Q.can you fertilise plums and cherries with a small brush
In the absence of insects such as bees can you fertilise plums and morello cherries with a small brush ? When is the right time i.e. what state of the blossoming?

Certified GKH Gardening Expert
If you are referring to pollination... Unless this is an indoor grown specimen, this will be completely unnecessary. Ant, flies, gnats, and just about any insect that is within the area will do the job for you. You will need a separate compatible pollinator, though, since most of the genus is not self-compatible.
If these are indoors, and you know for certain that this is a clean room lacking ANY insects, then you can pollinate one variety with another compatible one with the softest paint brush that you can find. It will be clear when pollen is forming, since it will easily collect on your fingertip upon touching the flower.
https://www.gardeningknowhow.com/garden-how-to/beneficial/insect-pollination-process.htm