Nematodes, mites, and insects cause galls to form in about 95% of cases. The other 5% of galls are formed by fungi, bacteria, and viruses. In either case, the parasite produces a growth-regulating chemical which stimulates the plant to form the gall.
- Abnormal warts, swellings, or knots on the plant: This swelling is known as a gall and is where the parasite is located.
- Reddish swellings: This is a sign of a newly-formed gall. The red colour is a result of exposure to sunlight.
- Dull brown, dry, hard knots: This is an old gall location.
- Preventive tools: Not applicable
- Physical tools:
- Prune and remove galls from affected areas.
- Biological tools: Not applicable
- Least toxic chemical tools:
- Apply dormant oil on the affected area if spotted early in the spring. Use dormant oil according to label directions