I have 3 beds in my garden and I was going to rotate plant families each year in the beds, especially the solanaceae (tomatoes/potatoes/peppers), because I have read that they can have more disease problems if they aren't rotated. However, I have a LOT of volunteer potatoes in one bed this year. I thought I harvested them all last summer, but obviously not. So, is the disease potential so high that I should rip the volunteer potatoes out and plant the tomatoes and potatoes in a different bed, as I was planning? Or should I keep the potatoes (it's always fun to welcome volunteer plants!) and plant tomatoes again in that bed this year?? I would hate to get diseases in my tomatoes.

I am not sure where these various diseases come from, initially. The starts I have are all from local, organic farms, so I would think they are quite healthy. I live in the Sierra foothills in Northern CA if that helps.

Just like pests and weeds and flowers, you get diseases that are native to an area or imported on seed stock. Where you get problems is when there is a soil imbalance. For instance, potatoes: They are heavy feeders. Eventually (how soon depends on the basic composition of your soil, assuming no amendments) the soil starts getting worn out, the potatoes are no longer as healthy, and diseases and bugs move in for the kill. This was the cause of the Great Potato Famine in Ireland, where potatoes had been introduced as a crop to stave off starvation a century or more earlier.

I would say if they are happy, and you are adding mulch and manure every year, no real reason to move them. (Of course, I've had volunteer potatoes in my garden for three years now and haven't bothered to move them.) I'm not sure I'd plant potatoes and tomatoes too close to each other, though. They sort of compete and can repress each other, as I understand it.

if the potatoes you dug last fall were clean, why get rid of the volunteers? I always have a few volunteers, but when I poke in new ones they go in a new spot, preferably the newest hugelkulture bed I've made as potatoes love hugelkulture..sometimes I put them where the newest sheet compost has gone in..i generally only rotate a crop away from an area IF IT HAD A problem there.