Rhododendron spp.
Anthracnose – Leafspot and Dieback (fungus – Glomerella cingulata): Symptoms on leaves are characterized by numerous small, round spots on both leaf surfaces. The spots are light to rusty-brown with a definite margin. Under severe conditions, the spots may coalesce and form large irregular spots. Premature defoliation may occur. Branches may suddenly wilt and die. Upon close examination, stem cankers that have girdled the stem may be observed. Pink pustules or spore masses may be seen in these cankers during moist periods. Plant in well drained soil and avoid excessive nitrogen fertilization. When dieback occurs, prune out and destroy diseased twigs. Preventive sprays with a foliar fungicide are effective.
Petal Blight (fungus – Ovulinia azaleae): The first evidence of infection is the appearance of small, brown irregular-shaped spots on the petals. Spots are white on colored flowers and brown on white flowers. Affected petals have a veined appearance and remain firm. Flowers may have a freckled appearance. The flower collapses and sticks to the foliage. Small, black fruiting bodies (sclerotia) form on blighted flowers. Where flower blight has previously occurred, spray soil and litter around plants with a recommended fungicide four weeks before bloom. Spray at regular intervals during the blooming period.
Leaf Spot (fungi – Cercospora sp., Phyllosticta sp.): Spots of various sizes and color on leaves throughout the year. Fungicidal sprays at 7-10 day intervals during periods of high humidity will prevent serious foliage damage.
Leaf Gall (fungus – Exobasidium sp.): Distorted leaf and bud growth developing in April and may. Affected parts later become covered with a whitish mold. Spray plants with protective fungicide just before flower buds open and at 10 day intervals until flowering ceases. Remove and burn affected parts.
Root and Crown Rot (fungus – Phytophthora cryptogea): Affected plants may wilt suddenly and die or they may be unthrifty if only a portion of the root system is infected. A brown discoloration of the wood may be seen in the basal portions of the stems and in the main roots. Plants are much more susceptible when planted in poorly drained areas or planted too deeply. Plant only in well-drained and well-aerated soil. Plant only as deep as the plants originally grew. Drench with a soil fungicide before plants are seriously affected.
Physiological Disorders: Leaf margins and leaf tips turn brown. Plant are unthrifty or fail to grow. Principal cause is improper soil moisture relationship; i.e., soil allowed to become too dry or soil waterlogged to exclude oxygen. Contributing factors may be salt toxicity from over-fertilization, “root-bound” plants unable to develop normal root systems, soil alkalinity due to leaching of lime from masonry of new residences or the use of alkaline water from certain deep well sources. Select healthy bushes with unrestricted root systems showing active growth for planting. Plant in well-drained soil avoiding sites exposed to full sunlight or to the west sun. Maintain high amount of organic matter in soil and adequate mulch to preserve uniform soil moisture in the root zone.
Chlorosis: See section on chlorosis.
