The hydrangea’s reliably charming and large flowers make it one of our beloved summer-flowering shrubs in the southern garden.
Many different species can be grown in our Louisiana landscapes. By far, the most commonly grown one is the bigleaf hydrangea, or hydrangea macrophylla, sometimes referred to as the garden hydrangea.
Two other remarkable hydrangeas we grow are hydrangea paniculata, also known as the panicle hydrangea, and hydrangea quercifolia, or the oakleaf hydrangea.
Though hydrangeas can be easy to grow, large amounts of rain can give rise to cercospora leaf spot, which attacks other plants as well.
Cercospora leaf spot is a foliar disease that rarely kills a plant. However, in severe cases, it will cause nearly complete defoliation. Mild cases of Cercospora leaf spot will cause hydrangeas to have unsightly foliage, can reduce the vigor of the shrub and possibly hinder flower buds from setting.
On the bigleaf and panicle hydrangea, the disease presents as small spots — ⅛- to ¼-inch in diameter with pale or light brown center and dark margins — which first appear on the bottom leaves. This pattern of leaf spotting is sometimes referred to as frogeye leaf spot. It will then progress to the top leaves.
On oakleaf hydrangeas, spots are angular instead of round and will be dark brown to purple. Similarly, the disease will progress from the bottom and move toward the top of the shrub.
Infected leaves that have fallen from the shrub can act as a source for spores of cercospora leaf spot. When the shrub receives any kind of overhead irrigation or rain, this will cause splashing of the spores from the fallen leaves to the underside of healthy leaves, allowing the disease to continue to spread throughout the shrub.
To control the disease, it's important to remove diseased leaves from the plant and those that have dropped. Do not put these leaves in your compost bin; discard them in the trash.
Chemical control options for cercospora leaf spot include spraying regularly with a product containing chlorothanil, such as Bonide Fung-onil, Ortho MAX Disease Garden Control or Daconil. While spraying will not cure the damaged foliage, it will prevent its spread to new foliage.
Also, fertilize lightly with a nitrogen fertilizer to help encourage new disease-free growth.