Apiosporina morbosa - AKA Sh*t on a Stick

Please excuse the crudeness of Sh*t on a Stick, but that was the first coined term I was taught 25+ years ago. It stuck and I will never forget it. lol

It certainly depicts poop on a stick, doesn't it?  Apiosporina morbosa (even known as Dibotryon morbosum) is also called Black Knot. It is a rather noxious, pervasive fungus that attacks the prunus species of plants. Cherries, Plum, Chokecherry, Apricot, Almonds and ornamental cherry trees and shrubs fall victim to it.

This wet spring and rainy summer has made it more visible than ever. I usually see it more in the winter, when no leaves cover the knots.

Here, I found it on a Chokecherry tree. Matter of fact, I think every Chokecherry tree in the vicinity had it.

When the lesions first appear, they are much smaller. Just a callous and swelling bark - usually green-brownish-black in colour. Hardly noticeable at first, and only enveloping one side of the stem.  Each following growing season, the callous gall grows in diameter and becomes black and gnarly. Can get as long as 30cm. The knot is woody and will eventually encase the entire stem/branch. Eventually girdling the branch and causing die-back.  Once black, the infection has been around for about 2-3 years already. If there are several knotty turd like lesions on the tree, you'll definitely notice brach die back before you see the black knot.

 The Black Knot becomes active during warm, wet weather. "Ascospores" are forcibly released from the ascostromata of the fungus. On this sample, it had rained a lot overnight with strong winds. You can see the knot has white masses. Beginning a new life cycle on the blackened gall. Those white spots contain the acospores.  These spores are spread by air currents and rain splashing. 

When wet weather persists, the fungus acospores disperse and an injury or susceptible spot on the branch bark is like an open wound. The inoculum spores of the fungus invade the wound and start the whole process.  More Black Knot galls on the tree = eventual total die back = dead tree.

Treatment: Removal is key.

1) Remove during the dormant season. When the spores and tree are not active.

2) Cut back at least 10" below the knot. Better yet, if possible, remove the branch further back to a secondary stem, so that no stubby ends are left. Cut back to a collar or side shoot. Make the cuts on an angle so that no water sits on the newly cut area.

3) With EACH cut, disinfect your pruners/loppers. This prevents any contamination on newly cut ends and spreading it from host to new host.

4) Destroy the pruned branches. DO NOT COMPOST. Burn or discard in garbage. Black Knot can still release spores several months after being removed from the tree.

Prevention:

1) Stressed out trees are susceptible to Black Knot. Love on them a bit more. Water during drought periods. Mulch trees, accordingly.

2) Prune regularly. One of the biggest issues is overcrowded canopies. No air circulating between the branches or branches banging into each other, causes perfect conditions for the fungus to adhere and  spread throughout the tree.

3) Don't plant a Prunus species tree within a area close to a tree with Black Knot.


Here's proof:

At the base of this Chokecherry are visible signs that the tree had been suckering from the base. This is very common for older Chokecherry trees. A true sign of stress.

With TLC and extra monitoring, the Prunus species of trees can survive Black Knot.