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.

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.

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.

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.

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.

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.