Severe Damage Especially In Winter Can Be Deer
For information about dealing with deer see the file Dealing With Deer
Leaves Turn Yellow means Chlorosis.
Hydrangeas cannot handle very alkaline soil. It causes their leaves to turn yellowish. In the absence of evidence of insect attack or disease, this may be the cause of leaf yellowing. Scratch some powdered sulfur or used coffee grounds into the soil to acidify it. Avoid excessive use of lime, which renders the iron in the soil unavailable to plants. A new way to cure chlorosis is to use iron chelates either as foliage sprays or soil drenches. See Yardner’s Tool Shed for a product called Ironite.
Leaves Skeletonized, Flowers Damaged Rose Chafers.
Rose chafers are grayish or fawn-colored and 1/2 inch long. These slow-moving beetles chew holes in leaves and damage flowers, especially white ones, by feeding on the petals and soiling them with excrement. Control them by hand-picking. For major infestations, spray beetles with a pyrethrum spray. Use it late in the afternoon to avoid harming beneficial insects and honeybees. If rose chafers are a chronic problem, cover plants with cheesecloth or fleece (Reemay) a week or so before their estimated appearance time. For long-term control, scratch milky spore powder (Bacillus popilliae) into the soil. This disease will attack their larvae over 2 or 3 years, reducing the problem substantially.
Leaves Curled And Distorted means Aphids.
Aphids are soft-bodied, pear-shaped sucking insects, about the size of the head of a pin. They cluster on tender new shoots and leaves, sucking plant sap. They retard and distort plant growth. Under their attack hydrangea leaves may turn yellow or brown, wilt under bright sunlight, or sometimes curl and pucker. Check leaf undersides for small groups of the pests. Hit them with a forceful spray of water 3 times, once every other day, in the early morning. If that does not work, spray them with insecticidal soap every 2 to 3 days until they are gone. A neem insecticide sprayed on shrub foliage 2 or 3 times at 7 to 10 day intervals will also control aphids.
For more information see file on Controlling Aphids.
Leaves Webbed Together indicates Leaftiers.
Hydrangea leaftier larvae are green caterpillars, 1/2 inch long, with dark brown heads. These caterpillars protect themselves while feeding by binding adjacent leaves together. Hydrangea foliage becomes ragged and unsightly, turns brown and dies. For minor infestations, crush the larvae in their rolled hideouts. To counter major attacks, spray the foliage of vulnerable shrubs with Bt (Bacillus thuringiensis) just as the caterpillars begin feeding. Repeat every week or 10 days while the caterpillars are feeding. They will ingest the bacteria as they feed and die in a day or two.
Leaf Margins Burned seems to be Mites.
Mites are about 1/50 inch long, barely visible to the unaided eye. They may be yellow, green, red or brown. Two-spotted spider mites cause damage on hydrangeas resembling sunscald. The leaves look burned, especially along their edges. Start control measures as soon as you notice evidence of burning on the leaves or delicate webbing near leaf stems. Spray the shrubs in the early morning with a forceful water spray to knock the mites from the leaf undersides. Repeat the water spray daily for 3 days. If that does not do the job, spray the mites with insecticidal soap every 3 to 5 days for two weeks. Spraying the bare stems with dormant (heavy) oil spray in early spring before leaves emerge destroys many over wintering mites.
For more information see file on Controlling Mites.
Small, Round Bumps On Twigs means Scale Insects.
Oyster shell scale sometimes attacks hydrangeas. They usually appear on the upper ends of the stems. Scale insects are covered by rounded waxy shells, which protect them while they feed. The shells may be white, yellow, or brown to black, and are about 1/10 inch in diameter. Spray infested shrubs with light, or "superior" horticultural oil that will coat the scale and smother them.
For more information see file on Controlling Scale.
Knotted Growths On Stems, indicates Roots Nematodes.
Nematodes are not insects, but slender, unsegmented roundworms. Most are soil-dwellers, less than 1/ inch long, and are invisible to the unaided eye. They have piercing-sucking mouthparts. Stem nematodes cause hydrangea stems to become swollen and split. As a result their leaves drop off. Infested plants look sickly, wilted, stunted, and have yellowed or bronzed foliage. They decline slowly and die. The root systems of affected plants are poorly developed, even partially decayed. To control these pests, add lots of compost or leaf mold, if it is available, to the soil around the hydrangea plants to encourage beneficial fungi that attack nematodes. Pour liquid fish emulsion into the soil as a drench to fertilize affected shrubs and to repel nematodes.
Buds and Flowers Spotted, Deteriorated shows Botrytis Blight.
A fungus that attacks dense hydrangea flower clusters during wet weather causes this blight disease. The flowers become spotted, and the spots coalesce into discolored, soggy blotches. Promptly remove all diseased parts, and spray shrubs with a copper fungicide when symptoms first appear. Repeat the spray every 10 days in wet seasons. Increase air circulation around the hydrangeas and avoid overhead watering which dampens their leaves and blossoms.
Leaf Undersides Covered With White Powder indicates Powdery Mildew.
Powdery mildew is caused by a fungus that covers leaf undersides with white mold. The upper surfaces of the infected leaves may stay green or turn purplish brown in color. Buds and new growth may also be attacked. Spray affected shrubs thoroughly with a wettable sulfur fungicide once or twice at weekly intervals starting as soon as the whitish coating appears. Collect and discard all aboveground refuse in the fall to prevent the fungus from over wintering on yard debris. Mulch shrubs to prevent rain from splashing fungal spores up onto leaves. Recent research suggests that spraying foliage vulnerable to mildews with anti-transpirant spray helps it resist infection, as the spores have difficulty adhering to the coated leaves.
For more information see file on Controlling Fungal Disease.
Leaves Spotted With Powdery Brown Pustules means Rust.
A rust disease caused by a fungus attacks certain hydrangea varieties. The leaves become brittle and spotted with many yellowish to rusty brown pustules, especially on the undersides. The disease over winters on old plant parts. Prune out and destroy any affected branches and spray with wettable sulfur at weekly intervals until the symptoms disappear.
For more information see file on Controlling Fungal Disease.