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Symptoms, Causes, and Cures
Scan either list of common plant disease and pest symptoms for conditions that match your garden’s problem, then click on the symptom to reveal the disease or diseases that may cause the symptoms or find a cure.
If the cure requires application of sprays or other treatments, read completely and follow exactly the application directions found on the treatment agent’s label for a control product that lists the specific pest or disease being treated.
Tree and Shrub Diseases
Symptom: Black, tan, or red spots on leaves; leaves yellow and drop. Black cankers on stems, with a general wilt of branch tips.
Cause: Anthracnose, a fungal disease that occurs in late spring and summer, typically after humid conditions.
Cure: Remove infected leaves and branch tips. Collect and destroy infected fallen leaves. Thin excessive growth to promote air circulation. Spray with Bordeaux mixture, available at many nurseries and garden retailers.
Symptom: Tree or shrub flowers wilt and foliage tips curl, yellow, and die. Oozing cankers form at base of shoots with main trunk or stem. Affected foliage may have a strong odor.
Cause: Blight and scab, several bacterial diseases caused when soil that contains bacteria is splashed onto cut and bruised foliage or is carried to flowers of trees or shrubs by pollinating insects.
Cure: Remove and dispose of affected trees or shrubs. Apply water at base of remaining trees or shrubs. Spray with Bordeaux mixture, available at many nurseries and garden retailers.
Symptom: Flowers and foliage collapse under heavy fuzz of brown or gray fungal spores.
Cause: Botrytis rot, also known as gray mold, a fungal disease.
Cure: Remove affected blossoms, foliage, or entire tree or shrub; space plantings for more air circulation; reduce nitrogen fertilizer.
Symptom: Brown stains and softened tissue near base of stem or crown of tree or shrub; leaves may yellow or drop.
Cause: Crown or stem rot. Look for decaying leaf stems. Usually associated with keeping soil overly moist.
Cure: Rarely curable; remove infected foliage, dipping pruning shears in isopropyl alcohol solution between cuts to sterilize them and avoid spreading the disease to healthy plants. Reduce watering. Take root cuttings of healthy growth.
Symptom: Gray, cottony masses shroud brown-spotted, dying foliage. Stems are discolored.
Cause: Downy mildew, a fungal disease common in cool, moist conditions.
Cure: Space trees and shrubs to allow good air circulation between neighboring trees and shrubs. Remove shading foliage. Apply water at base of tree’s or shrub’s root zones. Strip and destroy infected leaves.
Symptom: Tough twigs and branches die back, starting at tip ends. Leaves shrivel and turn brown. Tips of twigs curl to resemble a hook.
Cause: Fireblight, a bacterial disease common on ornamental pears, crabapples, and quinces. Infection occurs in early spring and is favored by wet conditions.
Cure: Prune out branches 6 in. (15 cm) below areas showing symptoms. Dip pruning tool in isopropyl alcohol after each cut to sterilize and prevent spreading infection. Avoid heavy nitrogen fertilization.
Symptom: Flowers of trees or shrubs wilt and foliage tips curl, yellow, and die. Gray, powdery spores and fibers sometimes are seen on leaf undersides.
Cause: Fusarium wilt, a fungal disease common in cool, moist conditions caused by spores in soil entering the tree or shrub through its roots. Most prevalent in plantings that are experiencing stress or mineral deficiencies.
Cure: Space trees or shrubs to allow good air circulation between neighboring trees and shrubs. Remove shading foliage. Strip and destroy infected leaves. Fertilize with 0–5–10 formula and mulch soil surface with organic compost. Apply water at outer foliage’s drip line on soil surface.
Symptom: Leaves turn light green and are puffy, swollen, and distorted.
Cause: Leaf gall, a fungal disease commonly found on azaleas, camellias and rhododendrons.
Cure: Pick off and discard or destroy affected leaves as soon as gall is noticed. Do not compost infected foliage.
Symptom: Powdery black or brown dusting on foliage and blossoms of trees and shrubs. Leaves may drop.
Cause: Leaf spot, a fungal disease. Common in low-light, crowded plantings of trees and shrubs.
Cure: Remove shading foliage. Increase air circulation. Spray with a fungicide containing sulfur.
Symptom: Streaked and mottled foliage, deformed blooms on trees and shrubs; stunting; loss of vigor.
Cause: Mosaic virus, an incurable plant disease.
Cure: Remove and destroy affected plants. Promptly control aphid, spider mite, and thrip infestations, which can spread viral infection. Plant mosaic-virus resistant cultivars.
Symptom: Tree and shrub leaves have white to grayish, powdery patches, as though they have been dusted with flour or gray powder.
Cause: Powdery mildew, a fungal disease prevalent when days are hot and nights are cool, with high humidity. Mostly attacks new leaves.
Cure: Thin branches to improve air circulation. Spray with a 0.5% solution of baking soda (sodium bicarbonate): 1 tsp. (5 ml) baking soda per quart [1 l] of water.
Symptom: Trees and shrubs lose vigor and wilt in hot weather. Leaves drop. Plants die.
Cause: Verticillium wilt, a fungal disease caused by spores in soil entering the trees or shrubs through their roots.
Cure: Remove and destroy affected trees or shrubs, or dispose of them in household garbage. Replant with resistant grafted rootstock.
Tree and Shrub Pests
Symptom: Leaves of trees and shrubs become curled and twisted, often with a black, sooty appearance. Deformed or stunted blooms on new growth.
Cause: Aphids. Look for soft-bodied, round, black, gray, green, or yellow insects, 1/16–1/4 inch (1.5–6 mm) long. Found on many tree, shrub, and plant species. Frequently found in combination with ants.
Cure: Wash off light infestations with a strong stream of water. If ineffective, dust with diatomaceous earth or spray with superior oil. Spray with botanical neem oil or pyrethrin as a last resort.
Symptom: Weakened and defoliated trees and shrubs. Small, silken bags about 1–2 in. (25–50 mm) long hang from branches.
Cause: Bagworms. Caterpillars hidden by silky bag. Common in North America, east of the Rocky Mountains. Found on deciduous plants and conifers; worst damage on arborvitae and cedars.
Cure: For light infestations, hand pick and destroy bags. For heavy infestations, spray with Bacillus thuringiensis (BT) as soon as young bagworms emerge and are noticed.
Symptom: Green parts of leaves of trees and shrubs are chewed, removed, or left in place stripped to veins. Later, leaves are dry and skeletonized.
Cause: Beetles, including Japanese beetles, elm-leaf beetles, willow-leaf beetles, and other beetle larvae. Hard-shelled beetles are most active in the heat of the day.
Cure: Hand pick beetles after shaking tree or shrub in early morning while they are inactive. Apply milky spore (Bacillus popilliae) for Japanese beetle grubs that develop underground beneath turfgrass lawns.
Symptom: Ripe and half-ripened fruit and berries of trees, shrubs, and vines bear numerous cuts and holes. Partially eaten produce covers the ground, accompanied by bird droppings.
Cause: Birds. Look for songbirds, crows, and other fruit- and berry-eating species that forage on or near the ground.
Cure: Mount silver-foil streamers or reflective pinwheels in landscape areas. Hang and frequently move owl and hawk silhouettes and decoys around the landscape.
Symptom: Foliage of trees and shrubs shows signs of eating, and stems are scored or eaten. Black or brown droppings, 1/20-in. (1.2-mm) wide are left on foliage. Moths and butterflies are seen hovering near or resting on plants.
Cause: Various larvae and caterpillars, white cabbage moth, wormlike larvae, and loopers. Look at undersides of leaves and along stems for pests during midday.
Cure: Hand pick caterpillars from center leaves of plant. Release parasitic wasps and flies. Spray with Bacillus thuringiensis (BT). Apply horticultural oil in summer.
Symptom: Uprooted plants; foliage of shrubs and trees eaten from ground level to a height of 8 ft. (2.4 m) or higher; roots eaten, leaving dying young trees or shrubs.
Cause: Deer and rodents. Look for hoof and paw prints, burrows, mounds, and tunnels.
Cure: Plant resistant plants. Install fence barriers or cages when planting, including beneath-soil barriers. Bait, live trap and remove rodents. Avoid strong-scented fertilizers containing bonemeal or fish byproducts.
Symptom: In summer and autumn, silky webs appear at the tip ends of branches, with larvae visible inside. Leaves are chewed.
Cause: Fall webworms. Adult larvae are about 1 in. (25 mm) long with a dark stripe down their backs. Common only on deciduous trees and shrubs.
Cure: Prune and destroy branches infested with webs. If necessary, spray the larvae with Bacillus thuringiensis (BT) after breaking open the web sacks so the control agent saturates them.
Symptom: Silvery spots on upper surfaces of leaves; black spots on undersides of leaves.
Cause: Lace bugs. Bugs are 1/8–1/4-in. (3–6 mm) long. Common during the growing season on rhododendrons, laurels, hawthornes, and elms.
Cure: Wash off light infestations with a strong stream of water. If necessary, spray with superior oil. Spray with botanical neem oil or pyrethrin as a last resort.
Symptom: White trails on or within leaves of trees and shrubs; papery yellow or brown blotches on foliage.
Cause: Leaf miners. Look for small, pale larvae and 1/6-in. (4.2-mm) tiny, green or black, flying insects active during the day.
Cure: Hand pick larvae. Remove infested leaves. Release lady beetles. Spray foliage with neem oil as a last resort.
Symptom: Stunted, discolored, spotted trees or shrubs with deformed roots, sometimes bearing swollen galls; loss of vigor.
Cause: Soil nematodes, microscopic wormlike creatures that live in soil and feed on plant roots.
Cure: Release beneficial nematodes. Remove and destroy affected plants. Replant with unrelated species. Solarize planting area prior to planting for 3–4 weeks by covering its soil with clear plastic and allowing sunlight to raise soil temperature to 140°F (60°C).
Symptom: Leaves of trees or shrubs with brown or black spots. Roots and shoots may be deformed.
Cause: Plant bugs. Look for greenish yellow or tan insects 1/4-in. (6-mm) long.
Cure: Hand pick. Spray with insecticidal soap. If infestation is severe, spray affected areas with pyrethrin or rotenone as a last resort.
Symptom: In spring and summer, silky webs, or tents, appear in the forks of small limbs, and leaves are eaten. In winter, masses of eggs encircle terminal branches.
Cause: Tent caterpillars. Adult caterpillars are 1–2 in. (25–50 mm) long with short, fuzzy bodies. Common only on deciduous trees and shrubs.
Cure: In winter, remove egg masses from bare branches or smother them by applying horticultural oil. If necessary, spray the larvae with insecticidal soap or Bacillus thuringiensis (BT).
Symptom: Brown-, silver-, or white-speckled leaves; may be gummy or deformed. Blooms of trees or shrubs are deformed and fail to open.
Cause: Thrips. Shake foliage and blossoms over white paper and look for moving, winged specks. Thrive in hot, dry conditions.
Cure: Release lady beetles. Remove and destroy infested foliage. Wash foliage with water; spray pests with insecticidal soap.
Symptom: Yellow leaves and stunted, sticky plants. When foliage of trees and shrubs is shaken, a cloud of white insects may fly up.
Cause: Whiteflies. Shake foliage and look for 1/20-in. (1.2-mm) mothlike, flying insects. Inspect leaf undersides for scalelike, gray or yellow eggs.
Cure: Catch with sticky traps. Spray with soap solution. Spray infested foliage with insecticidal-soap. Spray foliage with horticultural oil or neem oil extract solution. Spray with pyrethrin as a last resort.
Symptom: Chewed leaves and blossoms on trees and shrubs; silvery mucus trails.
Cause: Slugs and snails. After dark, look for shelled and unshelled mollusks on foliage or soil.
Cure: Remove leaf litter, which is used as a hiding place. Hand pick after dark; dust with diatomaceous earth; use beer-filled traps; use non-toxic baits containing iron phosphate; use bait gel.
> Next: Ornamental Trees