Pasquesi: Garden Info - Trees and Shrubs (Rhododendron and Azalea Plant Problems)

RHODODENDRON AND AZALEA PLANT PROBLEMS

Rhododendrons and azalea problems are often associated with Chicagoland’s poor soil quality and extreme temperature conditions. Pests are a common nuisance, too. Diseases, although less common in rhododendrons and azaleas, do occasionally occur.

Consistent effort and some basic rules will ensure problems are identified early.

Iron chlorosis

Iron chlorosis is a common threat to rhododendron and azalea plants grown in Chicagoland. Poorly nourished, low acidic soil causes the leaf tissue to yellow, while the veins remain green. Rhododendrons and azaleas thrive in soil that is well nourished. The correct pH range falls between 4.5 and 6.0. Check soil pH levels semi-annually. It is important to restore the soil pH and nutrient content to their appropriate levels if plants are to thrive.

Chicagoland soils are generally high in alkaline. To neutralize alkaline levels in soil, dig aluminum sulphate or cottonseed meal into plant beds. To create loamier soil and reduce concentrations of clay, add Canadian peat moss to the soil. Fertilize plants just after blooming each spring.

To provide a well-drained area, mix soil with sand and pine bark. Poor drainage is a major factor in the death of rhododendron and azalea plants. Ensure that the plant roots are growing out and away from the root ball. If they are not, gently disentangle them before returning plants to the ground.

Sun and frost damage

It is important to select a well-sheltered site when planting rhododendrons and azaleas. Often mistaken for diseases, environmental damage invites secondary infections to occur which will kill the plants.

Sun scold occurs in summer and winter. In summer, sun scold appears as brown spots on leaf tips and edges. In winter, sun scold appears as long, brown patches along the center vein of leaves. A deficiency of water is the most severe cause of this problem. Sun scold may be treated and prevented by providing adequate amounts of water and shade. Mulching the plant beds will help keep plants moist, also.

Winter damage is a more serious threat to rhododendron and azalea plants. If allowed to occur over several consecutive seasons, frost damage will severely affect a plant’s health. When planting rhododendrons or azaleas, avoid low or exposed sites. Situate beds near walls or hedges, and under trees for protection.

Winter damage appears on plant foliage, flower buds, stems and branches. Leaves will turn brown at the edges, flower buds will fail to bloom and the bark on stems and branches will split causing visible, open wounds.

Winter damage becomes evident after the damage has been done, usually during the growth season, when damaged branches wilt and die and stunted buds reveal blackened flowers.

Insects and diseases

Although much less common than soil or weather-induced problems, pests and diseases do sometimes stress rhododendron and azalea plants.

Symptom	Name	Explanation	Treatment
New plant growth is curled. There is a sticky honeydew substance on leaves.	Aphids	Tiny, green, soft-bodied insects that suck nutrients from the plant.	Spray plants with diazinon or malathion.
Leaves and buds wilt and die. Stems and branches shrivel from visible wounds.	Dieback	A fungal disease that infests the plant.	Prune damaged growth. Spray plant with a copper fungicide the year following blooming.
Branches wilt and die. Wood is discolored. (In severe cases, plants wilt and die).	Crown Rot	A fungus that attacks the root system in conditions of poor drainage.	Prune damaged branches well below infected area. Improve drainage. In severe cases, remove infected plants and surrounding soil.
Leaves are variegated yellow and green. Hard, shiny black droplets appear on the backs of leaves.	Lacebugs	Small, brown insects (mature insects have lacy wings) that feed on plants.	Spray with systemic insecticide Isotox; repeat seven to 10 days later.
Dry, flaky bark. Crisp circular spots appear on leaves.	Rhododendron borer	Yellow-white caterpillars that furrow into the bark.	Remove borer if possible. Cut dead and damaged wood.
Leaf edges have become scalloped.	Root weevil	Small gray beetles with long snouts that feed on leaves.	Spray foliage and soil with diazinon. Repeat twice in three-week intervals.

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