My beautiful oakleaf hydrangeas, those stately stalwarts that form the backdrop for many of my flower beds, look like they have a bad case of chicken pox, only the spots aren’t red.

Instead, the spots covering those magnificent leaves are brown or purple, somewhat angular in shape, and surrounded by lighter brown halos. These spots are the first symptom of a foliar disease known as Cercospora Leaf Spot that can strike all forms of hydrangea, the spores spreading with rain and wind.

And oh, how wet and windy was our spring, which prompted the earlier-than-usual onset of this common culprit.

But understanding how this fungus spreads and knowing its arrival was bolstered by our unusually wet spring doesn’t make it any less distressing to discover. And as I was soon to realize, it wasn’t just the large canopy leaves that were affected. So too were the new leaves underneath. 

With clippers in hand and a rake in tow, I began my clean up, snipping off the spotted leaves and raking up those that had already fallen.

It is actually the fallen infected leaves that are the main source of the fungus spores, so getting rid of them can help control the disease.

While there is no “cure” for this menace, there are other remedies besides sanitation that can be incorporated to reduce this disease, such as selecting resistant varieties for new plantings and keeping the foliage dry. While it’s difficult to outmaneuver Mother Nature (even umbrellas wouldn’t help as the rain splashes the spores up onto the plants, reinfecting new leaves), pruning can improve air circulation, and curtailing overhead irrigation can reduce some of the moisture.

Although not generally recommended, fungicides can be effective when used at the first signs of leaf spots, but obviously I missed that window of opportunity! Hopefully I’ll remember that early enough next year.

For suggested fungicides, consult the current OSU Extension circular E-832, “OSU Extension Agents’ Handbook of Insect, Plant Disease, and Weed Control.” During very rainy springs when leaf diseases become severe, two to three fungicide applications are needed for good control. Remember with all pesticides, always read and follow the label directions

And while this disease can be widespread on hydrangeas in the landscape, it is generally an aesthetic problem for homeowners because the disease rarely kills the plant. 


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