Chapter 5 -- Apple Insects And Diseases In The Southeast

Diseases of Apples

This chapter describes the symptoms, the cycles, and the control of diseases of apples in the Southeast. It describes fruit and foliar diseases, root and crown rot diseases, and diseases caused by viruses and mycoplasmas. It also covers monitoring and cultural considerations for disease management.

Fruit and Foliar Diseases

Many fruit and foliar diseases of apples in the Southeast can cause 100 percent losses if orchards are not treated. Fruit infections can reduce grade or yield; foliar disease can devitalize trees, reduce yield and quality of fruit, and affect return bloom next season. These diseases are characterized by lesions, which appear on foliage and fruit. Control of furit and foliar disease is accomplished by good orchard management practices and appropriate pesticide applications.

Black Rot And Frogeye Leafspot

Black rot and forgeye leafspot, Botryoshaeria obtusa (Schwein.) Shoemaker, are important diseases in orchards throughout the southeastern United States and can cause extensive fruit loss and defoliation if not controlled. Black rot cankers are important in the northern growing areas of the eastern United States but are not problems in most orchards in the Southeast.

Symptoms And Disease Cycle

The black rot fungus surivives on dead wood and mummied apples in trees and on the ground during the winter. In the Southeast, spores, produced from December to March, are washed into buds by rains. When buds start to swell (the silver tip stage), the spores germinate, infect the flower parts, and grow into the core area of developing fruit. The fungus remains dormant until the fruit starts to ripen; then it grows into the flesh and causes a firm brown rot, often at the calyx end ( Figure 5.1).

After petal fall, additional fruit infections can occur anywhere on the fruit. Infections which occur soon after petal fall appear as raised, black, pimplelike lesions while those later in the season are often black, irregularly shaped, and bordered by a red halo. Infections are common in late June, July, or August.

The optimum temperatures for fruit infection range from 60° to 75°F. Nine hours of wetting are required for infection at these temperatures.

Leaf infection can occur during the bud stage or anytime during the spring or summer when the foliage is wet. Leaf infections are brown, circular, 1/8 to 1/4 inch in diameter, and often bordered by a purple halo ( Figure 5.2). Leaf infections can occur in as few as 4 1/2 hours at 80°F. Sometimes leafspots are invaded by secondary fungi which enlarge lesions, giving them an irregular appearance.

Botryoshaeria obtusa can infect twigs killed by fire blight in the spring. By June, B. obtusa produces spores in fruiting structures on this dead wood. These spores can cause infections on the fruit and foliage throughout the summer.

Control

To effectively control black rot, remove all dead wood from the tree and ground, including current season pruning. This wood needs to be burned, removed to a landfill, or chopped and composted. Chopped wood should not be used as mulch in the orchard until it has been thoroughly composted. In the southernmost apple-growing regions of the Southeast, apply fungicides at silver tip to prevent infections in the bud. In the more northerly areas where bud infections are not as commom, apply fungicides on a 10- to 14-day schedule from petal fall to harvest.

Scab

Scab, Venturia inaequalis (Cke.) Wint., is one of the most important diseases of apples in the eastern United Sates. However, it is generally not as important in apple orchards in the southeastern United States. Cool, wet weather is necessary for apple scab infections. In the Southeast, by the time secondary infections are observed, it is usually too hot and dry for additional scab infection.

Symptoms

Scab is characterized by olive green lesions on leaves (Figure 5.3). Early season infections frequently occur on the lower leaf surface, but lesions can be found on the upper surface as well. Extensive infections can cause early defoliation and may reduce the next year's crop. Small, dark lesions occur on the fruit, often on the sepals or near the calyx end (Figure 5.4). The spots soon become black. Apple scab infections do not rot the fruit but may cause cracking as the fruit enlarge (Figure 5.5).

Disease Cycle

Venturia inaequalis overwinters on fallen leaves on the ground. In the early spring, the fungus produces spores in fuiting structures know as pseudothecia. The spores are discharged during rain and blow to the leaves and the developing fruit. Ascospore discharge usually starts about the time bud swell occurs and continues through petal fall. Leaf infections can occur in as few as 8 hours under favorable temperature and wetting conditions. Recently expanded leaves are more susceptible than older ones. Spores produced on these primary lesions are washed onto fruit and leaves, causing secondary infections, which can continue into the early summer if the weather is cool and wet. Fruit become more resistant to infection as they mature.

Control

The need to control apple scab is usually greater in the Carolinas and Virginia than in other growing areas of the Southeast. Where scab is a problem, fungicides need to be applied from green tip through petal fall or first cover. Additional applications may be needed in cool, wet seasons. Fungicide applications can be made on either a protectant or postinfection schedule.

Protectant sprays should be made on a 5- to 7-day interval beginning at silver tip or early green tip and continuing through petal fall. In most years the peak period of ascospore maturity occurs from tight cluster to bloom. However, during some years, spores mature by silver tip, and if conditions are favorable for infection, sepal infection can occur and result in misshapen fruit.

In the post infection approach, fungicides are applied only after an infection period and rely on the back action of certain fungicides to eradicate infection. Fungicides used in the postinfection approach have 24 to 96 hours of back action, depending on the particular fungicide used. Consult your apple spray guide for lists of fungicides with back action and their period of activity. The period of activity (back action) begins at the start of a wetting period. Whether or not an infection period has occurred can be determined by monitoring the temperature and hours of wetting (Table 5.1). Growers useing eradicant schedules need to consider other early season diseases that may be a problem (for example, black rot and rust diseases).

In addition to chemical control, remove as many leaves from the orchard as possible to reduce the amount of overwintering inoculum and aid in control. If removing leaves is not practical, spray them with urea at 2 pounds per 100 gallons to hasten their decomposition. Spray leaves on the ground as well as those on the tree. Spray leaves on the tree late enough in the fall to prevent the nitrogen from causing the tree to grow actively. Urea applications will also aid in the control of Brooks spot and Alternaria blotch.

More than 30 cultivars resistant to apple scab have been released worldwde; however, most are not adapted to the growing conditions in the southeastern United States.

Table 5.1 "Mills' Chart": Temperature and Moisture Requirement For Apple Scab Infection Periods As Determined By Mills And Modified By A.L. Jones.

 Hours Wetting Required For Infection

From Primary Inoculum (Ascospores)
Avg. Temp. (F) Light Infection Moderate Infection Heavy Infection From Secondary Inoculum (Conidia)
78 13 17 26 8.7
77 11 14 21 7.3
76 9.5 12 19 6.3
63-75 9 12 18 5.9
62 9 12 19 5.9
61 9 13 20 5.9
60 9.5 13 20 6.3
59 10 13 21 6.6
58 10 14 21 6.6
57 10 14 22 6.6
56 11 15 22 7.3
55 11 16 24 7.3
54 11.5 16 24 7.7
53 17 25 7.9
52 12 18 26 7.9
51 13 18 27 8.7
50 14 19 29 9.3
49 14.5 20 30 9.7
48 15 20 30 9.9
47 15 23 35 11.3
46 16 24 37 12.6
45 17 26 40 13.3
44 19 28 43 14.6
43 21 30 47 16.5
42 23 33 50 19.9
41 26 37 53 -
40 29 41 56 -
39 33 45 60 -
38 37 50 64 -
37 41 55 68 -
33-36 48 72 96 -

Cedar Apple Rust And Quince Rust

Cedar apple rust, Gymnosporangium juniperi-virginianae, and quince rust, G. clavipes, occur commonly on apples in the southeastern United States. Both damage apples through fruit infections which reduce grade or yield. Cedar apple rust also damages apples through leaf infections which either defoliate the tree, reducing size and quality of fruit, or devitalize the tree, reducing yield the following year.

Symptoms

In late May or early June, leaves infected with cedar apple rust develop pale yellow spots on the upper surface. These spots enlarge and become orange with age (Figure 5.6). In July and August, numerous tubelike protuberances known as aecia appear on the bottom of the leaves. If infections are numerous, extensive defoliation can occur. Infection often occurs on the calyx end of fruit but may occur on the sides or even the stems. Lesions on fruit are shallow and orange to yellow or dark green with an orange border (Figure 5.7).

Quince rust seldom affects leaves. Infection usually occurs on the calyx end of fruit. Dark green lesions appear, and necrotic tissues often extend to the core (Figure 5.8). Many fruit affected with quince rust drop prior to harvest.

Disease Cycle

Both G. juniperi-virginianae and G. clavipes overwinter on the eastern red cedar. Cedar apple rust infections on cedar foliage result in dimpled galls slightly smaller than a golf ball ( Figure 5.9). Quince rust infections result in spindle-shaped galls on cedar branches or twigs (Figure 5.10). Spores are produced on gelatinous orange-colored protuberances (called "telia" and often referred to as hornes), which extrude from the galls during wet weather in the spring (Figure 5.11). The spores are capable of infecting only apple fruit or foliage. The optimum temperature for infection of fruit is about 65°F with little infection occurring below 55°F. Spores produced in the aecia on the apple fruit or foliage complete the life cycle of these rust fungi by infecting the cedar in the late summer. Cedar apple rust infections produce cedar galls that will be active in the spring approximately 1 1/2 years later. Dry summer weather can reduce the number of infections on cedar and reduce the inoculum level.

Control

If the rust diseases are severe enough to require control, two approaches can be used. One approach is to remove all cedars within 1/2 mile of the orchard. Removal of all cedars will usually provide sufficient control in areas of the mountains where the eastern red cedar is not abundant. The other approach is to apply fungicides from early pre-pink through petal fall for fruit infections and from pre-pink through second cover for leaf infections. Of the cultivars planted in the Southeast, Rome Beauty, Gala, Mutsu (Crispin), and Jonagold are most susceptible to cedar apple rust. Delicious is not susceptible to cedar apple rust but is very susceptible to quince rust.

Fire Blight

Fire blight, Erwinia amylovora, occurs sporadically in most apple and pear orchards in the southeastern United States. Fire blight is a minor problem in some orchards each year, but serious epidemics generally occur every 10 to 15 years, causing extensive damage to scaffold limbs and tree death.

Symptoms

Blossom blight is usually the first symptom observed in the spring. A single blossom or the entire flower cluster may be diseased. Flowers are first water-soaked; then they wilt, shrivel, and turn brown or black (Figure 5.12). The bacterium moves down the peduncle into the fruit spur or stem, killing these tissues. Young fruit become infected, turn black, dry, shrivel, and remain attached to the tree. During warm, humid weather, droplets of fire blight bacteria ooze from the peduncle and fruit.

Symptoms on succulent twigs, waterspouts, and shoots are similar to those on flowers, but the pathogen moves more rapidly. Six to 12 inches of tissue can be killed in only a few days. Tips of twigs frequently droop to form a shepherd's crook ( Figure 5.13). Shoots also may be killed by infections that occur some distance back from the tip. These infections girdle the twig, killing all tissues beyond the point of infection. Foliage remains attached to dead twigs and shoots.

The pathogen advances downward from blighted flowers, twigs, and shoots to the scaffold limbs or the main trunk. As weather becomes unfavorable for disease development, cankers form at the base of the diseased tissue (Figure 5.14). Cankers vary in size, are slightly sunken, and are sometimes surrounded by cracks in the bark. Fire blight bacteria overwinter in these holdover cankers. If conditions favor disease development the next year, bacteria can move downward from the canker and may kill the scaffold limb or the entire tree.

Disease Cycle

The fire blight bacterium passes its entire life cycle in direct association with the host. This includes both primary and secondary disease cycles. Hosts include apple, pear, crab apple, quince, cotoneaster, hawthorn, pyracantha, blackberries, and mountain ash. Apple and pear are the economically important fruit hosts.

In the spring, holdover cankers from previous years exude bacteria, which are dispersed in the tree by splashing rain and insects, subsequently infecting flowers and twigs. Bacteria can be spread from diseased flowers to healthy ones by bees.

Throughout the summer, secondary infections can occur on leaves and twigs as long as there is new growth and the weather is favorable. Bacterial ooze produced on infected shoots and fruit and dispersed by rainfall is an important secondary inoculum source. Bacteria also can be spread by contaminated pruning tools.

There seems to be no connection between the amount of fire blight in an orchard from one year to the next. Factors which influence the amount of fire blight include the following:

Excessive levels of nitrogen, fall cultivation, and heavy winter pruning can lead to excessive succulent growth in spring. Trees that are properly fertilized, cultivated, and pruned will have slow but steady growth and will stop growing in midsummer. Maintaining a good balance between N-P-K and sufficient calcium will prevent excessive vegetative growth. Planting less vigorous rootstocks will also reduce excessive vegetative growth. Controlling vegetative growth is especially important on susceptible cultivars such as Jonathan, Granny Smith, and Gala.

Temperature and weather conditions can also influence the amount of fire blight in an orchard. In order for infections to occur, four criteria must be met:

  1. Open blossoms with intact petals must be present.

  2. Dew or rainfall must be greater than 0.01 inch. During bloom periods with dry weather and low relative humidity, very few infections occur.

  3. An average daily temperature of 60°F or greater must have occurred.

  4. At least 198 degree-hours greater than 65°F must have accumulated since the first blossoms opened. To calculate degree-hours, assume 6 hours at the maximum. Thus, for a day with a minimum of 60°F, a maximum of 80°F, and an average of 70°F, the degree-hours = 60 x 0 (degree-hours at 60°F, which is less than 65°F) + 12 x 5 (degree-hours at 70°F, which is 5° more than 65°F) + 6 x 15 (degree-hours at 80°F, which is 15° more than 65°F) = 0 + 60 + 90 = 150 degree-hours for that day.

Injuries caused by hail can be entry points for the fire blight bacterium. If hail injury is followed by weather suitable for fire blight, the risk of fire blight infections is great.

Control

The first step in fire blight control is orchard management to prevent rank, vigorous growth. This includes proper orchard fertilization and pruning. It also means avoiding highly susceptible cultivars on susceptible rootstocks such as M.26.

All fire blight affected tissues, including cankers, need to be cut out, removed from the orchard, and burned. Cuts should be made 8 to 12 inches below the diseased tissue. Pruning tools need to be dipped in 10% household bleach or Lysol between cuts to prevent spreading the bacterium. Bleach will cause tools to rust unless they are washed and oiled at the end of each day.

If additional control is needed, dormant copper sprays and sprays of streptomycin during bloom should be applied. The first streptomycin spray can be timed using the four criteria described above. Streptomycin sprays are effective for 3 days, and additional applications may be necessary if conditions remain favorable for fire blight. Sprays containing copper should be avoided after the silver tip stage to reduce the likelihood of fruit russet.

Sooty Blotch Flyspeck

Sooty blotch - Gloeodes sp. Colby, Leptodontidium sp., and other fungi - and flyspeck, Schizothyrium pomi (Mont. & Fr.) Arx, are the most common diseases of apple in the southeastern U.S. Without fungicidal control they will occur in 100% of the fruit and probably cause more losses to growers than any other disease. The fungi causing these diseases affect all cultivars of apple but grow only on the surface of the fruit, causing cosmetic damage. Both diseases often occur on the same fruit.

Symptoms

Sooty blotch colonies are olive green to black on mature fruit. They may be round or have feathery, diffuse margins, and infection may cover the entire fruit (Figure 5.15).

Symptoms of flyspeck are appropriately described by the name of the disease. Colonies have up to 50 small, shiny black fungal fruiting structures grouped in an irregular to circular pattern (Figure 5.16).

Disease Cycle

Sooty blotch is a disease complex caused by Gloeodes sp., Leptodontidium sp., and other fungi that grow on the waxy cuticle of the apple fruit. Gloeodes and the other fungi that cause sooty blotch survive from one season to the next on apple twigs as well as other perennial vegetation with a waxy cuticle. The fungus is dispersed by wind and in windblown rainwater to developing fruit in the spring and early summer. Secondary spread from these primary colonies occurs throughout the summer. Growth on fruit can begin 2 to 3 weeks after petal fall. The optimum temperature for Gloeodes is between 65° and 80° F. Growth does not occur at temperatures above 86°F. Gloeodes also will grow at relative humidities above 90% in the absence of free water. It usually takes 20 to 25 days for the colonies to appear on fruit after infection occurs. Little information is available on the other fungi in the sooty blotch complex.

Schizothyrium pomi, the fungus which causes flyspeck, also overwinters on apple twigs and other perennial hosts. The initial infection occurs from airborne ascospores which are produced for about 2 months beginning around bloom. Symptoms appear in the orchard about 3 to 6 weeks after infection. Secondary spread is by airborne conidia produced on infected fruit and twigs as well as numerous reservoir hosts which surround the orchard. The optimum temperature for fruit infection is about 65°F. Infection can occur at temperatures up to 82° F.

Control

Control of sooty blotch and flyspeck is achieved through dormant and summer pruning and tree training, which open trees and facilitate drying and fungicide deposition, and through preventive fungicidal sprays. Fungicides need to be applied on a preventive basis beginning about second cover and continuing at 10- to 14-day intervals until harvest. A minimum of 100 gallons of water per acre as a fungicide carrier is needed to assure good fungicide distribution throughout the canopy.

Bitter Rot

Bitter rot, Colletotrichum spp., is the most important summer rot disease of apples in the Southeast. In some seasons it has caused losses approaching 100% in just a few days. All cultivars of apple grown in the Southeast are susceptible to the disease. The fungus that causes the disease can also infect leaves and cause cankers on the tree; however, these phases of bitter rot are not as important as fruit infections.

Symptoms

Fruit infections can occur soon after bloom and appear as small gray to brown flecks, which may not enlarge until later in the summer (Figure 5.17 ). The most damaging fruit infections occur more than a month after petal fall. Small, sunken, brown lesions form, sometimes surrounded by a red halo. The halo is especially visible on green or yellow fruit. When lesions are about 1 inch in diameter, the fungus forms small, black fruiting bodies, about the size of a pinhead, around the interior of the diseased area (Figure 5.18 ). In wet weather, masses of cream to salmon pink spores are produced on the surface of the lesions.

As the lesions enlarge, the rot progresses to the core of the fruit in a V-shaped pattern (Figure 5.19). This pattern differs from bot rot which forms a cylindrical rot pattern extending to the core. Leaf spots, which are uncommon, start as small red flecks which enlarge to irregular brown spots about 1/16 inch in size. Infected leaves usually fall.

Disease Cycle

The bitter rot fungus survives the winter in dead wood on the ground and in the tree and in mummified fruit which hang on the tree. Spores are produced on overwintering sites during rainy periods in the spring and summer. The sexual spores are airborne while the more important asexual spores are waterborne. The optimum temperature for spore germination is about 80 °F. Infection can occur in 5 hours at this temperature. The amount of infection increases with increasing length of the wetting period up to 60 hours.

Fruit infection can occur from bloom through harvest; however, most infection occurs from midseason until harvest. The disease is frequently more severe on early maturing cultivars. Epidemics occur during prolonged periods of wet, warm weather. The most severe epidemics occur in seasons with warm, wet periods early in the season, followed by similar periods in mid to late season.

Control

Bitter rot control, like black rot control, is dependent on a good sanitation program. Dead wood, including the current year's fire blight strikes, needs to be removed from the tree and the ground and either burned or buried. Chopping with a flail mower removes the bark from the wood. All mummies hanging on the tree must be removed.

If practical, remove diseased fruit from the tree to reduce the rate of spread of the disease. Fungicides applied from first cover until harvest on a 2-week schedule are effective if a good sanitation program is followed.

Bot Rot (White Rot)

Bot rot, Botryosphaeria dothidea (Moug.:Fr.) Ces. & DeNot, is also known as white rot or botryosphaeria rot. Bot rot has caused losses up to 100% of the apple crop in some orchards in the Southeast. The fungus also causes a severe canker disease on apple trees. The canker phase is most severe in the piedmont areas of Georgia and the Carolinas but can occur throughout the Southeast during hot, dry summers.

Symptoms

Lesions on fruit begin as small, circular brown or tan spots, sometimes with a red halo on green or yellow fruit (Figure 5.20). The halo may be purple to black on red fruit. As the lesions enlarge, the rot progresses to the core in the shape of a cylinder ( Figure 5.21). This symptom can be used to distinguish it from bitter rot which forms a V-shaped rot extending to the core. Under warm conditions, the rot progresses rapidly. Rotton areas are watery and soft and a tan to brown color; fruit completely rot in a few days. Under cooler conditions, the rot is firmer and a darker brown color. It is difficult to separate bot rot and black rot based on fruit symptoms.

While limb and twig cankers can start in lenticels, most cankers start at pruning wounds, particularly stub cuts. Cankers can also start in frost cracks and areas scalded by the sun. Infected bark becomes depressed, and blisters form (Figure 5.22). Cankers develop very rapidly on drought-stressed trees. The fungus moves up and down limbs and trunks more rapidly than around them, leading to elongated cankers. The bark on the cankers peel in very thin layers similar to onion skin. Cankers can girdle limbs and trunks. The canker phase of the disease is most severe on Golden Delicious and Rome Beauty but can affect all cultivars.

Disease Cycle

Botryosphaeria dothidea overwinters on dead wood in the tree and on the ground. Spores are produced and released during the summer under rainy conditions. This fungus, like the black rot fungus, can invade fire-blighted tissues and produce spores by the end of June.

Spores are dispersed primarily by rain and can germinate in 90 minutes at 80°F. Fruit can become infected in as few as 2 hours at this temperature if moisture is present. All cultivars are susceptible; Golden Delicious appears to be most susceptible.

In Georgia, little or no infection occurs until the soluble solids content of the fruit reaches 10.5%, usually about 6 weeks before harvest. In contrast, fruit infections in North Carolina can occur as early as late May but remain latent until the soluble solids reach 10%.

Control

Control of bot rot starts with eliminating dead wood on which the fungus overwinters and produces inoculum. All dead wood, including the current year's fire blight strikes, needs to be pruned out of the tree. Prunings should be removed from the orchard and burned, buried, or chopped with a flail mower.

Chemical control of the fruit rot phase can be accomplished with sprays if the dead wood has been eliminated. In Georgia, sprays should be initiated 6 weeks before harvest or when the soluble solids reach 10.5%. Applications should be made every 2 weeks thereafter. In the Carolinas where the disease has been a problem, preventive fungicidal sprays should begin at second cover during wet years and continue on a 10- to 14-day schedule.

Control of the canker phase starts with a good sanitation program. Equally important are good pruning practices. Avoid stub cuts; proper pruning saves the branch collar which is essential for rapid wound closure. Remove cankers on limbs by making cuts 12 inches below the canker. During hot, dry weather, trees should be irrigated to lessen stress and reduce the likelihood of infections.

Brooks Spot

Brooks spot, Mycosphaerella pomi (Pass.) Lindau, occurs on susceptible cultivars throughout the Southeast.

Symptoms

On red fruit the spots are a deeper red; on green fruit they are a darker green (Figure 5.23). The spots are small and are frequently found on the blossom end of the fruit. Lesions are shallow and primarily associated with the peel. There is no corking beneath the lesions as found with cork spot or bitter pit (physiological disorders). The fungus infects leaves, but symptoms do not appear until late summer or early fall when small purple spots, often associated with the veins, appear (Figure 5.24).

Disease Cycle

The fungus overwinters in leaves on the ground and produces inoculum in these leaves in the spring. Spores are matured in the 30-day period after petal fall. They are discharged during rain and initiate infection. There are no secondary cycles during the growing season.

Control

Rome, Stayman, and Golden Delicious are the most susceptible cultivars widely grown in the Southeast. Infections occur on Delicious but are not as common. Where the disease is a problem, fungicides should be applied on a 2-week schedule from the period beginning at petal fall and extending through second cover. Because the fungus overwinters in leaves on the orchard floor, urea sprays in the fall will reduce the inoculum and aid in Brooks spot control (see "Fruit and Foliar Diseases, Scab.)

Black Pox

Black pox, Helminthosporium papulosum A. Berg, causes fruit spot, leaf spot, and limb and twig canker.

Symptoms

Fruit spots are small (1/4 inch in diameter), circular, sunken, and black (Figure 5.25). Leaf spots are light green with red halos and enlarge to about 1/2 inch in diameter. The center dies and becomes tan. Lesions on stems are conical, shiny black swellings.

Disease Cycle

The fungus overwinters in twig and bark cankers. Infections can occur as early as May and continue through the summer until harvest.

Control

Control is based on the use of a preventive fungicidal spray program, applied on a 2-week schedule, beginning in mid-May. Pruning to remove sprouts with cankers will reduce the inoculum and aid in control.

Powdery Mildew

Powdery mildew, Podosphaera leucotricha (Ell. & Ev.) Salm., is one of the most important apple diseases worldwide. Losses occur from the reduced grade of infected fruit and from stunting of leaves and defoliation, which reduce yield and can affect return bloom the next season.

Symptom

Infected terminals and leaves are stunted, distorted, and covered with a silver gray mat of mycelium (Figure 5.26). Severe infections can result in premature defoliation and fall blooming. Fruit infections are characterized by a net-like russet ( Figure 5.27).

Disease Cycle

The fungus overwinters as mycelium in dormant apple buds. The mycelium grows from the infected buds, and by the tight cluster of pink stage it produces spores on the leaf surface. These spores are blown to new leaves and fruit and initiate primary infections. Mildew infections are favored by warm, humid weather. Unlike scab and most other apple diseases, powdery mildew does not require free water (rain or dew) for infection. Secondary infections can continue through the summer until the terminal bud sets.

Control

The need for fungicides to control powdery mildew depends to a large extent on the susceptibility of the cultivar grown. Rome Beauty and Granny Smith are two of the most susceptible cultivars grown in the Southeast and require fungicides to manage the disease. Golden Delicious is moderately susceptible; Delicious is relatively resistant. When fungicides are needed, the first mildewcides should be applied at the tight cluster to pink stage, and they need to be continued until the terminal bud forms.

Alternaria Blotch

Alternaria blotch, Alternaria mali Roberts, is a serious disease of apple in Japan and has been found in North Carolina, South Carolina, Tennessee, Virginia, and Georgia in recent years. Severe infections result in up to 50% defoliation of trees by midsummer and reduction in yield and quality.

Symptoms

Alternaria mali affects leaves and twigs but seldom attacks fruit. Spots appear on the leaves in late spring and early summer. Initially, they are 1/4 inch in diameter, round, brown spots, often with a purple border (Figure 5.28). Some spots undergo secondary enlargement and become irregularly shaped and dark brown. Extensive infection can cause defoliation. Defoliation is more severe when mites are present. Stem lesions, which are somewhat sunken, round, blackish spots bordered by cracks, occur on susceptible cultivars such as Indo but have not been observed on Delicious.

Disease Cycle

The fungus overwinters on fallen leaves on the ground and in dormant buds and injuries on limbs and twigs. Primary infection occurs in late spring, and numerous secondary infections occur throughout the summer in hot, rainy weather. Infection can occur in 5 to 6 hours at favorable temperatures (70° to 75°F).

Strains of Delicious and Empire are very susceptible. Golden Delicious is moderately resistant but becomes infected when planted as a pollenizer in orchards of Delicious. Recently the disease has been reported on Redgold, Fuji, Mutsu, Jonagold, and Jonathan.

Control

Sanitation is important in control of the disease. Leaves in infected orchards should be removed by raking, or leaves on trees should be sprayed with urea in the late fall. To minimize defoliation in orchards with a history of disease, maintain mites below 10 per leaf. Additional control measures have not been developed.

Necrotic Leaf Blotch of Golden Delicious

Necrotic leaf blotch of Golden Delicious is a physiological disorder that affects only Golden Delicious and its sports.

Symptoms

The disease is characterized by irregular necrotic blotches on mature leaves. Blotches are 1/4 to 1 inch in size and are often bordered by veins. Numbers of blotches on affected leaves vary from one to many (Figure 5.29). Symptoms appear rapidly, within 12 to 24 hours of infection, and occur in distinct periods during the summer. Many affected leaves turn yellow and abscise within 1 week of the appearance of symptoms (Figure 5.30).

Disease Cycle

The exact cause of necrotic leaf blotch is not known although it is related to air temperature, light intensity, and soil moisture. The disease is most severe when a cool, rainy period is followed by hot, sunny weather.

Control

The incidence and severity of necrotic leaf blotch can be reduced by including ziram or thiram in the cover sprays beginning 1 month after petal fall.

Nectria Twig Blight

Nectria twig blight, Nectria cinnabrina (Tode:Fr.) Fr., is often a problem on strains of Rome in the southeastern U.S.

Symptoms

Symptoms somewhat resemble fire blight and are often confused with it. Infections originate in fruit pedicel scars or broken pedicels on last year's cluster bases and result in cankers, which girdle the stem (Figure 5.31). Cankers are orange-brown. Leaves often remain attached to dead shoots.

Control

There is no chemical control. Infected shoots should be cut out to reduce the inoculum present and to remove dead wood that can be a host for summer rot fungi.

Turner Sutton and Floyd Hendrix

Root Rot Diseases

Six root rot or crown rot diseases cause tree losses in the southeastern U.S.: Phytophthora crown rot, white root rot, Armillaria root rots (two types), black root rot, and southern stem blight.

Although they are caused by different species of fungi, the above-ground symptoms of all are somewhat similar. During the spring and summer affected trees are characterized by sparse foliage and small, light green leaves (Figure 5.32). Terminal growth is poor, and trees often produce a large crop of small fruit. As roots become rotton, trees may lean over following heavy rain or wind. In the fall, leaves on affected trees turn prematurely purple. This premature coloration is a good diagnostic characteristic of a weak tree and often is an indication of a root rot problem before other symptoms become evident. The symptoms described above are not specific to root rot diseases; they are also characteristic of trees injured by pine and meadow voles, machines, root borers, heart rots, etc.

Phytophthora Crown, Collar, and Root Rot

Phytophthora crown, collar, and root rot is the most widely distributed soilborne disease of apples in the southeastern U.S. It is caused by several species of Phytophthora which are common inhabitants of the soil in the Southeast. Phytophthora cactorum is the most common species found, but P. cambivora, P. cryptogea, P. citricola, P. syringae, P. megasperma, and P. drechsleri have also been associated with the disease.

Phytophthora crown rot is most severe in orchards planted in heavy and poorly drained soils and on trees propagated on size-controlling rootstocks. Losses are usually greatest 3 to 5 years after planting when trees first come into bearing. Losses of 40% or more have been recorded in some orchards 5 years after planting.

Symptoms

Infections on size-controlling rootstocks are most common in the crown where the main roots originate from the trunk. The inner bark of affected tissues is reddish-brown, and affected tissues are clearly separated from healthy ones by a sharp line (Figure 5.33). Collar and root infections are characterized by a similar appearance.

Control

Management of the disease is based on cultural and chemical controls. Only healthy, disease free trees should be planted. Numerous studies have shown that infected rootstocks and grafted trees obtained from nurseries are an important source of inoculum.

Trees should be planted only in well-drained sites. On flat or poorly drained sites, planting on a raised bed will reduce the likelihood of crown rot. Planting holes should be filled to prevent the soil from settling and forming a depression around the collar of the tree where water can stand.

Susceptible rootstocks should be avoided, particularly in heavy, poorly drained soils. Seedling and M.9 rootstocks are generally regarded as most resistant and MM 106 and MM 104 as most susceptible. MM 111, M.7, M.7a, and M.26 are moderately susceptible.

Several fungicides are registered for crown rot control and are most effective when used on a preventive basis.

White Root Rot

White root rot, Scytinostroma galactinum (Fr.)Donk (synonym Corticium galactinum) survives in many wild hosts and is a problem in orchards planted on newly cleared land.

Symptoms

Aboveground symptoms are similar to the other root rots, but early symptoms are often unilateral, affecting one or two scaffold limbs which may die back. Symptoms on roots of trees affected by the two Armillaria root rots are similar. The fungi grow within the bark as white to slightly yellow mycelial growth (Figure 5.35). This characteristic differentiates the mushroom root rots from white root rot which is characterized by white mycelium visible on the outside of affected roots. Fan-shaped mycelial mats are often produced between the bark and wood. Perforations may be observed in the mycelial mats produced by A. tabescens. Armillaria mellea is distinguished from A. tabescens by the presence of dark brown to black, root-like structures known as rhizomorphs, which are often found on dead roots, on the surface of live roots, or growing out into the soil. In the fall following rain, yellow to yellow-brown to brown mushrooms are often produced in clusters at the base of affected trees ( Figure 5.36). The stems or stalks of the mushrooms of A. mellea have a ring of tissue around them known as an annulus, which is absent in A. tabescens.

Control

The Armillaria spp. Also infect numerous forest trees and persist in the soil in infected roots for many years. As with white root rot, replanting is often not successful. All old roots should be removed from the site before any replanting is attempted. Several soil fumigants are registered for the control of Armillaria root rot; however, their use has not been widely investigated in the Southeast.

Black Root Rot

Black root rot, Xylaria mali Fromme and X. polymorpha (Pers.:Fr.) Grev., is a disease of mature trees. Affected trees usually decline over a period of 3 to 4 years or more and often lean or break off just below the ground.

Symptoms

Infected roots are covered with a black fungal encrustation. The wood of infected roots is cinnamon brown, dry, and brittle and marked with brown zonations (Figure 5.37). Black fruiting structures, known as dead man's fingers, are often produced at the base of infected trees or appressed to the collar (Figure 5.38). The initial source of inoculum is unknown although several forest trees may be hosts. Infection occurs when healthy roots contact infected root pieces left in the soil.

Control

There is no satisfactory control for black root rot. If possible, orchard sites with a history of the disease should not be replanted. If replanting is attempted, the site should be deep plowed and subsoiled, old roots removed, and the site allowed to remain fallow as long as possible. All rootstocks are susceptible; MM 106 and seedling rootstocks are less susceptible than MM 104. MM 111 is intermediate in susceptibility. Peaches are not susceptible, and peach orchards have been successfully established in infested sites.

Southern Stem Blight

Southern stem blight, Sclerotium rolfsii Sacc., is most severe in recently planted orchards and nurseries. One- to 3-year-old trees are most susceptible, and affected trees usually die suddenly. When trees die in the summer, leaves turn brown and remain attached to the tree.

Symptoms

The most distinct symptoms and signs occur at the collar of the tree. Small, round, light brown to yellow resting structures of the pathogen, known as sclerotia, can be found appressed to or in the soil adjacent to infected trees (Figure 5.39). If conditions are moist, a white web-like mycelial growth may also be present. Affected cortical tissues in the collar of the tree are often shredded.

Control

Sclerotium rolfsii affects many agronomic crops, including peanuts, soybeans, clover, and tomato; consequently, the disease tends to be most severe when apples are planted following these crops. Since dead organic matter serves as a food base for the fungus, clean cultivation will help reduce losses. Rootstocks vary somewhat in their susceptibility; M.9 and M.26 are most resistant. There are no chemicals registered for control of this disease.

Apple Replant Disease

Apple replant disease has been described in Europe since 1698 and refers to many problems relating to poor tree growth on sites previously planted to apple. The disease is a problem in virtually all major apple-growing regions of the world but has not been a major problem in the Southeast. However, as old orchards are removed and planted to high density orchards, the disease may become more important.

Apple replant disease is a disease complex with one or more biological components interacting with the soil environment. Two types of apple replant disease have been described. Nonspecific apple replant disease is characterized by poor apple tree growth regardless of the previous fruit crop and has been related to high numbers of parasitic nematodes, particularly the lesion nematode, Pratylenchus penetrans. Trees affected with nonspecific apple replant disease are often aggregated in the orchard. Specific apple replant disease, on the other hand, refers to poor apple tree growth only when apples are planted after apples. Various fungi and bacteria have been associated with specific apple replant disease; nematodes are not believed to be involved.

Symptoms

The principle symptom associated with apple replant disease is reduced growth of both aboveground and belowground portions of the tree.

Control

Management is based on cultural practices and preplant soil fumigation. Proper site preparation, including removal of old roots, subsoiling, liming, and fertilizing, is essential to promote good tree growth in replant situations. Crop rotations of 3 to 4 years have been used to manage the disease in some areas of the world. Soil fumigation will provide long-term control.

Turner Sutton

Diseases Caused by Viruses and Mycoplasmas

Apple Union Necrosis

Apple union necrosis, caused by a strain of tomato ringspot virus, is the only virus disease that is important in the southeastern U.S.

Symptoms

Aboveground symptoms are similar to those described for the root rot diseases except rootstock suckers are common. The bark of affected trees becomes thickened and disorganized at the graft union, and a row of pits and grooves occurs along the graft union (Figure 5.40). Brown, necrotic tissues extend from these pits across the union, weakening it so that affected trees break at the union. The disease is most severe on Delicious and its strains when propagated on MM 106. The virus is transmitted in the orchard from broadleaf weed reservoir hosts to apple by the dagger nematode, Xiphinema americanum.

Control

Control of union necrosis is based on the use of virus-free planting stock, trees planted on rootstocks other than MM 106, soil fumigation to control the dagger nematode, and control of broadleaf weed hosts.

Monitoring for Pathogens and Diseases

Monitoring for diseases is not as useful as monitoring for arthropods because once symptoms of most diseases are visible, the damage has already been done and the fruit may no longer be marketable. Monitoring can, however, be very useful to help determine whether or not a spray program is effective and if adjustments are necessary to improve the level of control.

When using the incidence or severity of a disease as an indication of the effectiveness of a fungicide control program or the need to modify it, growers need to understand the relationship between infection and the incubation period. Once conditions suitable for infection have occurred, the fungus (or bacterium) grows in the leaf or fruit tissue for some time (a few days or several weeks) before the lesions become visible. This period is known as the latent period. If a number of infection periods have occurred, then a large number of potential lesions may be incubating but not visible. For example, growers may monitor an orchard and determine that there is sufficient apple scab to warrant a special spray. On scouting the orchard several days after spraying, they may find that there are many more scab lesions present. These do not necessarily mean that the special spray applied earlier did not work. More than likely, the spray worked and has protected new leaves and fruit; the new lesions are just those that were incubating in the leaf or fruit from previous infection periods.

The important thing to remember is to make the appropriate modification in the spray program once a problem is detected. If the proper material is used at the correct times and rates, fruit and foliage should be protected from any new infections.

Monitoring is most useful for apple scab, sooty blotch, flyspeck, and bitter rot because these diseases have the potential to cause the greatest crop loss. Monitoring can also be useful for powdery mildew; however, losses are usually not great with this disease. The guidelines listed below have been empirically derived and are used in apple pest management programs in North Carolina. However, they have not been experimentally tested and should be used with caution.

Apple Scab

Although apple scab infections on cluster leaves can usually be observed prior to bloom, in the Southeast first visible infections are usually seen 7 to 10 days after petal fall on cluster and basal shoot leaves. Scouting should be initiated at pink. If scab has been a problem in a particular section of the orchard in past years, scout there first. Early season infections are most common on the underside of the leaf. Observe 10 groups of cluster leaves on a minimum of 10 trees of the most susceptible cultivar in the block (likely Delicious). Observe weekly; at petal fall include 300 fruit and 10 terminals per tree on a minimum of 10 trees in the sample. Early season fruit infections are most common on the sepals and around the calyx end of the fruit. If more than 1% of the fruit or 3% of the leaves are infected, begin monitoring at 3-day intervals to see if the disease is increasing. If the disease increases during the 3-day period, eradicant sprays may be necessary. Scab should be monitored weekly until July 1. Cluster leaves do not need to be examined after first cover.

Techniques have been developed for monitoring the maturation and discharge of spores of some apple pathogens. These techniques were developed primarily for apple scab. By monitoring the maturity of spores in the pseudothecia and the ascospore concentrations in the air, growers can determine when to initiate a spray program for apple scab, when peak ascospore discharge occurs, and when most spores have matured and discharged. These techniques require specialized equipment such as microscopes and spore traps, and they are time-consuming. Growers interested in obtaining more information on these monitoring techniques should contact their County Extension agent.

Sooty Blotch and Flyspeck

Sooty blotch and flyspeck infections can become visible as early as the first week in June. Monitor orchards weekly for sooty blotch and flyspeck by observing fruit in trees located in lower areas in the orchard and fruit which are in clusters in the centers of the trees. Observe 10 to 20 fruit per tree and a minimum of 300 fruit in the orchard. If more than 2 to 3% of the fruit are infected, a change in the fungicidal spray program may be necessary.

Bitter Rot

Bitter rot is the most serious rot disease and if uncontrolled can result in extensive crop losses. In orchards where this disease has been a problem, growers should monitor on a weekly basis, beginning in early June. Observe 300 to 500 fruit on trees in sections of the orchard where the disease has been a problem in previous years. Trees where fire blight strikes have not been pruned out should also be monitored because the bitter rot fungus can quickly colonize blighted tissues and become a good inoculum source. If any bitter rot is observed, a more intensive survey should be made to determine the extent of infection. If more than 3 infected fruit are observed on any tree, a change in the fungicidal spray program may be necessary.

Powdery Mildew

Powdery mildew can be monitored by determining the number of primary infections on shoots and flower clusters as well as secondary spread on terminal leaves. An assessment of the number of primary terminals per tree should be made at petal fall and first cover as an indication of the potential secondary mildew problem. Ten trees of the most susceptible cultivar should be selected and all primary infected terminals counted. An average of 0 to 5 primary terminals per tree indicates a minor problem, 6 to 20 a moderate problem, and more than 20 a potentially severe problem.

Monitoring for secondary mildew should begin at petal fall and continue weekly until most terminal buds have set. Examine 5 actively growing shoots on a minimum of 20 trees of the most susceptible cultivar per block. The 5 youngest expanded leaves on each shoot should be examined and the number infected with mildew recorded. If less than 8 percent of the leaves are infected, no additional control measures are necessary. If 8 to 20 percent of the leaves are infected, a mildewcide program should be initiated at once if mildewcides have not been applied, then rates may need to be adjusted or intervals modified. If more than 20% of the leaves are infected, a more intensive mildewcide program is needed.

Other Apple Diseases

While thresholds have not been established for the other apple diseases, scouting can be used as a guide to tell growers if the current spray program is working. If diseases are observed and increase significantly over a 3- to 7-day period, then some modification in the spray program is needed. Fungicide rates may need to be increased, the spray interval reduced, or a fungicide with a different spectrum of activity may be required.

Turner Sutton

Cultural Considerations for Disease Management

Cultural practices are very important in the management of apple diseases. Selecting cultivars resistant or partially resistant to various pathogens can reduce the need for fungicides or bactericides. Similarly, selecting the proper rootstock can help avoid certain root rots. Other cultural practices, such as pruning, are aimed at reducing the inoculum of the various pathogens or creating an environment less favorable for disease development.

Cultivars and Rootstock Selection

Cultivars vary greatly in their susceptibility to some diseases. Some of the more common cultivars grown in the Southeast and their relative susceptibilities to diseases are listed in Table 5.2. Most apple cultivars planted widely in the Southeast are susceptible to scab and require fungicide controls. More than 30 apple cultivars that are resistant to apple scab have been released wordwide, but most are not adapted to the Southeast. However, Liberty, Prima, Priscilla, and Redfree have performed well in most mountain locations in the Southeast. Freedom (not listed in Table 5.2) is very susceptible to bitter rot. Some cultivars, such as Delicious, are relatively resistant to powdery mildew and do not require mildewcide applications while others are very susceptible. Cultivars vary in their susceptibility to cedar apple rust and quince rust. Delicious is relatively resistant to cedar apple rust but very susceptible to quince rust. Cultivars similarly vary in their susceptibility to Brooks spot. Rome Beauty, Stayman, and Golden Delicious are very susceptible while Red Delicious is relatively resistant. Most cultivars are susceptible to the summer rot diseases and sooty blotch and flyspeck.

Cultural Practices

Cultural practices to help manage disease include removing dead wood, prunings, leaves, and alternate or reservoir hosts. Other cultural practices to control disease include pruning, training, and thinning.

Removing Dead Wood and Prunings

Dead wood needs to be pruned out of the trees and mummied apples removed. They are the principal inoculum source for fungi that cause bitter rot, bot rot, and black rot. Pruning can be accomplished in the dormant season, spring, or summer when leaves are on the trees and the dead wood is more apparent. Fire blight strikes can be removed at the same time. Current season prunings also can be colonized by rot fungi and produce inoculum by early to midsummer. A flail mower effectively grinds up the wood on the ground and removes the bark, which rapidly decomposes. Most flail mowers will grind limbs up to 4 inches in diameter. To effectively use a flail mower, throw or rake pruned and dead wood into the middles where it can be run over. An alternative to the flail mower is to remove dead wood and prunings from the orchard and burn them.

Removing Leaves

A practical way to significantly reduce the amount of overwintering leaf material and the fungi that survive on them is by spraying the foliage with urea just prior to leaf fall. This practice will help control scab, Alternaria blotch, and Brooks spot. Urea sprays applied earlier may stimulate tree growth and lead to increased likelihood of winter injury. Urea sprays will also give a stronger bloom the next season. A feed grade or low biuret form of urea should be used; the urea used to foliar feed cotton is suitable. Five pounds per 100 gallons should be used. One nozzle of the sprayer should be pointed at the ground to cover the leaves that have already fallen.

Removing Alternate or Reservoir Hosts

Removal of alternate or reservoir hosts can help reduce inoculum of some important apple pathogens. For example, the eastern red cedar is the alternate host for cedar apple rust and quince rust. In growing areas where the eastern red cedar is not abundant, it is possible to eliminate or reduce the need for fungicides for rust disease control by removing all cedars within 1/2 mile of the orchard.

In addition, woody wild plants that have waxy cuticles on their stems are reservoir hosts for the fungi that cause sooty blotch and flyspeck. Spores produced on these hosts are blown or washed into the orchard and infect apple fruit. Some of the hosts are blackberries, sassafras, sweetgum, smilax, dogwood, serviceberry, wild crab apples, and hawthorne. Blackberries are the most abundant and important host. Blackberries in the vicinity of the orchard should be killed with herbicides or mowed.

Pruning and Training

Pruning and training also aid in the management of some diseases such as sooty blotch and flyspeck. A good pruning job during the dormant season thins the tree canopy, allowing better light and wind penetration and permitting fruit to dry more quickly following dew or rain. The incidence and severity of sooty blotch and flyspeck can also be reduced by doing a good job of summer pruning to remove water sprouts. Good pruning increases pesticide penetration and distribution within the canopy and improves disease and insect control.

Thinning

Thinning to break clusters of fruit is important to ensure good fruit size. Thinning also aids in disease control. The inside of fruit clusters often stays wet following rain or dew and provides a favorable environment for the development of sooty blotch and flyspeck. Furthermore, thinned fruit clusters can be easily penetrated by pesticides.

Turner Sutton and Floyd Hendrix