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Crown Gall (bacterium - Agrobacterium tumefaciens): Crown gall is caused by soil-inhibiting bacteria. The bacterium often enters wounds caused by freeze injury or hail. It stimulates the cells in the plant tissues to grow rapidly, resulting in the formation of tumor-like, spongy overgrowths which develop around the crown of the plant. Rain can splash bacteria onto the aerial parts of the vine where the tumors are often called black knots. The pathogen can move in the vessels and spread throughout the vine. Sanitation is the best prevention. Avoid cutting into affected tissue when pruning. Chemical and biological controls are now available.