There are a number of species of bacteria that cause this disease including Corynebacterium michiganese pv. michiganese, Pseudomonas syringae pv. syringae and Pseudomonas syringae pv. morsprunorum. It can affect any part of the plant above ground and it is a particularly deadly tomato plant disease.
How It Spreads
The bacteria infects plant seeds so the seeds of diseased plants can produce infected seedlings. It can only survive for short periods of time in the soil but it may survive for longer periods in plant matter so if plants are left in the ground, are composted or used as mulch, they can be a source of infection for subsequent plantings.
For this reason, it is very important that diseased plants are never composted (except in a very hot compost heap - I wouldn't trust a home system) or used as mulch around the garden.
Bacterial canker affects tomatoes and stone fruit (such as nectarines, apricots and plums). Different species of bacteria tend to infect different types of fruit - Pseudomonas syringae affects stone fruit while Corynebacterium michiganese is the species that usually attacks tomatoes. Bacterial canker attacks mature plants, seedlings or anything in between. It is important to note that plants may not aways show any symptoms of this disease. For this reason, if you reuse potting mix, make sure you wait a few months before growing another plant in the used mix - sterilising the potting mix is also recommended. You should also be on the look out for signs of disease when planting new seedlings near established plants in case they're carrying the disease.
Initially, most plants infected with this disease will wilt, the leaves will turn brown, particularly at the edges and new leaves in particular may curl up. Often, these symptoms occur predominately on one side of the plant. Seedlings may die suddenly despite proper care and those that survive are usually stunted. Relatively large, white spots or black, dead patches may also be seen in leaves and stems. When spots occur on the tomatoes themselves, they may be dark brown in the centre with a surrounding white ring and they are usually slightly lumpy.
In later stages of the disease plant stems will form cankers hence the name bacterial canker. Fruit flesh will usually turn brown. Plants will also often become severely wilted and then die.
Unfortunately, effective chemical or organic treatments for bacterial canker have not been identified. In the very early stages of the disease (when it's difficult to know the causes of a plant's illness) regular sprays with chive, onion, garlic or ginger infusions may help to kill the bacteria but success is by no means guaranteed. It's much better to ensure your plants never get sick in the first place.
Prevention is the best cure for this disease given how easily it spreads and how difficult it is to eradicate. You can prevent your tomato plants from becoming infected with bacterial canker by firstly using only certified disease free seeds or seedlings. Similarly, if you're saving your own seed, never keep the seeds from any tomato plants that show any symptoms of illness, regardless of the symptoms and it can be a good idea to soak seeds in hot (not boiling) water prior to planting to kill bacteria that is present. Sterilising seed raising mix and pots with steam or hot water can also prevent the spread of the disease.
Whilst bacterial canker only survives in the soil for a short period of time, it's ability to survive for extended periods of time in plant matter means that you must always dispose of diseased material appropriately. This means putting it in your green waste bin if you have one or if you cannot remove green matter from your property, designate an area where you can put diseased material. Line this area with black plastic and always lay diseased material in thin layers on top of the plastic for at least a month so that the sun can help kill the bacteria that causes the disease. Even when this material then breaks down it should not be spread around the garden - bury it in your designated disease zone.
Companion planting with alliums that you trim regularly (such as chives) can help to prevent infections due to the antibiotic nature of these plants. Regular sprays of chive infusions can also help. Only spray early in the morning however, as night time spraying can leave foliage damp for extended periods of time and this may increase the likelihood of your plants succumbing to fungal diseases despite the anti-fungal properties of chives. Copper sprays can also be used as a preventative.
If you discover sick tomato plants in your garden and you suspect they may have bacterial canker, pull them out immediately. Rotating your crops can also help prevent bacterial canker as it improves the health of your soils, resulting in healthier plants that are more disease resistant.
If you prune your tomato plants, do it in the morning after watering to prevent the bacteria splashing into the wound from the soil. Make sure you always clean your secateurs before and after use to prevent infection of healthy plants due to having pruned sick tomatoes or stone fruit trees.