Rhododendrons in Trouble?
A Bay Weekly reader recently complained that her magnificent large rhododendron was dying after it had produced a super abundance of blooms this spring. After examining the plants closely, I knew that the cause of death was Phytophtora cactorum. This disease is often the primary cause for rhododendron dieback. It kills the plants starting at the ends of the branches, and works its way down the stem. If you can prune out the dying branches as soon as you spot it, you can often salvage the plant.
To identify the disease, look for chestnut-brown sunken cankers surrounding the stem immediately beneath the wilting flower. The stem just beneath the wilted flower or seed head will be green, but the lower part of that stem, where it had grown from the previous year’s stem, will be chestnut brown. You will note that the diseased stem originated near the stem that flowered the previous year.
The disease-causing fungi entered the stem as the old flowers wilted and dropped. Sometimes one or two branches are first affected. When this occurs, prune away the branch as close to the main stem as possible, sterilizing your pruners with rubbing alcohol between each cut. To prevent the disease from spreading, spray the plants with a mild fungicide such as Phaltan or Captan as the blossoms begin to wilt. However, fungicides are only a temporary protection.
The occurrence of this disease has been associated with low levels of magnesium in the soil. If you have rhododendrons that exhibit any signs of dieback, I strongly recommend you have the soil tested.
When taking soil samples from around shallow-rooted plants such as azaleas and rhododendrons, only sample from the upper three inches of soil. Ninety percent of roots from shallow-rooted species grow in that layer. Soil samples should be taken between the drip line of the branches and the trunk of the plant. Scrape away mulch before sampling.
Where this disease has been a problem, I recommend applying Epsom salts at the rate of one-half cup dissolved in two gallons of water and applied over 100 square feet. Apply every spring just before the plants start blooming.
To my knowledge, there are no rhododendron varieties that are immune to this disease. The best protection is frequent inspection of the new stems, pruning out diseased branches using sterile pruners, spraying infected plants as the blossoms are wilting and falling and proper nutrition following soil testing by a reputable lab such as A&L Eastern Agricultural Lab.
Ask Dr. Gouin your questions at [email protected]. Please include your name and address.