Rhododendron Pests and Diseases
Here is a summary of pests, disesases and problems you may find while growing rhododendrons and recommended solutions from various sources.
Rust appears as orange-red spots on the lower surface of the leaf. If rust appears good circulation may help. Avoid rhododendron varieties susceptible to rust. Also do not plant rhododendrons near Hemlock trees.
Mildew may have a different appearance on different plants. Some plants may have powdery material may appear on the leaves - most frequently on deciduous azaleas in late summer. Symptoms on rhododendrons may have yellowish leaves or may not be noticable until turning over the leaves to find brownish spots underneath. Since most leaves may fall off no action may be needed to control this problem. Good air circulation helps prevent mildew, as well as Benemyl or similar spray.
Flower buds turn brown and are covered with black bristels. Remove and destroy the affected buds. Freeze damage or other unknown factors will cause the buds to brown and fall off.
These are caused by a number of fungi which are most active soil where water does not properly drain and may puddle around the base of the plant. Plant in high, well drained soil and maintain drainage after planting. Rot fungus appears to be more active in warm sumer conditions. Good mulching to keep the roots as cool as possible may help limit the problem, as well as keeping dry leaves and debris out from under the plant. Typically by the time symptoms appear it is too late to save the plant. Symptoms include wilting of one limb or the whole plant as well as yellowing or browning of some of the leaves as the plant can no longer supply nutrients to the top. Spraying with metalaxyl or other similar chemicals may be helpful, but most of all ensure the plant has appropriate drainage and is cool as possible during hot weather, with good air circulation.
Brown spots appear on rhododendron and azalea flowers which then become soft and brownish. This occurs more as the temperature warms and while there is a great deal of moisture - especially in the Southeastern United States. Remove dying flowers and debris from around the plant. Do not use overhead sprinklers. Spraying with benomyl or triadimefon may help.
These small sucking insects feed on new growth as it begins to expand causing leaves to twist and curl. Spray plants with a heavy stream of water to wash aphids off plants. Natural predators such as ladybugs can help control aphids. If opting for pesticide, use Orthene, Malathion or Diazinon to eradicate these pests.
Caterpillars and Other Larvae
The larvae of several species of insects and bugs will produce similar results of chewing on new foliage. To control these pests they can be removed by hand, or use pesticide such as Orthene, Malathion or Diazinon.
There are a number of different types of white flies and they can be difficult to control. This small white fly sucks on the underside of the leaf, leaving white spots where it has been. When white flies are able to live in an area with no natural predators, a few flies can become thousands in a short period of time. To control white flies encouraging natural predators to hang around will help (green lacewings, lady bugs, minute pirate bugs, big eyed bugs, and damsel bugs) or if you choose the insecticidal route use Malathion, Diazinon or Orthene. Spraying leaves of plants with water and hand removal of egs on the plant may also help.
A small brown insect with transparent wings sucks on the bottom of the leaf causing a mottled appearance on the top with yellowish, brownish or white spots. The underside will show a brownish residue. Lacebugs perfer sunny areas so plant in more shaded areas if possible. Spray the young lacebug nymphs off the leaves as they hatch. You may choose to avoid insecticides and encourage natural predators or use a pesticide such as Malathion or Orthene.
Slugs and Snails
These pests feed on soft new growth and flowers. Slugs and snails leave evidence of their feeding behind on plants and may also leave trails of slime. Various methods are used to eradicate slugs and snails from pouring salt on them to drowning them or cutting them in half. In the past we've had ducks which like to eat slugs and that method worked pretty well, but not all gardeners have room for ducks! Some rhododendron gardeners recommend using Metaldehyde. Note that some snail and slug poison or bait, although will kill the slugs, may attract slugs to the places you don't want them.
Bark split occurs when sap in the plant freezes and splits the bark. This occurs most often in late spring or autumn frosts. Avoid feeding nitrogen to a rhododendron or azalea that could stimulate growth late in the summer. Use grafting wax to prevent fungal and insect damage. If left untreated, it is fatal.