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Blossom blast/bacterial canker

Pseudomonas syringae

Ed. note. Much of this reprinted from The facts about bacterial blast: How to spot it, prevent & treat it by Kate Marshall, Nov 17 2015 Bacterial/blossom blast/canker is particularly onerous in apricots, but also affects cherry. Peaches not so much. When growing apricots in New England, particular attention must be paid to this disease. JC

There are few diseases of fruit trees that can be as devastating as bacterial blast, the disease affecting stonefruit trees that is also known as Pseudomonas syringae, gummosis, bacterial canker or, simply, canker.

Plum, prune plum, apricot and cherry trees are most susceptible, though peach and nectarines trees, including dwarf types, can also be at risk.

Minor infections in branches can be controlled by hard pruning, but major trunk infections will often kill a tree.


Spring is the time of year when infections of bacterial blast can be most conspicuous, occurring on the branches, leaves and fruit.

Symptoms of the disease include branch dieback, rough, irregularly shaped cankers and amber-coloured gum – the most commonly noticed indicator. The oozing gum is filled with bacteria so is a source of further infection.

Sometimes stonefruit trees can leak sap that can be mistaken for bacterial blast. Take a whiff: if the gum smells sweet like golden syrup, then it is most likely the disease. If it has no aroma then it's probably just a bit of sap escaping.

Sometimes blossoms turn brown and cling on to the tree, leaves develop dark brown spots that drop out and fruitlets show sunken spots.

Sour sap is another term associated with this disease, where the branch or trunk exudes a sour odour from the fermentation of sugars by yeasts under the bark of the damaged area. 


For infection and development of any disease to occur, three conditions need to be met.

First, the presence of inoculum, or bacteria. Second, the plant must be at the susceptible stage of growth – that is, flowering and leaf buds opening in spring. And third, the environment must be conducive. In the case of bacterial blast, this means cool and wet spring conditions. 

The bacteria can infect trees in autumn, often if there is a sharp, early cold period. Sometimes this can cause dieback on branches that is not visible (because the infection is beneath the bark). The bacteria then overwinter in cankers, in buds and within the tree. During wet patches in spring, the bacteria multiply, ooze from the branch cankers and are spread by rain. 

The bacteria enter the plant through natural openings such as emerging buds and through pruning and other wounds, which can be as small as leaf scars. 

Favourable conditions for bacteria spread and infections are frequent periods of rain, cool temperatures and strong wind. 

The disease stops further development when temperatures increase in late spring and throughout summer. 


First, and most importantly, only plant healthy fruit trees. Inspect trees carefully upon receipt, you don't want to  introduce this disease to your orchard.

If there are any signs of gum oozing from the trunk – either fresh, or dry and hardened – disregard that tree and contact the nursery. Dieback of branches is more difficult to identify on young plants as bacterial blast, as this can sometimes also be caused by stress. Fruit trees that are growing strongly and healthily will be less susceptible to bacterial blast infection than weak or struggling trees.

Planting fruit trees in suitable sites with free-draining soil will encourage vigour and healthy growth. Provide the right kind of fertiliser and irrigation at the right times to encourage robust and productive growth.

Avoid overhead sprinklers for irrigation or frost control – the dispersal of water from a height can potentially drag bacteria down through the tree at the same time.  

And, apart from potassium/sulphate of potash, avoid fertilising fruit trees in late summer and into the autumn. The soft new growth that results will take longer to harden off and go into dormancy, creating more opportunities for bacterial infections over the autumn. 

Only prune your fruit trees during dry spells of weather – ideally, immediately after harvest. Tradition has it that winter pruning is best, but this is actually the worst time with a high risk of infections.

Whether pruning in summer or winter, always seal pruning wounds with a pruning paste, which encourages the cut to heal and provides an antibacterial and antifungal barrier.  

Prune trees to remove congested branches and to encourage air flow – a vase shape (or open centre form) is recommended for stonefruit trees. This also allows for better coverage when spraying.

Disinfect equipment between pruning each tree – whether the disease is evident or not – using bleach or methylated spirits. Keep a sprayer bottle of disinfectant along with a rag with your pruning gear so you don't forget.  

For most stonefruit trees, a spray programme is recommended to avoid problems such as leaf curl and brown rot. A copper-based fungicide applied in autumn and spring will also help to prevent the development of bacterial blast. 


For infections that are contained to a branch or two, prune aggressively to about 10cm below the obvious signs of infection. You may see brown staining in the wood – keep pruning down the branch until there are no signs of the disease in the wood. Unfortunately this may destroy the tree's form, but it should save the tree.

Take care with pruning hygiene, along with destroying all prunings. This plant material would not make good compost or mulch!

A light summer application of copper fungicide wouldn't go amiss, with a stronger dose in autumn as the leaves start to fall in order to clean up stray bacteria and prevent overwintering. 

Bacterial blast infections in the trunk will often cause the death of the tree. Small infections may be able to be controlled by using a copper-based fungicide programme to prevent the multiplication and spread of the bacteria into other parts of the tree, or even to other healthy trees. 

More information

BACTERIAL DISEASES OF STONE FRUITS: SPOTS AND CANKERS (Presentation by Kari Peter at 2015 New England Vegetable & Fruit Conference)