Things You'll Need
2 percent copper naphthalene
When conditions are warm and humid, several fungi can attack hydrangea shrubs, causing leaf spots and killing the flowers. Powdery mildew, for instance, covers the bottom sides of the foliage with gray mold. Not even well-protected greenhouse hydrangeas are safe. Gray mold or Botrytis blight has found a way of sneaking into the plant's sheltered environment. It targets the shrub's flowers. How you plant and maintain hydrangeas play a role in how susceptible to mold they are.
Ask the nursery staff what the spacing requirement is for your species of hydrangea. Plant accordingly to provide the shrub adequate air circulation, which creates an inhospitable environment for powdery mildew. Keep fans running if your hydrangea is indoors or in a greenhouse.
Treat powdery mildew at the first sight of spores on the hydrangea leaves, using a fungicide. Look for a thin layer of gray mold. Fungicides containing the chemicals azoxystrobin, fenarimol, paraffinic oil or thiophanate-methyl are labeled for controlling powdery mildew. Follow the manufacturer's instructions for application rate and frequency.
Disinfect greenhouse wooden surfaces with 2 percent copper naphthalene to kill Botrytis, the cause of gray mold in hydrangea flowers. Cover those surfaces with plastic to prevent ultraviolet rays from activating the spores.
Apply a fungicide containing iprodione, mancozeb or thiophanate-methyl before any signs of gray mold appear. Follow the manufacturer's instructions for the application rate. Spray the hydrangeas every one to two weeks on cloudy or rainy days.
Prune all infected plant parts and discard them in the trash can immediately to avoid transferring the powdery mildew and gray mold fungal spores to healthy tissue. Rake up any debris on the ground near the plant.