Cherry – Canker
Q: I have a cherry tree on which I noticed a place oozing sap about halfway up the main trunk. I looked it up in a reference book and it diagnosed it as bacterial canker. The book says to prune the infected branches, but this spot is on the main trunk. The tree is only six feet tall! Can I cut the tree in half and expect it to live? Is there another way to fight this bacteria or should I destroy the whole tree?
A: You are smart to know how to use a disease reference book and make your own diagnosis. My suspicion is that the problem originated with a weakened tree. Your job now is to figure out how the tree lost its vigor. When a cherry tree begins to decline, I wonder if the tree was planted too low initially. Trees that are purchased “balled and burlapped” are particularly prone to this problem. The digging machine compresses soil around the trunk a few inches higher than the initial soil level in which the tree grew. If the tree was then planted just an inch or two lower than it should have been, the soil around the tree now could be five inches higher than it should be.
Ornamental cherry, plum and peach trees are very sensitive to being planted too low. Their roots need to be close to the soil surface so the gasses they give off can dissipate quickly. If the cherry tree was planted too deep, in a relatively small hole surrounded by heavy clay soil, it might be weakened to the extent that bacterial canker could attack.
Bacterial canker is impossible to treat chemically; no anti-bacterial sprays or injections will solve the problem. Pruning out the canker, as you have recognized, is also impractical. Scientists have noted, however, that trees in the cherry family are very stressed when grown in acid soil. The low pH makes them more susceptible to disease.
If you have never added lime to the area around the tree, the soil pH is probably lower than it should be. You, and anyone else growing ornamental cherry or plum trees, can use the following technique to immediately raise soil pH and then to keep it high.
Regular garden lime takes years to counteract soil acidity. A quick method of raising soil pH in a limited area is to apply hydrated lime. This chemical is available at most garden centers. You’ll need one pound for every inch of tree trunk thickness. Mix one pound at a time in five gallons of water and sprinkle it evenly under the drip zone of the tree. Use caution when handling or mixing the stuff – hydrated lime powder is extremely irritating. Afterwards, scatter garden lime over the spot, again at the rate of one pound per inch of trunk diameter. Water everything into the soil thoroughly. If the tree survives the next few years, have the soil tested then by your local Extension office to make sure the soil pH is between 6.0 and 7.0.
If your tree was planted too low initially, not much can be done now. Pruning it in half is out of the question. However, raising the pH will give you something to do while nature runs it course and if the tree goes ahead and dies, a new cherry tree planted there will benefit from the lime.