Leaf and flower gall—Exobasidium vaccinii
This disease affects some plants in the family Ericaceae and is also called azalea gall, camellia gall, or leaf gall. Infection by this fungus causes plants to develop discolored, distorted foliage. Less commonly, flowers can be infected and distorted.
Identification
In the late winter or spring, a white coating of microscopic, spore-forming structures develops on plant tissue distorted and swollen (galled) by Exobasidium vaccinii. Under wet conditions, the spores cause galled tissue to develop as described below under Damage.
Life cycle
The fungus overwinters in the infected plant tissue. Fungal infection causes leaf and flower cells to multiply rapidly. When galled tissue becomes coated with microscopic, white, spore-forming structures, these can infect healthy tissue during prolonged foggy or rainy weather or if foliage is wetted by irrigation. Disease development can be minor unless wet conditions prevail for an extended time.
The severity of leaf gall symptoms varies depending on cultural practices and weather. The spores produced are dispersed by air movement and splashing rain or irrigation water. Infection generally occurs during cool, wet weather but the galls do not become visible until the following late winter or spring.
Damage
Azalea, camellia, huckleberry, manzanita, madrone (e.g., strawberry tree), and rhododendron are susceptible to infection by E. vaccinii. Infected hosts develop distorted, thickened, and initially pale green foliage. Infected blossoms or leaves may turn pink. Galling is generally most abundant on the lower portions of the plant. Late in the growing season the infected tissue turns brown, dry, and hard. During late winter or spring, infected tissue can become white when it is covered with microscopic spore-forming fungal structures.
Solutions
A modest amount of galling is harmless to otherwise healthy plants. But if unmanaged, the number of galls may increase during later growing seasons.
Hand pick or clip off and dispose of galled tissue before it develops a covering of white, spore-forming structures. Prune hosts or hand pick galls only when conditions and foliage are dry. Avoid overhead watering. Instead, irrigate using drip emitters or by flooding soil. Avoid crowding plants and otherwise provide good air circulation around them. If leaf and flower galls develop on crowded shrubs, consider removing some of the plants to improve air circulation around them.
Avoid planting cultivars that are highly susceptible to this disease and consider replacing such hosts with less susceptible cultivars. Azalea cultivars reported to be resistant to leaf and flower gall include Amoena, Aphrodite, Coral Bells, Eikan, Faker, Formosa, Glacier, Gloria, Hampton Beauty, Kow-Ko-Ku, Mrs. G. G. Gerbing, Nancy, New White, Pride of Summerville, R. Poukhanese, Sensation, Sunglow, Thinbegen, Treasure, and White Jade. Azalea cultivars reported to be highly susceptible include China Seas, Copperman, Herbert, Hinodeii, Mother's Day, Rosebud, and White Gumpo. The Purple Splendor and Roseum cultivars of rhododendron are also highly susceptible to this disease.
Fungicide application is generally not recommended for this disease in landscapes. However, it may be possible to reduce the extent of new infections by spraying foliage with a preventive fungicide such as Bordeaux mixture or another copper fungicide. Make any application before foggy conditions or rainfall. Note that these products may discolor flowers and foliage.
Adapted from Azalea Gall: Exobasidium vaccinii (PDF), Cornell University; Leaf and Flower Gall of Azalea and Camellia (PDF), Virginia Tech; and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM). |
Azalea leaves discolored and severely distorted and thickened by Exobasidium vaccinii.
A shoot discolored and distorted by Exobasidium vaccinii.
Galled leaves covered with white, spore-forming structures of Exobasidium vaccinii.
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