Control of rhododendron problems starts with proper planting and adequate soil quality. Place rhododendrons in well-drained soil with a pH value of 6.0 or lower. In the wild, rhododendrons are found in shaded areas of the forest floor, where decaying plant material provides the root system with plenty of organic nourishment. If you are unsure whether your soil is acidic enough, purchase a pH meter from a garden center. If you prefer, consult the local county extension office for soil testing instructions. Purchase healthy shrubs before planting.
Two fungi known as Phomopsis and Botryosphaeria cause dieback of rhododendron leaves and branches. It is spread by splashing rainwater during warm temperatures. Leaves develop olive-colored blotches, roll up and appear droopy. Smaller branches develop cankers before eventually dying back to a main branch. There is no control for infected branches. Prune back wilting twigs to a point where no brown discoloration is seen inside the stem. Keep pruning shears disinfected with a 10 percent bleach solution while working. Spray fungicides in early spring as new leaves emerge for added prevention.
A common fungus that attacks rhododendron, Phytophthora causes leaves to yellow and dry out a the edges. This is usually because the soil around the fine rhododendron roots is heavy and excessively wet. Dig around the base of the plants and inspect the roots. Those affected by root rot appear black and mushy. Dig out smaller plants, add organic material such as peat moss, and replant. Make sure the top of the root ball is at least 6 inches above the soil. For larger plants, dig a few feet around the root system and amend the soil with organics.
Rhododendron shrubs growing in alkaline soil (with a pH value higher than 6.0) may show yellowing between the leaf veins. Solve this by adding fertilizers containing iron or ammonium sulfate. Sphagnum peat moss also lowers soil pH. Trench a few feet around the base of the shrub and mix into the soil.
In mid to late spring, clearwing moths prefer to lay their eggs in cracks in rhododendron bark and the crotches of limbs. When the eggs hatch, the larvae bore into the branches, creating small holes and visible sawdust. Shrub leaves turn yellow and wilt as a result of the feeding larvae. An insecticide called Dursban may control the adults if sprayed per label instructions in late spring. Repeat the application once a week.
Black Vine Weevils
The most noticeable sign of a black vine weevil infestation is a series of notches around leaf edges. The adult weevils emerge in the evening hours from their nesting places around the rhododendron trunk at the soil line. They climb the plant and chew the leaves. Weevil larvae remain in the soil and feed on the roots and lower bark of the shrub. They eventually create a ring of missing bark and plant tissue that prevents the shrub from sending moisture from the roots to the leaves. The plant looks like it's drying out. Spray insecticides per label instructions in early spring before eggs are laid. Drench the soil with Merit or another applicable insecticide to control the larvae.