Cercospora leaf spot on hydrangea

Cercospora leaf spot is the most common fungal leaf disease of landscape hydrangeas, and I see it reliably on my bigleaf hydrangeas (Hydrangea macrophylla) every summer at my Long Island property -- always in August, always on the lower and inner leaves first, always worse in years with a wet July..

Cercospora leaf spot is the most common fungal leaf disease of landscape hydrangeas, and I see it reliably on my bigleaf hydrangeas (Hydrangea macrophylla) every summer at my Long Island property — always in August, always on the lower and inner leaves first, always worse in years with a wet July. It looks alarming the first time you see it, but in most years it is a cosmetic problem that the plant shrugs off without lasting damage.

Understanding what drives the disease — and where the line is between cosmetic problem and real plant health concern — saves a lot of unnecessary spraying.

The pathogen

Cercospora leaf spot on hydrangea is caused primarily by Cercospora hydrangeae. Per Clemson HGIC, the fungus overwinters in infected leaf debris on the soil surface. In spring and early summer, spores are released and splash-dispersed onto leaves; infection occurs during warm, humid weather with leaf wetness periods.

A related species, Cercosporella hydrangeae, can cause similar symptoms. Per Missouri Botanical Garden, both produce similar symptoms and respond to the same management.