Because Willows grow so rapidly, they are unfortunately quite vulnerable to many insect and disease problems. Because the list is so long, I have omitted the details for those insects and diseases often covered in other tree descriptions.
Break in Storms
Weak Wood - Willows have weak, brittle wood. They often split or break in storms, especially under the weight of ice or wet snow. There is no effective protection from this problem, except to site the trees in areas that are somewhat sheltered from wind.
Bark Attacked, Leaves Curled and Distorted
Aphids – Aphids are soft-bodied, pear-shaped insects that cluster on growing tips, new leaves and flowers of many plants. While they are usually about the size of a pinhead, a larger aphid attacks tree leaves, sucking their sap and causing them to turn yellow and curl. Aphid colonies on the lower branches can be dislodged with a strong stream of water from the garden hose.
Holes in Leaves and Girdled Twigs
Bagworm - Adult male bagworms are black, clear-winged moths. Wingless females and larvae (caterpillars) live in distinctive 2-inch long bags of tough silk, camouflaged with bits of vegetation that hang from willow branches like ornaments. The feeding caterpillars disfigure willow trees by feeding on their leaves and girdling twigs.
Branches Swollen and Distorted, Holes, Sawdust and Sap Stains at Base of Trunk
Borers - Adult borers are beetles of some sort. The borers or larvae are ½-inch long worms that damage the bark of living, injured, and dying trees. They make sinuous interlacing, flattened galleries in the inner bark and sapwood, girdling infested limbs or trunk and interfering with the tree's circulation. They prey particularly on trees that are low in vigor due to drought, transplanting, bark wounds from yard-care equipment, or insect attack. Try to locate the telltale holes in stems. They may have sawdust around the entrance.
Holes In Leaves
Caterpillars - Caterpillars come in lots of sizes and colors since they are the larvae of many different kinds of moth. When the hungry larvae of these moths, caterpillars, emerge to feed on tree foliage, they are capable of serious damage to the tree if present in sufficient numbers.
Leaves Consumed, Conspicuous Tawny Egg Masses Present
Gypsy Moth -Gypsy moth caterpillars grow from about 1/16 inch long at hatching to about 2-1/2 inches long by the time they become pupae. Mature larvae are covered with black hairs and have 5 pairs of blue spots, 6 pairs of red spots along the back, and voracious appetites. In July, they encase themselves in brown shells the trunks of trees to pupate. Gypsy moth larvae are often confused with the eastern tent caterpillar and fall webworm, both of which make silken tents in trees. Gypsy moths do not make tents. Seek out the distinctive sawdust-colored egg-masses on trunks, branches, under your roof eaves, and other protected spots. The eggs look like little gold pearls. Uncontrolled, gypsy moth caterpillars can defoliate a tree completely.
Leaf Undersides Damaged
Imported Willow Leaf Beetle - Imported willow leaf beetles are metallic blue beetles, about 1/8 inch long. They live through the winter under the bark scales and in the rubbish around willow trees. In early June they emerge and lay eggs. Adult beetles develop during July and produce a second brood in August. Their worm-like grubs feed on the undersides of willow leaves, leaving only a network of veins. These beetles cause a lot of damage to willows in the New York area.
Foliage Turns Pale or Mottled
Lace Bug - Willow lace bugs suck sap from leaf undersides. Adults are small square-shaped bugs, 3/16 inch long or less, with elaborately reticulated wings that resemble lacework. Willow leaves may be severely mottled and yellowed by this pest.
Leaves are Skeletonized
Sawfly - Adult willow shoot sawflies are wasp-like, but with thicker midsections, 5/8 to 1 ½ inches long. They have 2 pairs of transparent wings. Their larvae resemble caterpillars, and are, on the average, 1/2 inch long. These larvae skeletonize leaves of willows, some mine leaves as well. The tree may be defoliated. These pests are found from Canada south to Mississippi and Arkansas.
Thrips - Pear thrips and citrus thrips are pests of willows. Adult thrips are tiny, slender insects, 1/25 inch long, variously colored pale yellowish, black or brown. They have four long, narrow wings fringed with long hairs. Their legs are very short. Thrips larvae are usually wingless. They cause damage by rasping at plant cells and sucking sap from the injury. Willow leaf surfaces become flecked and whitened, leaf tips wither, curl up and die. The leaf undersides are spotted with tiny black specks of excrement.
Leaves Wilted, Discolored, Branches Cankered, Die Back
Bacterial Blight - A blight caused by bacteria causes willow leaves to turn brown and wilt. Blighted branches die back. These bacteria, that over winter in sores on the willow tree infects young leaves as soon as they emerge. Sometimes infected willow trees become entirely defoliated. Bacterial blight damage resembles frost injury and may be confused with it.
Tumor-like Swellings on Roots, Trunk or Branches
Crown Gall - A certain bacteria infect willow trees through wounds and stimulate its cells to form tumor-like swellings (galls). Ranging in size from peas to large burls growing to 1 or 2 feet in diameter, these galls have rough, irregular surfaces. The growth of affected trees slows and its leaves often turn yellow. Its branches or roots sometimes die. Cut down and destroy any affected trees. Be careful not to cause wounds in healthy trees with yard equipment through which infection can occur.
Leaves Covered With White Powder
Powdery Mildew – The ends of twigs, leaves, and blossoms covered with white to pearly-gray velvety mold, indicate Willow trees are infected with powdery mildew. Twigs are dwarfed, and the terminal bud is killed, which causes staghorn growth of side shoots.
Yellowish Spots and Dark Pustules on Leaf Undersides
Rust -Some rust diseases that are caused by fungi attack willow leaves. They are identified by lemon yellow spots on leaf undersides, that later develop into spore-bearing pustules. A severe infection will cause the willow leaves to drop. Although rust infections are not considered serious, they can defoliate young trees.