Soil, stone or grass around the base of trees?

Hi all,

I have a number of trees in my garden - see pic:



At the moment the majority of the trees are just surrounded by grass although the v large apple on the LHS near the shed has a 'flower bed' (only one rose and mostly weeds in there - I need to sort it out) and two of the none fruit trees have previously had stones put around them (mostly over grown again now with grass).

The trees near the dog are apple, plum (Victoria) and 2 x pear. I'm unsure what the none fruit trees are yet.

One of the apple trees is showing a bit of grey decease on the bark but the fruit is really good.

I was thinking if I could dig around the trees to expose soil I could put down manure or compost or something like that to maybe give the tree more nutrients? I think it might also look good / help me mow around the trees as its a bit of a pain. - any recommendations or just stay with the grass?



Thanks
Chris