Climbing hydrangea are hardy plants that are largely disease free. In fact, if your climbing hydrangea is suffering from disease, it may be a sign that its growing conditions are not ideal. Treat sudden outbreaks as soon as they are identified. However, if the disease comes back next year, it is time to find your hydrangea a new home.
Climbing hydrangea grown in excessively humid, hot environments where they receive limited direct sunlight are susceptible to developing powdery mildew. The mildew covers the leaves of climbing hydrangea in what looks like light-gray powder. This mildew is unsightly, but largely harmless. Occasionally, a particularly virulent infestation may mark the leaves with yellow or purple splotches or cause them to fall off. Powdery mildew is easily managed by removing affected leaves and leaf-litter. More persistent cases can be treated with a powdery mildew fungicide according to the manufacturer's instructions.
Fungal Leaf Spots
Climbing hydrangea prefer to be grown in partial shade. If planted in full sun, your climbing hydrangea may develop fungal leaf spot disease. Predictably, leaf spot fungus covers climbing hydrangea leaves and flowers with ugly brown or gray spots. Leaf spot fungus begins its attack in late summer, and gets progressively worse through fall. If the fungus is allowed to spread unchecked, the affected plant will stop growing, defoliate and produce sickly-looking flowers. Get rid of leaf spot fungus is relatively easy. Simply remove the affected leaves and blossoms, avoid wetting the foliage when watering the plant, and add nitrogen to the soil to encourage it to thrive and produce new growth. Recurrent cases can be treated with a fungicide.
Climbing hydrangea's roots cannot tolerate standing water. If your hydrangea is not planted in well-drained soil, its roots are susceptible to the fungus that causes root rot. Affected plants will suddenly wilt and will not perk up when watered. Once a plant is afflicted with root rot, it will die within a few weeks. As soon as you identify your plant's disease as root rot, uproot and destroy it.
Root rust is another climbing hydrangea fungus. It manifests in small, rust-colored spots on the underside of climbing hydrangea's leaves. When touched, the fungus will rub off onto your hands and disperse, spreading to other areas of your climbing hydrangea or neighboring plants. To rid your plant of root rust, remove any affected leaves or leaf litter as soon as the rust spots are sighted. If the disease comes back the following year, spray the plant with a fungicide according to the manufacturer's instructions.