Plant Health Problems

Diseases caused by Fungi:

Leaf spot, Phyllosticta cotoneastri.
Reddish-brown spots with a dark border appear on leaves in this infection.

Management can be achieved by raking and removing fallen leaves in autumn to reduce the amount of overwintering inoculum capable of infecting newly emerging tissues in spring. Chemical control is usually not necessary. However, fungicide sprays can be applied when new growth appears in spring. Several applications may be necessary during periods of extended wet weather. Among the compounds registered for use in Connecticut are Bordeaux mixture, ferbam, and chlorothalonil. Consult the label for dosage rates and safety precautions.

Diseases caused by Bacteria:

Fire blight, Erwinia amylovora.
This bacterial disease causes sudden shriveling of branches, which appear as though scorched by fire.

If infection is not extensive over the plant, some control may be obtained by pruning of affected branches; pruning tools should be disinfected by dipping in dilute alcohol or dilute bleach between cuts. Severity of disease may be reduced by avoiding excessive nitrogen fertilization. Some species of cotoneaster may be more resistant to fire blight than others. See Pear and the fact sheet on Fire Blight for more detailed discussions of this disease.

Insect Problems:

Black vine weevil, Otiorhynchus sulcatus.
The larvae of this weevil often injure ornamental plantings by feeding on the roots. The grubs devour the small roots and chew the bark from the larger roots, often girdling them. The tops of girdled plants first turn yellow, then brown, and the severely injured plants die. Large landscape plants tolerate root grazing quite well, but leaf notching by adults can be unsightly. The 1/2" long adult weevil is black, with a beaded appearance to the thorax and scattered spots of yellow hairs on the wing covers. Only females are known, and the adults are flightless. They feed nocturnally, notching the margins of the foliage. The legless grub is white with a brown head and is curved like grubs of other weevils. Adults and large larvae overwinter, emerging from May - July. The adults have to feed for 3-4 weeks before being able to lay eggs. Treating the soil with insect pathogenic nematodes may control the larvae, and should be the first line of defense in landscape plantings. Acephate and fluvalinate are among the compounds registered for control of this pest in Connecticut, and can be applied as soon as adult feeding is noticed to control them before they start laying eggs. The usual timing for these foliar sprays is during May, June and July at three week intervals. Insecticide resistance is very common; be aware that adults may appear to be dead following contact with fluvalinate, but may recover from poisoning within a few days. Consult the labels for dosage rates and safety precautions.

Hawthorn lacebug, Corythucha cydoniae.
This is a small lace bug that lives on the undersides of the leaves. See Chrysanthemum lacebug for details on the life cycle. When needed, insecticidal soap, ultra-fine horticultural oil or malathion, which are among the compounds registered for control of this pest in Connecticut, applied the last week in May and just after eggs have hatched, are highly effective. Spray should be directed from the bottom of the plant upward to ensure thorough coverage of the lower leaf surfaces. Imidacloprid, applied as a systemic to be taken up by the roots, will also provide season-long control. Consult the label for dosage rates and safety precautions.

Oystershell scale.
This scale often infests lilac. A dormant application of horticultural oil will control overwintering scales. Summer application of ultrafine oil can also be helpful. Spraying with malathion, ultrafine oil, or insecticidal soap, which are among the compounds registered for control of this pest in Connecticut, about June 15 kills the young crawlers. Consult the label for dosage rates and safety precautions. See also Apple.

Pear leaf blister mite, Phytoptus pyri.
Colonies of these microscopic mites may disfigure the unfolding leaves of cotoneaster by causing greenish yellow or reddish blisters that later turn brown. In severe infestations, the leaves may drop in summer. The mites spend the winter in the bud scales, and then develop their colonies within the leaf tissues. They can be controlled with a spray of horticultural oil, which is among the compounds registered for use against this pest in Connecticut, before the buds open in spring. Consult the label for dosage rates and safety precautions.

San Jose scale, Quadraspidiotus perniciosus.
Large infestations of the San Jose scale can cause branch and even shrub death as well as red spots on fruit. Partially grown scales overwinter under their circular gray covering or scale on the twigs and the branches. They begin to feed as the sap starts to flow. When apple trees bloom, the males emerge from under their scales to mate with the immobile females. Females are circular and cone-shaped, and their circular scales are about 1/16" in diameter, with a raised center or nipple. The males are smaller and elongate, with the nipple not centered on the scale. Females give live birth to tiny bright yellow crawlers in June, usually about 3-5 weeks after the flower petals drop. The young crawlers quickly settle, insert their long mouthparts into the twigs, and then suck sap from branches. As they grow, the crawlers secrete a waxy filament that becomes their scale or covering. Scales apparently have 2 generations per year, with the first in June and the second in August. Scales may be controlled by applying horticultural oil, which is among the compounds registered for control of this pest in Connecticut. Consult the label for dosage rates and safety precautions. To detect the yellow crawlers, wrap black tape coated with Vaseline around small branches. Adult flights may be detected with pheromone traps.

Sinuate peartree borer, Agrilus sinuatus.
Borers weaken shrubs, causing scars and sometimes completely girdling and killing shrubs. Larvae that are partly grown overwinter in their tunnels in the wood for 2 consecutive years. Second-year larvae form pupae in April, and the adult beetles emerge about 1 month later in late May or June. Adults, which are slender glossy bronze-brown beetles that are 1/3" long, feed on leaves. The females lay eggs in the crevices and under the edges of the bark. In early July, the young grubs or larvae hatch from eggs and excavate narrow sinuous tunnels in the sapwood just beneath the bark. Larvae are only partly grown by the time winter arrives.

Twospotted spider mite, Tetranychus urticae.
This pest infests the undersides of the leaves, which become light yellow in color, and the plants have a generally unhealthy appearance. Sometimes the mites form webs, which more or less enclose the upper as well as the lower leaf surface. Among the compounds registered for control of this pest in Connecticut are insecticidal soap and ultrafine horticultural oil. Spraying with insecticidal soap will give sufficient control if applied at least twice at 7-10 day intervals. The predatory mite, Neoseiulus fallacis, is most commonly found feeding where there are mite infestations. A single application of ultrafine horticultural oil (1/2 to 1% dilution) can be effective if predatory mites are present. Special care should be taken with soap or oil to obtain thorough spray coverage, because they only work on contact. Abamectin is an effective, restricted use product. Consult the label for dosage rates and safety precautions. Avoid applying carbaryl or pyrethroids, which tend to be much more toxic to the predators than to the pest spider mites.