Black knot of cherry

Yard and Garden: Dealing with Unusual Growths on Trees

Trees add value to the landscape, so homeowners may become concerned when their trees develop unusual growths. Iowa State University Extension and Outreach horticulturists explain what causes some of these growths. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or [email protected]

There are black growths on the branches of my chokecherry. What are they?

The black growths probably are black knot, a fungal disease that occurs on chokecherry, European birdcherry and several other wild and cultivated cherries and plums. The black growths (galls) can vary from a few inches to a foot or more in length.

Black knot is caused by the fungus Apiosporina morbosa. Fungal spores produced on 1-year-old and older galls initiate new infections. The knots are soft when newly formed and later become hard and black with age.

Black knot is difficult to control. If only a few galls are present, prune out the infected branches in late winter. When pruning, make the cut at least 3 to 4 inches below the gall. The pruned material should be removed from the area and destroyed. Several fungicide applications may help prevent future infections. The fungicide applications should begin just before bud break and continue until after fruit set.

Attempts to control black knot in badly infested trees are likely to be unsuccessful. When dealing with severe black knot infestations, the best options are to do nothing or remove the tree and replace it with a black knot resistant tree.

There is a large growth on the trunk of my maple tree. What is it?

The growth on the trunk of the maple tree is likely a burl. Burls are abnormal swellings or growths that develop on the trunks and branches of trees. Burls can be found on deciduous trees and evergreens. The exact cause is unknown. Possible causes include bacteria, fungi, insects, wounds or environmental stress.

Burls do not kill trees. However, they may reduce the tree’s vigor. On a positive note, the unusual swirling grain pattern found in burls makes them prized by woodworkers. Burls can be carved into bowls, furniture and other objects.

There are golf ball-sized growths on the branches of my pin oak. Each growth has several horn-like projections. What are they?

The growths on the pin oak are a type of gall. Galls are abnormal growths of plant tissue induced to form by mites, insects or other small organisms. The galls on the pin oak are called horned oak galls because of the horn-like projections that protrude from the surface of mature galls. Horned oak galls are caused by a tiny wasp (Callirhytis cornigera).

The life cycle of Callirhytis cornigera is unique. It consists of alternating generations that give rise to two distinct types of galls produced by two groups of female wasps. In spring, wasps emerge from mature horned oak galls. At this time, all of the wasps are female. The female wasps deposit their eggs on developing leaves. This initiates the development of tiny leaf galls along the leaf veins. Adult wasps emerge from the leaf galls in mid-summer. This group of wasps includes both male and female wasps.

Mated females lay their eggs on twigs, initiating the development of horned stem galls. Each gall houses anywhere from 1 to 160 larvae. Each larva is housed individually in a cone-like structure within the gall. Wasp development in twig galls takes almost three years. As the immature wasps reach pupation, the horns rise from within the gall to break through the surface and release the adult wasps.

Pin oaks with small infestations of horned oak galls are not seriously harmed. However, heavy infestations can cause tree decline and even death. Fortunately, heavy infestations of horned oak galls on pin oaks in Iowa are not common. There are no effective control measures for home gardeners.

Galls on Live Oak Leaves. Photo by Mike Merchant

Galls on trees are caused by insects laying eggs inside or feeding on the branches of leaves of trees and other plants. This usually occurs in the spring. The galls, or tumor-like growths, are produced by the tree in response to chemicals injected into it by an adult or larval gall-making insect. The shape of the gall is determined by the chemicals used by each species of gall-maker. Galls can be round and dense, woolly, fuzzy, veined, bullet-shaped or horned. Over 80% of galls reported in the U.S. grow on different oak species.

What insects cause galls to form?

Most insects that make galls are tiny wasps. Galls can also be caused by mites, insects, nematodes, bacteria or fungi.

Are galls harmful to trees?

Galls can have an ugly appearance. However, most do not seriously affect the health of a plant or tree. Heavy infestations may distort leaves or cause an early leaf drop. There is no need to remove the galls from a tree. The only sure way to prevent galls is to choose plants that are not hosts to gall-making insects and mites.

Sources for pictures and information:

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Black Knot of Prunus

Black knot of Prunus is caused by the native fungal pathogen Apiosporina morbosa.

Hosts

Black knot occurs on more than 25 species in genus Prunus (cherry, plum, chokecherry, peach, nectarine, apricot, almond, etc.), but is particularly destructive on ornamental cherry, plum and chokecherry in the landscape.

Symptoms & Disease Cycle

Black knot is a cankering disease that is sometimes highly destructive on landscape Prunus. New infections occur in the spring on succulent shoots with symptoms developing during the summer and early fall. In the months following infection, green stems become swollen but the most conspicuous symptoms are not yet apparent. As a result, the disease often goes undetected during the first growing season. Often times, the disease is first noticed early in the next growing season after infection (year two), after the twig/branch swellings have developed into thickened, abnormally-shaped, black-colored galls. The galls will continue to grow and expand as long as the stem tissue remains alive. The galls can either girdle the stems outright or cause the distorted stems to grow at odd angles. Gall and bark tissues splits open to allow for the release of fungal spores that are produced during the spring and early summer. This spore-bearing layer appears as an olive-green, velvet-like coating over the surface of the galls. The spores are splashed or blown to nearby stems to initiate new infection centers. Intact, green stems produced that season are primarily infected, although the fungus can also colonize wounds as well. Later in the summer, established galls may take on a white, dusty coating. This is the result of infection by a mycoparasitic fungus that is not a pathogen of the host (mycoparasites are fungi that attack other fungi). On certain landscape and forest trees, such as black cherry (P. serotina), very large galls develop over many years to decades on primary branches or the main trunk. Apiosporina is native in North America and is widespread in both forest and landscape settings. The disease can be found almost anywhere Prunus occur but is often more destructive on non-native trees and shrubs. If left unmanaged, black knot can be a destructive disease on plum and chokecherry, especially in shaded settings.

Management

The dormant season is the best time to scout for black knot when foliage is not present to obscure the galls. When detected early, black knot can be effectively managed by pruning out infected branches at least 6–12 inches away from the gall, if possible. The infected stems and branches should be removed from the site. In subsequent months and years, careful scouting of nearby branches should be performed regularly to determine if new infections have established. New, succulent shoots are especially vulnerable during rainy periods in the spring when temperatures are mild. Sanitize pruning tools with rubbing alcohol or lay in direct sunlight after pruning infected trees. Chemical management will not be effective in controlling the disease when the canopy is harboring numerous galls. When disease severity is low, fungicides may help to eradicate the pathogen in conjunction with pruning. Chemicals labeled for use against the pathogen include: copper hydroxide, copper sulfate, captan and thiophanate-methyl. Some fungicides are not labeled for use on fruit intended for human consumption. Read labels carefully. Fungicides are most effective in limiting new infections in the spring when the fungus is sporulating.

Black Knot of Plums and Cherries

Black knot of plums and cherries is a widespread and serious disease throughout the United States. Black knot is a common disease in Ohio on wild plums and cherries and in home orchards where pruning and spraying are not regularly practiced.

Figure 1. Hard, black galls caused by the black rot fungus on plum twigs.

The disease becomes progressively worse during each growing season and unless effective control measures are taken, it can stunt or kill the tree. The black knot fungus can infect American, European, and Japanese varieties of cultivated plums and prunes. Sweet, tart and Mahaleb cherries are also affected by the fungus, but are generally less susceptible than plum or prune. Occasionally, it may also infect apricots, peaches and other Prunus species.

Symptoms

The black knot fungus mainly affects twigs, branches and fruit spurs. Occasionally, trunks may also become diseased. Usually, infections originate on the youngest growth. On infected plant parts, abnormal growth of bark and wood tissues produce small, light-brown swellings that eventually rupture as they enlarge. In late spring, the rapidly growing young knots have a soft (pulpy) texture and become covered with a velvety, olive-green growth of the fungus. In summer, the young knots turn darker and elongate. By fall, they become hard, brittle, rough and black. During the following growing season, the knots enlarge and gradually encircle the twig or branch. The cylindrical or spindle-shaped knots may vary from one-half inch to a foot or more in length and up to 2 inches in diameter. Small knots may emerge from larger knots forming extensive galls. After the second year, the black knot fungus usually dies and the gall is invaded by secondary fungi that give old knots a white or pinkish color during the summer.

Smaller twigs usually die within a year after being infected. Larger branches may live for several years before being girdled and killed by the fungus. The entire tree may gradually weaken and die if the severity of the disease increases and effective control measures are not taken.

Causal Organism and Disease Development

Black knot is caused by the fungus Apiosporina morbosa. The fungus overwinters in knots on twigs and branches or in the infected wood immediately surrounding them. In the spring, the fungus produces spores (ascospores) in sacs (asci) contained within tiny fruiting bodies on the surface of the knots. These ascospores are ejected into the air during rainy periods and are blown for moderate distances by wind currents. Only succulent green twigs of the current season’s growth are susceptible to infection. Ascospores that land on them may germinate and cause infection if the twigs remain wet for a sufficient length of time.

Normal growth is disrupted in the infected regions, and a knot is formed as the fungus causes the plant to produce tumorlike growths. Knots may become visible by the late summer of the year of infection but often are not noticed until the following spring, when they begin to enlarge rapidly. New ascospores capable of spreading the disease may be formed in the young knots the year following infection but often are not formed until the second spring. The fungus continues to grow in infected wood during the spring and fall months, causing the knots to elongate several inches each year and eventually girdle affected twigs and branches.

Ascospores are potentially available from the time of bud break until terminal shoot growth stops, but the greatest number appear to be released during the period between white bud and shuck split. Although the precise environmental conditions required for infection are uncertain, only a few hours of rain apparently are required at temperatures above 55 degrees F (13 degrees C), whereas much longer rainy periods are required to produce infection at temperatures below this threshold.

Control

Most plum varieties grown in Ohio, including Stanley and Damson, are susceptible to this disease. It has been reported that Early Italian, Brodshaw, Fallenburg, Methley and Milton are somewhat less susceptible than Stanley; Shiro, Santa Rose, and Formosa are much less susceptible; and President is apparently resistant to black knot. Japanese varieties of plums are generally less susceptible than most American varieties.

When planting new plum or prune trees, avoid planting trees next to or downwind from an old or abandoned orchard with a significant black knot problem. Similarly, remove all wild plum and cherry trees (potential disease reservoirs) from fence rows or woodlands next to the orchard site.

Established orchards or backyard trees should be scouted or examined each year for the presence of black knot, and infected twigs should be pruned out and destroyed or removed before bud break. Chopping prunings with a flail mower (to strip infected bark and knots from wood) is an alternative method of destroying infected prunings if burning or burying is impractical. It is important to prune at least 2 to 4 inches (5 to 10 cm) below each knot because the fungus grows beyond the edge of the knot itself. If pruning is not possible because knots are present on major scaffold limbs or the trunk, they can be removed by cutting away the diseased tissue down to healthy wood and out at least ½ inch (1 cm) beyond the edge of the knot.

Fungicides can offer significant protection against black knot, but are unlikely to be effective if pruning and sanitation are ignored. The timing of fungicide sprays should be adjusted to account for inoculum levels and weather conditions. Where inoculum is high because of an established black knot problem or a neighboring abandoned orchard, protection may be needed from bud break until early summer. Where inoculum has been maintained at low to moderate levels, sprays are most likely to be useful from white bud through shuck split (the period of maximum availability of ascospores). Fungicides are most necessary and will provide the greatest benefit if applied before rainy periods, particularly when temperatures are greater than 55 degrees F (13 degrees C). In evaluating control programs, remember that knots often do not become apparent until the year following infection.

For the most current fungicide recommendations and spray schedules, backyard growers are referred to Bulletin 780, Controlling Diseases and Insects in Home Fruit Plantings, and commercial growers are referred to Bulletin 506, Midwest Fruit Pest Management Guide. These publications can be obtained from your county Extension office or the CFAES Publications online bookstore at estore.osu-extension.org.

Figure 2. Black knot disease cycle. We want to thank the New York State Agricultural Experiment Station for use of this figure. Taken from Tree Fruit IPM Disease Identification Sheet No. 6.

This fact sheet was originally published in 2008.

My ornamental plum tree has large black swellings on several of the branches. What is this and will it kill my tree?

Your tree had a disease called Black Knot, which is a widespread fungal disease that affects plum and cherry, and occasionally apricots, peaches and other plants in the Prunus genus, like chokecherry. Black knot is common throughout Nebraska in wild plum thickets. The disease in characterized by rough, hard, elongated, black swellings that persist on infected plants.

Black knot fungus infects fruiting spurs, stems and branches of susceptible plants, and occasionally the main trunk is affected. Infection occurs through splashing or wind blown spores when new growth is about 1 inch long. Fungal spores are discharged in moderate to heavy amounts during the pink blossom stage of cherry or plum, and ends about the time elongation of the new growth stops.

On infected plant parts, abnormal growth of bark and wood tissues produce small, light-brown swellings that eventually rupture as they enlarge. In late spring, the rapidly growing young knots have a soft (pulpy) texture and become covered with a velvety, olive-green growth of the fungus.

During summer, the young knots turn darker and elongate. By fall, they become hard, brittle, rough and black. The following growing season, the knots enlarge and gradually encircle the twig or branch. The cylindrical or spindle-shaped knots may vary from one-half inch to a foot or more in length and up to 2 inches in diameter.

Small knots may emerge from larger knots forming extensive galls. After the second year, the black knot fungus usually dies and the gall is invaded by secondary fungi that give old knots a white or pinkish color during the summer.

Smaller twigs usually die within a year after being infected. Larger branches may live for several years before being girdled and killed by the fungus. The entire tree may gradually weaken and die if the severity of the disease increases and effective control measures are not taken.

Avoid planting new plum or cherry trees next to or downwind from an old or abandoned orchard with a significant black knot problem. Similarly, remove all wild plum and cherry trees, which are a potential disease reservoir, from fencerows or woodlands within 600 feet of the orchard site. Established orchards or backyard trees should be scouted or examined each year for the presence of black knot, and infected twigs should be pruned out and destroyed or removed before bud break.

It is important to prune at least 2-4 inches below each knot because the fungus grows beyond the edge of the knot itself. If pruning is not possible because knots are present on major scaffold limbs or the trunk, they can be removed by cutting away the diseased tissue down to healthy wood and out at least 1/2 inch beyond the edge of the knot. Burn or bury the prunings before April 1st.

Fungicides can offer significant protection against black knot, but are unlikely to be effective if pruning and sanitation are ignored. Fungicides are most necessary and will provide the greatest benefit if applied before rainy periods, particularly when temperatures are greater than 55 degrees F. In evaluating control programs, remember that knots often do not become apparent until the year following infection.

For more information, view:
Black Knot of Plums and Cherries (pdf), Ohio State University

UConn Home & Garden Education Center

Black Knot Disease of Cherry and Plum

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Black knot, a serious disease of plums in Connecticut, is caused by the fungus Apiosporina morbosa (aka Dibotryon morbosum). Many American, Japanese and European plums are susceptible and it is also found on ornamental flowering cherry and plum trees. A major source of infection is the native American wild cherries that are common along the roadside. Severe infections can result in the death of branches and, eventually, the entire tree.
Symptoms
The black knot fungus causes elongated swellings or knots on infected branches. At first the knots, sometimes called galls, are greenish to light brown. Upon aging, they turn black and become quite hard. Initially, the knots are about an inch in diameter but, because they grow annually, they will eventually girdle the stem. This will result in the death of the upper portion of the stem. The knot will often be invaded by other fungi along with various boring insects.

Disease Cycle
Spores are discharged from mature knots during rainy periods in the spring. Young growth (under three years old) is very susceptible to infection. However, older branches can also be infected, especially if the bark has been damaged. Infection can be severe when temperatures are between 55º and 77ºF. The major infection period starts when the buds swell in the spring and continues until terminal growth stops or the supply of spores is exhausted.
Several months after infection has taken place, the knot begins to develop. At first it is a greenish swelling on the stem and by the following spring, the familiar black knot is present. The knot may produce spores at this time, but it usually takes two years for the knot to mature. In the spring and fall, the old knots are active, often growing several inches. This new growth is usually not noticed until the following spring. These knots will continue to produce spores until the branch is dead.
Control
On individual trees, a good control program consists of sanitation and protection. Because the wild cherries in the area are the primary source of inoculum, it is wise to remove them. Examine all the plum and cherry trees in late August and prune any knot found on the branches. Mature black knots should also be removed by cutting the stem 4 inches below the knot. All pruned material should be removed from the area and destroyed or buried. If left, the knots may be capable of shedding spores in the spring.

Spraying with a protectant fungicide is often necessary if the disease is to be controlled on the more susceptible varieties such as Stanley, Damson, Shropshire and Bluefre. Moderately susceptible varieties include Methley, Milton, Early Italian, Brodshaw and Fellenburg. Varieties slightly susceptible to black knot include Shire, Santa Rosa and Formosa, whereas President appears to be highly resistant.
Chemical control of the disease should start when the buds begin to swell, temperatures reach 55ºF or higher and the foliage remains wet for six hours or longer after a rain. It will be helpful to maintain a protectant layer of fungicide on the new expanding foliage and twigs until the terminals stop growing or the spores have been discharged. However, infection is greatest from the time the flower buds show pink until two weeks after bloom. During periods of heavy rain and/or rapid plant growth, it may be necessary to reapply the fungicide every seven days.
Despite good cultural practices, pests and diseases at times may appear. Chemical control should be used only after all other methods have failed.
For pesticide information or other questions please call toll free: 877-486-6271.
Edmond L. Marrotte, Consumer Horticulturist, & Dr. David B. Schroeder, Plant Pathologist
Revised by UConn Home and Garden Education Center 2019.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, the Dean of the College, Cooperative Extension System, University of Connecticut, Storrs. The Connecticut Cooperative Extension System is an equal opportunity employer and program provider. To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, Stop Code 9410, 1400 Independence Avenue, SW, Washington, DC 20250-9410 or call (202) 720-5964.

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