after 270 hours of leaf wetness have accumulated from 10 days past petal fall. Periods of frequent rainfall during above normal summer temperatures are particularly favorable to an outbreak of this disease.
• Management Options: A major decrease in incidence of this disease can be attained through cultural practices.
• Cultural Management Options: Pruning for air and light penetration and thinning of fruit clusters can reduce incidence of this disease. Mowing nearby ditches and banks to decrease the amount of nearby reservoir hosts may also be helpful.
• Chemical: see New England Apple Pest Management Guide

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Phytopthora Crown Rot, Collar Rot, and Root Rot

(Collar rot affects the scion portion of the tree, crown rot affects the rootstock portion of the tree. Root rot affects the root system away from the crown region.)

• Disease causing organisms: fungi in the genus Phytopthora
• Dissemination: Soilborn fungal pathogen. Pathogen survives in soil for several years as spores, especially in old orchard soils. May also be brought into the orchard on infected nursery stock.
• Symptoms: Cankers may be seen at or below the ground line, and may extend from the original site of infection into the root system and up the trunk to the bud union, and above the bud union if the scion is also susceptible. Infected bark is brown and often slimy when wet. When the bark is pulled away, the cambium and phloem will be an orange, reddish brown color. The cankers caused by the fungus girdle the tree, resulting in poor vegetative growth and chlorotic foliage that may turn purple in the autumn. A severely infected tree may die. Trees may be killed in one growing season or may linger for a number of growing seasons.
• Disease cycle: The fungus survives in the soil as thick-walled spores (oospores) that are resistant to drought and relatively resistant to chemical treatment. The fungus may also be brought in on infected nursery stock and in contaminated irrigation water. Mobile spores (zoospores) originating from the oospores move to the tree and colonize the apple bark tissue. The fungus may build up to high levels in the soil in a short period under favorable conditions - i.e. during wet, cool periods after harvest and in spring.
• Monitoring and Management:
• Key times for management: The best time to manage Phytopthora diseases is during the initial stages of orchard establishment -when selecting the orchard site, planting, site preparation and rootstock selection.
• Management Options: While there are some fungicides registered for control of Phytopthora, management is best achieved through cultural methods.
• Cultural Controls: Primary control of Phytopthora diseases is cultural. When planting a new orchard, select the site and rootstocks carefully. Be sure the orchard site has adequate drainage throughout the year. When irrigating the orchard, do not saturate the soil for prolonged periods. Use rootstocks resistant to infection by the disease. Scion cultivars may also be chosen for resistance.
• Chemical: see New England Apple Pest Management Guide

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