Issue 9, June 19, 2009

Azalea Exobasidium Gall

This disease is not a regular problem in Illinois. It is probably a direct result of the extremely wet conditions we have experienced so far this year. Azalea Exobasidium gall is a leaf and flower gall. The galls have a white, velvety appearance because of the spores, conidiophores, and basidia that form above the leaf surface. Mycelia grow between epidermal cells. Although Exobasidium fungi may cause swollen shoots, stem galls, witches' brooms and sometimes red leaf spots, we are most likely to see leaf galls as seen in this image of azalea leaves infected with Exobasium vaccinii. The galls appear to be balls of tissue, but closer inspection shows that they coat the leaf, which in turn curls up into a ball.

Spores are dispersed from the basidia on the leaf surface. Spread is by wind or splashing water. Spores may infect young leaves when water is present, but most reports of secondary spread are minimal. Wet, shady areas are more conducive to infection. Survival of the fungus over winter occurs in infected buds.

Exobasidium vaccinnii may infect plants in the Ericaceae family, including azalea, rhododendron, leucothoe, and Japanese andromeda.

This disease is usually managed by hand picking infected leaves. Do this as soon as possible to avoid further spread. Place infected tissue directly into a plastic bad before disposing. If fungicides are used, they should be sprayed in the spring just before bud break. They are used as preventive sprays before the disease appears. Check state pest management handbooks for registered products, but two commonly used products are Bordeaux mixture or Bayleton. A second spray may be needed, especially in wet seasons. If Bayleton is used, add a spreader sticker to obtain better coverage.--Nancy Pataky

Nancy Pataky

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