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IDENTIFY AND TREAT BACTERIAL CANKER


This article is about bacterial canker of plum and cherry trees. Because of the specific pruning methods used for these two trees, treatment of this bacterial disease described below is specific to plums and cherries. 

The technical name for this bacterial infection is Pseudomonas syringae and this particular bacteria has more impact in the world we live in than you may ever have imagined.


Background information on pests and diseases is always useful, the more we know about them the better informed we are about how to treat and prevent them. The next two paragraphs gives some startling facts about Pseudomonas syringae (the cause of bacterial canker) which some may find interesting. For those more interested purely in the identification, treatment and prevention of this disease click here to skip to next section of this article.

Pseudomonas syringae (now referred to as "the bacteria") is often present in hailstones and it is believed that it enables the formation of hail stones at higher temperatures than would normally be the case. The bacteria is frequently used to induce man-made snow.

Water in tree tissue does not normally freeze at 0°C / 32°F, the makeup of most trees prevents it freezing until it reaches -4°C / -24.8°F or much lower in some cases. However the bacteria which causes bacterial canker rises the temperature at which plants cells freeze by about 3 degrees centigrade. When plant tissue freezes it becomes damaged and damaged plant tissue is the entry point for the this bacteria. This is because the nutrients in the plant tissue become available for the bacteria to feed on when damaged.

Enough of just a few of the bizarre properties of Pseudomonas syringae, onto the more practical aspects!

There are three key sections to this article:

  1. The symptoms of Bacterial Canker.
     
  2. Treatment and prevention of Bacterial Canker.
     
  3. The life cycle of the bacteria.

SYMPTOMS OF BACTERIAL CANKER


Bacterial Canker is responsible for the demise of more young plum and cherry trees than any other disease. The key symptoms are:
  • Branches and stems have sunken and malformed areas on them. The size of the affected area can be as small as a two penny coin but can also spread over very large areas of the branches.
     
  • Damaged areas will often have a dark gum oozing from them which may harden to become almost solid after time.
     
  • Leaves turn prematurely yellow but do not shrivel. They drop off sooner than normal.
     
  • There may be small brown marks on the leaves which fall out leaving the leaves with small holes in them, this often referred to as shot hole

Young trees are particularly liable to be infected with the plum varieties Victoria and Laxtons being particularly vulnerable.

TREATMENT AND PREVENTION


Treatment of bacterial canker involves cutting away all infected branches and areas well back into good wood. Burn all prunings to avoid re-infection. See our detailed articles on pruning for the correct times of year to prune plums and cherries. Clear up leaves as they drop, burn these as well because they can also be infected. Sterilise (methylated spirits are good for this) all equipment before and after pruning.

If canker infects the central trunk of your tree it's probably time to say goodbye to your plum or cherry tree unfortunately. Dig it up and burn to avoid infecting nearby plants and trees. Replacement varieties which have some resistance to bacterial canker include Warwickshire Drooper and Marjorie's Seedling.

The copper fungicide Bordeaux Mixture has previously been recommended as a spray to prevent canker in fruit trees. However it has now (or soon will be) withdrawn from sale in the UK. Currently there are no alternatives which have been scientifically proven to be anywhere near as effective as Bordeaux Mixture

Various alternatives are suggested, for example aspirin solution or milk, but none have been proven to have any effect.

As far as removing an infected tree and planting a similar one in the same place goes, I suspect that the bacteria remains in the area of an infected tree for an unspecied period of time after it has been dug up. Primarily in surrounding weeds and vegetation. See the article below:

http://pcmg.ucanr.org/files/171588.pdf

But in general it is not a good idea to plant a similar tree in the ground as an infected one. A tree with bacterial canker will have been weakened by the infection and vulnerable to other diseases even if they are not apparent at the time the tree was dug up. Those diseases may well remain in the surrounding ground ready to infect the new tree.

LIFE CYCLE OF BACTERIAL CANKER


The infection starts on the leaves in the summer. When the leaves fall off the bark where the leaf was joined can become infected allowing the bacteria to enter the bark. This happens most frequently on smaller branches.

The infection will take hold through late autumn through to late spring. The cankers will slowly enlarge over this time. In summer the disease goes into temporary remission only to restart the process in late autumn.

If you have any questions about Bacterial Canker or general comments about this article, use the form below to send them to us. We will get back to you as soon as possible.

COMMENTS / QUESTIONS LEFT BY OUR READERS

Date: 7 July 2015 From: Peter V
My "Stella" cherry tree has bark peeling from the trunk which exposes a rust colored dust like power. I have also noticed some smaller branches have died and in certain places it is "bleeding" a amber sap, I suspect Canker. Finally I have Victoria plum tree close to the cherry can you recommend a spray I could use to reduce the risk of the canker (if that is what it is ) spreading

ANSWER: There is nothing you can use as a spray for canker now, copper fungicides such as Bordeaux Mixture have been withdrawn from sale in the UK. Your only option is to treat the canker in the cherry tree (now is a good time to prune a cherry tree) by pruning out all infected parts.

Keep a good eye on the plum tree and treat for canker as soon as possible if you see any developing. Clean up all fallen leaves regularly and burn them.


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