Peanut (groundnut)

Description

Peanut, Arachis hypogaea, is an herbaceous annual plant in the family Fabaceae grown for its oil and edible nuts. Peanut plants are small, usually erect, thin stemmed plants with feather-like leaves. The leaves are arranged in alternate pairs and have leaf-like attachments near the stalk. The peanut plant produces yellow, orange, cream or white flowers which produce 'pegs', characteristic floral structures which sink into the ground to grow the pod. The pods can reach up to 10 cm (4 in) in length and can contain between 1 and 5 seeds. The peanut plant can reach 0.6 m (2 ft) in height depending on the variety and as an annual plant, survives only one growing season. Peanut may also be referred to as groundnut, monkeynut or earth nut and originates from South America.


Uses

Most commercially grown peanuts are used for the extraction of their oil which is used in cooking. The by product of oil extraction is a pressed cake which is used as an animal feed and also in the production of peanut flour. Raw kernels are also commonly roasted and eaten as a snack food.


Propagation


Basic requirements
Peanuts grow very well in tropical and subtropical climates, requiring warm temperatures and a long growing season. Plants grow optimally at a temperatures between 30 and 34°C (86–93°F) although they will tolerate a range between 15 and 45°C (59–113°F). Temperatures above 34°C may damage flowers. The peanut plant grows best in a light, well draining sandy loam, but may also grow in heavier soils. The optimal pH range for peanut growth is with a pH of 6.0-6.5. Peanuts are resistant to drought and can withstand long periods without water albeit with a reduction in yield. For optimal production, a well distributed rainfall of between 500 and 600 mm of water over the course of the growing season is ideal.

Propagation
Peanut is usually propagated from seed. Seeds should be planted in a well prepared seedbed in soil that is loose and crumbly with no large clumps. The seedbed should be free from weeds which will compete with the peanut seedlings. Weeds may be removed by hand cultivating or through the use of an appropriate herbicide. Peanut seeds should be planted by hand to a depth of 3–5 cm (1–2 in). It is best to ridge the soil or use flat beds as this will make harvesting the peanuts easier. Peanuts can be grown a sole crop or intercropped with other crops such as corn (maize), cassava or soybean.

General care and maintenance
To achieve maximum yields, peanut fields should be kept as free as possible from competing weeds. Plants should be supplied with additional irrigation if dry conditions coincide with flowering and pod fill. Peanuts do not generally require the addition of supplemental nitrogen as their roots form symbioses bacteria which are capable of fixing atmospheric nitrogen in the soil. Peanuts require calcium in the soil for good pod fill. the calcium in the soil is absorbed directly by the pods in the pegging zone. Calcium deficient soils may result in empty or poorly filled pods.

Harvesting
Peanuts are ready for harvest between 85 and 130 days after sowing, depending on the variety. Pods can be harvested by hand or by mechanical means. Plants should be pulled from the ground and the pods removed. After harvest, pods are dried in the sun for 2–10 days until their moisture content is about 10%.


References

CABI Crop Protection Compendium. (2010). Arachis hypogaea (groundnut) datasheet. Available at: http://www.cabi.org/cpc/datasheet/6932. [Accessed 06 March 15]. Paid subscription required.

Duke, J. A. (1983). Arachis hypogaea L. Handbook of Energy Crops. unpublished. Available at: http://www.hort.purdue.edu/newcrop/du.... [Accessed 06 March 15]. Free to access.

Kokalis-Burelle, N., Porter, D.M, Rodríguez-Kábana, R., Smith, D. H. & Subrahmanyam, P. (Eds.) (1997). Compendium of peanut diseases. Available at: http://www.apsnet.org/apsstore/shopap.... Available for purchase from APS Press.

Wright, S. (2012). Peanuts. University of Kentucky College of Agriculture Cooperative Extension Service. Available at: http://www.uky.edu/Ag/NewCrops/intros.... [Accessed 06 March 15]. Free to access.



Common Pests and Diseases

Two-spotted spider mite
Tetranychus urticae

Symptoms
Leaves stippled with yellow; leaves may appear bronzed; webbing covering leaves; mites may be visible as tiny moving dots on the webs or underside of leaves, best viewed using a hand lens; usually not spotted until there are visible symptoms on the plant; leaves turn yellow and may drop from plant.
Cause
Arachnid
Comments
Spider mites thrive in dusty conditions; water-stressed plants are more susceptible to attack.
Management
Spraying plants with a strong jet of water can help reduce buildup of spider mite populations; if mites become problematic apply insecticidal soap to plants; certain chemical insecticides may actually increase mite populations by killing off natural enemies and promoting mite reproduction.

Charcoal rot
Macrophomina phaseolina

Symptoms
Water soaked lesions on stems of seedlings close to soil line; lesions girdle stem and kill seedlings; lesions in similar area may be present in older plants; lesions are initially water-soaked but turn brown; if lesions girdle the stem, plant wilts and branches die; infections beginning in the roots cause leaves to turn yellow and wilt and causes stems to be blighted.
Cause
Fugnus
Comments
Fungus survives in crop debris in the soil; disease emergence is favored by high soil temperatures which cause plants to be water stressed and more susceptible to disease; fungus can survive for prolonged periods in dry soils but are killed in wet soil.
Management
Rotating crop with rice for a period of 3-4 years can reduce the level of oioculum in the soil; providing the plants with adequate irrigation and fertilization reduces susceptibility to the disease; there are currently no resistant varieties of peanut; frequent irrigation to wet soil reduces the incidence of the disease.

Verticillium wilt
Verticillium spp.

Symptoms
Chlorosis of leaf margins; curling leaves; loss of leaf turgor; plants wilt and become stunted as the disease progresses; plants wilt rapidly during periods of water stress; vascular system becomes dicolored.
Cause
Fungi
Comments
Fungus can survive in the soil for several years; disease is spread to uninfested fields by movement of infested soil or contaminated tools and machinery; there are no peanut varieties known to have resistance to the disease.
Management
Irrigate plants frequently to reduce wilting of infected plants and allow them to reach maturity; eliminate weeds int he plantation which may allow inoculum to build up in the soil; remove and destroy infected crop residue after harvest to reduce inoculum in the field.

Botrytis blight
Botrytis cinerea

Symptoms
Numerous spots on upper surface of leaflets; entire plant or discrete parts may wilt and die; pods and stems become covered in fungal sclerotia.
Cause
Fungus
Comments
Disease emergence favors high moisture and high temperature; plants damaged by frost or other pathogens are particularly vulnerable to attack.
Management
Avoiding frost damage by planting early peanut varieties can help protect the plant from fungal colonization; application of appropriate foliar fungicides (e.g. benomyl), where available, can help to control the disease.

Cylindrocladium black rot
Cylindrocladium crotalariae

Symptoms
Leaves on main stem turning chlorotic and wilting; entire plant wilts very rapidly when there is a period of water stress following high moisture; clusters of red-brown fungal bodies occur on on stems, pegs and pods; roots destroyed; roots blackened and shriveled.
Cause
Fungus
Comments
Crops planted early are more susceptible to the disease as they are often exposed to cooler temperatures and higher soil moisture; disease is seed transmitted and also can spread over long distances by wind.
Management
The most effective method to control the disease is to plant peanut varieties that have some resistance to the disease; rotation of crop with nonhost such as corn , cotton or tobacco may help to reduce inoculum in the soil; application of appropriate soil fumigants in heavily infested fields can help to control the disease.

Early leaf spot
Cercospora arachidicola

Symptoms
Small chlorotic flecks on leaf petioles, stems and pegs which enlarge and turn dark in color; lesions on upper surface of leaves usually possess a yellow halo and are reddish brown on the underside of leaves.
Cause
Fungus
Comments
Disease emergence is favored by high humidity and warm temperatures; spread of the disease is promoted by pronlonged leaf wetness.
Management
If disease is present, a rotation away from peanut for a period of 2-3 years is advised but is insufficient to control the disease completely; peanut crop debris should be plowed into soil after harvest and any volunteers removed from the nonhost crop; fungicides should be applied with caution as they can exacerbate other foliar diseases where they are present.

Late leaf spot
Cercospora personatum

Symptoms
Small chlorotic flecks on leaf petioles, stems and pegs which enlarge and turn dark in color; symptoms may be very similar or identical to early leaf spot and can only be differentiated by examination of conidia under a microscope.
Cause
Fungus
Comments
Disease emergence is favored by high humidity and warm temperatures; spread of the disease is promoted by pronlonged leaf wetness.
Management
If disease is present, a rotation away from peanut for a period of 2-3 years is advised but is insufficient to control the disease completely; peanut crop depris should be plowed into soil after harvest and any volunteers removed from the nonhost crop; fungicides should be applied with caution as they can exacerbate other foliar diseases where they are present.

Rust
Puccinia arachidis

Symptoms
Characteristic orange pustules on undersides of leaves which become covered in masses of red-brown spores; pustules may form on pods.
Cause
Fungus
Comments
Peanut rust is highly specific to peanut; disease emergence and spread is favored by warm temperatures followed by leaf wetness.
Management
Allow field to fallow for at least one month between successive peanut plantings; remove any volunteer peanut plants during fallowing to reduce inoculum; sprays of appropriate fungicides such as Bordeaux mixture can effectively control the disease; such fungicides are often also effective at controlling leaf spot.

Sclerotinia blight
Sclerotinia minor

Symptoms
Tips of infected branches wilt or flag rapidly; early signs of infection include the presence of small water-soaked lesions at the base of the stems which turn yellow or bleached; leaves on infected branches turn chlorotic and then wither; fluffy white fungal growth may appear on infected tissues during periods of high humidity.
Cause
Fungus
Comments
Fungus can survive for prolonged periods in the soil, even in the absence of peanut; emergence of the disease in the peanut crop is favored by periods of cool weather, moist soil and high humidity.
Management
Plant seeds which are coated with protectants; avoid injuring plants with tools and/or machinery; application of appropriate fungicides can reduce crop losses when disease is present; avoid excessive irrigation during cool weather.

Southern stem rot
Sclerotium rolfsii

Symptoms
Lateral branches or main stem yellowing and wilting; white fungal mat developing on the stem close to the soil line; white to brown spore containing structures developing from the fungal mats.
Cause
Fungus
Comments
Pathogen has a large host range and attacks many crops including sugar beets; disease favors dry soils which crack deeply, allowing the penetration of oxygen.
Management
Plow crop debris deeply into soil after harvest of crop; crop rotation of 3-4 years are very effective at reducing soil inoculum in the case of severe infestations; applications of appropriate fungicides can help suppress stem rot but care should be taken with selection as some pesticides (e.g. benomyl) are known to increase the severity of the the disease.

Web blotch (Phoma leaf spot)
Phoma arachidicola

Symptoms
Circular, brown-black lesions on the upper surfaces of the leave; web or net-like brown lesions on leaves may form on leaves during periods of high humidity; as sisease progresses, lesions darken and develop a rough texture; lesions may cover entire leaf surface.
Cause
Fungus
Comments
Disease emergence is favored by cool, moist conditions and periods of prolonged leaf wetness.
Management
Remove and destroy infested crop debris to prevent build up of inoculum in soil; rotate crops away from peanut; foliar fungicide applications are largely ineffective if conditions are favorable for the disease.

Phyllostica leaf spot
Phyllostica arachidis-hypogaea

Symptoms
Circular lesions with red-brown margins and light brown or tan centers on leaves; centers of lesions may dry out and drop from leaf resulting in a "shot-hole" appearance.
Cause
Fungus
Comments
Phyllostica leaf spot is known to occur in the U.S., India, China, Argentina, Thailand, the Philippenes, Pakistan, Zimbabwe, Niger and Burkino Faso.
Management
Disease is held in check by fungicides applied to control early or late leaf spot.

Thrips (Tobacco thrips)
Frankliniella fusca
Frankliniella occidentalis
Thrips palmi

Symptoms
If population is high leaves may be distorted; leaves are covered in coarse stippling and may appear silvery; leaves speckled with black feces; insect is small (1.5 mm) and slender and best viewed using a hand lens; adult thrips are pale yellow to light brown and the nymphs are smaller and lighter in color.
Cause
Insect
Comments
Thrips transmit viruses to plants, including Tomato spotted wilt virus and Peanut bud necrosis virus.
Management
use adequate plant spacings to avoid overcrowding; provide plants with adequate irrigation and fertilization to encourage the fast establishment of seedlings and growth of a close canopy which is unattractive to thrips vectors.

Armyworms
Spodoptera spp.

Symptoms
Singular, or closely grouped circular to irregularly shaped holes in foliage; heavy feeding by young larvae leads to skeletonized leaves; shallow, dry wounds on fruit; egg clusters of 50-150 eggs may be present on the leaves; egg clusters are covered in a whitish scale which gives the cluster a cottony or fuzzy appearance; young larvae are pale green to yellow in color while older larvae are generally darker green with a dark and light line running along the side of their body and a pink or yellow underside.
Cause
Insect
Comments
Insect can go through 3–5 generations a year.
Management
Organic methods of controlling armyworms include biological control by natural enemies which parasitize the larvae and the application of Bacillus thuringiensis; there are chemicals available for commercial control but many that are available for the home garden do not provide adequate control of the larvae.

Velvetbean Caterpillar
Anticarsia gemmatalis

Symptoms
The larva causes damage to plant by feeding on leaves. The larva after hatching from egg starts feeding on leaves The first and second stage larva skeletonise the leaves by feeding only on soft parts. The later stage laraves consumes entire leaves. They will stripe the field just in four to seven days. The larvae may also feeds on tender stem, buds and pods.
Cause
Insect
Comments
Velvetbean caterpillars feeds on legume (like soybean, kudzu, velvetbean, horse beans, cowpea), cotton and several weeds. Generally moths lay eggs in mass under leaf surface.
Management
Encourage natural predators and parasites in the field; early or late planting of crops helps in escaping the insect attack; grow available resistant varieties; spray biocontrol agents like nuclear polyhedrosis virus (NPV) and the bacterium Bacillus thuringiensis (Bt). If the incidence is severe spray suitable insecticide.

Root knot nematode
Meloidogyne spp

Symptoms
Galls on roots which can be up to 3.3 cm (1 in) in diameter but are usually smaller; reduction in plant vigor; yellowing plants which wilt in hot weather.
Cause
Nematode
Comments
Galls can appear as quickly as a month prior to planting; nematodes prefer sandy soils and damage in areas of field or garden with this type of soil is most likely.
Management
Plant resistant varieties if nematodes are known to be present in the soil ;check roots of plants mid-season or sooner if symptoms indicate nematodes; solarizing soil can reduce nematode populations in the soil and levels of inoculum of many other pathogens.

Stunt
Peanut stunt virus (PSV)

Symptoms
Leaves curled and pale yellow to green; leaf petioles shortened; plants growth severely stunted; plants may produce very few pods if they have been infected early in the growing season.
Cause
Virus
Comments
PSV is transmitted by aphids; legumes such as white clover are the primary source of inoculum.
Management
Avoid planting peanut in close proximity to legumes such as clover; remove any infected plants from plantation to reduce inoculum.

Tomato spotted wilt virus & Peanut bud necrosis
Tomato spotted wilt virus (TSWV)
Peanut bud necrosis virus (PBNV)

Symptoms
Chlorotic spots or mottled patterns on leaves; drooping leaflets during hot weather; necrosis of terminal buds; stunted growth; new leaflets are reduced in size and may be puckered with mosaic mottling.
Cause
Virus
Comments
Both viruses have an extremely wide host range and are transmitted by thrips vectors.
Management
Use high quality seed and use adequate plant spacings to avoid overcrowding; provide plants with adequate irrigation and fertilization to encourage the fast establishment of seedlings and growth of a close canopy which is unattractive to thrips vectors; intercropping peanut with sorghum or millet can reduce disease incidence.