Cankers on your fruit tree are just one of the signs you may have Phytophthora or root rot disease. However, cankers can also be caused by many other diseases.

What are cankers?

Well, the example in the fruit tree above of sap on the bark is pretty typical. The word is also used to describe “sores” in the bark of the tree. Cankers will often have both sap and damaged bark together.

A canker in the bark of a peach tree
A canker in the bark of a peach tree

What causes cankers on fruit trees?

It can be very tricky to pin down the cause of a canker.

Two of the most common causes in fruit trees are Phytophthora and Bacterial canker. The symptoms can look very similar.

Cankers can also be caused by diseases like Blossom blight, or even by physical damage.

How to diagnose whether your canker is caused by root rot or Bacterial canker

There are a few giveaway signs that your fruit tree might be infected with Phytophthora:

  • Phytophthora cankers will smell sickly sweet if the infection is caused by P. cinnamomi (one of the three different species that cause root rot). Bacterial canker, on the other hand, smells sour;
  • Cankers caused by Phytophthora are more likely to occur in summer, and Bacterial Canker is more active in winter;
  • Phytophthora lesions start at the base of the tree and move upwards. Bacterial cankers appear at the top and move down the tree.

Though Phytophthora attacks the roots, another symptom that can help identify it is dieback of branches. This may start at the top of the tree, or on any of the branches.

A peach tree with dieback from Phytophthora
A peach tree with dieback from Phytophthora

Is there any treatment for Phytophthora?

Phytophthora is responsible for many fruit tree deaths. But fear not, in many cases it’s perfectly beatable.

Of course, the answer is in the soil!

A toad enjoying our healthy soil
A toad enjoying our healthy soil

Phythopthora is a fungus that lives in water in the soil, so it thrives in wet and waterlogged conditions.

But guess what eats it?

Other soil fungi, the non-disease causing kind!

Healthy, well-drained soil with lots of organic matter is the perfect breeding ground for these healthy soil fungi. Once established, they will out-compete the Phytophthora.

We learned a lot about this pervasive soil fungus after the flooding rains we had a few years back. A lot of trees died, but our soil improvement work helped us to save a lot of sick trees. We thought we would have to pull them out, but they recovered, as you can see in this photo below.

A peach tree showing lots of new growth as it recovers from Phythopthora
A peach tree showing lots of new growth as it recovers from Phythopthora

The key to beating Phytophthora is a healthy soil improvement regime. It should be based on (a) increasing the organic matter in the soil, and (b) making sure you have active (healthy) soil microbes doing their work.

Here’s some suggestions for what to use:

  • compost (just spreading compost on top of the weeds or grass is good enough, as the worms quickly take it into the soil);
  • compost tea — aim to put it on at least three times a year (though even more would be great if you have time);
  • microbe food — things like fish, kelp, and humates put through the irrigation system will feed your soil microbes;
  • occasional soil remediations, e.g. calcium if needed.

Soil improvement is a long, slow, continuous process. You’ll probably find you’re soon seeing improvements to help you know you’re on the right track.

You’ll find more information about how to identify and treat Phytophthora (and other common fruit tree diseases) in our short course “Keep Your Fruit Trees Free from Disease“.