• Topics: Plant Pathology  | Plant Disease Management Guide. Fruit and Nut Crops  | Momol, Timur M  | Ritchie, Laura  | Dankers, Wilbur A  | Pear Pest Management 

2007 Florida Plant Disease Management Guide: Pear (Pyrus communis)¹

BLACK ROT

Introduction

Black rot of pear occurs throughout the eastern United States but is most severe in the southeast. Losses from the disease come not only from fruit rot, but from the weakening of trees due to limb cankers and leaf spot-induced defoliation.

Symptoms

Leaf lesions begin as small purple flecks which enlarge and develop a tan to brown center, giving a "frog eye" appearance. Heavily infected leaves may become chlorotic and abscise. Fruit symptoms from direct infection begin as minute red flecks which develop into raised purple spots bordered by a red ring. Upon maturation of fruit the lesions will enlarge and form a series of alternating black and brown concentric rings (Figure 1).

Figure 1. 

Black spot on pear fruit.

Limb cankers begin as slightly sunken reddish brown areas in the bark and can lead to a superficial hardening of the bark or the canker may cause the wood to crack open and die. Fruit may develop rotting around the core, yet remain on the tree, ripening 3-6 weeks before harvest and often dropping before the rot appears on the surface.

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, 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 pears, providing an early-season source of inoculum. Mummified fruit, 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 fire-blighted twigs should be removed to lessen the colonization sites for B. obtusa. Mummified fruit and dead wood should be pruned and removed from the orchard or burned. Black rot is best controlled through chemical applications. See 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 southeast United States. Extensive fruit losses (50%) have been reported and cankers can cause the loss of scaffold limbs and possible 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, 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 loss of bark.

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 197µm). Conidia are nonseptate 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.

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 82-90°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 the periods of hot and dry weather.

Mycelium, 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. Severity of Botryosphaeria rot in the area and whether early-season latent infections are common are the deciding factors for chemical control timing. See Table 1.

CERCOSPORA LEAF SPOT

Introduction

Cercospora leaf spot is a minor foliar disease of apple which 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.

CROWN GALL

Introduction

Crown gall affects woody and herbaceous plants from over 90 families, including pears 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

Agrobacterium tumefaciens is the causal organism of crown gall. It lacks endospores, is rod-shaped, gram-negative bacteria, aerobic and motile by 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 essential 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, stimulating 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.

FIRE BLIGHT

Introduction

Fire blight is one of the most devastating bacterial diseases affecting pear, apple, and other rosaceous plants. This disease varies in severity from year to year, dependent upon 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 scorched by fire. Infected blossoms may exhibit ooze, and then change color from red to brown to black as the disease progresses. Infected leaves will turn brown to black and desiccate, yet remain attached to the branches. Vegetative shoots often wilt and take on the shape of a shepherd's crook (Figure 2), the pith of infected stems exhibiting a dark brown discoloration. The outer bark of infected branches and limbs are often sunken and darker than normal, whereas the inner tissues will be water-soaked with reddish streaks while the pathogen is active, later turning brown.

Figure 2. 

Fire blight of apple. Note blighted leaves and shepherd's crook symptoms.

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 which first appears clear or milky, later turning red to brown.

Causal Organism

Erwinia amylovora is the causal organism of fire blight. The rod-shaped bacterium is gram-negative and facultatively 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 fruits. 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, phonological stage 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 growth.

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 late-summer cultivation. There are forecasting models available to time chemical applications. Properly timed chemical applications can be highly effective against fire blight. See Table 1.

FLY SPECK

Introduction

Flyspeck is a common disease of pear that lowers fruit quality by fungal growth on the fruit surface. Due to the warm, moist weather, fungicide use is essential in the southeastern United States.

Symptoms

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

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 a 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 subhyaline 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 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 will 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 provides and allows for thorough coverage of fungicide sprays. Primary control of flyspeck is through fungicide sprays. See Table 1.

LEAF SPOT

Introduction

Various fungi cause leaf spot on pear. Due to the warm, wet summers, leaf spot is more prevalent in the southeastern United States.

Symptoms

Leaf spots vary in size, shape, and color, depending on the pathogen involved (Figure 3). Severely infected leaves will often turn yellow and abscise. Severe defoliation can reduce tree vigor and yield.

Figure 3. 

Leaf spot on apple leaf.

Causal Organism

Various fungi.

Disease Cycle and Epidemiology

The fungal pathogens for leaf spot overwinter in infected leaves, and twig cankers. In the spring spores are released and spread by rain or overhead irrigation. Secondary infections occur during warm, wet weather.

Management

Infected leaves should be removed from beneath the trees and buried or destroyed. Adequate spacing of trees and reduction of overhead irrigation can reduce infection. Control of primary inoculum improves productivity of fungicide applications. See 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; a white to light yellow mycelial growth within the bark of surface roots. 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 woodland (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

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, or 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. Fumigation of the soil can assist in control of mushroom root rot.

PINK LIMB BLIGHT

Symptoms

Pink limb blight produces a pale pink mycelium which often encircles limbs, twigs and trunks of trees. 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; pruning limbs a minimum of 4-6 inches below the external appearance of mycelium on twigs. Prunings 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 mycelium and spores (Figure 4) 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.

Figure 4. 

Powdery Mildew - asexual stage on apple leaf.

Infected flower buds will open 5-8 days later than healthy buds and exhibit reduced fruit set; flower petals will be distorted and pale yellow or light green. Pears affected during bloom may be covered by white mycelium until early summer when the mycelium sloughs off, leaving a russeted patch on the surface where cells died.

Causal Organism

Podosphaera leucotricha (anamorph Oidium farinosum) is the causal organism of powdery mildew on pear. Produced in long chains on thin, amphigenous mycelium, 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 perithecium, 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 8 ovate to elliptical ascospores.

Disease Cycle and Epidemiology

P. leucotricha overwinters as mycelium in dormant buds infected during the previous growing season. Conidia produced on the mycelium 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. See Table 1.

RUST

Introduction

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

Symptoms:

Quince rust appears initially as yellow lesions on the upper leaf surface (Figure 5). Lesions may later appear on the lower leaf surface. Darker lesions may appear on the calyx end of fruit, causing the fruit to distort.

Figure 5. 

Juniper-pear rust on pear leaves.

American hawthorn rust infects pear leaves, but rarely infects fruit.

Causal Organism

Various Gymnosporangium species are the causal organisms of rust on pear. Morphologies of fungal structures vary between species.

Disease Cycle and Epidemiology

On native cedars Gymnosporangium sp. will induce a gall, from which telial horns will 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 1 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 is best controlled using fungicides. See Table 1.

SOOTY BLOTCH

Introduction

Sooty blotch is a late-summer disease of pear that lowers fruit quality 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.

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, 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 mycelium 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 humidities above 90%.

Management

During normal to dry weather, well-pruned trees will 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 provides and allows for thorough coverage of fungicide sprays. Primary control of sooty blotch is through fungicide sprays. See Table 1.

SOUTHERN BLIGHT

Introduction

Southern blight occurs in orchards and nurseries on trees approximately 3 years old and younger. Due in part to the warm, humid weather, tree losses in some southeastern United 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 mycelium 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 mycelium and fall into soil, where they can survive for several years. Infection occurs directly through both injured and healthy bark. Warm summer temperatures (77-95°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.

INFORMATION SOURCES

• Simone, G. W. and R.S. Mullin. 1999-2000 Florida Plant Disease Management Guide. Vol. 3: Fruit and Vegetables

• Compendium of Apple and Pear Diseaes. 1990. American Phytopathological Society.

• CDMS Chem Search. http://premier.cdms.net/webapls. Accessed December 11, 2006.

Tables

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 extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer, Interim Dean.

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