Leaves Turn Yellow Because of Chlorosis
Sometimes soil is too alkaline for even tolerant oakleaf hydrangeas. In the absence of evidence of obvious insect attack or disease, this may be the cause of leaves turning yellow with green veins. Scratch some powdered garden sulfur, according to label instructions 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. For product information see the file in Yardener’s Tool Shed for Ironite.
Leaf Margins Burned May Mean 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 according to label instructions. Spraying the bare canes with dormant (heavy, or Volck) oil spray in early spring before leaves emerge destroys many overwintering mites.
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Leaves Skeletonized, Flowers Damaged is Caused by Rose Chafers
Rose chafers are grayish or fawn-colored and 1/2 inch long beetles. Slow-moving, they sometimes chew holes in leaves and damage hydrangea 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 pyrethrin/pyrethrum insecticide spray. Use it late in the afternoon to avoid harming beneficial insects and honeybees that may be in the area. If rose chafers are a chronic problem, cover plants with cheesecloth or white, polyspun garden fleece a week or so before their appearance time estimated from previous experience.
Leaves Curled And Distorted by Aphids
Aphids are soft-bodied, pear-shaped sucking insects, about the size of the head of a pin. They sometimes cluster on tender new hydrangea 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 clusters of these pests. Flush them from plant surfaces 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 according to label instructions.
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Leaves Webbed Together Signals 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 for a shelter. Affected 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 a product containing Bt (Bacillus thuringiensis) just as the caterpillars begin feeding according to label instructions. They will ingest the bacteria as they feed, stop eating and die in a day or two.
Small, Round Bumps On Twigs Are Caused by Scale Insects
Oystershell scale sometimes attack hydrangeas. They usually appear as bumps along the tender upper ends of the stems. The scale insects feed under the protection of these rounded waxy shells which 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.
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Knotted Growths On Stems, Roots Signals Nematodes
Nematodes are not insects, but slender, unsegmented roundworms. Most are soil-dwellers, less than 1/20 inch long, and are invisible to the naked 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, Deteriorate Because of Botrytis Blight
This blight disease is caused by a fungus which attacks dense hydrangea flower clusters during wet weather. The flowers become spotted, and the spots coalesce into soggy blotches. Promptly remove all diseased parts, and spray the entire shrub with a copper fungicide when symptoms first appear to discourage the spread of the fungus to healthy blossoms. Follow instructions on the product label. Increase air circulation around the your hydrangeas and, if possible, avoid overhead watering which dampens their leaves and blossoms.
Leaf Undersides Covered With White Powder Is Powdery Mildew
Powdery mildew is caused by a fungus which covers undersides of hydrangea leaves with a whitish coating. 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 garden 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 overwintering 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.
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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 overwinters on old plant parts. Prune out and destroy any affected branches and spray healthy plant parts with garden sulfur at weekly intervals until the symptoms disappear. For more information see the file on Controlling Fungal Disease