This is a common problem in acid-loving plants such as azalea and rhododendron. These plants prefer soil with a pH between 5.0 and 6.0. The soil is seldom truly deficient in iron, but iron is often found in an insoluble form that is not available to the plant, especially in soil with a pH above 7.0. A high soil pH can result from overliming or if lime has leached from cement or brick. Regions where soil is derived from limestone or where rainfall is low also have high-pH soils. Plants use iron in the formation of chlorophyll in the leaves. When iron is lacking, new leaves are yellow.
Fertilize with a plant food formulated for acid-loving plants. If your soil drainage is poor, add enough peat moss at a 50:50 ratio to native soil when planting. Never lime the soil around this plant.
Stephanitis species Populations of lace bugs are highest when rhododendrons and azaleas are grown in sunny rather than shady locations. The wingless, immature insects and the lacy-winged adults suck sap from the undersides of leaves. As they feed, droplets of black excrement accumulate around them. Damage is unsightly, and food production by the leaf is reduced, making the plant less vigorous.
Apply an insecticide labeled for this pest, following label directions.
Macrosiphum rhododendri Rhododendron aphids do little damage in small numbers, and plants can tolerate fairly large numbers without much effect. Aphids are extremely prolific, however, and populations can rapidly build up to damaging numbers during the growing season. Damage occurs when the aphid sucks the juices from the rhododendron leaves and buds. The aphid is unable to digest fully all the sugar in the plant sap and excretes the excess in a fluid called honeydew. The honeydew often drops onto the leaves below. A sooty mold fungus may develop on the honeydew, causing the rhododendron leaves to appear black and dirty. Ants feed on the sticky substance and are often present where there is an aphid infestation.
When you first notice aphids, apply an insecticide labeled for this pest, following label directions.
Root weevils are common pests of many garden plants. The adult weevils feed at night, notching leaf margins. This feeding detracts from the appearance of the plant but rarely causes serious injury. Severe damage and death of the plant may result from larvae feeding on roots if the weevils are left uncontrolled. The grubs feed unseen on the roots in the spring and from mid- or late summer into fall.
To control adults, apply an insecticide labeled for this pest, following label directions.
This plant disease is caused by several different soil-inhabiting fungi, also known as water molds. These fungi (Phytophthora and Pythium species) attack a wide variety of ornamental plants, where they destroy roots and work their way up the stem. If they girdle the stem, the plant wilts and dies. Very wet conditions favor the fungi, which are most common in heavy, poorly drained soils. Azaleas and rhododendrons need constant moisture, but they require good drainage.
Control of root rot is difficult. Improve soil drainage before replanting azaleas or rhododendrons in the same location. If drainage cannot be improved, plant in beds raised a foot or more above grade. Or, plant shrubs that are resistant to wilt and root rot.
Windburn and winter injury on rhododendron and azalea leaves are common on plants growing in windswept locations. In cold climates, where temperatures commonly fall below freezing, strong winds cause leaves to lose their moisture more rapidly than it can be replaced by the root system. The leaf edges dry out and die. This is most pronounced when water is unavailable because the soil is frozen. Leaf burning also occurs on exceptionally windy, dry days in summer.
Once leaf edges have been damaged, the injury remains. Where practical, pick off damaged leaves. To prevent winter injury and windburn, plant shrubs in locations protected from wind, or provide windbreaks. Fertilize plants rwith plant food for acid-loving plants. Water in late fall or winter to ensure adequate soil moisture. Mulch plants after they are dormant to reduce the depth of frost penetration into the soil.