i have a tall standard PG hydrangea in my front yard which was cut back quite a in the spring due to overgrowth. it is normally very healthy but i have noticed over the last month that there are black spots on the leaves and they fall off. Can you advise on how to rectify this? thanks
Mary Catherine Tierney (member of TBG)
Thank you for contacting the Toronto Master Gardeners.
This year’s wet cool spring has brought on an increase in fungal diseases. There are two types of fungal diseases that can affect hydrangeas, cercospora leaf spot and anthracnose. Cercospora leaf spot (Cercospora hydrangea) begins as brown or purple spots on the leaves at or near the base of the plant. Are the lower leaves of your PG affected? Anthracnose is caused by the fungus Colletotrichum gloeosporioides. It produces large brown spots on leaves that eventually develop lighter brown or tan centers. The spots may also appear on blossoms and may appear angular if bordering a vein. From your photograph it appears as if your hydrangea is affected with Cercospora leaf spot.
One of our earlier posts gives detailed instructions on how to treat this disease.
“Foliage and flowers affected by this fungus develop brown and purple spots. These spots first appear on lower leaves and works it way upwards as the disease progresses.Leaf spot fungus begins its attack in late summer, and gets progressively worse through the fall. If the fungus is allowed to spread unchecked, the affected plant will stop growing, defoliate and produce sickly-looking flowers. The fungal spores on the fallen foliage will transfer to healthy areas of the vine and onto other shrubs if the infected leaves remain on the ground.
The best way to manage the disease is to gather up and remove all affected leaves and blossoms. Applying nitrogen to the soil to encourage the vine to thrive and produce new growth and surface watering will help slow the development and spread of the disease. A fungicide purchased from your local garden center can be applied as soon as you notice the spots.
The following is a website with additional information on hydrangea leaf spot disease
Good Luck with your hydrangea.