An azalea which develops yellow leaves with green veins may be suffering from iron deficiency. This type of yellowing, known as chlorosis, will occur in new leaves. If the pH level in your soil is too alkaline, your azalea may be unable to absorb iron from the soil. The Azalea Society of America notes that iron chelate foliar spray temporarily improves symptoms. For long-term treatment, add iron fertilizer to the soil or add iron chelate or powdered sulfur to lower the soil pH level.
If your azalea has yellow leaves and has stopped responding to fertilizer and water, the stunt nematode (Tylenchorhynchus claytoni) may be the culprit. This tiny, soil-dwelling worm feeds on roots and causes visible symptoms in heavy infestations. There are no chemical controls for stunt nematodes; the best controls for this pest are prevention and maintenance. Prevent drought and cold stress in infested plants by adding a layer of mulch to the soil. Remove and destroy dead infested plants and plant new azaleas away from the infested area.
Leaf rust, which is caused by the Pucciniastrum vaccinii fungus, causes round yellow flecks on deciduous azalea leaves. During the summer, spore-producing reddish pustules form under the yellow flecks, particularly if the leaves are wet. Eventually the diseased leaves fall off; spores may form on damp fallen leaves. Prevent the spread of this disease by removing fallen azalea leaves and by purchasing resistant cultivars, such as Balzac and Gibraltar.
Phytophthora Root Rot
The Phytophthora cinnamomi fungus, which flourishes in warm, poorly drained soil, causes Phytophthora root rot. Plants with advanced infections develop chlorosis or small, yellow leaves with green veins. The leaves eventually droop and fall off and the plant produces minimal growth in the spring. According to the North Carolina State University Cooperative Extension Service, chemical treatment is usually not effective for symptomatic plants. Purchase azaleas with healthy green leaves and plant them in well-drained, disease-free soil. Resistant cultivars include Corrine Murrah, Formosa and Fred Cochran.