posted 13 years ago
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.