Unlike minor soil deficiencies and cultural problems that can turn your oakleaf hydrangea's leaves yellow, leaves that are turning black signify a more severe problem. While providing optimal care to your oakleaf hydrangea is key in avoiding pest and fungal disease issues, determining the cause and appropriate treatment for your plant as soon as you notice symptoms is essential in avoiding plant death.
Preventive care, at the very least, lessens the intensity of insect infestations and disease and at best, keeps them from affecting your oakleaf hydrangea. Vigorous plants are more likely to avoid and recover from pests and disease than weakened or stressed plants. Grow oakleaf hydrangeas in locations that provide full sun to shade. Preferred soil conditions include moist, well-drained soil with an acid pH level, though these hydrangeas will tolerate a higher pH.
Types of Problems
Fungal diseases are the most likely culprit when oakleafy hydrangea leaves turn black. Homeowners should look for abnormally shaped spots on leaf surfaces that, while they appear black from a distance, are likely dark brown or dark purple upon closer inspection. These spots are a symptom of the fungal infection cercospora leaf spot disease. Another cause is sooty mould, a black fungal growth that can cover leaf surfaces. Sooty mould develops as a result of insect feeding. Sucking bugs, like aphids, are a prevalent problem on hydrangea plants. These bugs feed on the inner fluid of tender plant tissue and, as they feed, they release a sticky-sweet substance called honeydew. The honeydew that drips onto leaves and other plants parts encourages the growth of sooty mould.
Deal with problems that turn oakleaf hydrangea leaves black promptly, as plants too far in decline are incurable. The weaker a plant is, the more difficult it is to treat. Cercospora leaf spots disease leads to the yellowing and early drop of leaves. Aphid infestations lead to stunted growth and diminished health, while sooty mould can fully cover leaves, blocking out necessary sunlight.
Natural management methods are the initial step in controlling black leaves on your oakleaf hydrangea. For cercospora leaf spot, gardeners should remove and destroy affected plant parts and fallen leaves. Apply a nitrogen fertiliser in addition to surface watering reduces the spread of the fungal infection. Disposal of affected plant parts is also advisable for aphid and sooty mould problems. Home gardeners can wash both aphids and sooty mould from hydrangea surfaces with a hard spray of cool water. Additionally, gardeners can release natural enemies, like parasitic wasps, that kill aphids. Predator bugs are available in garden supply stores.
For severe problems, chemicals provide effective control. Cercospora leaf spots on hydrangeas respond well to fungicides with the active ingredient chlorothalonil or mancozeb). For control of aphids and sooty mould, insecticidal soap and insecticides with the active ingredient pyrethrin are reliable.
- Alabama Cooperative Extension System: Diseases of Hydrangea; Austin K. Hagan and Jackie Mullen; 2001
- University of Missouri Extension: Aphids, Scales and Mites on Home Garden and Landscape Plants; Bruce A. Barrett; June 2008
- University of Wisconsin-Extension: Sooty Mold; Gayle L. Worf, et al., 1995
- University of Illinois Extension: HortAnswers - Oakleaf Hydrangea