Rhododendrons are popular landscape plants known for their attractive evergreen foliage and colorful blooms. Although they're basically hardy plants, rhododendrons are prone to contracting several diseases such as flower blight and dieback. The main cause of diseased rhododendrons is planting the wrong plant in the wrong spot, according to The Maryland Master Gardeners. Rhododendrons also battle pests such as black vine weevils. Plant these shrubs in the right locations to reduce the possibility of pest problems.
Flower blight is a disease where water-soaked blotches emerge on flowers and then become brown and soft, according to the Fraser South Rhododendron Society. This disease is more likely to occur in warm regions with significant amounts of moisture. The Fraser South Rhododendron Society recommends removing all spent flowers, since good sanitary practices are essential to control the disease.
Dieback, another common rhododendron disease is caused by different fungi, according to the Adams County Master Gardener Cherie Moyer. Parts of a stem begin to die starting at a stem's tip and then progress backward. Signs of dieback are leaves turning reddish brown or leaves with water soaked spots. The disease is easily spread by tools, rain and splashing water in which fungi invade a plant in a weakened area.
Black Vine Weevils
The black vine weevils are black beetles with yellow spots on their backs. Adult beetles feed on rhododendron leaves at night. A large infestation causes little or even no plant growth with yellowed foliage that can dry out. Plant leaves should be examined in early June for any signs of notching with interior leaves typically suffering from the most damage. Although it's hard to control these nocturnal pests, the problem can be reduced by planting rhododendrons in the shade.
To control black vine weevils, apply insecticides thoroughly to plants, and reapply about three weeks following the first application since this is when more adult beetles will emerge, according to the Penn State Cooperative Extension.
Lace bugs, which have a pale yellow body with two dark spots on their lacy wings, bother rhododendrons. Nymphs, which are smaller than adults, are black. According to Master Gardener Moyer, eggs that overwinter in leaf tissues, hatch in May and mature into adults by June. They lay eggs in late June and July with a second generation appearing in August. Both adults and nymphs suck plant juice from the undersides of leaves, resulting in a white silvery white discoloration called strippling. Beginning in May, examine leaves for signs of strippling damage. Applyhorticultural oil on leaf undersides and using systemic insecticides, which are a type of stomach poisoning, to help lower the damage.
Rhododendron Stem Bower
The adult rhododendron stem bower is a dark-colored beetle with long yellow antennae and two black spots behind its head. They feed on leaf undersides along midveins which cause leaves to curl. Larvae bore into leaves resulting in wilting and the final dieback of canes. The Connecticut Agricultural Experiment Station recommends that you examine lower branches for curled leaves, and cut open any wilted branches to detect any larvae present. Prune and destroy all wilting branches in late summer or early spring to help control the problem.