Most of the important sweet potato diseases attack the roots. Fungal leaf spots are common but seldom justify control measures. In North Carolina, the economically important diseases are viruses, nematodes, Fusarium root and stem rot, Fusarium surface rot, soil rot, Rhizopus soft rot, root know, scurf, and southern blight. Disease-control measures are carried out in the field, the plant bed (where transplant production occurs) and the packinghouse.
Feathery mottle and cork are caused by the feathery mottle virus and most Sweet Potato cultivars are infected. Leaves show light green patterns along veins and circular spots surrounded with a purplish ring (a) Roots may have radial and longitudinal surface cracks (b) and may develop internal corking (c) The virus is transmitted by aphids (insert) and planting stock. Management: Use tolerant cultivar, virus-free transplants, and isolate planting.
Black rot is caused by the seed-borne fungus Ceratocystis fimbriata. Symptoms include large circular, brownish to black, firm, dry rots on sweet potatoes. In plant beds symptoms include plant stunting, wilting, yellowing, leaf drop, and plant death. Rots may continue developing in storage. Infected roots have a bitter taste. Management: Avoid infected seed roots. Cut transplants above the soil line. Rotate with other crops in a 2 to 3 year rotation. Treat seed roots with a fungicide.
Ring rot is caused by the common, soil-borne fungus Pythium spp., which also parasitizes many other plants. Infected roots have sunken, chocolate colored lesions that tend to extend laterally and often form a ring around the sweet potato. The soft rot extends into the interior as illustrated. Losses generally occur late in the season during cool, rainy periods. Symptons may be confused with Rhizopus and bacterial soft rots and souring. Ring rot usually does not spread in storage. Management: Harvest prior to cool, wet periods.
Circular spot is caused by the soil-borne pathogen Sclerotium rolfsii, which also causes stem rots on many plant species. Lesions are circular with sharply defined margins, saucer-like in cross section, and yellowish-brown to brown. Lesions become dark and leathery as they dry and may be removed intact. The underlying tissue has a bitter taste. The disease does not develop in storage. Management: Avoid fields with history of southern blight.
Pox (Streptomyces root rot) is caused by Streptomyces ipomoea, a soil-borne bacterium that only parasitizes Sweet Potatoes. It may persist in soil for many years in the absence of sweet potato. Pox spots are circular, V-shaped in cross section, and composed of dark brown to black, corky tissue. Affected roots may be cracked, distorted and resemble dumbells. Feeder roots, especially tips, are rotted. Plant growth and yields are severely reduced. Management: Use a five year rotation; select resistant varieties; avoid light, sandy soils with high pH; inject chloropicrin in the row prior to planting; and keep soil moist.
Souring (Flooding Damage). Sweet potatoes are alive and constantly exchange oxygen and carbon dioxide. In waterlogged fields or airtight curing and storage facilities this cannot occur and roots die because ethyl alcohol accumulates in the root. Decay organisms rapidly invade affected roots. Initially, affected roots appear normal; internally the surface is dull, somewhat darkened, latex does not flow, and alcohol can be smelled. Souring may occur more frequently in well-drained, drought-prone fields than in fields that remain moist because these roots may have high gas exchange needs. Management: Select well-drained, non-drought-prone fields and provide irrigation when necessary. Ventilate storage and curing facilities with 2 to 4 air exchanges per day and double these rates for curing.
Scurf is caused by Monilochaetes infuscans. Dark brown to black blotches with diffuse borders develop on the surface of the sweet potato in the field. The stem end of sweet potatoes is more frequently affected because scurf usually develops from infected plants. Spots may coalesce and spread to cover the entire surface. Management: See black rot.
Foot rot, caused by the fungus Plenodomus destruens, may occur in plant beds, fields, or in storage. Lower leaves yellow, and plants gradually wilt and die with brown, necrotic lesions at or below the soil line. The disease may also extend into the root causing a dark brown, dry firm decay at the stem end of storage roots. In storage, the decay continues, but generally does not destroy the entire root. Management: See black rot.
Chilling. Sweet potatoes exposed to temperatures below 55 degrees may appear normal, but internally, the flesh may be spongy with dark vascular elements and latex does not flow. When chilled sweet potatoes are cooked, the central area of the root may be hard. The effects of chilling injury are cumulative with intermittent exposure to low temperatures. Management: Keep storage temperatures above 55 degrees. In late fall, remove roots immediately after digging.
Dry rot, caused by the fungus Diaporthe phaseolorum, is primarily a storage rot. Occasionally it is found in plant beds and fields. The firm, dry rot generally progresses from one end and causes the sweet potato to shrink or wrinkle. Affected tissue is light to dark brown externally and dark brown to black internally. Affected roots become mummified. Sprouts from infected seed roots may develop a reddish-brown to black decay at the base. Management: See black rot.
Fusarium surface rot, caused by Fusarium oxysporum, develops in the field and in storage. Lesions are circular, light to dark brown, firm, dry, and superficial. This rot is common and occurs primarily where there has been mechanical injury. Management: Minimize harvest and handling injuries, harvest when soil is dry, treat and cure immediately.
Fusarium root and stem rot, caused by the fungus Fusarium solani, is a common field and storage rot. The rot extends deep into the sweet potato and is firm and dark tan in color. Internally, elliptical cavities form in which a white mold develops. The soil-borne disease may be spread by infected transplants. The base of mature stems may become swollen and distorted. Management:See scurf and fusarium surface rot.
Rhizopus soft rot, caused by the common bread mold fungus Rhizopus nigricans, is a field and common post-harvest problem. Infection usually occurs before and during harvest through injuries on the surface of the sweet potato. Infected tissue rapidly becomes soft, stringy, and watery with a pleasant fermentation odor. In a few days, “whiskers” consisting of fungal strands and spores appear. Fruit flies are attracted to the rotting roots. Management: See fusarium surface rot.
Bacterial soft rot is caused by the “seed”-borne bacterium Erwinia chrysanthemi. The rot is largely internal, very wet, and mushy. Black streaks may appear in otherwise normal stem and root tissue. Diseased roots are seen at harvest, during curing, and in storage. Infected seed roots and transplants may be symptomless. After hot weather begins, random plants in the field may become yellowish, especially on the lower foliage, and the base of stems may have a black, shiny rot, which is the “black leg” stage. Management: Practice good sanitation in the seed program. Thoroughly clean and wash packing and grading equipment and rinse with water with at least 50 ppm chlorine. Water used for washing sweet potatoes should contain at least 300 to 500 ppm chlorine.
Fusarium wilt (not illustrated) is caused by the wide-spread, soil-and plant-borne fungus, Fusarium oxysporum batatas. The disease is frequently observed when susceptible cultivars are grown. Vines are stunted, older leaves yellow, and plants wilt and die. The base of vines may turn brown to purple and pith may decay. Vascular elements in stems and roots turn brown. Storage roots may appear normal but vascular tissue may be discolored. If these roots are used for plant production, wilt will occur in the field. Management: Use resistant cultivars, rotate crops, select seed roots from wilt-free fields, and use cut transplants.
Growth cracks are caused by uneven growing conditions, usually uneven watering, and are sometimes associated with secondary disease problems. Cracks are most common on large roots and on nematode-infested roots. Certain viruses also increase cracking.
Mutations. Sweet potatoes have an unusually high rate of mutation. Multicolored roots, called chimeras, have areas of differently colored skin or flesh.