Azaleas are perennial members of the Rhododendron genus, commonly grown for their showy, spring blooming flowers. Like any plant in the garden, azaleas are susceptible to a range of specific diseases which may mar the beauty of the plant. Leaf rust is a common azalea fungal disease that, though rarely serious, causes unattractive foliage spots.
Certain hybrids and azalea cultivars are more susceptible to rust than others. Deciduous azaleas such as the pinxterbloom azalea (Rhododendron periclymenoides), plumleaf azalea (Rhododendron prunifolum), and swamp azalea (Rhododendron viscosum) are especially susceptible to leaf rust caused by fungi such as Pucciniastrum vaccinii and Pucciniastrum myrtilli. Azalea varieties such as the Knap Hill and Exbury Hybrids are also susceptible to leaf rust.
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Leaf rust causes small, circular yellow marks on the upper side of the azaleas leaves. The undersides of leaves produce orange or rust colored pustules flecked with yellow. The spores from the pustules spread easily to other leaves on the shrub, especially in damp or wet conditions. Severe cases may lead to premature leaf drop or stunted growth. The first symptoms of the disease typically appear in mid-summer, while orange spores may not appear until late summer or fall.
Leaf rust rarely requires treatment with fungicides, although in severe cases gardeners may wish to use registered fungicides such as those that include sulfur and Bordeaux. Follow fungicide directions carefully. Remove heavily diseased leaves from the plant, and clean up fallen debris around the plant that may harbor the fungus. In warm climates, the fungus may grow in fallen leaves even through the winter, infecting new foliage again in the spring.
Leaf rust is best prevented by growing rust resistant cultivars. Cultivars such as 'Balzac,' 'Red Letter' and 'Gibraltar' have proven to be especially resistant against the disease, while cultivars such as 'Klondyke,' 'Ilam Primrose' and 'Rufus' have a high risk of developing rust. When growing multiple azalea shrubs, space plants properly to avoid crowding and to promote air circulation. Poor air circulation harbor the disease and contribute to the spread of rust from a healthy plant to an infected plant.