The disease, also known as black knot, is caused by Agrobacterium tumefaciens. The major symptom is the growth of fleshy galls (tumours) on the lower trunk which can girdle the trunk and portions of the vine above may die. At one time it was believed that the bacterium lived in the soil like its relative that causes galls on other plants. However, crown gall of vines lives inside the vine itself and so is spread at planting. Control of crown gall is difficult. Research in the early 1990s suggested that hot water treatment of dormant cuttings (50 °C/122 °F for 30 minutes) reduces the bacterium, and tissue culture offers total elimination by producing nursery stock free of crown gall. Avoiding the disease in new plants can also help to reduce winter freeze injury to trunks. In the north eastern united states, the growers train the vines with up to five trunks so that there is always a young healthy trunk to replace dead or dying ones. See training systems.