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2011 Florida Plant Disease Management Guide: Apple (Malus domestica) 1

Mathews L. Paret, Tim Momol, Laura Ritchie, and Hank Dankers2

APPLE SCAB

Introduction

Apple scab is an economically important disease resulting in direct loss from fruit or pedicel infections. Indirectly, repeated defoliation reduces tree growth and yield. Apple scab severity is inconsistent in the southeastern United States due to the necessity of cool, wet weather for infection.

Symptoms

Young leaf lesions are olive green with indistinct margins. As the infected leaf ages, several lesions may coalesce and tissues adjacent to lesions thicken, causing the leaf to become curled, dwarfed, or distorted (Fig. 1).

Figure 1. 

Apple scab on apple leaf caused by Venturia inaequalis


Credit:

Ohio State University Extension


[Click thumbnail to enlarge.]

Fruit lesions are similar to those of leaves. As infected fruit enlarge, the lesions become brown and corky. Early-season infection may result in uneven fruit development, causing cracks in the skin and flesh (Fig. 2). Late-season infection may result in pinpoint scabs where circular lesions are rough and black.

Figure 2. 

Apple scab on fruit caused by Venturia inaequalis


Credit:

Clemson University - USDA Cooperative Extension Slide Series (IPM Images, The Bugwood Network)


[Click thumbnail to enlarge.]

Causal Organism

Venturia inaequalis is the causal organism of apple scab. Pseudothecia, produced in overwintered leaves or fruit, are separate, dark brown to black in color, and spherical, with a short beak and distinct ostioles with single-celled bristles at the apex. Asci are fasciculate, cylindrical, short stipitate, eight spored, and have thin, bitunicate walls. Ascospores are yellowish green to tan and unequally two celled. Conidia are olive, have one or two cells, are ovate to lanceolate, and are produced sequentially by a series of abscission ridges on the conidiophore.

Disease Cycle and Epidemiology

V. inaequalis overwinters in infected fruit and leaves on the ground. As infected fruit and leaves become wet, asci discharge ascospores, which are disseminated by the wind and initiate primary infections on new growth. Free moisture is required for the germination of ascospores on the new leaf or fruit. After initiation, germination continues as long as the relative humidity is 95% or greater. After fungal penetration of the cuticle, conidiophores and conidia are produced in a visible lesion. Conidia are then disseminated by rain and wind to new leaves and fruit, initiating a secondary infection cycle.

Management

Removal of fallen leaves and fruit in the fall can reduce disease inoculum. Attention to weather forecasts, particularly those of extended wet periods, can assist in chemical control (Table 1).

BITTER ROT

Introduction

Bitter rot is an important summer disease of apples grown in the southern United States. During periods of warm, moist weather, bitter rot can destroy an entire crop within a few weeks.

Symptoms

Fruit infections begin as small, circular, tan to light brown spots that expand rapidly under warm, moist conditions. As the lesion sinks into the fruit surface, it is covered with rings of spore masses that are creamy and salmon to pink in color (Fig. 3). The fruit may become soft and watery, developing a sour rot odor. The fruit dries and has a dark, leathery appearance.

The pathogen can form cankers on twigs and cause leaf spots. Although not as common, lesions on leaves start as small red flecks that enlarge to irregular brown spots. Severely infected leaves often abscise.

Figure 3. 

Bitter rot lesion on apple fruit caused by Colletotrichum gloeosporiodes. Note creamy spore massses on lesion surface.


Credit:

Clemson University - USDA Cooperative Extension Slide Series (IPM Images, The Bugwood Network)


[Click thumbnail to enlarge.]

Causal Organism

Colletotrichum gloeosporioides (teleomorph Glomerella cingulata) is the causal organism of bitter rot. Acervuli are produced beneath the cuticle, which is ruptured by the growth of conidiophores and conidia. Conidia are uninucleate, vary in size and shape, and are produced in pink to salmon-colored sticky masses. Perithecia can be solitary or aggregated, dark brown to black, and vary in size and shape. Asci vary in size and are clavate to cylindrical; ascospores vary in size and are slightly curved and unicellular.

Disease Cycle and Epidemiology

Fruit infection, through direct penetration or through wounds, is most common from midseason to late in the growing season. Lesion expansion is most rapid in warm weather (around 85°F). Prolonged warm, wet weather can lead to epidemics and extensive losses. A warm, wet early season can be the most severe because the primary infection provides abundant secondary inoculum.

Perithecia and acervuli of C. gloeosporioides survive from one season to the next in mummified apples, dead wood, and cankers. Mummified apples and stems left on the tree provide overwintering sites; mummified fruit on the ground can serve as a source of inoculum.

Management

Management of bitter rot is based on sanitation and a fungicide spray program. Removal of mummified fruit, cankers, dead wood, and blighted branches from the previous season as well as throughout the current growing season removes potential inoculum sources. Removal of fruit infected with C. gloeosporioides during the growing season also helps slow the spread of disease.

No commercial cultivar is resistant to bitter rot to negate the need for sprays. For fungicide application details, see Table 1.

BLACK ROT

Introduction

Black rot of apples occurs throughout the eastern United States but is most severe in the Southeast. Losses from the disease can be because of fruit rot, but losses can also be attributed to the weakening of trees due to limb cankers and leaf spot-induced defoliation.

Symptoms

Leaf lesions begin as small purple flecks that enlarge and develop a tan to brown center, giving a frog-eye appearance (Fig. 4). Heavily infected leaves may become chlorotic and abscise. Fruit symptoms from direct infection begin as minute red flecks that develop into raised purple spots bordered by a red ring. Upon fruit maturation, the lesions darken and enlarge (Fig. 5), often with alternating black and brown concentric rings.

Limb cankers begin as slightly sunken reddish-brown areas in the bark and can lead to a superficial hardening of the bark. Canker may also cause the wood to crack open and die. Fruit may develop rotting around the core, yet remain on the tree, ripening three to six weeks before harvest and often dropping before the rot appears on the surface.

Figure 4. 

Frog-eye appearance of black rot lesions on apple leaf


Credit:

Clemson University - USDA Cooperative Extension Slide Series (IPM Images, The Bugwood Network)


[Click thumbnail to enlarge.]

Figure 5. 

Fruit decay on apple infected with black rot


Credit:

Clemson University - USDA Cooperative Extension Slide Series (IPM Images, The Bugwood Network)


[Click thumbnail to enlarge.]

Causal Organism

Botryosphaeria obtusa (anamorph Sphaeropsis malorum) is the causal organism of black rot. Pycnidia, commonly found on infected wood and fruit, are globose, solitary or botryose, and stromatic with papillate ostioles. At maturity, conidia are nonsepate, ovoid, and melanized with a rough or faintly echinulate wall. Asci are bitunicate and eight spored; ascospores are fusiform and occasionally one septate. Pseudothecia are rare in the southeastern United States.

Disease Cycle and Epidemiology

B. obtusa can survive between seasons in tree cankers and mummified apples, providing an early-season source of inoculum. Mummified apples, wounded bark, and fire-blighted twigs are rapidly colonized by B. obtusa and provide inoculum during the growing season. Conidia and ascospores are released from fruiting structures during rainfall throughout the year in the southern United States. Infection can occur through stomata of leaves or fruit (early season), and through wounds and cracks in the fruit cuticles (later season).

Management

Throughout the season, blighted twigs should be removed to limit the colonization sites for B. obtusa. Mummified apples and dead wood should be pruned and removed from the orchard or burned. Black rot is best controlled through chemical applications (Table 1).

BOTRYOSPHAERIA ROT/WHITE ROT

Introduction

Botryosphaeria rot, also referred to as bot rot or white rot, has symptoms similar to black rot. This disease can have a severe impact in the southeastern United States. Extensive fruit losses (50%) have been reported, and cankers can cause the loss of scaffold limbs as well as tree death.

Symptoms

Fruit lesions begin as small, slightly sunken brown to tan spots, often surrounded by a red halo (halo may appear purple to black on red cultivars). The rotted area extends in a cylindrical manner to the core as the lesions expand in diameter (Fig. 6), forming a v-shaped lesion in cross section. This criterion can be used as a distinction between black rot and Botryosphaeria rot.

Limb and twig infections begin as small, sunken, oozing lesions, often red in color. The lesions enlarge, exhibiting rings of black pycnidia and bark loss (Fig. 7).

Causal Organism

Botryosphaeria dothidea (anamorph Fusicoccum aesculi) is the causal organism of Botryosphaeria rot. Pycnidia, found on infected wood and fruit, are typically compound and spherical (153 X 197 µm). Conidia are non septate and hyaline. Ascostroma are solitary and scattered, botryose, ostiolate, and spherical. Asci are cylindrical, eight spored, and bitunicate; ascospores are hyaline, one celled, and ovoid.

Figure 6. 

Botryosphaeria rot/white rot on apple


Credit:

The Bugwood Network, Insect and Disease Indentification Guide for IPM in the Southeast, The University of Georgia, Cooperative Extension Service Bulletin 849, September 1981


[Click thumbnail to enlarge.]

Figure 7. 

Botryospaeria canker on apple limb


Credit:

Michael A. Ellis, The Ohio State University


[Click thumbnail to enlarge.]

Disease Cycle and Epidemiology

Although ascospores and conidia are produced throughout the growing season in the southeastern United States, inoculum production is dependent on temperature (optimum 82°F–90°F), and number of spores released is dependent upon the amount and duration of rain. Infection by spores is most common through wounds in fruit, twigs, and limbs, although twig and limb infection is often associated with periods of hot, dry weather.

Mycelia, pycnidia, and pseudothecia of B. dothidea survive between seasons in cankers, colonized dead bark, and mummified fruit. B. dothidea can colonize in current-season mummified fruit and fire blight strikes to provide a secondary source of inoculum.

Management

Dead limbs, cankers, and mummified fruit should be pruned and removed from the orchard. Current-season fire blight strikes should be removed to lessen colonization of B. dothidea. Limb and branch infections can also be reduced by irrigation during hot, dry periods. Timing for chemical control is determined based on the severity of Botryosphaeria rot in the area and whether early-season latent infections are common (Table 1).

CERCOSPORA LEAF SPOT

Introduction

Cercospora leaf spot is a minor foliar disease of apple that may cause early defoliation.

Symptoms

Lesions on infected leaves are round to oblong, often with a zonate appearance.

Causal Organism

Pseudocercopsora mali is the causal organism of Cercospora leaf spot. Produced in clusters, conidiophores are dark in color, sparingly branched, and septate. Conidia are typically hyaline, long and slender, and septate.

Management

Fallen infected leaves should be removed from the area and destroyed. See Table 1 for chemical control.

CROWN GALL

Introduction

Crown gall affects woody and herbaceous plants from over 90 families, including apples grown for fruit production and ornamental use. Crown gall is variable in severity but gradually lowers tree vigor and may lead to tree death.

Symptoms

Galls, varying in size, form on the crown, roots, trunk, or limbs. The texture of a gall can range from soft and spongy to hard, depending on the amount of vascular tissue it contains. Careful diagnosis of smaller galls is important, as they may be confused with excessive callus growth around wound sites or with nematode or insect-induced galls.

Causal Organism

The bacterium Agrobacterium tumefaciens is the causal organism of crown gall. It is a rod-shaped, gram-negative, aerobic, motile bacterium having one to six flagella. A large extrachromosomal piece of DNA, commonly referred to as a tumor-inducing (Ti) plasmid, is carried by A. tumefaciens.

Disease Cycle and Epidemiology

Wounds are necessary to the infection process and initiation of the disease cycle. A. tumefaciens enters through a wound, attaches to a susceptible plant cell, and inserts transfer DNA (T-DNA) from the Ti plasmid into the plant cell chromosome. Expression of the T-DNA results in overproduction of plant hormones, which stimulates plant cells to divide, enlarge, and form a gall. The pathogen may move from galls to surrounding roots and soil, then disseminate to new plants or planting sites by rain, irrigation water, wind, insects, tools, and plant parts used for propagation.

Management

Good cultural and sanitation practices are key deterrents to crown gall. These include choosing a rootstock with low susceptibility, budding rather than grafting, developing management practices that minimize wounding, removing young infected trees as well as older galled trees, and dipping shears in rubbing alcohol for 10–15 seconds between cuts. Planting sites where galled plants were grown should be left fallow for several years.

Effectiveness of chemical control through soil fumigation and rootstock dipping varies.

FABREA LEAF SPOT

Introduction

Fabrea leaf spot usually begins in early summer. In other references, this disease may be referred to as Entomosporium leaf spot.

Symptoms

Initially, leaf lesions appear as small, purplish, circular spots. Lesions become sunken with a raised dark purple to black border and a gray to white center. Within the center is a small, dark, mound-like fruiting body (acervulus).

Causal Organism

Fabraea maculata/Entomosporium mespili is the causal organism of Fabrea leaf spot. Conidia have a distinct insect-like appearance and are composed of four cells (lateral cells smaller than central cells) and bristlelike appendages on all but the basal cells. Asci, which contain eight hyaline, clavate, two-celled ascospores, extend above the leaf cuticle surface when the ascospores are mature.

Disease Cycle and Epidemiology

The fungus overwinters in infected leaves and twig cankers as ascospores and conidia. Rain and overhead irrigation spread the spores to susceptible tissue. Severity increases when wet springs follow mild winters.

Management

Infected leaves and twigs should be removed from the area and destroyed by burial or composting and burning. Adequate spacing between rows and avoidance of overhead irrigation allow good airflow and rapid drying of the foliage. Chemical control may be difficult if primary leaf and twig infections are abundant (Table 1).

FIRE BLIGHT

Introduction

Fire blight is one of the most devastating bacterial diseases affecting apple, pear, and other rosaceous plants. This disease varies in severity from year to year, depending on temperature and precipitation. Additionally, fire-blighted wood can provide a suitable site for other diseases, such as black rot and white rot.

Symptoms

Plant parts affected by fire blight appear as if scorched by fire. Infected blossoms may exhibit ooze and change color from red to brown to black as the disease progresses. Infected leaves turn brown to black and desiccate but remain attached to the branches. Vegetative shoots often wilt and take on the shape of a shepherd's crook (Fig. 8), the pith of infected stems exhibiting a dark brown discoloration. The outer bark of infected branches and limbs is often sunken and darker than normal, whereas the inner tissues are water soaked with reddish streaks while the pathogen is active, later turning brown.

Fruit infected during the early season remain attached to the cluster base, yet remain small and appear shriveled and dark, whereas fruit infected as the disease progresses from the branches appear less shriveled and dark. Fruit infected following injury often develop red, brown, or black lesions and may exude an ooze that first appears clear or milky and later turns red to brown.

Figure 8. 

Fire blight on apple branch


Credit:

Tim J. Smith, Washington State University


[Click thumbnail to enlarge.]

Causal Organism

Erwinia amylovora is the causal organism of fire blight. The rod-shaped bacterium is gram negative and facultative anaerobic. Isolation and tentative identification can be made using several selective or differential media; rigorous identification requires additional biochemical and molecular testing.

Disease Cycle and Epidemiology

E. amylovora overwinters in small twig cankers and dead wood to provide an initial source of inoculum early in the next season. Transferred by rain or insects, the bacterium penetrates host tissue at wounds or natural openings. Inoculum produced as ooze from fresh infections can serve as a secondary source of disease for later-season vegetative shoots, blossoms, and fruit. Lesion extension slows in late summer to autumn in response to less favorable conditions.

Severity of fire blight varies from season to season, dependent upon the interaction of a susceptible plant, a virulent pathogen, and favorable weather conditions. Plant susceptibility varies with plant age and horticultural practices employed; strains of E. amylovora vary in virulence toward plant genotypes. Weather conditions, particularly temperature and moisture, affect vector activity (primarily bees) and pathogen multiplication.

Management

Overwintering cankers should be removed during the dormant season. Active lesions should be pruned out at least 6–8 inches below obvious discoloration. Pruning tools must be disinfected between cuts to reduce spread of the bacterium.

Susceptible cultivars and rootstocks should be avoided, as should the use of nitrogen fertilizers early in the season and during late-summer cultivation. There are forecasting models available to time chemical applications. Properly timed chemical applications can be highly effective against the blossom phase of fire blight (Table 1).

FLY SPECK

Introduction

Flyspeck is a common disease of apple whereby fruit quality is lowered by fungal growth on the fruit surface. Due to the warm, moist weather prevalent in the southeastern United States, fungicide use is important.

Symptoms

Flyspeck colonies on fruit surfaces are well-defined groupings of shiny, black, superficial pseudothecia (Fig. 9). Colony size varies from 1–3 cm and from round to irregular. Conidiophores and conidia are produced within the colonies of pseudothecia during warm, moist weather.

Figure 9. 

Flyspeck colonies


Credit:

Michael A. Ellis, The Ohio State University


[Click thumbnail to enlarge.]

Causal Organism

Schizothyrium pomi (formerly Microthyriella rubi; anamorph Zygophiala jamaicensis) is the causal organism of flyspeck. Pseudothecia vary in size and have irregular margins. Embedded in centrum tissue, asci are spherical to oval and bitunicate, each ascus containing eight hyaline, two-celled ascospores. The upper cells of ascospores are shorter and wider than the basal cells. Conidiophores consist of a sub hyaline basal cell; a smooth, thick-walled, brown sector; an angular, subhyaline terminal cell; and two hyaline conidiogenous cells. Two-celled conidia are thin walled and elliptical to obovate.

Disease Cycle and Epidemiology

S. pomi overwinters as pseudothecia on infected apple twigs and woody reservoir hosts. Airborne ascospores are released by the pseudothecia just prior to bloom and germinate from 60.8°F to 82.4°F. Under optimum conditions (63.1°F, relative humidity above 96%), conidia may be produced in 10–12 days. Airborne conidia are released after sunrise as the relative humidity declines and the twigs dry, providing the secondary inoculum.

Management

During normal to dry weather, well-pruned trees usually have less disease. During wet seasons, well-pruned and poorly pruned trees may be equally diseased. Proper thinning of fruit can lessen the microclimate for disease development that tightly clustered fruit provide, and it also allows for thorough coverage of fungicide sprays, which are the primary means of control for flyspeck (Table 1).

MUSHROOM ROOT ROT

Introduction

Mushroom root rot is also known as Armillaria root rot, oak root fungus disease, and shoestring root rot.

Symptoms

Foliage may turn yellow, then brown, and dry rapidly. Dark brown to black rhizomorphs, or "shoestrings," appear at the soil line around the trunk of the tree. A creamy-white layer of fungus is often present between the bark and the wood. Honey-colored mushrooms may form in groups around the drip line of the tree or next to the trunk during moist periods.

Causal Organism

Armillaria tabescens is the causal organism of mushroom root rot. Lacking an annulus on the stipe of the mushroom (basidiocarp), it is easily distinguished from other Armillaria species. Blackish, hardened mycelial extrusions are produced on the bark of infected roots.

Disease Cycle and Epidemiology

In recently cleared woodlands (particularly that which had oak cover), rhizomorphs and mycelial strands of the fungus can remain on infected roots in the soil for many years, serving as an initial source of inoculum for new orchard trees. Rhizomorphs also spread between trees, attaching to the roots of a new host and entering through pressure and enzymatic activity.

Management

The location of orchard plantings is important. New trees should not be planted in recently cleared woodland (particularly that which had oak cover), near existing stumps or buried debris, nor where trees have recently died from mushroom root rot unless the root system has been removed in its entirety and several years of fallow have passed. Soil fumigation can assist in control of mushroom root rot.

PINK LIMB BLIGHT

Introduction

Pink limb blight is a common disease during warm, moist climatic conditions.

Symptoms

Pink limb blight produces a pale pink mycelium, which often encircles limbs, twigs, and trunks. The foliage distal from the affected area wilts and dies, eventually killing the limb or twig.

Causal Organism

Erythricium salmonicolor (anamorph Necator decretus).

Management

Infected tissue should be removed immediately by pruning limbs a minimum of 4–6 inches below the external appearance of mycelia on twigs. Pruned plant parts should be collected and destroyed.

POWDERY MILDEW

Introduction

A persistent disease, the severity of powdery mildew and resulting economic loss varies with environmental conditions, cultivar susceptibility, and management practices. Powdery mildew can be especially damaging in nursery production.

Symptoms

Infections on leaves first appear on the lower surface as grayish-white patches of mycelia and spores with chlorotic spots on the upper surface. Infections may spread to the upper surfaces, covering the entire leaf and eventually turning brown. Leaves infected along the margin may curl, while severely infected leaves may fold longitudinally, become brittle, and abscise.

Infected flower buds open five to eight days later than healthy buds and exhibit reduced fruit set; flower petals are distorted and pale yellow or light green. Apples affected during bloom are stunted in growth and covered with a network pattern of cork cells (russet) that may be so closely woven as to appear as a solid patch (Fig. 10).

Figure 10. 

An apple from a flower bud infected with powdery mildew


Credit:

Clemson University - USDA Cooperative Extension Slide Series (IPM Images, The Bugwood Network)


[Click thumbnail to enlarge.]

Causal Organism

Podosphaera leucotricha (anamorph Oidium farinosum) is the causal organism of powdery mildew on apple. Produced in long chains on thin, amphigenous mycelia, conidia are ellipsoidal, truncate, hyaline, and contain fibrosin bodies. Perithecia are densely gregarious and subglobulose with apical and basal appendages. Apical appendages are three to seven times longer than the diameter of the perithecia, brown basally, and widely spreading or erect fasticulate. Rudimentary basal appendages are pale brown, short tortuous, and simple or irregularly branched. Asci are oblong to subglobulose and contain eight ovate to elliptical ascospores.

Disease Cycle and Epidemiology

P. leucotricha overwinters as mycelia in dormant buds infected during the previous growing season. Conidia produced on the mycelia serve as primary inoculum. Healthy buds often open earlier than infected buds, thus providing susceptible tissue upon conidia development. Limited germination occurs at high temperatures or in free water. High concentrations of spores are released in the air from midday to early afternoon. Infection of young or mechanically damaged leaves, blossoms, and fruit serve as a secondary source of inoculum.

Management

Dormant-season pruning may remove infected buds and reduce the level of primary inoculum. During the growing season, severely infected shoots should be pruned and destroyed. Prevention of new spores and reduction of spores produced on new lesions may be accomplished through fungicide sprays (Table 1).

RUST

Introduction

Several related rust fungi infect apple in the southeastern United States, including cedar apple rust, quince rust, and American hawthorn rust. These fungi involve two host plants in their life cycle, usually requiring a juniper species as an alternate host. The various fungi differ in life cycle complexity and whether they affect fruit, leaves, or both.

Symptoms

Cedar apple rust affects leaves, petioles, and fruit, beginning as small yellow lesions (upper surface of leaves), which may be surrounded by chlorotic halo or red band. Small orange-brown pustules (pycnia) develop within the lesions, producing watery orange drops. Later, yellow-brown lesions form on the undersurface of leaves and produce small, dark, tubular structures (aecia) that fracture to release red-brown spores (Fig. 11). Fruit lesions are usually superficial, causing a brown necrosis 1–5 mm into the flesh (Fig. 12). Cedar apple rust on alternate host juniper often forms a gelatinous horned gall (Fig. 13).

Figure 11. 

Symptoms of cedar apple rust on leaves


Credit:

Clemson University - USDA Cooperative Extension Slide Series (IPM Images, The Bugwood Network)


[Click thumbnail to enlarge.]

Figure 12. 

Cedar apple rust on apple fruit


Credit:

Lorraine Berkett, University of Vermont


[Click thumbnail to enlarge.]

Figure 13. 

Cedar apple rust on alternate host, juniper


Credit:

Hank Dankers, Uiversity of Florida, IFAS, NFREC,Quincy


[Click thumbnail to enlarge.]

Quince rust does not affect apple leaves, but it does affect fruit. However, obvious rust symptoms (such as pycnia and aecia) are often not present. American hawthorn rust infects apple leaves but rarely infects fruit.

Causal Organism

Various Gymnosporangium species are the causal organisms of rust on apple. Fungal structure morphologies vary between species.

Disease Cycle and Epidemiology

On native cedars, Gymnosporangium species induce a gall from which telial horns emerge under wet conditions. During rains, telia swell and appear jellylike, releasing teliospores, which then germinate to produce basidiospores. Basidiospores are immediately discharged into the air and can travel more than a mile on air currents. Those landing on susceptible apple tissue may germinate and infect the host if a film of water is present for a suitable length of time. Aeciospores are later released from aecia during dry weather and may germinate and infect native cedars.

Management

Removal of infected native cedars within close proximity may reduce infection pressure; however, elimination is unlikely as basidiospores can travel great distances. Rust disease can be effectively controlled by fungicide application (Table 1).

SOOTY BLOTCH

Introduction

Sooty blotch is a late summer disease of apple whereby fruit quality is lowered by fungal growth on the fruit surface. Due to the warm, moist weather, fungicide use is essential in the southeastern United States.

Symptoms

Sooty blotch colonies appear as olive green, soot-like smudges on mature fruit. Large portions of the fruit surface may be covered by colonies due to secondary spread on the fruit (Fig. 14).

Figure 14. 

Sooty mold on apple


Credit:

Clemson University - USDA Cooperative Extension Slide Series (IPM Images, The Bugwood Network)


[Click thumbnail to enlarge.]

Causal Organism

Gloeodes pomigena is the causal organism of sooty blotch. Produced in the thallus, pycnidia are dark brown, scattered or aggregate, and dimidiate. Conidia are variable in length and generally cigar shaped with slight constrictions at the point of septation. Spores in mass are cream to pinkish in color.

Disease Cycle and Epidemiology

Sooty blotch survives between seasons as mycelia and pycnidia on infected twigs of apple and reservoir hosts (woody plants common to hedgerows and woodlots). Spores are released during spring and early summer rains. Infection can occur any time, but is most noticeable during late season. Mycelial growth is possible despite a lack of free water at relative humidity above 90%.

Management

During normal to dry weather, well-pruned trees usually have less disease. During wet seasons, well-pruned and poorly pruned trees may be equally diseased. Proper thinning of fruit lessens the microclimate for disease development that tightly clustered fruit provide, and it also allows for thorough coverage of fungicide sprays, which are the primary means of control for sooty blotch (Table 1).

SOUTHERN BLIGHT

Introduction

Southern blight occurs in orchards and nurseries on trees approximately three years old and younger. Due, in part, to the warm, humid weather, tree losses in some southeastern states have reached 30% due to this disease.

Symptoms

A coarse, white mycelial mat is often found at the base of an infected tree, progressing upward. Small, white sclerotia develop within the mycelium, later turning tan to brown. Leaves of an infected tree may exhibit a reddish or grayish purple discoloration, later drying and turning brown as the fungus girdles the crown and the tree dies.

Causal Organism

Sclerotium rolfsii is the causal organism of southern blight. Grown on a wide range of media, the fungus produces white mycelia and reddish-brown to dark brown or tan, hard, round sclerotia. No asexual spores are produced.

Disease Cycle and Epidemiology

Sclerotia are easily dislodged from mycelia and fall into soil, where they can survive for several years. Infection occurs directly through both injured and healthy bark. Warm summer temperatures (77°F–95°F), high soil moisture, good soil aeration, and plentiful organic debris promote a high incidence of disease.

Management

Delay placement of apple trees where legumes or solanaceous crops have been grown until the area has been deep plowed, fallowed for a season, and fumigated.

References

CDMS Chem Search. 2011. http://premier.cdms.net/webapls/formsloginRef.asp?/webapls.

Jones, A. L., and Aldwinckle, H. S., eds. 1990. Compendium of Apple and Pear Diseases. St. Paul, MN: American Phytopathological Society.

Simone, G. W., and Mullin. R. S. 2000. 1999-2000 Florida Plant Disease Management Guide Volume 3: Fruit and Vegetables. Gainesville: University of Florida Institute of Food and Agricultural Sciences.

Tables

Table 1. 

A short list of fungicides approved for disease management of apple in Florida

   

Max rate/acre

     

Chemical (a.i.)

Fungicide group1

Application

Season

Min. days to harvest

Disease

Remarks2

Captan 50WP, Captan 50W, Captan 80 WDG

Captec® 4L

(Captan)

M4

2.5–8 lb

4 qts

40–64 lb

32 qts

0

0

Bitter rot, black rot, Botryosphaeria rot, flyspeck, powdery mildew, scab, sooty blotch

Use at higher rates or in drenching sprays; may cause a necrotic spotting on tender, immature leaves. See label for use in combination with or closely following, or in alternation with, wettable sulfur products on sulfur-sensitive varieties.

Basic Copper 53, Cuprofix® Ultra 40 Disperss®

(Basic copper sulfate)

M1

1–5 lb

32 lb

0

Bitter rot, fire blight, flyspeck, powdery mildew, scab, sooty blotch, white rot

See label for use with hydrated lime.

Champ® DP Dry Prill, Champ® WG, Kentan® DF, Champion® WP, Kocide 2000®, Kocide® 3000, Nu Cop® 50 WP, Nu Cop® 50DF, Nu Cop®HB, Champ® Formula 2F, Nu Cop® 3L

(Copper hydroxide)

M1

See Label

0.75–16 lb

 

1

Fire blight, scab

Varying rates for desired fruit finish. See label for use with hydrated lime.

Dithane® DF Rainshield®, Penncozeb® 75DF

M3

3–6.4 lb

21–25.6 lb

77

Fabrea leaf spot, flyspeck, rusts, scab, sooty blotch

 

Dithane® F45 Rainshield®, Manzate® Flowable, Penncozeb® 4FL

M3

2.4–4.8 qt

16.8–19.2 pt

77

Fabrea leaf spot, fire blight, flyspeck, rusts, scab, sooty blotch

See label for use. If recommended for fire blight, mix with copper.

Dithane® M45, Manzate® Pro-Stick, Penncozeb® 80WP

(Mancozeb)

M3

3–6 lb

21–24 lb

77

Fabrea leaf spot, fire blight, flyspeck, rusts, scab, sooty blotch

See label for use. If recommended for fire blight, mix with copper.

Maneb 75DF, Maneb 80WP

(Maneb)

M3

3–6 lb

21–25.6 lb

77

Fabrea leaf spot, flyspeck, rusts, scab, sooty blotch

 

Eagle® 20W, PROKZ® Hoist, Rally® 40WSP, Sonoma 40WSP

(Myclobutanil)

3

See label

 

14

Powdery mildew, rusts, scab

 

Agri-Mycin® 17, Firewall

(Streptomycin)

25

24–48 oz

 

50

Fire blight

 

Kumulus® DF, Microfine Sulfur, Microthiol® Disperss®, Sulfur 90W

(Sulfur)

M2

     

Powdery mildew, rust, scab, sooty blotch

Do not use on sensitive apple varieties. Do not use within two weeks of an oil-spray treatment.

Thiophanate Methyl 85 WDG, Topsin® M 70WP, Topsin® M WSB

(Thiophanate methyl)

1

0.6–1.5 lb

3.3–4.0 lb

 

Bitter rot, black rot, flyspeck, powdery mildew, scab, sooty blotch, white rot

 

Ziram 76DF

(Ziram)

M2

6–8 lb

56 lb

14

Bitter rot, flyspeck, rust, scab, sooty blotch

 

1FRAC code (fungicide group): Numbers (1–44) and letters (M, NC, U, P) are used to distinguish the fungicide mode of action groups. All fungicides within the same group (with same number or letter) indicate same active ingredient or similar mode of action. This information must be considered for the fungicide resistance management decisions. M = multisite inhibitors, fungicide resistance risk is low; NC = not classified and includes mineral oils, organic oils, potassium bicarbonate, and other materials of biological origin; U = recent molecules with unknown mode of action; P = host plant defense inducers. Source: FRAC Code List 2009; http://www.frac.info/frac/index.htm [September 7, 2011].

(FRAC = Fungicide Resistance Action Committee).

2Information provided in this table applies only to Florida. Be sure to read a current product label before applying any chemical. The use of brand names and any mention or listing of commercial products or services in the publication does not imply endorsement by the University of Florida Cooperative Extension Service nor discrimination against similar products or services not mentioned.

Footnotes

1.

This document is PDMG-V3-05, one of a series of the Plant Pathology, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date June 2006. Revised July 2011. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Mathews L. Paret, assistant professor; Tim Momol, professor; Laura Ritchie, biological scientist; and Hank Dankers, senior biological scientist, North Florida Research and Education Center (NFREC), Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.