Sweet potato

Description

Sweet potato, Ipomoea batatas, is an herbaceous perennial in the family Convolvulaceae grown for its edible storage roots. The sweet potato plant is a branching, creeeping vine with spirally arranged lobed, heart shaped leaves and white or lavender flowers. The plant has enlarged roots called tubers which act as an energy store for the plant. The tubers can be variable in shape and can be red, yellow, brown, white or purple in color. Sweet potato vines can reach 4 m (13 ft) in length and the plant is usually grown as an annual, harvested after one growing season. Sweet potatoes may also be referred to as yams or Spanish potatoes and originate from Central America.


Uses

Sweet potato tubers are eaten cooked as a vegetable or may be processed into flour or starch. The leaves can be eaten fresh or after cooking.


Propagation


Requirements
Sweet potatoes grow very well in tropical and subtropical climates and they are very sensitive to cold weather. They grow best at temperatures in excess of 25°C (77°F) in well-draining, loamy soil with a pH of 5.6–6.6. Sweet potatoes should be planted in full sun and require plenty of space as the vines will spread over large areas.

”Slips”
Sweet potatoes are grown from plants called slips. Slips are plants which have been grown from a mature sweet potato tuber. Slips can be purchased or grown at home. The easiest way to grow slips is to place a sweet potato in a jar of water using toothpicks to support it, place it in a bright, sunny window and and allow it to sprout. Some of the sprouts will develop roots, others may not. The shoots that have roots can be snapped from the main tuber and planted in individual pots. Those without should be snapped off and placed in a jar of water until they begin to develop roots.

Planting
Sweet potato slips should be planted outside after all danger of frost has passed, usually about a month after the last frost date in your area. Prepare the planting site beforehand by working in about an inch of compost. Plant the slips in 15 cm (6 in) deep holes spaced 30–45 cm (12–18 in) apart, allowing 0.9 m (3 ft) between rows.

General care
Weed the plants about two weeks after planting by gently pulling weeds by hand. Weeding is not necessary if using black plastic mulch. Sweet potatoes require watering weekly especially during dry periods and the plants will benefit from a side dressing of fertilizer. Stop watering the plants 3 to 4 weeks before the anticipated harvest.

Harvest
Sweet potatoes are usually harvested around the time of the first fall frost. Use a garden fork to carefully dig up the roots being careful not to bruise the tubers. If weather permits allow the harvested tubers to dry out for a few hours by leaving them on top of the soil. Cure sweet potatoes for 10 to 14 days prior to use by storing in a warm, dry place. The tubers will keep for about six months in storage.


References

CABI Crop Protection Compendium. (2008). Ipomoea batatas (sweet potato) datasheet. Available at: http://www.cabi.org/cpc/datasheet/28783. [Accessed 16 April 15]. Paid subscription required.

Collins, W. W. (1995). Sweet potato. Purdue University Center for New Crops and Plant Products. Available at: http://www.hort.purdue.edu/newcrop/cr.... [Accessed 16 April 15]. Free to access.

Lerner, B. R. (2001). The sweet potato. Purdue University Cooperative Extension Service. Available at: http://www.hort.purdue.edu/ext/ho-136.... [Accessed 16 April 15]. Free to access.

Wright, S. (2014). Sweet potato. University of Kentucky Cooperative Extension Service & University of Kentucky College of Agriculture, Food and Environment. Available at: http://www.uky.edu/Ag/NewCrops/intros.... [Accessed 16 April 15]. Free to access.


Common Pests and Diseases

Bacterial soft rot
Erwinia chrysanthemi

Symptoms
Brown to black water-soaked lesions on stems and petioles which expand rapidly and and cause large areas of soft rot on the stem; stem may collapse causing several vines to wilt; entire plant may die; storage roots may develop areas of soft rot which is initially colorless, but eventually turns brown with a black margin
Cause
Bacterium
Comments
Symptoms develop after hot weather; can effect stored tubers
Management
Avoid wounding storage roots at all stages of growth; plant only disease-free seed material; discard any stored roots which become infected with the disease; vines for transplanting should be cut above the soil surface; plant sweet potato varieties which are resistant to the disease

Pox
Streptomyces ipomoea

Symptoms
Poor growth of plants; reduced yield; circular dark brown, corky lesions on tubers which are V-shaped in cross section; cracked and distorted tubers which resemble dumbells; rotting feeder roots
Cause
Bacterium
Comments
Disease emergence favors light, sandy soils
Management
The most effective method of controlling the disease is through the use of resistant varieties of sweet potato; if resistant varieties are not being used then the soil should be maintained at a low pHwhich is unfavorable to the pathogen; sweet potato should be rotated with other crops which are non-hosts to prevent build up of the pathogen in the soil; fumiigation of the soil prior to planting can be an effective method of reducing the severity of the disease

Bacterial wilt
Ralstonia solanacearum

Symptoms
New sprouts wilting and have water-soaked bases which turn yellow-brown to dark brown in color; vascular system of the sprouts is discolored brown; infection of healthy transplants causes the lower portions of the stems to become water-soaked and turn a similar color to infected sprouts; yellow-brown streaks may develop inside storage roots and, if infection is severe, gray-brown water-soaked lesions may be present on the root surface
Cause
Bacterium
Comments
The bacteria causing the disease is an important pathogen of other crops but in sweet potato it is only severe in certain regions of China where it can cause 70-80% reductions in yield
Management
Quarantine procedures have been put in place in regions of China where the disease is severe; only disease-free storage roots should be used for planting and planting should only be done in sites free of the disease; rotating sweet potato with a flood crop such as paddy rice or a non-host such as corn or wheat can be beneficial; growing sweet potato during cooler periods of the year allows some avoidance of the disease

Black rot
Ceratocystis fimbriata

Symptoms
Stunted plants; wilting plants; yellowing plants; dropping leaves; plant death; circular brown-black patches of rot on tubers
Cause
Fungus
Comments
Rot continues to develop in stored tubers
Management
Only disease-free seed material should be planted; sweet potato should not be planted in sites where sweet potato has been grown during the previous 3-4 years; transplant material should be collected from plant by making cuts above-ground; seed material should be treated with an appropriate fungicide prior to planting

Fusarium root and stem rot
Fusarium solani

Symptoms
Swollen and distorted base of stems; deep, dark rot extending deep into tuber and forming elliptical cavities; growth of white mold
Cause
Fungus
Comments
Disease can be spread by infected transplants
Management
Disease is generally not a problem if good sanitation is implemented; select only disease-free roots for seed; use cut transplants rather than slips; practice crop rotation; treat seed roots with an appropriate fungicide prior to planting

Alternaria leaf spot & Leaf and stem blight
Alternaria spp.

Symptoms
Brown lesions on leaves with concentric rings resembling a target; lesions are usually restricted to the older leaves and may be surrounded by a yellow halo; small gray-black oval lesions with lighter centers may occur on stems and leaf petioles and occasionally on leaves; stem and petiole lesions enlarge and often coalesce resulting in girdling of the stem; defoliation may occur
Cause
Fungus
Comments
Stem and leaf petiole blight is much more destructive than leaf spots caused by Alternaria; stem and petiole blight is a severe disease of sweet potato in East Africa and has also been reported from Asia, South America and Cuba
Management
Destroy all sweet potato crop residue immediately following harvest; plant resistant or tolerant sweeet potato varieties where available; plant only disease-free seed material

Leaf and stem scab
Sphaceloma batatas

Symptoms
Small brown lesions on leaf veins which become corky in texture and cause veins to shrink which in turn causes leaves to curl; lesions on stem are slightly raised and have purple to brown centers with light brown margins; scabby lesions form on stems when lesions coalesce
Cause
Fungus
Comments
Leaf and stem scab is one of the most severe diseases of sweet potato; disease is most severe in regions where there is frequent fog, rain or dew accumulation and is common in Asia and Australia
Management
Avoid the use of overhead irrigation; rotate sweet potato with other crops; use only disease-free planting material; destroy sweet potato crop residue immediately after harvest; application of appropriate fungicides can help to control the disease, good control of the disease can be achieved with benomyl and chlorothalonil where licensed for use

White grub
Phyllophaga ephilida

Symptoms
Grub feeds on underground parts including main stem and roots. They also feed on tubers by making tunnels. The infected plant become wilted and die eventually.
Cause
Insect
Comments
White grub are the larvae of scarab beetles commonly called as May and June beetles. The grubs are white in color and appear C shape. They generally feed on soil, organic matter and plant materials.
Management
Cultural practice: Deep summer ploughing to expose grub and pupa present in soil. Provide proper drainage to soil to avoid excess moisture. Follow crop rotation with soybean to reduce grub population. Application of biocontrol agents like Bacillus popilliae and B. lentimorbus bacteria kill the grubs.

Sweet Potato stem borer
Omphisa anastomosalis

Symptoms
Larvae bores into the stem leading to the storage roots. Feeding in the crown region leads to wilting, yellowing and dying of plant. The borers can be easily identified by the presence of fecal matter on the soil surface and holes on the stem.
Cause
Insect
Comments
Larva is light-purple and/or yellowish-white in color.
Management
Keep the field free from weeds especially Ipomoea spp. Fallow the land for few season if infestation is more. Use insect free planting material. Use pheromone traps to monitor and control the insect.

Sweet potato virus disease (SPVD)
Sweet potato feathery mottle virus (SPFMV) and Sweet potato chlorotic stunt virus (SPCSV)

Symptoms
Sweet potato virus disease is a disease complex caused by two viruses; sweet potato chlorotic stunt virus (SPCSV) and sweet potato feathery mottle virus (SPFMV). The symptoms are sever stunting of infected plants, stunting, distorted and chlorotic mottle or vein clearing of the leaves. It is confirmed that SPCSV enhances the accumulation of SPFMV.
The symptoms caused by SPCSV alone is negligible. Where as symptoms caused by SPFMV is localized, mild and often asymptomatic and won't cause significant damage to the plant. Common symptom include appearance of feathery, purple patterns on the leaves.
Cause
Virus
Comments
It is estimated that SPVD causes yield loss up to 80 - 90%. The disease was first reported in 1939 from eastern Belgian Congo (present Democratic Republic of Congo). SPCSV is crinivirus of Closterviridae and SPFMV is potyvirus belong to Potyviridae.
SPFMV is transmitted by a wide range of aphid species. SPCSV is transmitted by white flies (Bemisia tabaci).
Management
Use healthy cuttings for planting. Remove the infected plants and burn them. Follow crop rotation. Spray suitable insecticides to control aphids and white flies.