Cankers are the evidence of certain kinds fungal and bacterial diseases. Cankers are most common on woody plant material. They appear as sickly, abnormal areas on a cane or branch's surface, sometimes seeming as though the branch is rotting from within and other times seeming as a swollen abscess that may crack open. Bacterial cankers attack some berries and tomatoes. Fungal cankers attack a wide range of plants, including landscape and fruiting trees, roses, and shrubs.