Colorado Potato Beetle

On a small scale you can simply hand pick off any beetles you find and scrape off the tiny orange egg masses from under the leaves (and any newly hatched larvae). The larvae are eaten by many predators, though the adults are fairly poisonous.

Verticillium wilt

This fungus shows itself by the tops dying off prematurely (it’s also known as Early dying fungus). You may still get a small crop of potatoes from affected plants, but they won’t store well. This disease can last for years in the soil. To eliminate it, don't plant potatoes in the same spot for at least 4 years. Other members of the Solanum family are also affected, so they shouldn’t be grown either during that period (except for a few resistant varieties).

Late blight

(Phytopthora infestans) This is the disease that caused the famine that depopulated Ireland, by killing one and a half million people and causing another million to emigrate. It is called Late Blight because it likes warmer weather and usually occurs after Tomatoes (which it also affects) have flowered. It doesn’t much bother early crops. This fungus first manifests itself as spots on the lower leaves in cool weather, but then the leaves die and brown patches appear on the tubers. The only thing you can do is dig the tubers 2 weeks after the tops die down and use them (then spores from tops won’t be transferred to the tubers). This disease affects yield, but doesn’t affect storability (of course you wouldn’t use infected potatoes for seed). Many modern varieties have some resistance to Late Blight.

Pests

The Potato has more than its fair share of disease and insect pests. Fortunately these aren’t found everywhere; in some favored areas Potatoes have few problems and are very easy to grow. The severity of potato pests varies from year to year, with different growing conditions. In some years they do little harm, in other years they can be devastating. Warm humid conditions are the worst for Potatoes.

Scab

(Streptomyces scabies) Soil bacteria cause this very common disease. It is undetectable above ground and the damage is mainly cosmetic, so it is not very serious unless you are growing for market (it reduces their marketability). Alkaline soil (above 6.0 pH) and lack of moisture are the main causes of scab. It persists in the ground for several years and can also infect other root crops such as carrot, beet and turnip. The best ways to prevent scab is to rotate annually, keep the soil somewhat acid (don’t lime it) and to use resistant varieties. Abundant water may reduce damage from Scab.

Viruses

These aren’t always an obvious problem, but they can reduce yields considerably. You may avoid them by using certified seed and not saving your own tubers for planting.

Removing viruses: It is possible to get a virus-free plant from an infected tuber (this is only worthwhile if you have a special variety). You plant the tuber in a container of sterile potting mix and keep it in a warm place to grow. When the shoot reaches 6˝ to 8˝ high you cut it off 2˝ to 3˝ above the soil line (it should never touch the soil or the rest of the tuber). The shoot can then be rooted in another container of sterile potting mix and it will hopefully be virus-free. Just hope that when you plant it out your garden is also free of viruses.