The Q&A Archives: Azaleas

Question: My azaleas have tiny black specks on the backs of the leaves and the front of the leaf which is suppose to be green looks like it has a white coating on it. What do I need to use to get rid of this? Also my Rhododendrons have brown around the edges of the leaves - not the newest ones but as they get older they develope the same sympton and finally turn dark yellow then brown and fall of. Can you help?

Answer: Your azaleas have a fungal disease called azalea leaf gall. Exobasidium vaccinii causes leaves and flowers to become swollen, curled, waxy and fleshy. The swollen plant tissues or "galls" are made up of abnormal plant tissue. Infected leaf tissue is usually pale green in color during the early stages of the disease; infected flowers are usually pinkish. Later in the season, a white spore layer covers the infected plant parts. Galls eventually turn brown and harden as the season progresses. Lower leaves on plants are usually the most seriously damaged, but under humid conditions and in shaded locations galls may occur at the ends of upper branches.

The occurrence and intensity of the disease depends on weather conditions and on the source of the causal fungus. Spores produced in the whitish mold on the surface of galls in late spring to early summer are blown and washed to leaf and flower buds where they cause new infections. Galls form the following spring. Cool, wet weather favors dispersal of the spores.

When only a few plants are involved, as in a home planting or a small greenhouse area, the disease is easily controlled by hand picking the galls and burning or burying them. To prevent new infections, it is important to pick the galls before the white spore layer appears.

Since your plants are already showing the white coating, the spores are probably already spreading to the rest of your plant. I would hand pick the galls now and then use a preventative fungicide next spring to keep the problem from returning. Two applications of a fungicide containing mancozeb (e.g. Dithane), one made just before leaves unfurl in spring and one 10 days later, will help prevent new infections.

Browning on the margins and tips of the leaves of rhododendrons occurs when loss of water from the leaf surface is greater than the ability of the roots to absorb water. This happens when roots are injured by recent transplanting, cultivation, insects, fungi, or other mechanical agents.

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