Pesticides updated by Joey Williamson, HGIC Horticulture Extension Agent, Clemson University, 10/16. Originally prepared by Marjan Kluepfel, HGIC Horticulture Specialist, James H. Blake, and Anthony P. Keinath, Extension Plant Pathologists, Clemson University. Revised by Joey Williamson, HGIC Horticulture Extension Agent. New 04/00. Revised 02/12.
Many diseases of Irish and sweet potatoes can be prevented in the home vegetable garden by using the following cultural controls:
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 fungus Phytophthora infestans. Late blight is especially damaging during cool, wet weather. The fungus 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 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:
If disease is severe enough to warrant chemical control, select one of the following fungicides: chlorothalonil, copper fungicide or mancozeb. 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 mancozeb or copper fungicide gives fair control. See Table 1 for examples of products. Begin applications when disease shows up on leaves and stems or at flowering. Spray every seven to 10 days. 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. 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 properly, and keep the plants growing vigorously. Remove 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. Mancozeb gives good control of early blight, and mancozeb or copper fungicide gives fair control. See Table 1 for examples of products.
Common Scab: This disease is caused by the 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 and 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 growth of plants and poor color of foliage. 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:
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:
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 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. Disease usually develops in localized areas associated with poor drainage. The bacterium attacks many different plant families, but most susceptible hosts are in the Solanaceae. 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 potato. The varieties Sebago and Green Mountain have tolerance to bacterial wilt.
Bacterial Soft Rot: The bacterium Erwinia carotovora 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. Erwinia is almost always present in 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 digging the potatoes carefully 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 potato. 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.
|Fungicide||Examples of Brand Names & Products|
|Chlorothalonil||Bonide Fungonil Concentrate
Ferti-lome Broad Spectrum Landscape & Garden Fungicide Concentrate
GardenTech Daconil Fungicide Concentrate
Hi-Yield Vegetable, Flower, Fruit & Ornamental Fungicide Concentrate
Monterey Fruit Tree, Vegetable & Ornamental Fungicide Concentrate
Ortho Max Garden Disease Control Concentrate
Southern Ag Liquid Ornamental & Vegetable Fungicide
Tiger Brand Daconil
|Mancozeb||Bonide Mancozeb Flowable with Zinc Concentrate
Southern Ag Dithane M-45
|Copper Fungicides||Bonide Copper Fungicide Spray or Dust (wettable powder with copper sulfate)
Monterey Liqui-Cop Fungicide Concentrate (a copper ammonium complex)
Southern Ag Liquid Copper Fungicide (a copper ammonium complex)
Bonide Liquid Copper Concentrate (a copper soap)
Camelot Fungicide/Bactericide Concentrate (a copper soap)
Natural Guard Copper Soap Liquid Fungicide Concentrate
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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.