Victoria Plum tree not giving as much fruit as usual

[Woody4life]

My Nan's victoria Plum tree is about 54yrs old and this year isn't producing the same yield as the same time last year I've found some fungus(?) on it and was hoping for advice on whether it is the cause or if it's just getting old or maybe something else. Has a few holes around the top of the trunk too but we assumed these were Woodpeckers

Many thanks