The bacterium Agrobacterium tumefaciens is a soil borne pathogen that causes crown gall on many types of plants. In greenhouse crops, Crown Gall has been diagnosed in recent years on mums, argyranthemum, osteospermum and lobelia.
Symptoms of Crown Gall are white masses of callus tissue or small swellings appearing on roots, at the base of the stem and occasionally on leaves or anywhere wounds occur. Gall formation may be seen about 8-12 days after infection.
The bacterium can be moved by water splash, but wound or natural opening is required for the bacterium to penetrate the plant tissue. Propagation tools may move the bacterium between plants and also provides wounds for bacterium to enter. Infection can also occur through the roots through infested substrate or irrigated from contaminated water source.
Crown gall management begins by carefully monitoring plants for symptoms and practicing good sanitation techniques if the disease is found. Discard infected plants as soon as disease has been diagnosed. Do not compost since the bacterium can survive in the soil. Use disinfestants to clean surfaces and any tools used on the crops. Never use cuttings that have galls and do not propagate from plants with galls. If one cutting in a tray has symptoms, others in the tray could be infected especially if the plants have been pinched or sheared. It is recommended to bag and discard the entire tray, substrate and cutting.
There is no cure for infected plants and chemical prevention is ineffective.
Resources
Crown Gall Agrobacterium tumefaciens, Colleen Warfield, Ball Horticultural Company
Crown Gall: Still Confounding Scientists and Growers Alike, Grower Talks, Sept. 2015
Photos: Crown gall on many crops, Oregon State University