Plant Health Problems
The many species of oak are grouped as white and chestnut oaks; red, black, and pin oaks; and live oaks. They are subject to the same diseases.
Diseases caused by Fungi:
Anthracnose, Apiognomonia veneta and Discula umbrinella.
White oak leaves brown and curl from the edges inward, progressing upward from the bottom of the tree. On black and red oaks, infection shows as small or large irregular brown spots that often enlarge along the veins.
Rake and dispose of fallen leaves to reduce the chance of infection in the following season. This disease is usually not a serious problem for the health of the trees. Fertilize trees in the spring and water during dry weather to maintain tree vigor. Chemical controls are usually not necessary.
Leaf blister, Taphrina caerulescens.
This disease can occur on all the oaks except for those in the white oak group. Light green or whitish, raised-puckered areas, 1/4 to 3/4 inch in diameter, appear on newly expanding leaves. These spots eventually turn brown and the symptomatic leaves remain attached to the tree giving it an off-colored appearance from a distance.
Maintain the vigor of the trees with regular watering during drought and fertilizing in the spring. Chemical controls are usually not necessary.
Powdery mildew, Microsphaera alni.
Whitish mealy appearance of upper surface of leaves is characteristic of infection by this fungus on species in all the groups. In the fall, small black dots which are the fruits of the fungus may be found embedded in the white mycelium.
Although widespread and common, this fungus is not a serious problem for the health of the trees. It is helpful to maintain the vigor of the trees with regular watering during drought and fertilizing in spring. Chemical controls are usually not warranted.
Twig canker and dieback, Diplodia longispora.
Twigs and small branches are killed. Leaves dry up but remain attached to the tree. Infected sapwood shows black streaks and black fruiting bodies may occasionally be found on the twigs. This disease is apt to occur on oaks in poor condition for some other reason, such as any changes in the stand, a severe drought, or attack on the roots by the shoestring fungus.
Correction of the fundamental trouble may be impossible, but pruning and removing dead wood will reduce further damage. Fertilize trees in the spring and water during dry weather to maintain tree vigor.
Strumella canker, Conoplea globosa.
This fungus, commonly known as "strumella," occurs most frequently on forest trees but may become important on ornamental red and scarlet oaks. Infection usually is first noticed as a depressed area at the base of a branch stub which enlarges in a targetlike fashion with concentric callus ridges until the trunk is either girdled or considerably bowed on the opposite side to the canker. The original infection on the branches, consisting of yellowish-brown depressed or raised areas on the bark, usually passes unnoticed. The fungus travels rapidly down the branch into the trunk, forming the canker which withers, deforms, or kills the tree.
Removal of trees as soon as infection is noticed helps protect remaining trees because the fungus fruits primarily on dead trees. Fertilize trees in the spring and water during dry weather to maintain tree vigor.
Chestnut blight canker, Cryphonectria parasitica.
Many oaks are infected by this pathogen, but the most serious damage is done to live oaks. Scarlet oaks are frequently infected at the base, causing a swollen distortion of the trunk which makes the tree unmarketable as timber.
There are no chemical controls for this disease, but see Chestnut for a discussion of biological control.
Nectria canker, Nectria galligena.
Cankers start as depressed areas which quickly girdle branches. Leaves, beyond the point of attack, wilt but remain attached to the tree. Eventually the branch dies. Nectria may be suspected in cases where one branch of a tree turns red in the fall before the rest of the tree.
Pruning well back of cankered area and disposing of infected wood removes a source of new infection. Fertilize trees in the spring and water during dry weather to maintain tree vigor.
Shoestring root rot, Armillaria mellea.
This fungus penetrates the bark of collar and roots at and below the ground level, eventually killing the tree. The crown may have small leaves and produce little or no new growth. This is followed by dieback of branches. The growth increment of the trunk is slight. The fungus may be recognized by the rhizomorphs which look like black shoestrings but are actually cables of fungus threads protected by a rubbery black cover. They are produced in the ground or along or under the bark of the roots or collar. White mycelial fans on the inside of loose bark are also indicative of infection by this fungus. The mushrooms, or fruiting bodies, are honey colored with a pebbly top, about 2 to 6 inches in diameter, and appear in clumps near the base of the tree in wet fall weather. Attack by this fungus is apt to occur on older trees or trees stressed by drought, particularly where environmental conditions have changed, such as cutting surrounding trees, near construction, or areas with a change in drainage.
There are no effective controls, but maintaining the vigor of the trees with adequate water during dry periods and fertilization in the spring may help to keep the trees alive. The fungus persists in roots for at least 30 years. Only resistant trees, such as catalpa, gingko, sweet gum, or scots pine, should be used for replacements.
Hypoxylon canker, Hypoxylon atropunctatum.
This fungus invades wounds in trees stressed by drought or other environmental problems, and kills the cambium. The bark sloughs off exposing a thin layer of fungal tissue with brownish, dusty masses of spores. Later small black dots are formed which contain the sexual spores.
Pruning well back of cankered area and disposing of infected wood removes a source of new infection. Maintain vigor of trees with adequate water during dry periods and fertilization in the spring.
Asiatic oak weevil, Cyrtepistomus castaneus.
These 1/4" weevils are dark brown, with whitish scales on the body and yellowish brown legs. The snout is very short. Adults are the most damaging stage, feeding on the edges and thereby scalloping the leaves of many plants. The adults then lay their eggs in the soil, where the small legless larvae develop on roots. In high populations, adults are attracted to lights, where they may become a nuisance inside houses. Apply acephate or fluvalinate to foliage if the adult feeding is intolerable.
Broadnecked root borer, Prionus laticollis.
The grub of this beetle is a borer in decaying roots and stumps of oak and several other kinds of trees. It is a white fleshy grub without legs and when mature reaches a length of nearly 3". The adult is a dark brown, longhorned beetle about 1 1/2" long. They emerge in July. No control measures are required.
Cankerworms, Alsophila pometaria and Paleacrita vernata.
Cankerworms often feed on oak as well as many other kinds of fruit, shade, and woodland trees. Also called measuring-worms or inch-worms. When abundant, they may defoliate the 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, spinosad or carbaryl, which are among the compounds registered for control of this pest in Connecticut. Consult the labels for dosage rates and safety precautions.
Forest tent caterpillar, Malacosoma disstria.
The larvae of this tan moth are capable of defoliating deciduous trees. They do not form a tent like the Eastern tent caterpillar but do form silken mats on the uppermost branches of host trees. They move out as a group when young to strip foliage from one branch at a time. With increasing size, they become solitary feeders. Winter is passed as a tubular egg mass encircling a host twig. Eggs hatch as leaves are unfolding. The mature caterpillar is dark gray with blue longitudinal stripes on either side of a line of cream-colored spots resembling high heel shoe prints. They form light yellow cocoons in folded leaves or on nearby plants. The moth, with a wing spread of almost 2", is nocturnal and attracted to light. There is one generation per year. Among the compounds registered for control of this pest in Connecticut are acephate, carbaryl, insecticidal soap, spinosad and Bacillus thuringiensis. All can be applied to the foliage in early May to young larvae to achieve the best control. Two sprays, ten days apart may be needed. Consult the label for dosage rates and safety precautions.
Golden oak scale, Asterolecanium variolosum.
This is a green, glossy scale partly sunken in the bark, that occurs on the English oak and chestnut oak. This circular scale, about 1/16" in diameter, winters in a nearly mature condition, and the young appear in May. A dormant horticultural oil spray, applied in the spring, will control this scale. Crawlers may be controlled with malathion, which is among the compounds registered for use against this pest in Connecticut. Spraying the bark in May is useful. Consult the labels for dosage rates and safety precautions.
Gypsy moth, Lymantria dispar.
Oak is a preferred host of the gypsy moth, and solid stands of oak are subject to periodic defoliation. 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 tubercles bearing hairs. 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. Individual shade trees may be sprayed.
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 needed to infect any future populations of 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.
There are several leafminers on oak, one of the most conspicuous being the white blotch leafminer, Lithocolletis hamadryadella. Other species are L. fitchella and L. tubeiferella. Imidacloprid, which is among the compounds registered for use against this pest in Connecticut, applied as a systemic to be taken up by the roots, will give satisfactory control when treatment is made in early spring. Abamectin is an effective restricted use product. Consult the label for dosage rates and safety precautions. Raking up all leaves and shredding them for compost in the fall will help to control the overwintering insects.
Mites often infest oak leaves, causing a rusty brown appearance. Mites can be controlled by spraying with insecticidal soap or ultrafine horticultural oil, which are among the compounds registered for use against this pest in Connecticut. Abamectin is an effective restricted use product. Consult the labels for dosage rates and safety precautions.
Oak trees are subject to galls and many different species are found on them. Only a few of the more conspicuous galls can be mentioned here. The large oak apple is a globular gall from 1 to 2" in diameter, occurs on black, red and scarlet oaks; the gouty oak gall appears as rough cylindrical galls surrounding the twigs on black, red and scarlet oak; the oak bullet gall is a smooth spherical gall ½" in diameter, occurring on white oak; the oak seed gall is a mass varying from ½" to nearly 2" in diameter on white oak; and the white oak club gall, a woody swelling at the ends of twigs on white oak. Control is usually unnecessary.
Oak gall scale.
Several species of gray or yellow globular scales occur on the twigs of various species of oak, usually in the axils of leaves or buds. Control has been obtained by a dormant horticultural oil spray applied in early spring. Consult the labels for dosage rates and safety precautions.
Oak lecanium scale, Parthenolecanium quercifex.
These convex scales are 1/6 to 1/3" long. They are rare in Connecticut and control usually is not required.
Orangestriped oakworm, Anisota senatoria.
This is a caterpillar striped lengthwise with black and orange, with short black spines on some of the segments, and two longer fleshy protuberances or horns back of the head. It reaches a length of about 1 3/4" and feeds on scrub oak and some other species, becoming mature in August. It overwinters as pupae in the soil and the moths emerge during June. There is one annual generation. The adults are a tawny orange color with a purplish diagonal line on each wing, and have a wingspread of about 1 3/4". The feeding caterpillar can be controlled by sprays of carbaryl, malathion, spinosad or Bacillus thuringiensis var. kurstaki, which are among the compounds registered for use against this pest in Connecticut. Consult the labels for dosage rates and safety precautions.
Twig pruner, Elaphidionoides villosus.
Small twigs are constantly dropping on the ground under the oak trees in July and August. Some of them remain on the tree, hanging with dried leaves. The larva of this beetle has cut them off. The eggs are laid in July on the smaller twigs, and the young grubs work for a time under the bark, then tunnel along the pith in the center of the twig. When nearly mature, the borer cuts off nearly all of the wood, and the twig, with the borer inside, usually breaks off in the wind and falls to the ground. The insect overwinters in the twig and the beetle emerges the following summer. The beetle is grayish brown, with body about 5/8" long and long slender antennae. Gathering and burning the fallen twigs immediately, if allowed, will destroy the larvae. If further control is necessary, spraying with permethrin, which is among the compounds registered for control of this pest in Connecticut, during July should control the beetles before eggs are laid. Consult the label for dosage rates and safety precautions. Maintaining adequate irrigation may also prevent injury by this pest.
Twolined chestnut borer, Agrilus bilineatus.
This borer commonly infests oak. The white, flat-headed grubs of this beetle make tortuous and interlacing galleries under the bark of chestnut and oak. This grub is about 1/2" long, and the parent beetle is 3/8", black with two narrow converging longitudinal grayish stripes on the wing covers. The beetles appear in May and June, feed somewhat on the foliage and then deposit eggs. The grubs work in the inner bark and outer sapwood, pupating in cells in the wood. There is one generation each year, and mutilated, weakened and dying trees are often infested and killed by this beetle. Because these beetles attack mainly weakened plants, keep trees healthy with adequate fertilizer and water. Carbaryl and permethrin, which are among the compounds registered in Connecticut for use against this pest, may be sprayed on trunks and main limbs for control. Consult the label for dosage rates and safety precautions.
Walkingstick, Diapheromera femorata.
Oak trees are occasionally partially stripped in September by green and brown stick-like insects called walkingsticks. Walkingsticks are related to crickets, grasshoppers and katydids. They feed chiefly on oak and there is one generation each season. If too numerous, this insect can be controlled by spraying foliage with carbaryl or malathion, which are among the compounds registered for use against this pest in Connecticut. Consult the labels for dosage rates and safety precautions.