Leaf Spots (fungi – Cercospora spp., Phyllosticta hydrangeae and others): Circular-to-irregular shaped spots are produced on leaves. Use foliar protectant fungicides as needed.
Powdery Mildew (fungus – Erysiphe polygoni): A white, powdery substance appears on leaf surfaces. Yellow spots that later turn brown may appear on the upper surface. If allowed to develop, it may get into newly developing buds and cause stunting. Use a foliar fungicide that has the ability to control powdery mildew.
Botrytis Blight (fungus – Botrytis cinerea): Botrytis is probably the most serious fungus attacking this plant because it causes severe problems in the flower buds. Infected flower parts may drop on leaves and cause infection of those structures. Cool, damp conditions with an abundance of fungal spores encourage the condition. This problem is especially severe at times in commercial greenhouses. Dispose of any weak or decaying plant tissue and use appropriate protectant fungicides if necessary.
Ring Spot (virus – Hydrangea ring-spot virus): Pale colored rings may appear on leaves and some varieties may have distorted leaves. Dispose of diseased plants. Commercial plant producers should use tolerant varieties.
Nematodes (nematodes – Meliodogyne spp., Pratylenchus spp., and Ditylenchus spp.): Nematode damage occurs when this plant is set in infested beds. Plants grown in pots would not be affected if nematode-free propagating stock and sterilized soil is used.