Problems of Tall Cotoneaster

Silvery, Webbed Leaves
Spider Mite - Adult mites are reddish brown or pale tan and 1/150 to 1/50 inch long. They are more easily visible through a hand lens. Mites lay their eggs on leaves and buds or at the base of plants. Spider mites turn cotoneaster leaves silvery or yellow, giving them a parched appearance. The leaves may curl and become covered with fine webbing, especially on undersides, and fruit is deformed. These pests are more common in some regions than others. They favor hot, dry locations. Contol them by spraying infested plants early in the morning with a forceful jet of cold water. Repeat the water spray daily for three days. If that doesn't do the job, spray the mites with insecticidal soap every three to five days for two weeks. Dormant oil spray will destroy many overwintering mites. Spray with 1 part flowable sulfur fungicide in 10 parts of "superior" type dormant oil before growth begins in spring. For more information go to Controlling Mites

Foliage Turns Pale Or Mottled
Hawthorn Lace Bug - Lace bug adults are small square-shaped bugs, 3/16 inch long or less, with elaborately reticulated wings that resemble lacework. This sucking insect sometimes appears in late June and in the summer on the undersides of cotoneaster leaves. They suck sap from the undersides of leaves, causing the foliage to turn pale or mottled. Crush the bugs by drawing the leaves between your thumb and forefinger. Control larger infestations by spraying the visible bugs with insecticidal soap, a nicotine spray, or a pyrethrum spray. Do this three times, once every 3 to 5 days. For more information see the file Dealing With Lacebugs

Leaves Curled, Discolored And Distorted
Aphid - Aphids are spindly-legged, pear-shaped insects little bigger than the head of a pin. They suck sap from leaves and stems, causing cotoneaster foliage to curl, pucker, and turn yellow, while reducing the plant's vigor. Also called "plant lice," these pests attack tender branches and flower clusters. Often ants, attracted by the aphids' honeydew secretions, wander over the plants and protect the aphids from natural predators. Check leaf undersides for small groups of aphids. For light infestations, spray leaf undersides vigorously with water three times, once every other day, in the early morning. Eliminate nearby ant nests if possible. Spray visible aphids with insecticidal soap every two to three days for heavier infestations. As a last resort, use pyrethrum spray. For more information go to Controlling Aphids

Leaves Turn Yellow, Become Sticky
Scale - Scale insects appear as clusters of somewhat flattened, waxy reddish gray or brown bumps a bit bigger than the head of pin. The bumps are protective shells, under which the pests insert thread-like sucking mouthparts into the plant and suck the sap. They often cover plants with white cottony, cushion-like masses. Cotoneaster leaves turn yellow and may drop. A shiny or sticky material (honeydew) may cover the leaves, especially on the undersides. Scale outbreaks can be triggered by pesticides used against other pests or by environmental stresses such as too much or too little water. Overuse of nitrogen fertilizer can encourage the growth of scale populations. Avoid this by using very little fertilizer. If caught early on, scale can be scraped off plant surfaces with a finger nail or a cotton swab dipped in rubbing alcohol. Spray heavily infested plants with a mixture of alcohol and insecticidal soap every three days for two weeks. Add 1 tablespoon of alcohol to a pint of ready to use commercial soap spray. For more information go to Controlling Scale

Webbing Holds Leaves And Twigs Together
Cotoneaster Webworm - The cotoneaster webworm caterpillar is blackish with white spots, and is nearly 1 1/2 inches long when full grown. It weaves together leaves and twigs with webbing, forming a nest within which it feeds. This webbing usually starts after midsummer, and the nests remain on the bushes during the winter. Some of the nests of webworm can be handpicked and destroyed. To more effectively control webworm, spray BT (Baccillus thuringiensis) on the foliage as soon as you spot small worms starting to feed. Making three applications over a three week period so that all emerging caterpillars eat it. They will sicken and die in a matter of days. If that doesn't work try a spray or dust of pyrethrum .

Spots On Leaves; Twig Dieback And Blackening
Fireblight - Fireblight is the biggest problem that cotoneasters have. It is caused by a bacterium which overwinters on cankers and on large branches of apple and pear trees which are relatives of cotoneasters. The disease is spread from plant to plant by insects and splashing raindrops. It causes dark spots or "shotholes" in leaves, death of flowers and twigs and girdling of main stems and branches. Infected twigs wilt from the tip and turn black. Cankers form on the bark, becoming sunken and cracked. Fruit shrivels and turns black. The bush appears to have been scorched by fire. A sticky ooze may appear on any infected part. There is no cure for fireblight. To control this disease, prune all blighted plant parts 4 inches below visible cankers and burn them. Be sure to disinfect pruners and other tools in between cuts with a 10% solution of household bleach and water or denatured alcohol. Spray affected plants with lime sulfur, or, a more sophisticated antibiotic when they are about to bloom or are blooming. While this will not kill fireblight germs that are already deep in wood tissues, it helps in preventing new infection from becoming established. In the east 3 applications are usually sufficient. In California it may take 5 applications.

Swollen Bumps On Stems
Canker - Fungi and bacteria cause cankers on cotonester stems. These are localized, swollen and discolored dead areas resulting from infection of the soft tissue just under the stem's surface. They commonly split open, exposing underlying tissues, and sometimes bleed a gummy exudate. The germs and spores that cause them are spread by rain splashes, handling of plants, or contaminated tools. Infection usually occurs as a result of wound injury, or is hastened by some other problem that weakens plants, such as nutrient deficiency, severe winter conditions, or nematodes and insect pests. Remove and destroy infected plants or plant parts as soon as you notice the symptoms. Cut affected stems back several inches below the cankers. Prompt pruning of infected plants is the best control. Disinfect clippers and other tools after using them on cankered plants. In the fall gather and destroy all aboveground infected parts to reduce chances of the fungus overwintering. A spray with Bordeaux mixture may give some control, and possibly reduce the spread of the disease to adjacent plants. For more information see the file on Controlling Fungal Disease

Foliage Burned
Dog - Dog urine is always a potential problem for ground covers like spreading cotoneaster plants. It may discolor foliage and even kill entire branches. Spraying foliage with an anti-transpirant gives some protection. Screen the plants or spray them with an aerosol pet repellant. For more information go to Dogs and Cats

Bark of Stems and Roots Gnawed
Rodent - Small rodents sometimes gnaw bark off main stems of cotoneasters, causing injury that allows disease organisms to invade. Keep mulch away from shrub stems during the growing season, removing it altogether during fall cleanup. Spread a winter mulch only after the ground is frozen solid so that rodents will not be tempted to build nests in it. Dealing With Mice

The following questions were asked by visitors who viewed this page:
see all questions...

Do you have a gardening question? Ask Nancy