Foliage Diseases of Potatoes

Fusarium and Verticillium Wilts

• The symptoms of fusarium and verticillium wilts are very similar and require laboratory tests or expert evaluation to differentiate them. Infected plants have droopy yellow lower leaves. The disease travels up the plant and in the case of verticillium wilt the leaves fall off the plant. In mild cases, the vines may produce a crop of small potatoes. Fusarium wilt thrives in high temperatures and favors crops planted late, while verticillium wilt takes hold in cool temperatures.

Management of Wilts

• Fusarium and verticillium fungi live in the soil and persist for several years. Wait four to six years before planting potatoes in soil where tomatoes, peppers or eggplant previously grew. Several types of weeds host verticillium wilt, so keep the garden free of weeds. After harvest, remove and destroy remaining plant material. Encourage strong plants that can resist disease by regularly fertilizing and irrigating them, but take care not to over-water.

Early Blight

• Early blight causes brown spots on mature potato leaves, later spreading to young leaves. As the spots enlarge they develop concentric rings, giving the lesion a "target" appearance. Early blight is a disease of weak, unhealthy plants, so start your potato crop off right with a good fertilization program and proper watering.

Late Blight

• Late blight is a fungal disease that causes small green spots that appear to be water-soaked or greasy. The spots begin on lower leaves and over time turn brown and enlarge so that the entire leaf turns brown. The disease spreads quickly during cool, moist weather. Late blight spreads through diseased tubers, so purchase certified seed potatoes and don't allow volunteers, or potatoes that come up on their own where a crop was previously grown, to mature. Moisture in the presence of cool temperatures encourages late blight, so avoid watering when temperatures are cool or at night, and water just the soil, keeping the foliage dry as much as possible.

Viruses

• Some mosaic viruses cause mottled, crinkled leaves with pale veins while others have less obvious symptoms such as stunted growth. Leafroll virus begins as red leaves that eventually roll upward. Both viruses are spread through infected seed potatoes, and you can avoid the disease by purchasing tubers that are certified disease-free. Once the plants are infected, the disease is spread by aphids feeding on the plant. Remove and destroy plants infected with viruses to prevent the spread of disease.