Plant Health Problems

Diseases caused by Fungi:

Leaf spot, Fabraea maculata.
This is the same fungus that causes leaf spot on pear. Leaf spotting generally occurs in June. Small angular purple dots are produced by this fungus. Leaves may turn yellow and drop.

Overwintering infected leaves are a main source of spores in the spring. Removal of the fallen leaves before budbreak will lessen the chances of new infections. Control can also be achieved with the use of fungicide sprays. Among the compounds registered for use in Connecticut are chlorothalonil, thiophanate-methyl, and mancozeb. Consult the label for dosage rates and safety precautions.

Leaf rust, Gymnosporangium globosum.
The rust spots appear brown or orange-yellow on the upper side of the leaves, but from the underside long slender whitish tubes break open to shed orange-brown spores. Infected leaves fall prematurely. Damage to hawthorn is slight. The alternate host is red cedar or juniper species.

Removal of these alternate hosts from the vicinity (1 mile radius) will cut down on infection, but this alternative is rarely if ever practical. Control can also be achieved with the use of fungicide sprays applied when the gelatinous orange telial horns are evident on the junipers (usually mid-May). Among the compounds registered for use in Connecticut are chlorothalonil, fenarimol, triadimefon, and mancozeb. Consult the label for dosage rates and safety precautions.

Stem rust, Gymnosporangium clavipes.
Leaves, twigs, and fruit are attacked by this fungus. Orange spots are produced on leaves which soon fall. Fruit and twigs may be deformed. Spores that infect the hawthorn are produced on cedars during long spring rains, but infection does not spread from hawthorn to hawthorn. The fungus is perennial in the cedars so that infected trees remain a threat to hawthorns year after year.

Control as for leaf rust.

Powdery mildew, Podosphaera oxycanthae.
This fungus appears as a white mold that covers leaves and new growth much in the manner of apple powdery mildew, often deforming and killing young shoots.

Affected new growth should be pruned out and burned or otherwise removed from the premises. Control can also be achieved with the use of fungicide sprays applied when it appears. Among the compounds registered for use in Connecticut are thiophanate-methyl, triadimefon, and sulfur. Consult the label for dosage rates and safety precautions.

Diseases caused by Bacteria:

Fire blight, Erwinia amylovora.
Leaves on branches suddenly shrivel and turn blackish-brown as though scorched by fire. Discoloration and death of wood follows. The causal bacteria are carried by bees and inoculated into flowers at blossom time. The disease is most severe on pears but occurs also on apples, mountain ash, and Pyracantha.

Infected wood should be cut out well below the damaged part and burned or otherwise removed from the premises, making sure that pruning cuts are at least 10-12 inches below the visible symptoms. Pruning tools should be disinfested between cuts. Avoid excessive vigor, which can be caused by excessive nitrogen fertilizer. For more information, see the fact sheet on Fire Blight.

Insect Problems:

Aphids, Aphis pomi, Rhopalosiphum fitchii, Amphorophora crataegi.
Several species of aphids frequently infest hawthorn. The rosy apple aphid, Dysaphis plantaginea, sometimes curls the leaves. All species secrete honeydew which supports sooty mold fungus. Sprays of ultrafine horticultural oil, insecticidal soap or malathion, which are among the compounds registered for use against this pest in Connecticut, will control these aphids. Several applications may be required to provide adequate protection. Imidacloprid, applied as a soil drench, will provide season-long systemic control. Consult the labels for dosage rates and safety precautions.

Cankerworms, Alsophila pometaria and Paleacrita vernata
Cankerworms feed on hawthorn foliage during the spring. They are also called measuring-worms or inch-worms and when abundant, may defoliate trees. In early spring, caterpillars hatch from the eggs laid on the trees in late fall or early spring. Older caterpillars are black or greenish with stripes. The male moths are gray with a wingspread of 1"; the female moths are wingless. Each species has only one generation a year. The abundance of cankerworms varies in cycles. The caterpillars can be controlled with one or more springtime applications of azadirachtin, carbaryl, Bacillus thuringiensis var. kurstaki, or spinosad, which are among the compounds registered for control of this pest in Connecticut. Consult the labels for dosage rates and safety precautions.

Cottony maple scale, Pulvinaria innumerabilis.
This is a brown, oval, soft scale on the bark of the branches in winter, but in June the large egg masses are formed, and their wax covering resembles a tuft of cotton. The young crawl in July and some of them live for a time on the leaves, but return to the twigs to pass the winter.

Gypsy moth, Lymantria dispar.
The caterpillars feed on the leaves of hawthorn. When fully grown, the caterpillars are between 2 and 3" long, dark gray or brown with prominent light brown hairs. Some have a light narrow stripe along the back and all have two rows of hair bearing tubercles. From the head, the first five pairs are blue, and the remaining six pairs are brick red. They feed during May and June, and do most of their feeding at night.

Caterpillars pupate in cracks or crevices spinning a very small amount of silk. The moths emerge in about 2 weeks. The female is buff with narrow zigzag lines across the forewings. The wingspread is about 2", and the body is so heavy that the female cannot fly. The male is reddish-brown with variable light gray and dark brown markings and a wingspread of 1 to 1 1/2". The males fly freely.
Eggs are laid on the bark of trees, on stones, or lumber. They are laid in masses of about 400 eggs and covered with buff hairs from the body of the females.

A fungus, Entomophaga maimaiga, discovered by Station scientists in 1989, is giving natural control of larvae. Larvae killed by the fungus characteristically remain on the tree with their head hanging down. As these are the source of fungal spores to infect future gypsy moth caterpillars, do not destroy them. The fungus originated in Japan and was introduced to the Boston area via infected gypsy moth larvae in 1910. The fungus was never recovered, despite attempts in subsequent years, and so it was thought not to have established. This fungus grows best in warm, humid weather.

Should chemical controls become necessary, sprays can be applied when caterpillars are young, about 1/4" long. Carbaryl, malathion, methoxychlor, spinosad, and Bacillus thuringiensis var. kurstaki, which are among the compounds registered for control of this pest in Connecticut, are all effective treatments. Consult the labels for dosage rates and safety precautions.

Hawthorn lacebug, Corythucha cydoniae.
This is a small lacebug that lives on the undersides of the leaves. See Chrysanthemum lacebug for details on the life cycle. When needed, insecticidal soap, ultrafine 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 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 labels for dosage rates and safety precautions.

Hawthorn leafminer, Profenusa canadensis.
This sawfly overwinters in the soil as pupae. Adults emerge just as hawthorns are beginning to leaf out. Eggs are laid singly in the upper epidermis where the petiole and leaf blade meet. After hatching, the larvae feed on inner leaf tissue, moving along the leaf margin toward the tip. There may be multiple larvae per leaf. The larvae are flattened with three pairs of true legs and are 3/8" long when mature. In mid-June they cut a hole through the lower epidermis and drop to the ground to pupate.

There are two wasp parasites that may provide control. If damage is severe, use imidacloprid, which is among the compounds registered for use against this pest in Connecticut, as a soil drench for season-long, systemic control. It should cause the least harm to the populations of beneficial wasps. Avoid planting Crataegus crus-galli, C. persimillis and C. erecta since they are the most susceptible species.

Roundheaded appletree borer, Saperda candida.
This insect is also a borer in hawthorn. Larvae of the roundheaded appletree borer tunnel deeply into the trunks of the trees from 4" below ground to 1-2' above ground. Borers mainly injure young trees, weakening or girdling them. Adults lay eggs from June to August. The adults are slender, long-horned beetles that are about 3/4" long and brown with two conspicuous longitudinal white stripes on the wing covers. The larvae may take up to 3 years to develop to maturity. Applications of permethrin or carbaryl, which are among the products registered for use against this pest in Connecticut, to the trunk may kill adults before they lay eggs. Consult the labels 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. Partially-grown scales overwinter under their circular gray covering or scale on the twigs and the branches of trees. 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. 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. Overlapping generations are present from June through September. Scales may be controlled by applying horticultural oil at a dormant timing or in June when crawlers are active. Malathion or insecticidal soap, which are among the compounds registered for control of this pest in Connecticut, can also be applied against the crawlers in June. Consult the labels 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.

Scurfy scale, Chionaspis furfura.
These whitish or light gray scales infest hawthorn and other trees and shrubs, reducing tree vigor. Scurfy scales pass the winter as purplish eggs under the shell of their mother. In Connecticut, the yellow crawlers hatch from eggs about the last week of May. They soon settle on the bark and insert their long mouthparts to suck the sap. The adult females are pear-shaped and about 1/10" long. The males are much smaller, long and narrow, with 3 longitudinal ridges or carinae. Dormant treatments with horticultural oil control these scales. A spray of insecticidal soap, which is among the compounds registered for control of this pest in Connecticut, early in June also will destroy the crawlers. Consult the labels for dosage rates and safety precautions.

Spotted tentiform leafminer, Phyllonorycter crataegella.
The adult of this blotch leafminer is a small moth whose wingspan is less than 1/4". The young larvae are flat with thoracic segments being wider than the abdominal segments. They cut cell walls and suck sap. As they grow, they do more damage by eating entire cells. Mines initially may be visible only from the underside of the leaf. Larvae spin silken threads between the lower and upper epidermis, which shrinks as it dries and causes the characteristic tent-like ridge. Pupation occurs in the mine. There may be more than one larva per mine and, depending on weather, there may be three generations per year.

Next year's population will be reduced if leaves are destroyed in the fall. Healthy earthworm populations assist in reducing overwintering populations of tentiform leafminers by dragging leaves underground, from where adults the next year cannot successfully emerge. When needed, acephate can be applied in early May and again in early June to control young leaf miner larvae. Imidacloprid, also registered in Connecticut for control of this pest, will provide season-long systemic control if applied in early spring as a soil drench. Consult the labels for dosage rates and safety precautions.

Thorn limb borer, Saperda fayi.
The larva of this beetle is a borer in the smaller branches and twigs of hawthorn, where it causes swellings about 1" long with four or five longitudinal scars. Infested twigs break off in the wind. The beetle is ½" long, brown, with two white crescent shaped spots near the middle of the wing covers and two smaller circular spots near the apex. The thorax has a white stripe on each side extending on the base of the wing covers. There is one annual generation and the beetles appear in June. Removing and destroying the infested twigs will provide some control.

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 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 - 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 and hexythiazox, of which the former is a restricted use product, are also effective. Consult the labels 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.