MM106 - Another one bites the dust

[FB.]

Yet another sorry story of crown rot/canker killing the rootstock of a several-year-old MM106 apple tree.

The root system is quite large and loppers had to be used to free the tree from the ground (tree is several years old - just about to come into good cropping), but all of the roots from ground level to about six inches below ground have rotted away and the crown rot canker is now spreading up the trunk - notice the discoloured (purple-rusty-brown-ish) cracked and peeling bark; the greenish cambium layer exposed by the rotten, papery, peeling bark.

The lower roots and trunk no longer have a functional sapflow between the two; the infection managed to sneak-up below ground, un-noticed and the roots are so badly diseased that the tree is 99% certain to die.
I've never had a MM106 survive from this once infected.
M27, M9, M26 don't seem to suffer from it.
M25 is intermediate in susceptibility - nowhere near as susceptible as MM106 but more than other rootstocks. I have had a few M25's survive after being infected, probably as a result of the great vigour of M25 allowing it to outgrow moderate attacks.
M116 and MM111 aren't usually affected and those that are often recover.

My recommendation: choose M116 or MM111 instead, which are not too much different in vigour to MM106, but are able to be planted into the same hole as a dead MM106 comes out of; hence have good resistance to the crown rot.

Pictures of the latest crown rot infected MM106 below:

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[FB.]

Two more pictures.

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[BUFFS]

sorry to see the stricken plant,as you are in a much drier area,is this canker/crown rot a common problem?,we got canker on the braeburn,but thats the first time we have seen it on any of our trees,and i will be finding some scions to graft onto whats left of it next year..

[FB.]

Crown rot of rootstocks is a common problem, particularly for commercial growers using MM106. Apparently many of them expect to lose up to 10% of their MM106 trees every year to this disease.

I think the reason for crown rot problems with MM106 are:

1. It is so widely grown that, like most "commercial" apple varieties, the diseases have long since evolved ways to attack it efficiently (same as over-use of antibiotics encourages resistance).

2. MM106 has relatively thin bark, which is easier for fungi to break through the physical defences.

3. It produces a lot of burrknots, which are splits in the bark where roots emerge; these splits allow fungi to invade - I've sometimes seen crown rot canker and nectria canker enter through above-ground burrknots, killing the lower stem and blocking sapflow between canopy and roots.
In my experience MM106 produces many more burrknots than any other rootstocks - hence why it will root very easily from 2-3yr old woody cuttings where other rootstocks don't root as easily.
Of course, rooting from cuttings is a desirable characteristic for nurseries who need to mass-produce trees. In the nursery, diseases are kept quiescent by chemical sprays.

4.
MM106 does not do well where soil moisture is variable - especially not doing well in light soils.
Where the soil dries during the summer its roots will not grow at all, so it cannot repair any damage from pests or diseases. The longer the dry spell the more damage the diseases cause before the tree can try to defend itself by growing new roots to replace those killed by disease attack. After a dry spell, MM106 roots are slower to restart growth than most rootstocks.
A scientific study showed that even the dwarf M9 rootstock produces two or three times the number of roots as MM106 after a dry spell or transplanting. Only when soil hadn't dried for three consecutive months did MM106 finally have more root growth than a dwarf such as M9. Unfortunately, the soil here is only damp for a few days to a week after heavy rain, and we often go for months in summer without heavy rain. The heaviest rain we've had in a day in the last three months is about 9mm. The last time we had more than 10mm of rain was 26th April.
In an entire average year, we only get about a dozen days with 10mm (half inch) or more of rain.
On the other hand, too much moisture which causes waterlogging (such as a heavy clay) and the roots become even more prone to fungal attack; not helped by their thin bark.

[FB.]

The canker on your Braeburn is a different disease to that which kills roots.
However, it is occasionally possible for a rootstock to get canker just above ground level.

Common canker (Nectria) usually attacks branches.
Crown rot canker (Phytophthora) usually attacks roots but can also attack fruits, causing a form of brown rot and an opportunity for the disease to hide in the environment on fallen fruit.

Pictures below of:

1. Common (Nectria) canker - usually enters a leaf scar during a wet autumn; in this case the leaf-scar bud grew into a branch, but later a latent canker developed, or possibly the narrow (bad) branch angle made it easier for the canker to invade.

2. Crown rot (Phytophthora) canker spreading up the trunk - usually enters wounds or burrknots at, or up to several inches below, ground level. The wood just under the cankered bark is often orange-stained and feels like cork when you push your thumb nail into it.

3. Burrknots becoming pink, above-ground roots in the damp grass around a MM106. The burrknots are emerging from splits in the bark - these splits making it easier for insects or fungi (crown rot or canker or even fireblight) to enter the tree at ground level.
Hence my phrase "hit below the belt" which describes the death of a super-disease-resistant variety which died because it was grafted onto a disease-prone rootstock which later became diseased.

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[BUFFS]

my canker was just like the first picture,it was 3ft up the stem,with the broken,swollen fissure as in the picture,i lopped it off 2ft below and although it is basically just the rootstock,it does seem to be staying healthy.i will let it continue to grow and get some scions in the spring ,to graft on.thanks for explaining the different cankers etc,very much appreciated as it saves me worrying about the rootball,thanks for the help..