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Problem
(factsheet)
Plant NameTypeDescription 
Bacterial soft rot and blacklegPotatoDiseaseSoft rot and blackleg are both caused by bacteria. The soft rot symptoms typically appear on tubers. The small to large circular lesions are sunken, tan or brown, water-soaked areas on the tuber surface. The affected internal tissues are white to tan and develop a soft, almost odorless rot. An amber liquid may ooze from rotting areas. Blackleg symptoms are generally limited to leaves and stems. Leaves curl upward and turn yellow. Soft, light brown to black lesions develop at the base of stems and may spread into a black, slimy rot of tubers. In dry weather, the stem discoloration may be internal only, but under favorable conditions for disease development (warm, moist weather), plants wilt and die. The bacteria are carried in contaminated plant debris, water, soil, and seed pieces and are easily spread by splashing water, insects, and equipment.
Late blightPotatoDiseaseLate blight of potato is caused by a fungus which also causes disease on tomato, eggplant, and other members of the potato family (Solanaceae). Gray-green, water-soaked spots appear on leaves and stems. These quickly enlarge into dark blotches which may be surrounded by a pale green margin. During moist weather, a sparse, whitish fungal growth may be seen on the underside of leaf lesions. Tubers may be infected either by spores washed into the soil or during harvest. Infected tubers show areas of somewhat metallic brown or purple discoloration followed by a brownish dry or wet rot. Disease development is favored by cool, rainy weather and may be more severe under sprinkler irrigation. The fungus overwinters primarily on potato tubers and is spread by wind.
Potato leafroll mosaic (Leafroll)PotatoDiseaseLeafroll and net necrosis symptoms are caused by a virus. The virus is transmitted by aphids and is also carried in infected tubers. Leaves of plants grown from infected seed pieces show a distinctive upward rolling of the leaf margins and have a brittle or leathery texture. Symptoms first appear at the base of the plant and progress upwards. These plants are generally slightly stunted and yellowish. Plants that are infected by aphid transmission of the virus may show rolling of leaves beginning at the top of the plant, but are seldom stunted. Infected tubers appear normal on the outside, but may display a net necrosis inside the tissues. This is seen as a brownish network of fine lines at the stem end. Net necrosis often develops in storage. The variety 'Russet Burbank' is highly susceptible to net necrosis.
Powdery scabPotatoDiseasePowdery scab is a fungal disease affecting potato roots and tubers. Tubers develop slightly raised, purplish or brown lesions or pustules on the skin. Initially, the lesions are closed and blisterlike. They later split open to reveal powdery, dark brown masses of spores surrounded by the torn edges of the ruptured skin. In storage, a dry rot may be associated with the lesions. Powdery scab lesions are small (less than 1/8" in diameter) and nearly circular. These characteristics help distinguish them from common scab lesions. The roots of potatoes and related species may develop small galls or wart-like structures. Disease development is favored by cool, moist soils. The fungus can survive 3 to 10 years in the soil.
Rhizoctonia canker (Black scurf)PotatoDiseaseRhizoctonia canker is caused by a fungus commonly found in the soil. On potatoes, young underground tissues may be attacked. The reddish-brown to brown lesions may result in girdling of sprouts, stolons, or roots. A powdery, gray coating of fungus may be seen on the stems near the soil line. Infection may also result in leaf curl on more mature plants. Tissues become more resistant to infection as they mature and as they emerge from the soil. Tubers develop "scurfy" patches of hard black fungal structures on the skin. These patches do not extend into the tissues, but are difficult to remove. The fungus may also cause a mild netting or scurf of the skin without forming the black structures. Tuber symptoms are often more severe in soils high in organic matter. Aerial tubers commonly form on infected plants.
Scab (Common)PotatoDiseaseCommon scab is characterized by the development of brownish, corky or scabby spots on the surface of the tubers. The spots may be up to 1/4" or more in diameter and slightly raised, or may develop into pitted, corky areas. Severe infections may deform tubers, but the damage is typically aesthetic. Common scab occurs more frequently in slightly alkaline soils, or soils which have been amended with fresh manure or wood ashes. The disease is less frequent on potatoes planted in moist soils. Smooth-skinned varieties are more susceptible than russet varieties. Beets are also susceptible.
Verticillium wilt (Potato early dying)PotatoDiseaseVerticillium wilt is caused by a fungus commonly found in the soil. Many species of plants are affected by Verticillium, but tomato and potato are favored hosts. Typically, the fungus attacks the roots and moves throughout the plant via the vascular system. Infected plants may wilt, sometimes along only one side. Other symptoms include premature yellowing and death of plants beginning at the base and progressing upward ("early maturity"). Discoloration of the vascular tissues is noticeable when cuts are made into the stem. Some tubers from infected plants may show a light brown discoloration in the vascular tissues. When tubers are cut across the stem end, this discoloration shows as a discontinuous ring of brown about 1/4"-1/2" beneath the skin.
Colorado potato beetlePotatoInsectThe adult Colorado potato beetle is a broad (1/4" wide), yellow-and-black striped beetle up to about 3/8" in length. The adults overwinter in the soil, emerging in the spring. They lay clusters of yellow eggs on the underside of the leaves of host plants. The humpbacked larvae are reddish-orange and have two rows of black spots along each side. Both larvae and adults chew holes in leaves and leaf margins. They may cause serious defoliation of potatoes. Other plants which may be attacked include pepper, tomato, petunia, and other potato relatives including weeds such as nightshade and groundcherry. They occur only east of the Cascades.
Potato flea beetlesPotatoInsectFlea beetles are small, brown to metallic black beetles with a habit of jumping like fleas when disturbed. They feed on many plants including potato, tomato, radish, spinach, and many weeds. Adults eat small round holes in the leaves, usually early in the season. The larvae typically feed on underground portions of the host plants, causing minor to severe damage in the form of winding grooves on the potato surface or as pinholes bored 1/2" or less into the potato. The holes can serve as entry points for tuber rots and other problems.
SlugsPotatoInsectSlugs are common garden pests in western Washington. They resemble snails, but lack shells. They may vary in size from as little as 1/4" up to several inches in length, depending on age and species. Foliage of older plants is raggedly chewed, while younger plants may be totally consumed. Slugs leave behind a characteristic slime trail, which appears silvery when it dries. Slugs typically feed at night and do more damage during cool, moist weather.
WirewormsPotatoInsectWireworms are the larvae of click beetles. The adult beetles are hard-shelled, tapered, and gray-brown to black in color. When turned on their back, they flip over with a sharp "click". The larvae live in the soil, feeding on seeds, underground stems, and small roots and burrowing into root crops. Typical damage to potato tubers consists of dark, 1/8"-1/4" holes and tunnels. The damage is often only cosmetic, although rots and other problems may attack via the wounds. The larvae are initially white with dark jaws, then turn dark yellow to brown as they mature. They are 1/4"-3/4" long and have six legs. They may resemble millipedes, which have many legs. Wireworms may require several years to reach maturity.