This year about one in ten of my potatoes has a groove rather like an old wound that has healed without the sides kniting up. I didn't cause it by, for example, I didn't cut them with a spade when they were half-grown. I've been growing potatoes in this garden for a dozen years without this effect. All I can observe that is different this year is lots of rain early, and growing the potatoes in a bed with lots of quack-grass in it. Any ideas as to what the cause might be? Regards, Peter.