Gall Formation on Savannah Holly: Causes and Management

Diagnosing Woody Galls on Holly

Large knots, holes, and lumpy branches on Savannah holly (Ilex × attenuata 'Savannah') indicate gall formation—abnormal growths caused by pathogens or pests. The two primary culprits are fungal pathogens and the bacterium Agrobacterium tumefaciens (crown gall).

Fungal Galls

Several fungi (Phomopsis, Botryosphaeria, Nectria species) induce galls on stressed hollies. These typically appear as rough, woody swellings on branches and trunks. Entry occurs through wounds—pruning cuts, freeze cracks, mechanical damage. The fungus colonizes cambial tissue, disrupting vascular function and causing dieback beyond the gall.

Bacterial Crown Gall

Agrobacterium tumefaciens transfers DNA into plant cells, forcing them to produce opines (nutrients for the bacterium) and proliferate uncontrollably. Galls are initially soft and light-colored, becoming hard, dark, and fissured with age. The bacterium persists in soil for years, entering through fresh wounds.

Differentiation and Diagnosis

Definitive identification requires laboratory analysis. Submit samples (including healthy/unhealthy transition zones) to your state's plant diagnostic clinic through the Cooperative Extension Service. Microscopic examination and culturing distinguish fungal from bacterial causes.

Management Strategies

• Sanitation: Prune out galled branches 6–8 inches below visible symptoms. Sterilize tools between cuts (10% bleach or 70% alcohol).
• Prevention: Avoid wounding; mulch to prevent mower/trimmer damage; maintain plant vigor with proper irrigation and fertilization.
• Soil management: For crown gall, avoid replanting susceptible species (roses, fruit trees, euonymus, willows) in infected sites for 3+ years. Soil solarization may reduce bacterial populations.
• Biological control: Agrobacterium radiobacter strain K84 (commercial products: Galltrol, Norbac) prevents crown gall on new plantings but doesn't cure existing infections.

When to Consult an Arborist

If galls affect the main trunk, structural stability may be compromised. A certified arborist can assess risk and recommend removal if failure potential threatens property or people.