Willow (Salix)

Plant Health Problems
Diseases caused by Fungi:

Willow blight.
The combined effect of the two pathogens below--a scab and a canker fungus:

Willow scab, Venturia saliciperda.
Infects unfolding leaves in early spring, killing and blackening them as it grows into the petioles and new shoots, which also become blackened and may develop cankers.

Black canker, Glomerella cinguilata.
Later in the season, this fungus infects and behaves much like the scab fungus, often attacking foliage missed by the latter, or adventitious foliage that formed following the killing of earlier shoots by the scab fungus.

Defoliation can be virtually complete, and trees die from repeated infections.

The diseases may be reduced by pruning and disposing of infected wood in early spring. In autumn, rake and dispose of fallen leaves to reduce the chance of infection in the following season. Fertilize trees in the spring and water during dry weather to maintain tree vigor.

Cankers, Botryosphaeria berengeriana, Valsa sordida.
Small, black, sunken cankers appear on twigs, branches, and trunks of willows. These may kill a tree within a period of a few years.

No control is known. Vigorous trees may resist infection better than those weakened by blight or winter injury. Fertilize trees in the spring and water during drought to improve plant health and reduce stress.

Rusts, Melampsora sp.
The uredial and telial stages of these rusts appear as orange and red blisters on the underside of the leaves and are self-perpetuating. The alternate hosts of these different rusts are balsam fir, larch, and Ribes spp. The willow-larch mycelium can overwinter in terminal buds, young stems, or catkins.

Control is usually unnecessary, but around young trees, elimination of alternate hosts and prompt pruning and destruction of infected wood, in the case of willow-larch rust will be found helpful.

Tar spot, Rhytisma salicinum.
Thick, black, raised spots like a blob of tar appear on leaves.

Raking and disposing of fallen leaves gives sufficient control.

Diseases caused by Bacteria:

Crown gall, Agrobacterium tumefaciens.
Irregular swellings are found at the base of the trees or on roots of nursery trees.

No control has been found, although wounding is thought to be the means by which the bacteria enter the tree. There are no registered chemical controls.

Diseases caused by Physiological/Environmental Factors:

Winter injury.
Alternate freezing and thawing of bark on the south or southwest side of a tree during late winter causes long, vertical cracks which may be later invaded by insects or fungi. This is more common on trees that have been fertilized late in the growing season and have not properly hardened off before freezing occurs.

A wide board leaning against the tree on the exposed side will prevent the sun’s rays from warming the bark on days in late winter when night temperatures drop below freezing. Painting the trunk with white latex paint will accomplish the same result.

Insect Problems:

Aphids.
Several species of aphids infest the willows. One of the most common is Clavigerus smithiae. The purplish aphids are on the small twigs in the fall, where they lay eggs that overwinter. Another species, Chaitophorus viminalis, has also been recorded on willow from Connecticut. These aphids may be controlled by spraying with malathion or insecticidal soap, which are among the compounds registered for use against this pest in Connecticut. Dormant applications of oil can kill overwintering eggs. Imidacloprid, applied as a systemic to be taken up by the roots will give at least one season of control. Consult the labels for dosage rates and safety precautions.

Fall webworm, Hyphantria cunea
The fall webworm damages willow leaves by both feeding and web-building. Webworms overwinter within cocoons located in protected places, such as crevices in bark or under debris and fences. Adult moths emerge in summer. They have a wingspan of about 1 1/4", and vary from pure satiny white to white thickly spotted with small dark brown dots. Females lay white masses of 400-500 eggs on the undersides of the leaves. The caterpillars hatch in 10 days, and all from the same egg mass live together as a colony. They spin webs that enclose the leaves, usually at the end of a branch, to feed upon them. After they have defoliated a branch, they extend their nest to include additional foliage. When caterpillars are mature, they leave the nest to seek a place to spin gray cocoons. The mature caterpillars are about 1 1/4" long with a broad dark brown stripe along the back, and yellowish sides thickly peppered with small blackish dots. Each segment is crossed by a row of tubercles with long light brown hairs. In Connecticut, there is one complete generation and a partial second one. In light infestations, the webworms can be destroyed by pruning the nests and destroying them. In larger infestations, caterpillars can be controlled with foliar sprays of acephate, carbaryl, Bacillus thuringiensis var. kurstaki or spinosad, which are among the compounds registered for use against this pest in Connecticut. Consult the labels for dosage rates and safety precautions.

Gypsy moth, Lymantria dispar.
The caterpillars of the gypsy moth feed on willow. 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 two 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 to infect any future gypsy moth caterpillar populations, 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.

Imported willow leaf beetle, Plagiodera versicolora.
Dark metallic blue beetles (1/8" long) and their larvae feed on the leaves of willow, especially the glossy leaf varieties. Adult beetles overwinter under bark and in other sheltered places and emerge in April and May. Females lay yellow eggs in clusters on the undersides of the leaves. These eggs hatch in 4 or 5 days and the grubs feed on the leaves, eat away the lower tissue and leave only the veins and upper epidermis. The grubs are bluish-black, alligator-like larvae that skeletonize the foliage, but the beetles may eat holes through the leaves. There are two generations each season. Among the compounds registered for control of this pest in Connecticut are Bacillus thuringiensis var. san diego, horticultural oil, carbaryl, acephate, spinosad and imidacloprid. Control of willow leaf beetles with Bacillus thuringiensis var san diego may be improved when combined with ultrafine horticultural oil and timed to coincide with the activity of young larvae. Imidacloprid applied as a systemic to be taken up by the roots may give at more than one season of control. Consult the labels for dosage rates and safety precautions.

Mite galls, Eriophyes sp.
These are very common on willow, but little is known about them. Spraying with malathion (registered for control of this pest in Connecticut) when the leaves are developing in the spring should be effective. Consult the label for dosage rates and safety precautions.

Oystershell scale, Lepidosaphes ulmi.
The oystershell scale commonly infests willow. Heavy infestations of the oystershell scale reduce tree vigor because the scales suck water and nutrients from the tree. These insects sometimes kill limbs or entire trees. Eggs overwinter under their mother scale on the bark of trees. Crawlers hatch in late spring and settle on the bark, usually 2-3 weeks after bloom. They insert their mouthparts and then form a waxy coating over their bodies. As they grow, they assume an oystershell-like shape, becoming elongate with one end enlarged. The gray scales mature late in the summer, and at that time, the females lay eggs under their waxy covers. A dormant application of horticultural oil will control overwintering scales. 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. To determine when crawlers are active, wrap black tape around infested limbs and coat the tape with Vaseline. The light-colored crawlers are easily seen on the tape. Consult the labels for dosage rates and safety precautions.

Poplar and willow curculio, Cryptorhynchus lapathi.
This insect is a borer in willow and has killed many fine specimens of pussy willow in the past. This weevil infests and destroys all of the larger pussy willows and also infests the smaller stems and branches of Carolina poplar and probably other species. The adults emerge in midsummer and lay eggs in punctures in the bark. The mature grub is about half an inch long, white, and without legs. The adult is about 1/3" long, black with the last third of the wing covers white. Badly infested trees or parts thereof should be cut and disposed of before the beetles emerge.

Poplar tentmaker, Clostera inclusa.
Caterpillars of this insect are present on willow in May and June and again in August and September, indicating that there are two generations each season. They make small nests or tents near the tips of the twigs. The mature caterpillar is about 1 1/2" long, dark brown, striped lengthwise with narrow yellow lines, and black head. On each of the fourth and eleventh segments there is a pair of pointed tubercles. The adult moth has a wing spread of 1 1/4", and is brownish gray with the apical third of forewings darkened with reddish brown and marked with fine white lines. This insect seldom defoliates trees, but when it is abundant, the trees may be sprayed with 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.

Spiny elm caterpillar, Nymphalis antiopa.
The mourning cloak butterfly lays eggs in cylindrical clusters on the small twigs of elm, poplar and willow. The larvae hatching from the eggs feed together in a cluster and soon strip the branch. The mature caterpillar is about 2" long, with the back sprinkled with small white dots, giving it a frosted or grayish appearance. Along the back is a row of somewhat diamond-shaped red spots. Each segment bears several black branched spines arranged approximately in transverse rows. The cocoon, suspended by the tail, is about 1" long and has a row of spine-like projections along the under surface. The butterfly has a wingspread of 2 1/2 - 3" and is purplish brown with yellow wing margins. There are two annual generations. Clipping off the twig as soon as caterpillars are noticed and crushing the caterpillars will control this pest.

Terrapin scale, Mesolecanium nigrofasciatum.
This small, oval, convex scale occurs on the small twigs of willow often killing them. It varies from 1/16 - 1/8" long and is usually reddish-brown mottled with black. Eggs are deposited in June under the old shells and there is one generation each year. Among the compounds registered for control of this pest in Connecticut are horticultural oil, malathion and imidacloprid. Horticultural oil may be applied as a dormant spray or when crawlers are active. Malathion may be applied in April when crawlers are active. To determine just when crawlers are active, wrap black tape around infested limbs and coat the tape with Vaseline. The light-colored crawlers are easily seen on the tape. Imidacloprid may also be applied as a systemic to be taken up by the roots. Consult the labels for dosage rates and safety precautions.

Willow cone gall, Rhabdophaga strobiloides.
This is a gray or brown cone-shaped gall about an inch long on the tips of willow shoots. The egg is laid by a small fly or midge in early May and the gall reaches maturity in June. There appears to be one generation each year. If control is required, spraying with malathion, which is among the compounds registered for use against this pest in Connecticut, when the adults are emerging in April and May should be effective. Consult the label for dosage rates and safety precautions.

Willow shoot sawfly, Janus abbreviatus.
This wasp is about 1/2" long. It appears in early spring and lays eggs in the new shoots of willow, girdling the stem below the egg. The larvae are borers in the shoots, sometimes tunneling them for 2'. They reach maturity in November and pupate in the shoots. Cutting and disposing of the wilted shoots will be helpful in controlling this insect.

Willow slugs, Nematus spp.These sawfly larvae feed on willow and poplar leaves. They usually have pale bodies with rows of black or yellow spots along each side of the body. Control can be achieved by spraying larvae with carbaryl, spinosad or malathion, which are among the compounds registered for use against this pest in Connecticut. Consult the labels for dosage rates and safety precautions.