I agree with all the never too late comments. Mulching may help with disease prevention especially of any soil borne diseases. Won't help with diseases that are airborne. But I don't think of that as the main reason to mulch. Main reasons are that it suppresses weeds, so you won't have nearly as much weeding to do and it conserves moisture in the soil. Eventually your organic mulch breaks down and adds fertility to the soil.
Grass clippings are great if you have them. i usually don't, because I leave the grass clippings in the lawn to help feed it. So pulled weeds also work and yard trimmings, shredded paper, wood chips, fall leaves (in the fall I collect bags of leaves for my compost pile and for mulching), straw (in summer once the fall leaves are all used up, I usually buy one bale of straw to feed in to my compost pile and to renew all the mulches). This year I am trying to do brown and green mulching, just as I put brown and green in the compost pile. So a layer of fall leaves or wood chips covered with a layer of green yard trimmings and weeds.
The other day I was working on my community garden plot. When I walked up and looked at it, I immediately noticed one little patch of it that was full of weeds. I realized that was the little patch where I ran out of mulch (and energy!) when I was mulching the bed. So all of the mulched area, no weeds. The one little un-mulched spot, thick with weeds. It was a great demonstration!