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 and osteospermum and this year it has been found in some of Selecta's lobelia production. Selecta found symptomatic plants in four varieties of Magadi Lobelia: Blue, Compact Dark Blue, White and Basket White. Selecta sent information out to their customers and is discontinuing shipments of Lobelia for this spring.
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. The first signs of gall formation may be seen about 8-12 days after infection.
The bacterium can be moved by water splash, but a wound or natural opening is required for the bacterium to penetrate the plant tissue. Propagation tools move the bacterium between plants and also provide wounds for bacterium to enter. Infection can also occur through the roots through infested substrate or by irrigation from a 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 also be infected especially if the plants have been pinched or sheared. It is recommended to bag and discard the entire tray, including the substrate and cuttings.
There is no cure for infected plants and chemical prevention is ineffective.
Tina Smith, UMass Extension
Photos: Crown Gall on many different plants, Oregon State University
Crown Gall Agrobacterium tumefaciens, Colleen Warfield, Ball Horticultural Company
Crown Gall: Still Confounding Scientists and Growers Alike, Melodie Putnam, Oregon State University Plant Clinic, Grower Talks Magazine. Sept. 2015.