I have a 12 year old Newport Plum tree that has been struggling for the past two years. Last year there was definitely less new shoots growing and this year very few. I noticed there is a black fungus growing on the branches. It’s in a sunny location, the type of soil is clay like. I fear it is dying. Anything we can do to save my favourite tree?
What your photo reveals is a common disease of Prunus trees including plums… Black Knot.
You don’t say how many branches are infected, but you do need to remove those cankers as much as possible, including removing infected branches completely when it will not destroy the structure of the tree.
Immediately, before leaves emerge, prune the cankers out, back quite close to the main stem. Pruning too shallow retains the undetectable, developing canker on the tree and does nothing to limit the disease. And it is important to do a thorough clean up, destroying all pruned twigs, not leaving any debris around the tree.
When the leaf buds start to break spray with lime sulfur. Later on spray with a sulfur spray and repeat at approximately 2 week intervals until late July. Observe the no spray interval before harvest as stated on the product label.
Black Knot spreads easily, so have a look at your neighbours’ yards and if you see the disease anywhere, show them this solution and encourage them to take action. If there is a badly diseased tree adjacent to your property, your tree will be re-infected.