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Cankers on your fruit tree fall into the broad category of “gummosis”, and are often seen with dieback.

There are quite a few potential causes, but it may be a sign that your tree has a fungal disease called Phytophthora, which is sometimes called root rot disease.

However, cankers can also be caused by many other diseases. So let’s start with finding out a bit more about cankers.

What causes cankers on fruit trees?

First, let’s check out what cankers actually look like. The word conjures up all sorts of nasty images of disease.

The example in the photo at the top of the blog of a fruit tree with sap on the bark is pretty typical. A canker is often just a blob of gum or resin (which is actually the tree’s sap) on the bark.

The word is also used to describe “sores” in the bark of the tree, where the bark may have lifted, look diseased, or be cracked. Cankers will often have both sap and damaged bark together.

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 Phytophthora or Bacterial canker

There are a few 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 limbs. This may start at the top of the tree, or on any of the branches.

Is there any treatment for Phytophthora?

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

Of course, the answer is in the soil!

Phythopthora is a fungus that lives in the 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 soil is the solution to most fruit tree cankers

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.

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.

How to make your soil healthier

Here are 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“.