Hydrangeas (Hydrangea spp. and cvs., Zones 3–9) are timeless favorites of gardeners everywhere, but these beloved shrubs are also favored by a wide variety of pathogens. Fortunately, there are many ­preventive measures we can take to keep our plants safe, and knowing the signs and symptoms of common diseases can help us make a prompt and accurate diagnosis when disease does strike. With a little planning, research, and appropriate action when needed, you can keep your hydrangeas healthy and flowering for many years to come. Here are some of the most common hydrangea ailments, their signs and symptoms, and recommended management strategies.

Bacterial spot

Cause: Xanthomonas campestris bacteria

Symptoms and signs: Angular leaf spots first appear in the interior, lower canopy of the plant, often when the weather turns warm and wet. Unlike fungal leaf spots, the discolorations are black and angular, and they occur on both top and bottom sides of the leaf. The spots sometimes have a yellow halo. ­Bacterial spot is commonly misidentified as Cercospora leaf spot (p. 22) on oakleaf hydrangea (H. quercifolia, Zones 5–9). Microscopy or more advanced laboratory techniques may be needed to correctly diagnose this problem.

Management: Bigleaf hydrangeas (H. macrophylla and cvs., Zones 5–9, smooth hydrangeas (H. arborescens and cvs., Zones 3–9), and oakleaf hydrangeas are common hosts, but all members of the genus are susceptible. This bacterium is spread by water and enters injured tissue. To reduce its spread, avoid overhead watering and working around wet plants. Copper-based products are often suggested for treatment but are only marginally effective; this heavy metal will build up in the soil over time and can cause phytotoxicity problems if aluminum is used to acidify the soil. For these reasons, it is best not to use copper-based products to control bacterial spot. 

Botrytis blight, or gray mold

Cause: Botrytis cinerea fungi

Symptoms and signs: Tan to brown blotches appear first on bracts and flowers; these discolorations may become covered in light gray or taupe mildew under high humidity. In severe cases, the fungus spreads from blighted flowers to shoots and stems. 

Management: Botrytis blight tends to be most problematic when the weather is cool and wet, often in spring and fall. Avoid overhead watering, excessive nitrogen fertilization, and injuring plants, which predispose hydrangeas to infection. While bigleaf hydrangeas are often impacted most severely, most species are susceptible. Thinning and pruning plants to open up the canopy improves airflow and discourages botrytis infection. Fungicides that contain captan or chlorothalonil are effective; to protect pollinators, avoid applying these to flowering plants. 

Powdery mildew 

Cause: Several species of fungi, including Erysiphe spp., Golovinomyces spp., and Microsphaera spp. 

Symptoms and signs: The first signs to appear are small, fuzzy, white patches on the upper leaf surface. Upon closer examination, threadlike growth (hyphae) is visible on the leaf surface. Later in summer, small black “balls” develop within the hyphae that protect the fungal offspring in a thick shell that will survive the winter. In severe instances, infected leaves will turn yellow and drop. 

Management: Bigleaf hydrangeas are most susceptible to powdery mildew, while oakleaf hydrangeas appear to be more resistant. Keeping plants well watered to minimize drought stress and senescence also protects against powdery mildew. Plants perform best when they are located in a site with morning sun and afternoon shade. Effective fungicides for controlling powdery mildew include sulfur, myclobutanil, tebuconazole, and triticonazole. 

Cercospora leaf spot

Cause: Cercospora hydrangeae fungi

Symptoms and signs: Symptoms of Cercospora leaf spot are most dramatic on bigleaf hydrangeas, but smooth, panicle (H. paniculata, Zones 3–9), oakleaf, and mountain hydrangeas (H. serrata and cvs., Zones 6–9) are also hosts. As with many foliar diseases, the symptoms are first observed on the interior leaves near the base of the plant. On bigleaf hydrangeas, these spots have a round, reddish-purple halo surrounding a tan to gray lesion, whereas reddish-purple angular leaf spots develop on oakleaf hydrangeas. If an infection becomes severe, heavily afflicted leaves may turn yellow before falling off. Unlike many foliar diseases, Cercospora is worse on plants grown in full sun. 

Management: Proper placement of plants—specifically, providing afternoon shade—is a critical component in managing this disease. Avoid overhead watering to reduce the spread of the pathogen. Researchers at ­Tennessee State University found that these varieties of bigleaf ­hydrangea were tolerant of or resistant to Cercospora: ‘Ami Pasquier’, ‘Ayesha’, ‘Forever Pink’, ‘Fuji Waterfall’, ‘Seafoam’, ‘Taube’, ‘Tricolor’, and ‘Veitchii’. Resistant mountain hydrangea varieties include ‘Bluebird’ and ‘Miyama-yae-Murasaki’, which can even be grown in full sun. Although integrated management using fungicides is possible, most gardeners don’t want to apply fungicides every few weeks or whenever wet weather occurs. Fungicides containing chlorothalonil, copper, triticonazole, or propiconazole are effective when applied before symptom development to prevent infection. To protect pollinators, spray susceptible foliage only and avoid any flowers.

Viruses

Cause: At least 17 reported viruses, including aster yellows

Symptoms: Plants with a virus infection may display leaf yellowing (chlorosis), mosaic, chlorotic and/or necrotic ringspots, leaf mottling and distortion, blotches, stunting, and sometimes flower virescence (flowers reverting to leaf development). Some viruses, like tobacco rattle virus or tomato spotted wilt virus, may impact the appearance and health of the plant. However, many viruses are latent (endemic to the plant) and cause only slight symptoms or none at all. A latent viral infection may even protect against other, more serious viruses. 

Management: There is no treatment or cure for hydrangeas infected by a virus, but it is important to identify which virus is infecting a plant and to reduce the spread of nonlatent viruses. Some nonlatent viruses, like tobacco rattle virus and tomato spotted wilt virus, have broad host ranges and affect hundreds of plant species. Insects and nematodes act as vectors, spreading the virus to other plants in a garden. If you suspect a plant has a viral infection, work with your local diagnostic lab to identify the virus so you can then implement an appropriate management strategy (either tolerating the infection or removing the plant). Alternatively, you may choose to simply remove and destroy the infected plant to eliminate any risk of spread by insect, nematode, or human vectors. Although this strategy sounds harsh, I have been in gardens where almost every plant is infected with tobacco mosaic or tobacco rattle virus. Losing one plant is heartbreaking, but it will be a worthwhile sacrifice if it protects the rest of your garden.

More hydrangea resources for diagnosing and treating diseases

• Disease-resistant hydrangeas information from Cornell 
• Purdue Plant Doctor hydrangea page 

Janna Beckerman, Ph.D., is a professor and extension plant pathologist at Purdue University specializing in diseases of horticultural crops.