Bacterial blast

Pseudomonas Syringae
Also known as Canker, Bacterial Canker or Gummios.

Bacterial blast is a disease that presents itself by creating open oozing ‘cankers’ on branches or trunks of trees. This causes sudden decay of flower buds and dieback on branches and, in major cases, the entire tree.

Bacterial blast lives on plant surfaces and is spread by splashing water, usually occurring from rainfall. This disease will grow in periods of damp and wet conditions with low temperatures, and in Auckland usually presents itself in autumn to early spring.

Affected branches will slowly die, fail to bud and won’t be able to produce new growth in spring.

Disease development stops when temperatures rise.

Stone fruit trees are the most susceptible to this disease, but the disease can also spread to other trees through open wounds or via the use of a pair of dirty secateurs.


If caught early, bacterial blast can be managed.

Spring is when symptoms of the disease are more obvious which can show on any part of the tree.

If you are pruning plants over winter, always make sure you wait for dry weather. If the pruning cut you make is larger than 1cm, immediately apply Grosafe Organic Prune N Paste to help prevent infection.

Equipment hygiene is important as tools that have infected sap on them can spread the disease. Clean tools in between pruning each tree with methylated spirits.

Implementing a regular spray program is essential to protect against diseases. Spray with Copper Oxychloride at leaf drop, and then twice through spring at bud movement, then again ten days later.

Certain rootstocks have been known to have better resistance to bacterial canker. Always choose a fruit tree from a garden centre, as stone fruit trees grown from seed are more likely to contract diseases.


If the infection is minor on outer limbs, cut back the diseased wood and remove it from the property (do not use the diseased wood for any type of compost or mulching, remove it immediately). Prune to 10cm below obvious signs of the infection. Often the wood will have a brown stain to it, so prune back past this as this is a sign of infection.

Dispose of all infected clippings in the rubbish, do not compost or use as mulch as this can spread the disease.

After pruning spray infected plants with Grosafe Free Flo Copper

If the infection has spread to the trunk, you will need to dispose of the tree, as heavily infected plants cannot be treated.

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