Crown gall is a plant cancer generally caused by a species of bacteria called Agrobacterium tumefaciens though other related species of bacteria may cause the disease.
How It Spreads
The bacteria enters wounds on plant roots or stems at soil level. It lives in the soil so often digging injures plants and transfers the bacteria to the resultant wound. Infected secateurs can also spread the disease and grafted tomatoes may be infected via this method. This means you may purchase a healthy looking plant that could develop symptoms sometime after transplant.
Crown gall mostly commonly affects stone fruit and some ornamentals but almost 100 different plants, including tomatoes, may be attached by this disease.
Initially infected plants will have raised, rough lumps but as the disease progresses, these lumps grow in large 'galls' which are ball shaped and usually dark and knobbly. These galls can occur on the roots or stems of plants. Usually, they appear at huge soil line. This may be the only symptom apparent on plants with mild infections but where there are a large number of galls, plants may become stunted. During times of water stress and drought, plants may die.
There is no known cure for this disease but regular watering can prevent plant death.
If you can find certified disease free seeds or seedlings, your prevention goal will be off to a good start. Crop rotation is also important for preventing a build up of bacteria in your soil. Avoid digging near tomato plants so that you don't accidentally injure the roots. If you're pruning your tomato plants, be sure to clean your secateurs both before and after use to ensure you don't spread the bacteria from plant to plant.