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Pear Black Rot Info: What Causes Pear Black Rot

Pear Black Rot Info: What Causes Pear Black Rot


By: Mary Ellen Ellis

If growingpears in the home garden, be aware of the signs of a fungal diseaseknown as black rot. Black rot of pear is not a major commercial issue, but itcan ruin a small harvest and weaken trees. Look for this disease especially inthe eastern U.S. It is rare in western states.

What Causes Pear Black Rot?

Pears with black rot have been infected by a fungus called Physalospora obtusa (syn. Botryosphaeria obtusa). It overwinters in cankers on trees and in leaf matter, old fruit, and twigs on the ground. The prime conditions for infection are warm and wet weather in the spring.

Trees are likely to become infected through sites where theyhave been wounded, mechanically, by insects, or by other diseases. The fruitcan get infected through the calyx end, even if the overall tree is notinfected.

Pear Black Rot Information – Symptoms

The characteristic sign of black rot on pears is a brownspot on the fruit that darkens and widens with age. When the rot sets in whilethe fruit is on the tree, you may see concentric brown rings as the rotdevelops. Some fruit may not show signs of rot until in storage. The rottenspot is firm and in advanced stages will develop dark pustules in the center.

Signs of the disease on the tree usually begin with theleaves. They develop small purple spots that develop into larger purple markswith brown centers. The leaves may eventually yellow and drop. On twigs lookfor sunken brown or red spots, and on larger limbs and the trunk these spotswill form larger cankers.

How to Control Pear Black Rot

There are two main ways to control this disease in pears:use good sanitation and clean up practices to prevent its spread, and if necessary,use a fungicide to treat trees.

Remove and destroy leaf matter, affected twigs and limbs,and rotted fruit. Keep the ground under trees clear of debris and sanitizetools after working on an infected tree.

Fungicidesare effective in managing black rot of pear. Application is typically inspring, but check with your localextension service to find out which fungicide is best and how andwhen to apply it to your pear trees.

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How to Manage Pests

Apple and Pear Scab

Leaf infected with apple scab.

Pear scab appears as velvety, dark olive to black spots on leaves and leaf stems.

Apple scab lesions on the undersurface of a leaf.

Apple scab infecting flower stems.

Fruit scabs caused by apple scab infection.

Fruit damaged by pear scab.

Apple and pear scab are two different diseases that look very similar and are controlled in similar manners in home gardens and landscapes. Both cause spotting and scabbing of fruit, especially during wet springs but different fungi cause them.

The fungus Venturia inaequalis causes apple scab. Apple scab is a serious disease of apples in California, resulting in loss due to severe surface blemishing of fruit. It is most severe in coastal and foothill areas where spring and early summer weather is cool and moist. However, it can be a problem wherever apples grow when conditions are favorable for pathogen development. Apple scab also is a problem on ornamental crabapple.

Pear scab, which the fungus V. pirina causes, results in similar blemishes on pear fruit.The disease is most prevalent in the North Coast production area. However, V. pirina won’t affect apples nor can the apple scab fungus cause problems on pears. Both have quite limited host ranges.

IDENTIFICATION

Scab first appears as yellow, or chlorotic, spots on leaves. As the disease progresses, dark, olive-colored spots form on leaves, fruit, and—in severe cases—stems. Spots on the undersurface of leaves sometimes look velvety due to fungal growth. Affected leaves might twist or pucker in minor cases, this will affect only a few, irregularly scattered leaves, but if the disease is severe, all foliage could show symptoms. Severely affected leaves often turn yellow and drop.

When scab affects flower stems, it can cause flowers to drop. Scabby spots can appear on fruit later in the season. These begin as velvety or sooty, gray-black (and sometimes greasy looking) lesions that sometimes have a red halo. The lesions later become sunken and tan and can have areas of olive-colored spores around their margins. Severely infected fruit becomes distorted and usually drops from the tree. Fruit also can crack, which allows entry of secondary organisms.

LIFE CYCLE

Both apple and pear scab pathogens overwinter primarily in infected leaves on the ground. Rainfall or sprinkler irrigation is necessary to release the spores. In spring, air currents or splashing water carry these primary spores (ascospores) from the infected leaves to flowers, leaves, or fruit where they germinate and cause primary infections. Pear scab also can overwinter in lesions on pear twigs in high rainfall areas.

Secondary spores, or conidia, are produced on infected leaf or fruit surfaces 8 to 17 days following primary infection. In the case of pears, this process also occurs on twig lesions. The disease continues to spread until conditions become dry or the plant tissue becomes more resistant to infection.

Infection occurs most rapidly between 55° and 75°F, and leaves or fruit must remain wet continuously for a minimum of 9 hours for initial infection to occur at these temperatures. If spring weather is dry from the green tip stage of bloom (when flowers are still green and petals aren’t showing yet) through fruit set, scab usually won’t be a problem.

DAMAGE

Scab can destroy an apple or pear crop. Young, infected flowers or fruit can drop, or the fruit can become malformed, cracked, and unsightly, rendering it unusable. Defoliation follows severe, early leaf infection. Late-season infections generally can be tolerated in backyard trees, because peeling the fruit will remove the pinpoint-sized scab lesions.

MANAGEMENT

Several techniques are available for controlling scab. Advantages of one method over another depend on the number of trees you are managing and whether conditions are ideal for disease development.

Cultural Control

For a single, backyard tree, removing—then composting or destroying—its dropped leaves in autumn or winter can limit the disease to tolerable levels. In plantings of several trees, additional steps might be necessary to effectively control this disease, especially in cool, moist coastal areas. These include applying zinc and fertilizer-grade urea (or some other nitrogen source) to leaves in autumn to hasten leaf fall and adding lime to leaf piles beneath the tree. In pears, apply urea by itself, because zinc can be phytotoxic.

If you are using sprinklers that wet any of the tree’s foliage, irrigate between sunrise and noon to allow adequate drying time, or reduce the angle of the sprinkler.

Disease-resistant Cultivars

Table 1 lists the relative susceptibility of different apple varieties to apple scab. Major breeding efforts for disease resistance are ongoing in New York, where Enterprise, Liberty, Prima, Priscilla, and many newer varieties appear to be resistant to scab. Scab-resistant crabapples also are available.

European pear cultivars with negligible scab risk include Arganche, Barnett Perry, Batjarka, Brandy, Erabasma, Harrow Delight, Muscat, Orcas, and Passe Crassane. Because Asian pears (Pyrus pyrifolia) are a different species, they are less susceptible to scab than European pears (P. communis).

TABLE 1. Susceptibility of Apple Varieties to Apple Scab.
Susceptible Highly resistant
Bellflower Easy-Gro
Blushing Gold Enterprise
Fuji Florina
Gala Freedom
Golden Delicious Goldrush
Granny Smith Jon Grimes
Gravenstein Jonafree
Grimes Liberty
Ida Red Mac-free
Jonathan Prima
Monroe Priscilla
Mutsu Pristine
Paula Red Redfree
Red Delicious Sir Prize
Rome Beauty Spigold
Stayman Winesap Williams Pride
Winesap
Yellow Newtown
York Imperial
Chemical Control

Fungicide sprays are necessary only if the weather is rainy and leaves are likely to remain wet for 9 or more hours. Fungicide applications require careful attention to timing, as preventing early infection is the most important step toward successfully controlling later fruit infections. It is difficult to prevent secondary fruit infections once primary infections occur.

Unlike peach leaf curl, treatments for scab made when trees are completely dormant aren’t effective and aren’t recommended. If treatments are needed, the generally recommended time is between when buds begin to break and a month after petal fall.

If rain threatens, it is important to apply a fungicide as soon as you see the tips of the leaves emerge. A second application might be needed 10 to 14 days later if it is still rainy, once you can see blossom clusters but before they have opened. If rainy weather continues, apply a third spray toward the end of the bloom period, when most of the petals have fallen.

The surfaces of the fruit and foliage become more resistant to infection as the season progresses, although extended wet, foggy weather can lead to an infection period due to secondary spores that develop on leaves and fruit. If no scab infections are evident 1 month after petal fall, secondary infections probably won’t be a problem, and fungicide sprays can stop. However, continue to watch for pinpoint scab symptoms, especially if late rains occur.

Several fungicides are available for controlling apple and pear scab. These include fixed copper, Bordeaux mixtures, copper soaps (copper octanoate), sulfur, mineral or neem oils, and myclobutanil. All these products except myclobutanil are considered organically acceptable.

Generally copper or Bordeaux sprays should be used only from green tip to full bloom. Later applications increase the risk of fruit russetting, a chemical burning of the fruit skin, although in some years this occurs even if you’ve used these materials only before full bloom. Fixed copper products include Lilly Miller Kop-R-Spray concentrate and Monterey Liqui-Cop. Bordeaux mixture is a combination of copper sulfate and hydrated lime that must be mixed just before application. For more information about how to prepare this fungicide, see Pest Notes: Bordeaux Mixture.

You can apply wettable sulfur through bloom and early fruit set. When using sulfur-containing compounds such as wettable sulfur, never apply them within 3 weeks of an oil application or when temperatures are near or higher than 90°F. Bordeaux has a narrower application time frame than other sulfur-containing products, because it contains copper, and shouldn’t be applied after full bloom.

Myclobutanil (Spectracide Immunox Multipurpose Fungicide Spray Concentrate) is a synthetic fungicide that is effective against apple scab. You can apply it any time from green tip until after petal fall.

REFERENCES

Broome, J. C., and C. A. Ingels. Dec. 2008. Pest Notes: Peach Leaf Curl. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7426.

Flint, M. L. 1998. Pests of the Garden and Small Farm: A Grower’s Guide to Using Less Pesticide, 2nd ed. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3332.

Giraud, D. D. 1989. Apple Scab Control for the Home Orchardist. Univ. Calif. Coop. Exten. Publ., Humboldt Co.

Ingels, C. A., P. M. Geisel, and M. V. Norton. 2007. The Home Orchard: Growing Your Own Deciduous Fruit and Nut Trees. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3485.

Ohlendorf, B. L. 1999. Integrated Pest Management for Apples and Pears, 2nd ed. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3340.

Postman, J. D., R. A. Spotts, and J. Calabro. 2005. Scab resistance in Pyrus germplasm. Acta Hort. 671:601–608.

Swezey, S., P. Vossen, and J. Caprile. 2000. Organic Apple Production Guide. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3403.

UC Statewide IPM Program. Nov. 2000. Pest Notes: Bordeaux Mixture. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7481.

PUBLICATION INFORMATION

Pest Notes: Apple and Pear Scab (formerly titled Apple Scab)
UC ANR Publication 7413

Authors: D. D. Giraud, UC Cooperative Extension, Humboldt Co. R. B. Elkins, UC Cooperative Extension, Lake/Mendocino Co. and W. D. Gubler, Plant Pathology, UC Davis.

Produced by UC Statewide IPM Program, University of California, Davis, CA 95616

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Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2019 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

Agriculture and Natural Resources, University of California


How does black knot survive and spread?

The black knot fungus overwinters in the galls on branches and trunks.

Spores are released during wet periods in the spring. The wind carries these spores to trees where they infect young green shoots or wounded branches.

The fungus grows within the branch for several months with no outward symptoms of disease.

As the fungus grows, it releases chemicals that make the tree grow extra plant cells that are unusually large. This unusual growth results in the swollen, woody galls.

Galls are made up of both plant and fungal tissue.

One year after infection, galls can be seen as a swollen area of the branch with a velvety olive green covering of fungal growth.

Two years after infection, the gall has turned black and hard. These galls release spores in spring when wet.

Sometimes, the branch and the gall die after spores are released in early spring. If the branch lives, the knot keeps getting bigger and produces new spores every spring.

The gall can completely encircle and girdle a branch. When this happens, the leaves beyond the gall wilt and die.

Although the black knot fungus will not cause the trunk to rot, the cracks from the infection can let in other wood rotting fungi.

How to manage black knot

In areas where there are many wild Prunus sp. infected with black knot, avoid planting landscape and edible Prunus sp.

If only a few infected wild Prunus sp. are present, prune out existing galls or completely remove infected plants to reduce the amount of fungal spores present before planting landscape and edible Prunus sp.

Choose trees that have some tolerance to black knot. (See tree list above under “Trees affected by black knot in Minnesota”.)

Inspect all trees and shrubs for black knot before purchasing them from the garden center. Do not purchase any trees with black knot galls.

Black knot galls can be removed from infected trees through pruning. This will make ornamental plants look better and reduce the amount of fungal spores produced within the tree canopy each spring.

Pruning out galls is not necessary in trees where black knot galls do not result in wilt and death of leaves and young branches.

Pruning cuts should be made in late winter (February or March) when temperatures are below freezing. This will prevent black knot spores from infecting the pruning wound.

Make the pruning cut at least 4 inches below the black knot gall.

Infected branches should be removed from the area and burned, buried, or disposed of. Infected branches left below the tree will continue to release spores that can start new infections in the tree.

To maintain a black knot free tree, it will be necessary to inspect the tree and prune out any new galls each winter.

Cracked and oozing galls on trunks or large branches should be inspected by a certified arborist to determine if the tree is stable. Black knot will not rot wood but wood decay fungi can enter through cracks caused by black knot. These fungi cause wood rot that weakens the tree.

Fungicides can be used to protect young trees or trees that will be severely affected by black knot.

Prune out any existing galls in late winter before applying fungicides in spring.

Fungicide sprays must be applied in early spring to protect young green shoots.

Begin fungicide treatment when flower buds are just beginning to open.

Repeat sprays according to label instructions until shoots mature or weather is consistently warm and dry.

Sprays work the best when applied before a rain event when temperatures are warmer than 60°F.

Before applying fungicide read the label carefully.

The plant to be treated MUST BE listed on the label or the fungicide cannot be used on that plant.

Not all fungicides registered for use on ornamental Prunus varieties can be used on edible Prunus varieties.

For large trees, high-pressure spraying equipment is needed in order to get complete coverage. Hire a professional arborist who can safely operate all necessary equipment.

Fungicides with one of the following active ingredients are effective in protecting trees from black knot:


Disease Control for Home Pear Orchards

By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106

Telephone: (203) 974-8601 Fax: (203) 974-8502
Email: [email protected]

There are a number of diseases that commonly occur year after year in both commercial and home plantings of pears. These diseases do not infect at the same time but appear in a fairly regular sequence depending on the weather and the development or phenology of the pear host, beginning at dormancy and continuing until fruit are harvested. Consequently, a season-long program for disease management is often necessary in order to harvest a high percentage of useable fruit. The diseases that are common in pear include fire blight, pear scab, Fabraea leaf spot, and sooty blotch. Weather conditions greatly influence both the occurrence and severity of plant diseases. Therefore, diseases are generally most difficult to control in years of prevailing high temperature, high humidity, and abundant rainfall and cloudcover.

I. CONTROL STRATEGIES

Pear diseases can be effectively managed through the combined use of culture, sanitation, resistance, and fungicide sprays. This integrated approach to disease control minimizes the reliance upon one type of control over the others and usually results in a high percentage of quality fruit.

A. Culture:

Cultural methods include maintaining tree vigor by proper planting, fertilizing, and pruning and by following general practices that help to minimize tree stress.

B. Sanitation:

Sanitation involves pruning and removing affected or dead portions of the tree and removing diseased foliage or fruit, which are often important sources of inoculum for the next season.

C. Resistance:

Resistance involves selection and planting of varieties with genetic resistance to specific diseases. This effectively reduces or eliminates occurrence of the disease in question.

D. Fungicide Sprays:

Proper selection, timing, and application of these sprays are important. Thorough coverage of all parts of the tree is necessary and sprays should be applied until runoff. The fungicide label will contain information on plant hosts and diseases, dosage rates, days-to-harvest intervals, and safety precautions.

II. COMMON DISEASES

A. Fire Blight:

Fire blight, cause by the bacterium Erwinia amylovora, is the most devastating disease of pear. Fortunately, this disease is not a problem every year and, when it does occur, it is often isolated to specific geographical locations. However, when infection occurs, the disease develops quite rapidly and can destroy individual trees or even orchards in a single season.

The bacteria survive the winter in old cankers on pears and other plants and in healthy pear buds. This disease can occur in four phases: canker blight, blossom blight, shoot blight, and trauma blight. As the weather becomes favorable for growth in the spring, the bacteria begin to multiply rapidly and can be seen oozing out of tissues. This creamy, bacterial ooze is attractive to insects and they pick it up and carry it to open flower buds where infection occurs. The bacteria are also carried by wind and rain to open pear blossoms. Infected tissues are characterized by their blackened, "burned" appearance, hence the name "fire blight."

The most effective method for control of this disease in home plantings is sanitation. Any cankered or infected branches or twigs should be cut back to healthy wood during the dormant season. All pruning cuts should be made at least 8-12" below visible symptoms. All tools should be disinfested with 10% bleach (1 part bleach: 9 parts water) or 70% alcohol. Prunings should also be removed from the vicinity of the tree. In addition to these practices, it is important to scout for new infections and remove blighted tissues as soon as they appear. The effects of this disease can also be minimized by maintaining overall tree health by following proper cultural practices that avoid excessive vigor. Pear cultivars vary with regard to their overall susceptibility to fire blight, so "less susceptible" cultivars (e.g., Comice, Winter Nelis) can be selected for planting.

B. Fabraea Leaf Spot:

Fabraea leaf spot, also known as leaf blight and black spot, is caused by the fungus Fabraea maculata. This disease usually appears late in the growing season but can occasionally develop in late May and early June. Fabraea leaf spot attacks leaves, fruit, and twigs of pear. Symptoms first appear as brown to black spots on the leaves. Heavily infected leaves often yellow and drop prematurely. Severe defoliation can substantially reduce tree vigor and yield, especially if trees are defoliated several years in a row. Lesions on fruit appear similar to those on leaves but become slightly sunken as fruit expand. Severely infected fruit may also crack. Once established in a tree or planting, this disease is difficult to control since significant amounts of fungal inoculum overwinter on infected leaves. Spores of the fungus are easily spread by splashing rain and wind in the spring.

Effective control includes a good sanitation program. Since overwintering infected leaves are a major source of spores in the spring, removal of all fallen leaves during the dormant season significantly reduces the chances for new infection. In addition, properly selected and timed fungicide sprays are important for disease control (refer to Spray Guide below).

C. Pear Scab:

Pear scab, caused by the fungus Venturia pirina, is a disease that is quite similar to apple scab. The fungus causes circular, velvety, olive-black spots on leaves, fruit, and sometimes twigs. As the lesions age, they become gray and cracked. The fungus overwinters on dead, fallen leaves and produces spores (primary) in the spring that can infect during periods of rain. Infection from these primary spores can take place anytime after pear growth begins until mid to late June if suitable weather conditions exist. During the summer, a different spore (secondary) is produced by the fungus that is capable of inciting more new infections when splashed onto leaves and fruits by rain.

This disease is effectively controlled by a good sanitation program in which diseased leaves and fruit are removed from the vicinity of the tree. This significantly reduces sources of inoculum in the spring. Scab can also be controlled with properly selected and timed fungicide sprays (refer to Spray Guide below).

D. Sooty Blotch:

Sooty blotch, caused by the fungus Gloeodes pomigena, is recognized by black, sooty smudges on the surface of pear fruit. This disease is particularly severe when rainy weather occurs early in the season and continues into the summer. Sooty blotch develops gradually during periods of high humidity. It is favored by frequent showers, prolonged cloudy weather, and poor air circulation.

Since the fruit infections are superficial, they can often be removed with vigorous washing and rubbing. In addition, practices that promote air circulation, such as pruning and mowing the grass around the tree, are usually enough to keep this disease in check. Fungicide sprays can be applied if the tree has a history of severe disease and blemish-free fruit are important (refer to Spray Guide below).

III. SPRAY GUIDE

A. Pesticides:

Several fungicides are effective for control of many of the common diseases of pear. These include:

1. Ferbam: used alone or in combination with thiophanate methyl for control of scab, Fabraea leaf spot, and sooty blotch.
2. Mancozeb: for control of scab and Fabraea leaf spot.
3. Thiophanate methyl: use in combination with mancozeb or ferbam for control of scab, sooty blotch, and Fabraea leaf spot.

CAREFULLY READ THE LABEL ON EACH PESTICIDE BEFORE USE.

B. Spray Schedule:

GROWTH STAGE

MATERIALS/CULTURE

DORMANT: during the winter or dormant season

Prune any cankers or infected wood at least 12" below symptoms disinfest tools

GREEN CLUSTER BUD: after the blossom buds are fully exposed but before they separate from the cluster

Scab and Fabraea leaf spot

Thiophanate methyl in combination with mancozeb, or mancozeb

WHITE BUD: approximately 7 days after Green Cluster Bud

BLOOM: when 25% or more of the blossom buds are open

PETAL-FALL: when 90% or more of the petals have dropped

FIRST COVER: 7 days after Petal-Fall

Scab, Fabraea leaf spot, and fire blight

Same as above and scout for blighted leaves or twigs

SUMMER COVER SPRAYS: apply on a 10- to 14-day schedule until harvest depending upon the weather refer to "days-to-harvest" interval on fungicide label

Scab, Fabraea leaf spot, and sooty blotch

Thiophanate methyl in combination with mancozeb or ferbam

Summary

Pears in the home orchard are commonly affected by several diseases every year. These are scab, Fabraea leaf spot, sooty blotch, and occasionally fire blight. The symptoms, causes, spread, and control strategies for these diseases of pear are discussed.

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