Potatoes small. Causes: Poor growing conditions; calcium deficiency. See "Culture" on page 188 for growing guidelines. If soil test indicates calcium deficiency, amend soil as needed.
Cause: Exposure to light. Tubers exposed to light often turn green and develop a toxic substance known as solanine. Peel off green tissue before eating. Prevent greening by hilling plants with soil or mulch and renewing it as necessary. Store potatoes in the dark.
Potatoes with brown or black spots or patches on skin. Cause: Scurf. Peel off spots before using tubers. Soak seed pieces in compost tea before planting to help suppress fungal diseases. Do not plant spotted tubers: The resulting plants will produce only small tubers, many of which will rot.
Potatoes with rough, corky spots on skin. Cause: Scab. Him out spots before using tubers. Prevent this fungal disease by keeping soil pH below 5.5 and planting resistant cultivars such as 'Beltsville', 'Norland', 'Onoway', 'Pungo', 'Rhinered', 'Russet Burbank', 'Russian Banana', and 'Superior'.
Potatoes with wartlike bumps. Cause: Root knot nematodes. See "Leaves yellow; plant stunted and may be wilted" on page 189 for controls.
Potatoes knobby. Cause: Uneven soil moisture. If tuber growth is interrupted because of lack of soil moisture, tubers are often deformed. Keep soil moist, but not soggy, and never let it dry out.
Potatoes with a black, rotted ring at stem ends. Cause: Bacterial ring rot. Tubers have a soft, light brown ring in the flesh near the skin. See "Leaves discolored and puckered or curled" on page 188 for controls.
Potatoes with dark blotches on skin; flesh with dark, corky areas. Causes: Early blight; late blight. Trim out corky areas before using; discard severely affected tubers. See "Leaves with gray-brown, concentrically ringed spots" and "Leaves with water-soaked brown spots" above for controls.
Potatoes with gray to black areas in flesh; centers may be hollow. Causes: Waterlogged soil; extreme temperatures; potassium or phosphorus deficiency; mechanical injury; viral disease. Potatoes grown in wet, poorly drained soil often grow too fast and may develop a discolored and/or hollow area in the center.
Prevent problems by keeping soil moist, but not soggy.
Extremely hot weather or cold snaps before potatoes are harvested can cause similar symptoms. Prevent problems by keeping tubers well-covered with hilled-up soil or mulch.
Phosphorus or potassium deficiency can cause spots or patches of dark flesh. Oversize tubers with hollow centers may also indicate potassium deficiency. If deficiencies are suspected, have soil tested and amend as needed. If plants show symptoms of phosphorus deficiency, raise soil pH to 6.0 so the mineral will be more available to the plants.
Rough handling can cause mechanical injury, such as bruising, that appears as discolored areas on the flesh. Handle tubers gently when harvesting and storing to prevent problems. Plants suffering from viral diseases may produce deformed or discolored tubers. See "Leaves discolored and puckered or curled" on page 188 for controls.
Potatoes riddled with tunnels. Causes: Tuberworms; wireworms. If tubers have browned, silk-lined tunnels, look for tuber-worms. These pinkish white larvae are Vi" long and feed in tubers, stems, and leaves. Destroy infested tubers and plants. Prevent problems by keeping tubers hilled with soil as they grow and by removing the dead vines before digging tubers. Cover plants with row cover to prevent adult moths from laying eggs.
Wireworms are yellow to reddish brown, hard, segmented larvae up to l'A" long that tunnel into tubers and chew on roots. Adults are dark-colored, elongated click beeties. Apply parasitic nematodes to the soil before planting to control them. Avoid planting potatoes where sod or grain grew the previous season, because wireworms are often numerous there.
Potatoes spoil in storage. Cause: Bacterial or fungal rots. Poor growing conditions or improper curing or storage may encourage various rot diseases. See "Culture" on page 188 for growing guidelines.
Let vines yellow and dry before carefully digging tubers. Sort out any bruised, cut, cold injured, or diseased potatoes and keep them cool (around 40°F) until they can be used. Cure healthy tubers by storing them between 50° and 60°F for 2-3 weeks, then store them between 35° and 45°F in a humid place.
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