For most hydrangeas grown in landscape settings -- endless summer hydrangea, vanilla strawberry hydrangea, or little lime hydrangea -- they grow relatively pest-free, though there are certain conditions that contribute to diseases and infestation of insects.
For the big leaf hydrangea, the primary disease problem is powdery mildew. Powdery mildew is common on plants that grow in the shade or under high-humidity conditions. Hydrangea leaves that are infested with powdery mildew are coated with a light gray and powdery-looking substance. Purple splotches on the leaf may also appear. Powdery mildew very rarely kills plants, but it renders them unattractive. Powdery mildew is most severe on the big leaf hydrangea though it may also be present on other species.
Leaf spot organisms which are fungal in origin also attack hydrangeas. The fungal attack doesn’t kill the plant but the leaves turn a very unattractive brown to gray splotches surrounded by purplish halos. This commonly happens during late summer and in early fall. Moreover, sunny locations also contribute to the growth of fungal spots. Almost all hydrangea species are susceptible to fungal spots.
Oak leaf hydrangeas are the most susceptible to root rots, specifically the Armillaria root rot. The infested hydrangeas will wilt and will not recover even when they are watered afterwards. Hydrangeas with root rot will eventually die. Planting hydrangeas on soils that drain poorly will increase the risk of root rot. Thus, avoid soggy soils that retain too much water.
Fungal rust is also a problem. This appears on the back side of hydrangea leaves as small, orange spots. If you rub the back of the leaves, it releases the dust-like spores of the fungi. Fungal rust rarely kills hydrangeas. It usually occurs towards the end of the growing season.
Japanese beetles are known to eat oak leaf hydrangeas; they rarely attack other hydrangea varieties. These insects can be controlled by spraying or by dusting with insecticides. This is not advisable though since the infestation of Japanese beetles is not severe enough to merit the use of insecticides.
Aphids also attack new growths of all varieties of hydrangeas, but they are easily controlled by simply washing the plant with an insecticidal soap. If you see ants on the leaves, then that means you may have an aphid infestation. The ants eat the sticky excrement called the honeydew which is left by the aphids. Aphid problems usually disappear as the hydrangeas mature.
Finally, mites, which are too small to be seen by the naked eye, can also cause problems on hydrangeas. Mites cause damage in new shoots. Their activity worsens during hot and dry weather. To prevent mite damages, water your hydrangeas adequately during hot weather. Don’t control mites using insecticides when simple watering can solve the problem.