Irish & Sweet Potato Diseases

Many diseases of Irish and sweet potatoes can be prevented in the home vegetable garden by using the following cultural controls:

• Always purchase certified disease-free potato tubers. Saving your own seed potatoes is generally not worthwhile because viruses and other diseases often show up the next year.
• Select varieties recommended for South Carolina, especially those with some degree of disease resistance.
• Keep the garden and surrounding area free of weeds, which may harbor insects that can spread viruses.
• Avoid overhead sprinkler irrigation and keep the leaves dry when watering.
• Remove plant debris from the garden after harvest since many diseases survive on plant debris from year to year.
• Do not grow potatoes in the same area year after year, but rotate with crops other than tomatoes.

Irish Potato Diseases

For advice on how to grow potatoes (Solanum tuberosum), see HGIC 1317, Potato. Common diseases of Irish potatoes in home gardens are described below.

Late Blight: This is a potentially serious disease of potato and tomato, caused by the water mold Phytophthora infestans. Late blight is especially damaging during cool, wet weather. The pathogen can affect all plant parts. Young lesions are small and appear as dark, water-soaked spots. These leaf spots will quickly enlarge, and a white mold will appear at the margins of the affected area on the lower surface of the leaves. Complete defoliation (browning and shriveling of leaves and stems) can occur within 14 days or less from the first symptoms. Infected potato tubers have a dry, corky rot that may be brown or reddish. Tubers are symptomless at the initial stages of infection but often develop symptoms in storage. The fungus produces a foul odor when the infection is severe.

Fungal spores are spread between plants and gardens by rain and wind. A combination of daytime temperatures in the mid-70’s with high humidity is ideal for infection.

Prevention & Treatment: The following guidelines should be followed to minimize late blight problems:

• Keep foliage dry. Locate your garden where it will receive morning sun. Allow extra space between the plants and avoid overhead watering, especially late in the day.
• Purchase “certified disease-free seed potatoes.”
• Use resistant varieties. The Irish potato variety Kennebec is resistant to late blight. Other varieties include Elba, Sebago, Rosa, and Allegany.
• Destroy volunteer potato and tomato plants, which may harbor the fungus.
• Do not compost rotten, store-bought potatoes.
• Rotate with crops other than tomatoes.
• Remove diseased plants and tubers of all sizes from the soil and garden area.

If the disease is severe enough to warrant chemical control, select one of the following fungicides: chlorothalonil, phosphorus acid, mancozeb, or copper fungicide. Wait 14 days after spraying mancozeb before harvest and 7 days after spraying chlorothalonil before harvest. Chlorothalonil gives very good control of late blight, and phosphorous acid, mancozeb, or copper fungicide gives good to fair control. Begin applications when the disease shows up on leaves and stems or at flowering. Spray every seven to 10 days. See Table 1 for examples of products.

Follow the directions on the label.

Early Blight: This disease is caused by the fungus Alternaria solani, and is first observed on the plants as small, black lesions, mostly on the older foliage. Spots enlarge and concentric rings in a bull’s eye pattern can be seen in the center of the diseased area. The tissue surrounding the spots may turn yellow. If high temperature and humidity occur at this time, much of the foliage is killed and falls off. Tubers may be infected with brown, corky, dry spots. The fungal spores spend the winter in plant debris left in the garden and on other solanaceous hosts, such as tomato, eggplant, and black nightshade. Tubers are frequently infected through wounds inflicted during harvest. Allow tubers to mature fully before digging. Infected tubers are inedible.

Prevention & Treatment: Use crop rotation, eradicate weeds and volunteer potato and tomato plants, fertilize and lime based on soil test results, and keep the plants growing vigorously. Remove and dispose of diseased plants immediately after harvest. Use disease-free seed potatoes. The varieties Kennebec, Rosa, and Sebago are resistant to early blight. For fungicide application, use mancozeb, chlorothalonil, or copper fungicide. Wait 14 days after spraying mancozeb before harvest and 7 days after spraying chlorothalonil. Chlorothalonil gives good control of early blight, and mancozeb or copper fungicide gives good to fair control. Chlorothalonil has a 0 day pre-harvest interval, and mancozeb has a 5 day pre-harvest interval. See Table 1 for examples of products.

Common Scab: This disease is caused by the filamentous bacterium Streptomyces scabies, which persists in the soil for long periods. Brown corky scabs or pits occur on potato tubers. These spots enlarge and merge together, sometimes covering most of the tuber. Leaves and stems are not affected. Scab is most severe in dry soil with a pH above 5.5 and in soil low in nutrients. Tubers infected with scab are edible; however, when blemishes are removed, much of the tuber may be wasted.

Prevention & Treatment: Soil pH should be between 5.0 and 5.2. Therefore, avoid alkaline materials, especially lime and wood ashes, but also avoid mushroom compost and leaf compost. All of these will raise the soil pH. Sulfur can be applied to garden soil to lower the pH. Examples of products include Southern Ag Soil Acidifier, Espoma Garden Sulfur Natural Acidifier, Hi-Yield Soil Sulfur, and Bonide Soil Acidifier.

Scab is favored under low soil moisture conditions, so the garden soil must be kept moist during the active growing period of the tubers (particularly 4 to 9 weeks after planting). Although potatoes are heavy feeders, high nitrogen levels can increase scab severity. Additionally, high ratios of calcium to potassium can increase disease. Do not use manure on potatoes because the bacterial spores can pass intact through the digestive tracts of animals.

Do not plant potatoes in the same area of the garden more than once every three to four years. Do not plant Irish potatoes after beets, carrots, radishes, parsnips, rutabagas, or turnips, which are also susceptible to scab. Use certified seed pieces that are resistant to scab. The following potato varieties are scab tolerant: Superior, Goldrush, Red LaSoda, Red Gold, Caribe, Dark Red Norland, Butte, Carola, Russett Burbank, and Sebago. There is no reliable chemical control.

Root-Knot Nematodes: Root knot is caused by the southern root-knot nematode, Meloidogyne incognita. Nematodes are microscopic worms that live in the soil. They are not related to earthworms. Nematodes feed on plant roots, damaging and stunting them. The first evidence of a nematode problem is poor plant growth and poor foliage color. The damaged roots cannot supply sufficient water and nutrients to the aboveground plant parts, and the plant is stunted or slowly dies. Symptoms are more pronounced during dry weather. Infested tubers are unsightly but edible if peeled. The root-knot nematode causes small inconspicuous root swellings or galls to develop.

Nematodes are found throughout the United States but are most severe in the South. They prefer moist, sandy-loam soils. Nematodes can move only a few inches each year on their own, but they may be carried long distances by soil, water, tools, or infested plants.

Prevention & Treatment: Testing roots and soil is the only positive method for confirming the presence of nematodes. There are no chemicals available to homeowners to kill nematodes in planted soil. When nematodes are present, consider the following practices:

• Relocate the garden to a nematode-free area.
• Use root-knot nematode-resistant varieties.
• Establish a rotation system with certain varieties of marigolds in a solid planting (no more than 7 inches apart on all sides). Marigold varieties Tangerine, Petite Gold, Petite Harmony, Single Gold (sold as Nema-Gone), and Lemon Drop will reduce root-knot nematode populations in soils. Reduction occurs by starving the nematode rather than the release of a toxic substance by the marigold roots. Keep marigolds free of weeds and grass to prevent nematodes from feeding on roots other than those of the marigolds. Rotate marigolds in the same area a minimum of every other year as long as root-knot nematode problems exist.
• Use a fallow period with summer tilling. The populations of root-knot nematodes can be reduced significantly in one season by repeated tilling (every 10 days) of the garden soil during the hot, dry summer. The nematodes are brought to the surface to be killed by the drying of the sun.

See HGIC 2216, Root-Knot Nematodes in the Vegetable Garden for more options and details.

Where root-knot nematodes are not present, the following procedures are recommended:

• Move the garden location every other year.
• Examine roots of transplants carefully for signs of very tiny knots. Avoid planting those that look suspicious. Look for bumps on the surface of Irish potatoes. It is best to purchase certified disease-free transplants and seed potatoes.
• Destroy roots by pulling up or plowing up immediately after harvest is complete.

Brown Rot or Bacterial Wilt: This disease is caused by the bacterium Ralstonia solanacearum. Symptoms include wilting, stunting, and yellowing of the foliage. Wilting of leaves and the collapse of stems may be severe in young, succulent plants. In young potato stems, dark, narrow streaks are visible. Upon cutting the stems, glistening beads of a gray to brown slimy ooze are visible. Tubers from infected plants may or may not show symptoms, such as a distinct, grayish-brown discoloration. When tubers are cut in half, grayish-white droplets of bacterial slime ooze out of them. The eyes become grayish brown and exude a sticky substance, causing soil particles to adhere to the tuber surface.

Prevention & Treatment: Infected seed potatoes are an important factor in the distribution of this disease. The disease usually develops in localized areas associated with poor drainage. The bacterium attacks many different plant families, but the most susceptible hosts are in the Solanaceae family. Use disease-free tubers and disinfect the cutting knife. Some crop rotation sequences, such as those including sweet corn, reduce disease severity. They may act indirectly by reducing populations of root-knot nematodes that enhance bacterial wilt disease infection in potatoes. The varieties Sebago and Green Mountain are tolerant to bacterial wilt.

Bacterial Soft Rot: The bacterium Pectobacterium carotovorum infects potatoes, tomatoes, peppers, parsnips, carrots, and many other vegetables. The bacteria rarely infect potato leaves, but they severely infect the tubers, rendering them inedible. The tubers may be infected either in the ground or in storage. Bacteria can rot tubers completely in three to 10 days. Pectobacterium is almost always present in the soil, but the bacterium only infects potatoes that have been wounded.

Prevention & Treatment: Plant potatoes in well-drained soil and ridge or hill the plants to encourage excess water to flow away from them. Wait until the leaves turn yellow and die before carefully digging the potatoes to avoid bruising them. Do not store blemished tubers.

Sweet Potato Diseases

Information on how to grow sweet potatoes (Ipomoea batatas) can be found in HGIC 1322, Sweet Potatoes. Disease problems can be reduced by following a few important practices. Do not plant on the same land for at least two years, and do not use transplants that show spots of black rot on the lower stems. Potatoes should be certified disease-free. Avoid bruising sweet potatoes during digging and storage.

Scurf: Damage from this disease, caused by the fungus Monilochaetes infuscans, is primarily cosmetic. Dark brown to black spots develop on the roots. The spots enlarge and may coalesce. Scurf lesions continue to enlarge when sweet potatoes are placed in storage.

Most scurf infections result from the use of infected propagating material. The pathogen also survives in soil for one to two years. Disease severity is greater in fine-textured soils and when the organic matter content of the soil is high. The use of animal manure may increase the incidence of scurf.

Prevention & Treatment: Only scurf-free sweet potatoes should be used as seed roots. Seed roots should be bedded only in soil free of the pathogen. Sweet potatoes should be rotated with other crops in a three- to four-year rotation. Remove all plant debris after harvest. Chemicals are not available for home garden use.

Black Rot: The fungus Ceratocystis fimbriata causes black rot. Infected fleshy storage roots develop a firm, black, dry rot. Lesions enlarge throughout storage. Black cankers form on the underground parts of the stem. Secondary symptoms vary depending on the severity of infection, and may include stunting, wilting, yellowing, and dropping of leaves, much as with Fusarium wilt (see below).

The fungus is usually introduced through the use of infected seed potatoes. It survives in crop debris in the soil.

Prevention & Treatment: Use only certified disease-free seed potatoes. Soak potatoes in a borax solution (⅔ cup borax per gallon of water) for 10 minutes immediately before planting. Sweet potatoes should not be planted in the same field more than once every third or fourth year. It is critically important for transplants to be cut at least 1 inch above the soil line, to exclude infected underground portions of the stem. Remove all plant debris after harvest.

Fusarium Wilt: This disease is caused by a strain of the fungus Fusarium oxysporum, which only infects sweet potatoes. Symptoms include yellowing of older leaves followed by wilting, leaf drop, and stunting or dying of the plant. Discoloration of the vascular tissues of the stem occurs earlier. Frequently, the symptoms are one-sided. They are more severe when soil moisture is low. The fungus persists in the soil for many years.

Prevention & Treatment: The most effective means of control of Fusarium wilt is the use of resistant cultivars, such as Jewel, Beauregard, Regal, Excel, and Sumor. A combination of cultural practices can be employed to reduce Fusarium wilt: rotation, sanitation, and the use of certified disease-free seed roots. Raising the soil pH to 6.5 – 7.0 and using nitrate nitrogen rather than ammoniacal nitrogen will retard disease development. However, raising the soil pH may greatly favor soil pox, a disease of the tubers caused by Streptomyces ipomoea. There are no chemicals available for home garden control of Fusarium wilt.

Root-Knot Nematodes: For symptoms, prevention and treatment, see root-knot nematodes in Irish potatoes. Some sweet potato varieties resistant to the southern root-knot nematode are Carolina Bunch, Excel, Jewel, Regal, Nugget, and Carver. Hernandez is moderately resistant. Liberty is a bonito, or tropical sweet potato, which is not as sweet and used for baking and is highly resistant to root-knot nematodes.

Table 1. Fungicides for Sweet & Irish Potato Disease Control.

Pesticides are updated annually. Last updates were done on 6/21 by Joey Williamson.

Originally published 4/00