For the last year, my plum tree has not borne fruit and has produced small knotty branches on the top. How should I deal with this please.
It sounds like your tree may have Witches Broom. This article will explain more:
https://www.gardeningknowhow.com/ornamental/trees/tgen/witches-broom-disease.htm
What is the sticky substance on that plums that looks like dew dripping off
This is Honeydew, which is the residue from pests.
These links will help you.
https://www.gardeningknowhow.com/garden-how-to/info/what-is-honeydew.htm
https://www.gardeningknowhow.com/edible/fruits/plum/common-plum-tree-pests.htm
How do we dispose of a diseased plum tree so it doesn't spread to other species i.e. berry bushes. Thank you.
Most Counties have a site to drop diseased trees and plant material.
Contact your County Extension Office, City or County offices to locate the area closest to you.
They also will provide information on how to transport the plant material to the site.
https://www.gardeningknowhow.com/extension-search/
Dear Sir/ Madam, I have plum tree with some kind of disease, I dont know what it's called or how I should treat it , I have attached photos so please could you help me and advise me what to do. Thank you. Maysa
This is not a disease, but Lichen growing on the bark of the tree.
Lichen is not generally harmful to the host plant, though it can be an indication of growing conditions for the tree.
https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm
https://www.gardeningknowhow.com/edible/fruits/plum/growing-plum-trees.htm
most of the fruit is going rotten from green they turning dark brown iam no good with photos and uploading can you help me please.
This sounds like Brown Rot Plum disease.
https://www.gardeningknowhow.com/edible/fruits/plum/treating-brown-rot-in-plums.htm
can it be prevented
You may be referencing Brown Rot.
This article has more information.
https://www.gardeningknowhow.com/edible/fruits/peach/brown-rot-fungus.htm
I have a young plum try which I planted last spring. I lost two branches which developed a green powdery coating on the branches. The leaves and fruit died off .i sprayed the tree with bug spray and also a fungicidal spray as I could not identify the cause. This year, I have had a beautiful display of blossom and lots of tiny plums but I have lost another three branches with fruit to the same problem. Can you help me identify the cause?
It is probably powdery mildew. Here is a link that will help: https://extension.psu.edu/plants/tree-fruit/diseases/stone-fruit-diseases/powdery-mildew