My plum trees leaves have started to turn yellow & are dropping what could be the cause & is it likely to die off?

Could it be drought?

just copied this from the RHS website

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Plum trees seem to grow in a very untidy fashion. They seemingly will never look as neat as an apple tree after pruning. That said the following tips should yield you some nice fruit.

If your tree has rather yellow leaves it is probably short of mineral such as iron or magnesium or needs a general fertilizer. Apply in a foliar spray to the leaves before flowering or once the fruitlets are bigger ie in late May,June or even after harvest.

If you have silvery leaves on a branch this is a sign that the tree has silver leaf. This is bad news! Without wasting time cut off the affected branch and any dead branches and burn said branches. You will see the dark diseased wood as you cut in the centre. Try and cut back until it all looks good. The feed with fertiliser and repeat each year. Unfortunately the chances of saving a diseased tree are small but this will prolong its life and stop the disease spreading to other plum trees.

Ideally always cut out any dead branches in the spring.

Pick fruit the day before the flies arrive! They are more of a pest than wasps. If you have wasp or fly damage you have left the fruit too long on the tree.
or like Louloubelle said may be thirsty?

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