There is a very old rhododendron in my backyard. It is located near a pagoda dogwood, an eastern redbud and several evergreen shrubs. The soil is sandy and acidic. The area is well sheltered from strong winds.
I have noticed some red spots on some of the leaves. Would you know what is afflicting it?
Thank you for contacting Toronto Master Gardeners regarding your Rhododendron.
Congratulations on your Rhododendron reaching its “very old age” – a testament to your care. Rhododendron originates mostly from western China, the Himalayas and Myanmar (Burma). It is a broadleaf evergreen that, besides looking beautiful, is attractive to a variety of pollinators. You did mention its sandy acidic soil but, to keep your plant in good health and if it has been sometime since the soil was tested, it may be warranted to retest the soil to ensure that the soil pH is in the range of 5.0 to 5.5.
From your photo, it is difficult to give a definitive answer. A dark, circular spot can be seen but also some leaf yellowing. Rhododendrons are susceptible to many insect and disease problems, leaf spot being a common problem. Early treatment is advised as it has the potential of debilitating your plant. Several fungal diseases cause leaf spot. It is suggested that you take a leaf to a knowledgeable staff person at your local garden centre who would be able to suggest a treatment for your plant. Although the degree of yellowing is difficult to see, it may be due to iron deficiency in the soil, caused by too high soil pH requiring soil amendments to lower the pH. In addition to your regular care, you may want to review general care strategies at for rhododendrons below.
For more information, please see: https://www.rhododendron.org/plantcare.htm
All the best with your Rhododendron.
April 24, 2021