The Online Gardener's Handbook
Chapter 6: Ornamental Plants
Disease Control on Ornamental Plants
Table of Contents
- Aster Yellows
- Autumn Shed of Conifers
- Bacterial Soft Rot
- Black Spot
- Botrytis Blight
- Cytospora Canker of Spruce
- Crown Gall
- Dutch Elm Disease
- Fire Blight
- Juniper Blight
- Leaf Blotch, Leaf Blight, Anthracnose
- Leaf Spot
- Needle Cast of Spruce
- Needle Cast of Pine
- Powdery Mildew
- Root Rot
- Armillaria Root Rot
- Sooty Mould
- Tar Spot of Maple
- White Mould
- Bacterial Wilts and Blights
- Learn More
In this chapter, a description of various pests of ornamental plants will be provided along with suggested management options. These management options will not include the use of pesticides. Some biopesticides and certain reduced risk pesticides are still available to the homeowner for controlling weeds and pests in lawns and gardens. For more information, refer to Chapter 2 of this handbook and the Ministry of the Environment's website. For suggestions on managing specific weeds and pests, consult local horticulturalists, Master Gardeners or your local garden supply centre.
NOTE FOR TREE OWNERS: There is an exception under the ban that allows you to hire a licensed exterminator authorized to use commercial pesticides to maintain the health of your tree. This exception applies only to pests that threaten the tree's health. For example, the exception cannot be applied to a pest that impacts the quality of the fruit but will not kill the tree itself. To obtain this exception, licensed exterminators are required to obtain a written opinion from a professional tree care specialist that a pesticide is necessary to maintain tree health. For more information, contact the Ministry of the Environment.
Note that many trees can tolerate some damage, particularly to the foliage, without suffering lasting impacts. Pest descriptions below include suggestions for cultural controls however in many situations these may not be necessary.
Aster Yellows is caused by a mycoplasma-like organism, with symptoms similar to viral diseases. It affects China aster, calendula, coreopsis, cosmos, delphinium, daisies, marigolds, petunias, phlox, zinnia, and many other flowers and weeds. Carrots, celery and lettuce are also attacked. Plants are stunted, and the top leaves turn yellow; blooms are also stunted and may rot, taking on a greenish, leaf-like form. The disease is spread by the six-spotted leafhopper.
Prompt removal and destruction of affected plants is of some help. Controlling leafhoppers may help reduce spread of the disease.
Autumn Shed of Conifers
This natural phenomenon is not a disease. All conifers undergo annual needle shed to varying degrees. Beginning in early autumn, needles toward the inside of the tree (closest to the trunk) turn yellow or brown and fall. Cedars and many junipers have scale-like needles covering tiny branchlets. These trees also shed the oldest branchlets in the fall.
Trees of low vigour, or under stress from factors such as drought, transplant shock, poor drainage or soil compaction may lose more than one year's quota of foliage. In such cases, identifying the problem and alleviating the stress will minimize autumn shed.
Bacterial Soft Rot
This bacterial disease attacks ornamentals, particularly iris, dahlia, chrysanthemum and geranium. Soft rot of iris usually follows borer infestation. The bacteria enter through the bore holes, causing a rapid, wet rot with an offensive odour. The interior of the rhizome disintegrates into a vile, yellow mass while the exterior remains intact. Leaves wither, and the bottoms appear wet and shredded.
Divide infected irises, replanting only the healthy rhizomes. Do not plant too deeply. For other plants, remove as soon as observed and put in garbage. The best defense is to control the iris borer.
Black spot is common in roses. Red to black spots appear on the leaves, and the leaves, beginning at the base of the canes, turn yellow and drop. If the leaves are repeatedly wet by dew, rain or watering, the disease soon becomes serious. The loss of leaves reduces the number and quality of blooms and may so weaken a plant that it cannot survive the winter. The fungus overwinters on infected leaves and in cracks in the bark of canes.
If no mulch has been applied, gather fallen leaves regularly during summer and fall. If mulch is used, level the mounded soil that acted as winter protection before applying a fresh layer early in spring. Avoid watering the leaves; water the soil instead. If this cannot be done, irrigate in the morning, so the leaves will dry quickly. Some nurseries list varieties that are less susceptible to black spot than others.
The Botrytis fungus attacks the flowers of a wide variety of ornamental plants, including begonia, tulip, peony, petunia, geranium, gladiolus and rose. On tulips, the disease begins as small beige to brown spots. During wet weather, it rapidly kills both flowers and leaves, growing down into the stalk and bulbs and often rotting them completely. This is why Botrytis is also known as tulip fire.
Botrytis typically attacks open or senescing (ageing) flowers and leaves, or enters through wounds. It quickly produces grey-brown fuzzy mould with numerous spores. Affected areas on the plant appear soft and brown, and eventually rot. Hard, black structures - sclerotia - often appear late in the season and overwinter in the soil to promote re-infection the next year when another susceptible crop is grown.
Remove all fading, senescing, and diseased plant parts promptly, particularly before predicted periods of wet weather. Improve air circulation around plants.
Cankers kill patches of bark on the trunks and branches of many species of trees and shrubs. The infected sites usually appear different in colour from the surrounding bark. The canker area may be depressed, having bark that is cracked, split, or peeling away. Small, dark fruiting bodies, which are usually reddish, orange or black, not more that 0.5-1 mm dots, may be apparent, especially during moist weather when an ooze of white, amber, orange or black spores is produced. If the canker girdles the trunk, the plant dies.
Healthy plants can usually defend themselves against infection by canker-causing fungi. However, if poor soil or environmental conditions weaken the tree or shrub, the fungi can more easily take hold. Any wounding of the bark by insects, pruning, or lawnmowers creates an easy opportunity for fungi to penetrate. Cankers can also be caused on the south side of a tree by warm, sunny temperatures during the day and severe freezing at night.
Poplars, especially Lombardy poplars, are often severely affected. In fact, Lombardy poplars should not be expected to last more than 15-20 years due to canker death. The leaves on slender branches turn brown first, then the tree gradually dies from the top down. Diseased shoots and branches exhibit small, oval or elongated swellings in the ruptured bark. During damp weather, amber coloured tendrils release spores which are splashed by rain to healthy branches.
Remove and destroy badly infected trees. When partly dead trees are cut near the ground and destroyed, the stump will sprout quickly into a new tree, but it will be re-infected in a few years.
Cytospora Canker of Spruce
This disease damages Colorado blue and Norway spruce, but will also attack white spruce, balsam fir, and eastern hemlock. It badly disfigures but seldom kills the tree. Stresses such as drought and compacted soil predispose plants to attack.
In spring and early summer, the needles on lower branches of trees at least 10 years old turn yellow and then brown. They fall during the winter, leaving unsightly bare branches. This is an indication that cankers have completely girdled the affected branch. A clear amber resin usually oozes from the cankers, dripping onto lower branches and hardening to a white crust.
Remove the infected branches to avoid the spread of spores by wind and rain. Canker wounds on the trunk should be cleaned by cutting away diseased parts. Care must be taken to remove all brown or stained inner bark and to ensure that the edges of cuts are smooth. An oval-shaped cut with smooth edges helps the wound close more quickly. Disinfect the pruning tools after each cut by dipping into a mixture of 1 part chlorine bleach to 3 parts water. Treating wounds with tree dressing or pruning paint is not necessary. Create the best possible growing conditions by root feeding and watering. Be sure to water especially well during periods of extended drought.
Crown gall is caused by a soil-borne bacterium present in most garden soils. It commonly attacks roses, Euonymus, forsythia, fruit trees, raspberries and many other plants. Conifers are not susceptible. The bacteria gain entrance through wounds, and galls or woody swellings develop on the roots, base or stem of the plant.
Avoid unnecessary wounds. Do not buy plants with galls on roots. Replace soil where a severely affected plant has grown before replanting or replant with a non-susceptible species.
If seedlings, especially those growing in pots or flats indoors or in a cold frame, fall over and die, the seedlings have been attacked by one of the damping-off fungi. If stems turn brown or black and appear wire-like near the soil line, Rhizoctonia is involved. If the roots and lower stem are water soaked, soft, and tan brown in colour, Pythium or Phytopthora is involved. Most seedlings are susceptible to these soil-borne fungi.
Prune the dead tips and cones, and remove dead needles that hang in the trees and harbour the fungus. Do not crowd trees; maintain adequate sunlight and air circulation.
Dutch Elm Disease
When leaves, branches or entire limbs of American elm quickly wither and die, the tree is infected by Dutch Elm disease. The fungus is introduced by elm bark beetles as they feed on twigs; it spreads quickly, clogging the vascular system of the tree. One or more branches may be affected initially, but the entire tree usually dies within one or two years.
Destroy the wood of diseased elms.
This bacterial disease causes sudden dying of young twigs and branches, leaving a scorched appearance. Susceptible ornamental plants are cotoneaster, crabapple, hawthorn, mountain ash, firethorn, quince, ornamental pear and serviceberry. Fire blight infection occurs during flowering, when splashing rain or insects carry the bacteria to the plant.
Remove and destroy infected twigs and branches 30 cm below the scorch line. If pruning shears are used, disinfect blades after each cut in a solution of 1 part chlorine bleach to 3 parts water. Nearby trees may be the source of infection. Avoid heavy application of nitrogen fertilizer.
Several fungi cause twigs on junipers, particularly sabina, virginana and scopulorum types, to dieback. The presence of tan-grey cankers and corresponding dieback from the canker to the tip of the twig are characteristic symptoms. At times, symptoms can easily be confused with winter injury. These fungi infect at different times of the year and so are difficult to control without a proper microscopic examination.
Prune out diseased tips to prevent re-infection. Avoid shearing plants in the fall. Provide good air circulation. Do not encourage soft, succulent growth through excessive pruning and fertilization. Avoid drought conditions.
Leaf Blotch, Leaf Blight, Anthracnose
These common and similar diseases affect maples, London plane tree, sycamore, ash, oak, hawthorn, horse chestnut and catalpa. They are most severe when May and June are cool, wet and cloudy. Trees affected in successive years become weakened and branches gradually die.
Infection starts in early spring, when spores released from fallen leaves or infected buds and twigs attack new leaf growth. By May, the first symptoms appear: buds fail to expand and leaves develop small spots. By early summer, the small spots have become large black or dark brown blotches. The affected leaves dry out and drop prematurely.
When a few trees are fairly isolated it is helpful to gather all fallen leaves, as they carry the infection until the following spring. Burn them or put them in the garbage. This is not very effective, however, if nearby leaves are not gathered or if the fungus survives in cankers on the bark of the tree, as is the case with anthracnose of sycamore. Root-feeding and heavy watering are helpful. Maintain good air circulation.
Several fungi and bacteria cause leaf spots on plants such as begonia, coleus, impatiens, marigold, salvia, chrysanthemum, delphinium, zinnia, iris, lily-of-the-valley and phlox. A different variety is found on catalpa and hawthorn. The disease weakens but seldom kills the plant, though its effects are enhanced by wet weather. Many of the leaf spot infections begin as small dark brown to black spots on lower leaves, and then progress up the plant. Spots increase in size, forming large black areas. The dead leaves remain on plants.
Remove all infected plant parts to avoid carry-over into the next growing season. Promote quick drying of leaves by adequate spacing and removal of weeds. Avoid watering in the evening.
Needle Cast of Spruce
Rhizophaera needle cast is a disease of blue and white spruce causing premature death and shedding of needles. The fungus attacks young needles on the lower branches first, usually in May and June. By late summer, these needles show a yellow mottling, later turning brown or purplish-brown. Rows of tiny black dots on the needles are also noticed. The disease overwinters in infected needles on the tree or on the ground, and is spread the following spring by wind and splashing water.
Do not crowd trees; maintain adequate sunlight and air circulation.
Needle Cast of Pine
With needle cast of pine, the previous season's needles turn red in late winter and early spring and drop in late spring to early summer. Black fruiting bodies develop on the fallen needles. Only the current year's needles remain on the tree. This disease severely defoliates Austrian and Scots pine.
Do not crowd trees; maintain adequate sunlight and air circulation.
The first symptoms of this disease are white powdery spots on stems and leaves. If widespread, the entire leaf may become infected. On plants such as roses, the young shoots may be infected first. On many plants such as phlox, the disease starts on lower leaves. Other susceptible plants include begonia, chrysanthemum, crabapple, delphinium, lilac, Norway maple, privet, snapdragon and zinnia.
Powdery mildew can be expected after midsummer, when the days are sunny and dry and the nights are cool. This leads to extreme fluctuations in humidity levels, which promotes the development of the disease. Powdery mildew is unsightly, and will reduce the health and vigour of the plants if left untreated.
Plant sensitive plants in in areas with good air circulation. Planting in sunny locations can reduce incidence of powdery mildew, for those plants that can tolerate full sun. The use of overhead watering during the day has been shown to reduce the spread and development of powdery mildew. Avoid overhead watering in late afternoon. Remove all plant residue in the fall to reduce the source of inoculum in the following spring. Select resistant varieties where possible, if powdery mildew has been a problem in the past.
Fungi such as Pythium, Phytophthora, Rhizoctonia, and Fusarium cause root and lower stem rot of herbaceous ornamentals and some woody plants. These soil-dwelling fungi are more common when contaminated soil is used to grow transplants, or when soil is wet for extended periods of time.
Remove and destroy roots and surrounding soil near infected plants. Use only sterile soil to grow transplants. Do not plant transplants or woody plants too deeply. Supply adequate water and good drainage, and avoid overcrowding. Do not plant bulbs with sunken or discoloured spots.
Armillaria Root Rot
Armillaria root rot mainly attacks woody plants weakened by drought and other stresses. The roots and lower trunk show white fans of fungal growth underneath the bark, and the inner wood is discoloured. Black shoestring-like strands called rhizomorphs are found on the root surfaces, and these spread the fungus from plant to plant as they grow through soil. Clusters of honey-coloured mushrooms may appear at the base of infected plants in the fall.
Avoid planting trees too deeply or watering too much or planting in an area with poor drainage.
Rusts are cool weather diseases that require free water for germination and infection of leaf tissue. The fungi are host specific, with hollyhock, alpine currant, geranium, pine, rose, fuchsia and snapdragon the most commonly attacked plants. Orange to reddish-brown spots or lesions with powdery masses of spores appear by midsummer on the undersurface of leaves and on stems.
Control is difficult, though it is recommended that you avoid overhead watering late in the day and destroy all infected leaves or plant debris.
Cedar-Apple Rust, Quince Rust and Hawthorn Rust
These diseases are caused by fungi with complex, two-year life cycles. They require two hosts: the red cedar juniper and a deciduous tree or bush. With cedar-apple rust, apple or crabapple serve as the second host. With quince rust, apple, crabapple, quince or serviceberry are the alternate hosts and with hawthorn rust, hawthorn serves as the alternate host.
The fungus overwinters as kidney-shaped or pitted galls on the twigs or leaves of the juniper. During warm spring rains during mid-to-late May the galls swell, producing bright orange jelly-like horns. Spores released from the horns are blown by wind to the deciduous host.
Infection on the deciduous host initially appears as small, pale yellow spots. With cedar-apple rust, orange cup-like structures with horny growths appear on the underside of the leaves by midsummer. Quince rust infects only the fruit, causing the calyx end (the remains of the flower) to become puckered and dark green in colour. In late summer, spores from these structures are blown back onto the junipers, and the cycle begins again.
Remove galls from the junipers. Remove alternate hosts where possible.
Control can only be achieved if all common mallow and round leaf mallow weeds are removed from the area because of their susceptibility to the same rust.
Most garden roses are susceptible. Masses of yellow to bright orange powdery spores are produced on the underside of leaves and on the canes. Upper leaf surface will show reddish brown spots. Heavily infected leaves will yellow, dry and fall. Frequent rain or dew and temperatures of 18-22°C favour development of this disease.
Remove infected leaves and destroy.
Western Gall Rust on Pine
Western gall rust commonly attacks jack, Austrian and mugho pine. It causes the formation of pear-shaped woody galls on branches and main trunk. Galls are perennial, enlarging each year, reaching 10-30 cm before dying. Branches show significant dieback and witches brooming.
Remove infested trees or branches.
White Pine Blister Rust
This rust requires two hosts to complete its life cycle. The first is the white pine; the second is the red or black currant. The fungus initially invades branches and slowly spreads into the trunk, eventually killing the tree. It infects the bark, turning it orange as the stem swells and blisters. The blisters on the pine release spores that blow to currant leaves. In late summer, spores are released from the infected currant plants to infect healthy pines, and the cycle starts over.
Prune out infected branches from trees. Do not plant white pine and currants in close proximity.
This disease affects crabapple and firethorn. Olive-green spots appear on the leaves, later turning dark brown and black. Dark, irregular spots also appear on the fruit. If the disease is severe, the skin cracks and the fruit is deformed. Leaves may drop and the following year's crop may be reduced.
The fungus that causes the scab overwinters on dead leaves on the ground. Spring rains cause the fungus spores to be released to the air and they infect young plants. If the quality of the crabapple fruit is not important, control need not be as effective as for the fruiting apple tree. However, serious damage to leaves should be prevented as the health of the tree can be jeopardized.
Proper pruning to improve air movement is beneficial. Resistant varieties of firethorn and crabapple are available.
Black sooty mould is caused by fungi growing on the honeydew secretion of sucking insects such as aphids, mealybugs and scales. It may partially or completely cover the needles or leaves of trees and shrubs.
Controlling the insect will stop the production of sooty mold. A strong jet of water may help dislodge the honeydew and some insects.
Tar Spot of Maple
The characteristic symptoms of this disease are small, irregular, black tar-like spots that become evident midsummer on the upper surface of infected leaves. The surface beneath the tar spot turns brown. Leaves with multiple infections will drop prematurely. The fungus overwinters on the fallen diseased leaves, infecting new leaves the following spring.
Silver, red and Norway maple are most susceptible, but sugar and sycamore maple can also be attacked. Young trees are more likely to be weakened by leaf drop.
Although the spots detract from the appearance of the leaves, spraying with a fungicide is rarely justified. Raking up and destroying infected leaves in the fall will reduce the chance for infection the following spring.
This disease, caused by the Sclerotinia fungus, affects densely growing annual plants such as marigolds, vinca and salvia during periods of wet weather or frequent watering. The symptoms are rapid browning and rotting of lower stems and leaves, with white cotton-like tufts of mycelium and round, black spore bodies - the sclerotia - embedded in the tufts. The sclerotia will remain viable in the soil for long periods of time.
Remove and dispose of diseased plants and fallen leaves as soon as you notice them. Do not compost them or work them into the soil. Avoid splashing water on the leaves. When planting, allow adequate spacing for good air circulation.
Bacterial Wilts and Blights
For a full discussion of bacterial wilts and blights, see the Diseases Causing Plant Injury section of Chapter 1.
This soil-borne fungus is the cause of many rots and wilts in herbaceous ornamentals. One type attacks the bulb of the lily, tulip, narcissus and crocus. The base of the bulb rots, leaving a purplish-brown tissue with pink threads. A second type attacks dianthus, while yet another affects garden chrysanthemum. These both show yellowing of leaves on one side of the stem or a complete wilting of a branch or shoot. The stem at the soil line is reddish-brown. Stunting may occur.
Dig out and remove all infected plant parts including bulbs or roots. Replace with plants that are not susceptible to the same type of Fusarium. Improve soil drainage to prevent water logging. Increase the organic matter content of the soil to avoid undue moisture stress during the summer.
This soil-borne fungus attacks many ornamental trees, shrubs and flowers. It enters through the roots and gradually plugs the water conducting tissue. Susceptible plants include maple, ash, linden, catalpa, dahlia, aster, chrysanthemum, impatiens, phlox and snapdragon. Once the soil is infested, it will remain so indefinitely.
Verticillium wilt can have acute and chronic phases. Sudden wilting, curling and yellowing or blackening of leaves followed by premature shedding are the first symptoms. An entire tree may wilt suddenly after many years of growth, or only one limb may die while the others remain healthy. Black, dark brown or olive green streaks may be found in the outer growth rings of the wood, resulting in long cankers. If the disease is restricted to the wood of one season, compartmentalization occurs (see Wood Decay below).
Improve the vigour of infected trees through fertilization and watering. Remove infected branches. If Verticillium wilt kills a tree, replant with a non-susceptible tree or shrub such as walnut, oak, European mountain ash, sycamore, all conifers, boxwood, pear or mulberry.
Many different wood decay fungi attack trees and shrubs, usually entering through wounds caused by insect or mechanical damage. Healthy plants normally close wounds with a scar-like tissue and prevent the fungi from penetrating into the wood by producing chemical barriers. This process is called compartmentalization. However, when stress factors such as drought, defoliation and soil disturbances reduce the health of the plant, these normal defence mechanisms are weakened.
Wood decay fungi grow vertically in the column of wood associated with the wound, causing discoloration, rotting, and sometimes complete disintegration of the wood. Structurally weak or hollow trees can result, presenting a hazard to nearby buildings or property. After growing extensively inside the plant, the fungi produce reproductive structures on the bark or lower trunk and roots: mushrooms, shelf-like bracket fungi, corks, or flattened pore-filled growths are common.
Infected trees should be examined by a competent arborist for structural soundness, and dead wood pruned out properly. Reduce any stresses to the tree, provide adequate water, and fertilize occasionally. Avoid wounding trees as much as possible, and surgically remove loose bark and jagged edges from wounds. Research has shown that tree wound dressing or pruning paint are of no value in promoting wound closure or in preventing tree decay.
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