Learning to deal with plant disease is easier than you think. So why is the lush Rhododendron you bought at a greenhouse now sickly with veined leaves? You can thank Japanese beetles. Or perhaps a fungal disease floated in the air and landed on a leaf to germinate, spreading wilting, dark spots. To identify any issue, start with examining your plant's leaves and flowers.
Phytophthora Root and Stem Rot Disease
Plants that sit in wet soil are affected. It looks like yellowing foliage, according to Rodale’s Illustrated Encyclopedia of Organic Gardening. The stems die, leaves drop off and flowers wilt. Trees will die within 3 to 10 years. To prevent this from happening, replace the soil. There is no cure for phytophthora, but it’s suggested to plant rhododendrons in well-drained areas.
Rhododendron Bud Blast Disease
Rhododendron leafhoppers spread a bud-blast disease; buds die and are covered with spiny, black fungal spores. It happens during the late summer, when flower buds are developing. According to the Reader’s Digest Guide, you should spray the plant with insecticidal soap to prevent this problem. If it’s too late, get rid of the infected buds and apply garlic spray to deter the leafhoppers.
Ray Blight or Petal Blight Disease
Ray blight or petal blight are dark water spots on the flower petals that cause rot. This happens is during wet, cold summers. To avoid this issue, spray plants with copper before flowers open.
Azalea Gall Disease
Azalea gall affects small-leaf rhododendrons by plumping up the leaf, turning it pale green or pink and then brown. This occurs in poorly drained soil. Cut off the dying leaves to save the plant.
Pluck off dead flower heads to increase how many buds your plant will produce. According to the Reader’s Digest All-New Illustrated Guide to Gardening, give your plant only half the recommended fertilizer bags normally suggest.
You may notice holes in your flowers and a skeleton of the leaves. Japanese beetles are the cause. They are thick, metallic green bugs that can consume an entire rhododendron. The beetles lay their eggs midsummer, and later dirty, brown-headed grubs emerge. By spring of the next year, they feed on your plant. According to Rodale's, you must shake the beetles off your plant every morning to prevent this destruction. You may kill them with soapy water. Rodale's best advice is to "plant pollen and nectar-rich flowers to attract native species of parasitic wasps and flies."
- Rodale’s Illustrated Encyclopedia of Organic Gardening, The Complete Guide to Natural and chemical free gardening; Pauline Pears, Editor; 2002
- Reader’s Digest The All-New Illustrated Guide to Gardening; Fern Marshall Bradley and Trevor Cole Editors; 2009