This common fungal disease is mostly associated with potatoes, but it can also infect beet, carrot, parsnip and turnip (and occasionally also cabbage, eggplant, spinach, onion).
You can’t tell if a plant has Scab by looking at the above ground plant, as it is only visible as rough scabby patches on the skin of the tubers. Fortunately the damage is usually only skin deep and not serious (it is just peeled off with the skin before cooking), though occasionally it may provide a way for other infections and rots to enter the tuber. If you are growing for market it can be a problem in that it reduces their marketability. Though the scab fungus is present in most soils, it becomes less of a problem as the soil gets more acidic. The best way to prevent scab is to make the soil (or keep it) somewhat acid, ideally the pH should be no higher than 5.5 (never lime the soil before planting potatoes). You should also add plenty of organic matter. A lack of moisture is another important factor in causing scab, so keep the plants well watered too. Some potato varieties are resistant to scab.
Image: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org