What Are Plant Galls
Causes and Characteristics of Galls
A wide variety of plant pathogens produce chemicals that act like growth regulators on plants and lead to galling or other types of misshapen growth. Galls are plant tumors, usually roundish swellings much larger than normal growth. Galls can form on roots, stems, leaves, flowers, seeds or, basically, any plant part.
Exobasidium leaf gall on horse sugar produces lumpy galls as large as a small potato.
Anthony Keinath ©2016 Clemson Extension
Understanding Exobasidium Fungi
How Exobasidium Forms Leaf Galls
Exobasidium is the scientific name of a gall-forming, worldwide group of simple fungi in the broader category of mushrooms. Exobasidium, however, doesn’t make mushrooms. It grows and produces a layer of spores directly on the outside of young leaves, flowers, or fruits of woody plants. This layer of spores gives the galls a white, dusty look.
When and Where Galls Appear
Exobasidium leaf galls form on plants that leaf out and bloom when the weather is cool because Exobasidium prefers low to moderate temperatures. This seasonality means galls appear in spring in South Carolina.
Leaf Galls on Horse Sugar (Symplocos tinctoria)
Symptoms and Identification
Exobasidium simploci forms large, obvious galls on a unique native plant, horse sugar (Symplocos tinctoria). The galls in the photo with this article were found by a homeowner in the Charleston area in May 2016, about the time the plant was putting out new leaves. The leaf buds become infected and turn into oval to round, lumpy, pale green galls that are covered with a thick, white layer of spores.
History and Uses of Horse Sugar
Horse sugar is also called sweetleaf; both names refer to how much livestock and deer love the taste of this small tree. Another old name is yellowwood, as this plant is a natural source of yellow dye. The Latin species name “tinctoria” means “used as a dye.”
Other Plants Affected by Exobasidium
Horse sugar is found throughout South Carolina, so presumably, the galls also appear all around the state. Other species of Exobasidium form leaf galls on camellia, azalea, rhododendron, and blueberry. All five plants are distantly related, which may explain why they are susceptible to Exobasidium leaf galls.
Further Reading and Resources on Leaf Galls
For more information on Exobasidium leaf gall on ornamental plants, see HGIC 2050, Azalea and Rhododendron Diseases.
