Diseases of Groundnut | Plant Diseases

Here is a list of fourteen major diseases found in groundnut: 1. Groundnut Pod Rots 2. Collar Rot/Crown Rot and Stem Rot 3. Afla Root 4. Crown Rot 5. Pod Rot/Root Rot/Wilt 6. Groundnut Yellow Mold 7. Rust 8. Early and Late leaf Spots 9. Bud Necrosis Virus 10. Groundnut Rosette 11. Peanut Clump 12. Root-Knot Nematode 13. Root-Lesion Nematode 14. Ring Nematode.

Diseases of Groundnut:

1. Groundnut Pod Rots 
2. Collar Rot/Crown Rot and Stem Rot
3. Afla Root 
4. Crown Rot
5. Pod Rot/Root Rot/Wilt
6. Groundnut Yellow Mold
7. Rust
8. Early and Late leaf Spots
9. Bud Necrosis Virus 
10. Groundnut Rosette
11. Peanut Clump
12. Root-Knot Nematode
13. Root-Lesion Nematode
14. Ring Nematode

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Disease # 1. Groundnut Pod Rots:

Most of the pod rots are due to the combined attack of several fungi. Important pathogens of the pod rot complex are Fusarium solani, F. oxysporum, Macrophomina phaseolina, Rhizoctonia solani, Sclerotium rolfsii, and Aspergillus niger.

The occurrence of pod rots may be severe due to attacks of nematodes, termites, and millipedes on pods, thus predisposing the invasion by fungi and bacteria. High levels of soil Nitrogen and phosphorus tend to increase the pod rot.

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Disease # 2. Collar Rot/Crown Rot and Stem Rot (Sclerotium rolfsii):

Host range:

Sclerotium rolfsii, an omnivorous, soilborne fungal pathogen, causes disease on a wide range of agricultural and horticultural crops. Although no worldwide compilation of host genera has been published, over 270 host genera have been reported in the United States alone.

Susceptible agricultural hosts include sweet potato (Ipomea batatas), pumpkin (Cucurbita pepo L.), corn (Zea mays), wheat (Triticum vulgare) and peanut (Arachis hypogea). Horticultural crops affected by the fungus are included in the genera Narcissus, Iris, Lilium, Zinnia, and Chrysanthemum.

Morphological and Diagnostic Features:

Colonies of S. rolfsii are readily distinguished on plant material or artificial media by gross morphological characteristics. Rapidly growing, silky-white hyphae tend to aggregate into rhizomorphic cords.

In culture, the whole area of a Petriplate is rapidly covered with mycelium, including aerial hyphae which may cover the lid of the plate. Both in culture and in plant tissue, a fan-shaped mycelial expanse may be observed growing outward and branching acutely.

Sclerotia (0.5-2.0 mm diameter) begin to develop after 4-7 days of mycelial growth. Initially a felty white appearance, sclerotia quickly melanize to a dark brown coloration.

Four zones in the mature sclerotium:

(i) Thick skin,

(ii) Rind of thickened cells,

(iii) Cortex of thin walled cells, and

(iv) Medulla containing filamentous hyphae.

Sclerotia forming on a host tend to have a smooth texture, whereas those produced in culture may be pitted or folded. Serving as a protective structure, sclerotia contain viable hyphae and serve as primary inoculum for disease development.

Epidemiology:

Sclerotia serve as the principle overwintering structures and primary inoculum for disease. Persisting near the soil surface, sclerotia may exist free in the soil or in association with giant debris. Those buried deep in the soil may survive for a year or less, whereas those at the surface remain viable and may germinate in response to alcohols and other volatiles released from decomposing plant material.

Thus, deep plowing serves as a cultural control tactic by burying sclerotia deep in the soil. High temperatures and moist conditions are associated with germination of sclerotia. High soil moisture, dense planting, and frequent irrigation promote infection.

Since S. rolfsii does not produce spores, dissemination depends on movement of infested soil and infected plant material. Use of contaminated equipment and machinery may spread sclerotia to uninfested fields.

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Disease # 3. Afla Root (Aspergillus flavus):

Introduction:

Aspergillus flavus is responsible for the disease of peanuts and corn commonly known as yellow mould. This disease does not reduce the yield, but the the quality of the produce is very poor. In the early 1960s, aflatoxin, a toxic metabolite of A. flavus was found in peanut meal. Feed prepared with this meal caused the death of 100 000 turkeys in Great Britain.

A very small amount (10-20 ppb) can produce fatal liver cancer in young animals. Yellow mould is more severe in the tropics, with symptoms appearing both early in the growth of the peanut seedlings, and near harvest time on pods and seeds in the soil.

Disease Symptoms:

Symptoms first appear as spots on the cotyledons of the seedlings. Seedlings and un germinated seeds shrivel to become a dried brown to black mass covered by yellow or green spores. Plants that survive germination and emergence appear chlorotic due to the presence of aflatoxin throughout the plant. The roots are stunted and lack a secondary root system, a condition known as aflaroot.

The leaves are small and pointed with a thick and leathery texture. Infected seedlings may survive infection in optimal growing conditions, then yellow mould of peanut pods and seeds may occur, especially in dry conditions. Following harvest, further infections may develop, with fungal growth covering the seed surface and invading the seed itself.

A yellow to brown discolouration, and weight loss occurs as a result. Aspergillus flavus is commonly associated with boll rot of cotton. The fungus stains and weakens the lint fibre. Seed infection results in reduced quality and viability and the production of aflatoxin.

Description of the Pathogen:

Aspergillus flavus produces hyphae that are colourless, septate and branched. A vesicle is borne at the end of each long conidiophore. On this vesicle, rows of sterigmata develop, that bear chains of yellow-green to blue-green conidia.

The conidial heads, each containing a mass of conidial chains, appear as masses up to 600 µm in length and 100 µm in diameter. Sterigmata of A. flavus are borne in single or double series on an elongate to subglobose vesicle. The radiate heads split with maturity. A. flavus may also produce sclerotia.

Host Range:

Yellow mould has been recorded on peanut, corn, cottonseed, coconut, pistachio nuts, and to a lesser extent on soybeans, rice, pecans, walnuts, almond and cassava.

Epidemiology:

The extent of yellow mould damage and aflatoxin production is dependent on the environmental conditions and production, harvesting and storage practices. The pathogen is seedborne and soilborne, and active in high humidity (90-98%) and low soil moisture. Temperatures conducive to growth are 17-42°C with aflatoxin production between 25-35°C.

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Disease # 4. Crown Rot (Aspergillus niger):

Disease Symptoms:

Seedlings and young plants are more susceptible to the pathogen, and in these cases, the most obvious symptom is a sudden wilting of the young peanut plant. Diseased areas of the plant are covered in dark fungal growth. Infection of seedlings commonly occurs soon after germination. The disease progresses rapidly, and most affected plants will die within 30 days of planting.

Description of the Pathogen:

Upon isolation from diseased tissue, the fungus will grow on a variety of media, most optimally at 25°C, producing large black conidial heads (up to 700-800 um in diameter). These clusters of black spore heads can usually be seen without magnification.

Epidemiology:

This fungus is very common in the atmosphere and in the soil and is often found as a saprophyte. Infection will usually occur though damaged tissue. The disease is usually more severe on those crops grown in a continuous rotation of susceptible hosts. Warm, moist conditions favour the growth of this fungus.

Adverse weather conditions, extreme fluctuations in soil moisture, poor seed quality, seedling damage by pesticides and other factors that delay seedling emergence have been shown to increase susceptibility of plants to the pathogen. Older plants are more prone to infection, especially in dry soil. Seeds may be infected causing damping-off, and the fungus is also spread on seed.

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Disease # 5. Pod Rot/Root Rot/Wilt (Rhizoctonia solani):

Rhizoctonia solani, the most widely recognized species of Rhizoctonia is a basidiomycete fungus that does not produce any asexual spores (called conidia) and only occasionally will the fungus produce sexual spores (basidiospores).

In nature, R. solani reproduces asexually and exists primarily as vegetative mycelium and/or sclerotia. Unlike many basidiomycete fungi, the basidiospores are not enclosed in a fleshy, fruiting body or mushroom. The sexual stage of R. solani has undergone several name changes since 1891, but is now known as Thanatephorus cucumeris.

Epidemiology:

Pod rot is a term used to describe the decay of pods in soil. There are several causes of pod decay and discoloration in peanuts and several pathogens are a leading cause of pod rot. Advanced stages of pod rots result in complete decay of the pod and kernels.

Yield losses can be substantial where a large percentage of the pods are affected. The fungi that cause pod rot are normally found at some level in most peanut soils. Likewise, plants with a few rotted pods are found in most fields.

Severe outbreaks of pod rot are less common, but can be devastating where they occur. Pod rot tends to be severe in sandy soils and in fields intensively cropped to peanuts. A complex of factors in addition to the two fungi are probably responsible for these severe outbreaks.

These factors include excessive soil moisture, wide fluctuations in soil moisture, calcium deficiency, insect and nematode feeding, and irrigation with poor quality (salty) water.

R. solani can survive for many years by producing small (1 to 3 mm diameter), irregular-shaped, brown to black structures (called sclerotia) in soil and on plant tissue. Certain rice pathogens of R. solani, have evolved the ability to produce sclerotia with a thick outer layer that allows them to float and survive in water.

R. solani also survives as mycelium by colonizing soil organic matter as a saprophyte, particularly as a result of plant pathogenic activity. Sclerotia and/or mycelium present in soil and/or on plant tissue germinate to produce vegetative threads (hyphae) of the fungus that can attack a wide range of food and fiber crops.

Management:

i. Use cereal-cereal-groundnut crop rotation and seed treatment with thiram at the rate 3 g/kg seeds.

ii. Avoid drought at pod formation and maturity.

iii. Application of soil amendments like gypsum, rice hull, fish meal reduced pod rot and increased groundnut yields.

iv. Avoid heavy irrigation before harvesting. It will lead to pod rot if harvesting is delayed.

v. Tolerant varieties to stem and pod rots: ICGV 87157, or ICGV 86590; Virginia bunch type cultivar VB 186, ICGV 86034, 86124, 86252, 87160 are lower susceptibility pod rot and ICGV 86885 and R 8972.

vi. Virginia bunch type TCG 1525, P1269710, NCAc 38 and 2 spanish type-NCAc 18019 and RR5290 highly resistant to pod rot.

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Disease # 6. Groundnut Yellow Mold:

The yellow mold fungus, Aspergillus flavus, is commonly found in the seed of both rotten and apparently healthy pods of groundnut. Many strains of this fungus are capable of producing aflatoxins that render the seed unacceptable due to high toxicity for human or animal consumption.

Aflatoxin contamination in groundnut can occur in the stems of seedlings, pods and seeds. The fungus is capable of invading groundnut seeds before harvest, during postharvest drying and during storage.

The A. flavus groups of fungi are facultative parasites. They invade plant tissues directly or attack tissues that have been predisposed by environmental stresses such as dry weather or damages caused by insects, nematodes, natural cracking and harvest equipment.

Aflatoxins are carcinogenic and produced by the Aspergillus flavus group of fungi that have been identified as B1, B2, G1, and G2. The maximum aflatoxin level for groundnut acceptable in USA is 20 ppb.

Symptoms:

Yellow mold first appears on groundnut cotyledons after the emergence of seedlings. Necrotic spots become covered with masses of yellow-green spore heads of the A. flavus group of fungi. Fungus toxins are translocated throughout the seedling in the transpiration stream.

Infected plants generally become stunted with symptoms of vein clearing chlorosis on the leaflets. Such seedlings lack a secondary root system, a condition known as “aflaroot” Yellow-green Aspergillus colonies develop on over mature and damaged seeds and pods.

Management:

i. Harvest at proper maturity and discard the wilted and dead plants as such plants are likely to have seeds infected by Aspergillus flavus.

ii. Dry the groundnut pods to 6-8% moisture content immediately after harvesting

iii. Avoid damage to the testa while decorticating. Prevent drought stress, and also prevent water logging (40-80% of field capacity) at late stages of growth.

iv. Manage nematode because they may predispose plants to attack by fungal pathogens.

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Disease # 7. Rust (Puccinia arachidis):

It has now become a disease of major economic importance in almost all the groundnut- growing areas of the world.

Symptoms:

Rust (Puccinia arachidis) is identified by the appearance of orange pustules (uredinia) on the abaxial (lower) surface of leaves and reddish brown urediniospores (uredospores). Symptoms are mainly confined to leaflets but pustules can be seen on all the aerial parts of a plant except the flower.

Brown to dark reddish-brown pustules appear on the lower surface with the upper surface developing yellow, chlorotic spots with necrotic brown areas in the center.

At a late stage, the primary pustules are surrounded by secondary sori. The uredinias are usually circular, 0.3 mm-1.0 mm in diameter, and can develop on all the aerial parts of the plant except flowers and pegs. If rust associated with early and late leaf spots, the loss may be up to 70 per cent.

Epidemiology:

Rust becomes devastating under conditions of high rainfall and humidity. In India, a continuous dry period characterized by high temperature (>26°C) and low relative humidity (<70%) is reported to delay rust occurrence and severity, whereas intermittent rain, high relative humidity and 20 to 26°C temperature favor disease development.

Mayee (1987) observed that average temperature of 20-22°C, relative humidity above 85% and 3 rainy days in a week, if prevailed for 2 weeks, an outbreak of rust is likely in Parbhani region of Maharashtra, India. In the same region, from the long-term observations of rust and weather conditions, guide-lines similar to the above study for outbreak of rust were outlined.

If these conditions prevail for a week, rust outbreak is likely to occur in next 15 days. It is observed that incubation period of Puccinia arachidis causing groundnut rust was prolonged as the mean or maximum temperature increased while it is negatively correlated with relative humidity.

Multiple regression analysis of different environmental factors combinations including rainfall, evaporation rates explained more than 96% of the observed variation in incubation period. The rust epidemic commonly occurs during prolonged dry spell after heavy showers.

In their study on the influence of rainfall, temperature and relative humidity on groundnut leaf rust epidemiology, Lokhande (1998) observed that rainfall of about 200 mm, temperature between 23.5 to 29.4°C and relative humidity in the range 67 to 84% are congenial weather conditions for initiation and development of this disease.

Management:

i. A cereal-cereal-groundnut crop rotation and removal of volunteer groundnut plants from the field will help to check rust inoculum build-up.

ii. Adjust the sowing time to avoid the most conducive environmental condition for rust development (i.e., high humidity, cloudy weather) to help reduce damage caused by rust.

iii. Sprays of dithiocarbamate have been found effective to control rust and late leaf spots. Chlorothalonil 0.2%, hexaconazole at the rate 0.1%, propiconazole at the rate 0.1% spray have been found effective against rust and late leaf spot, when sprayed 30 days after germination till 15 days before harvesting at regular 10-15 day intervals.

However, this schedule could be modified using a suitable disease forecast system based on temperature, humidity, cloudy weather, and rainfall pattern to save the fungicide and reduce the spray cost. Calixin is effective against rust but not against leaf spots, whereas benomyl is effective against leaf spots but not against rust.

iv. The spray combination of carbendazim at the rate 0.1% + Mancozeb at the rate 0.1% effective in reducing leaf rust disease with highest yield 20.34 Q/ha followed by hexaconazole at the rate 0.1%.

v. Grow resistant cultivars: ICGV 87160 or ICGV 86590. The genotypes R-2001-3 and R 8808 showed moderate resistance to rust.

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Disease # 8. Early and Late leaf Spots:

The disease caused by Cercospora arachidicola Hori (Early leaf spot) and Phaeoisariopsis personata (Berk & Curt) V. Arx. (Late leaf spot).

Symptoms:

i. Early Leaf Spot:

It is caused by Cercospora arachidicola Hori. It develops small necrotic flecks, that usually have light to dark-brown centers, and a yellow halo. The spots may range from 1 mm-10 mm in diameter. Sporulation is on the adaxial (upper) surface of leaflets (Fig. 8.4).

ii. Late Leaf Spot:

It is caused by Phaeoisariopsis per sonata (Berk & Curt) V.Arx. It develops small necrotic flecks that enlarge and become light to dark brown. The yellow halo is either absent or less conspicuous in late leaf spot. Sporulation is common on the abaxial (lower) surface of leaves.

Comparisons of early and late leaf spots are in Table 8.3. Due to excessive spotting of leaves there is defoliation and general weakening results in lower yields. The loss varying from 20-50 per cent. (Fig. 8.5).

Disease Cycle:

The fungus survives in the conidial or mycelial state on diseased plant debris and on the shells of the seed. Conidia are disseminated through wind. These germinate by germ tubes and cause infection directly or through stomata.

Epidemiology:

Two weather variables that strongly affect the infection process in late leaf spot are temperature and leaf wetness. The percentage of conidia that germinate is known to decrease with increasing temperature above 20°C and at 20°C it takes about 12 h for 50% of the conidia to germinate. Shew (1988) found slightly more infection at 20°C than at 24°C. and much less at 28°C.

They also found that infection increased with the number of hours that plants were exposed to high relative humidity (RH. >93%) each day for a 6-days period. They wetted the leaves at the start of each daily period of exposure to high RH. Lannou & Blizoua Bi (1989) examined techniques to infect groundnut plants with P. personata and found that the infection process took longer than 6 days.

They also found greater infection efficiency with alternate high and low RH. They observed free water on the leaves in the high Relative Humidity treatment, which suggests that leaf wetness may be an important variable.

Cook (1981) observed germ tube growth of P. personata towards stomata from the first day after inoculation of detached groundnut leaves. Warm and moist weather conditions encourage development of epidemics. Optimum temperature is 24-28°C with a period of 3 days of high humidity. The incubation period is 8-15 days.

Management:

i. A crop rotation of cereal-cereal-groundnut and burying all groundnut crop residues by deep ploughing will reduce initial inoculum. Adjust the date of sowing to avoid conditions favorable for rapid disease development. Sowing date advanced to June or even earlier helps in reducing severity of the leaf spots.

ii. Multiple applications of a fungicide such as benomyl, captafol, chlorothalonil, copper hydroxide, mancozeb, or sulphur fungicides may control early and late leaf spot. However, carbendazim (0.05%) controls both leaf spots very effectively.

iii. Three sprays of 0.2% chlorothalonil at intervals of 10-15 days starting 40 days after germination up to 90 days provides effective control to early and late leaf spots, and rust. The spray combination of carbendazim at the rate 0.1% + Mancozeb at the rate 0.1% effective in reducing leaf rust disease.

iv. The systemic fungicide benomyl at the rate 1 g/lit spray at 7-14 days interval reported to control leaf spots. Carbendazim at the rate 0.1 per cent is better than copper fungicides for control of leaf spots as well as rust in groundnut. Protective fungicides having curative action difenconazole at the rate 0.1% is also effective in controlling leaf spots.

v. Grow cultivars tolerant to late leaf spot: ICGV 87160 or ICGV 86590. The genotypes R-2001-3 and R 8808 showed moderate resistance to leaf spot.

The fungicidal control of diseases is more effective if a disease forecasting system based on temperature and relative humidity during the growing season, as has been developed in Georgia, is used. The fungicides are applied when the temperature and leaf wetness conditions are favorable for disease development.

Indiscriminate application of fungicides to control early and late leaf spots results in non-desirable effects. For example, use of excessive chlorothalonil for control of foliar diseases increases the severity of Sclerotinia blight.

Normally, groundnut leaf spot diseases are scored using the 1 to 9 scale, as described below (Table 8.4).

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Disease # 9. Bud Necrosis Virus:

Bud necrosis disease (BND) is caused by two serologically distinct viruses, bud necrosis virus (BNV) and tomato spotted wilt virus (TSWV). BND was first recorded in Brazil in 1941, and significant crop losses by this disease have been reported from Australia, India, – and the USA.

Virus Transmission:

The virus is transmitted by thrips Frankliniella occidentalis, Thrips pami, which have a wide range of hosts. The virus survives in these hosts and acts as a source of inoculums for the vector. The thrips are carried by wind.

The population of vectors increases rapidly from January-March and August-September Kharif and hence the crop suffers a heavy loss in both the seasons. A prolonged dry spell favours the multiplication of thrips and spread of the virus.

Symptoms:

Initial symptoms are concentric rings or chlorotic spots on young leaflets. Subsequently terminal bud necrosis occurs especially when day temperatures exceed 30°C. Plants infected at early stages are severely stunted. Occasionally, necrosis may spread to the petioles and then to the stem leading to death of the plant.

Later infected plants may only show bud necrosis on a few branches and axillary shoot proliferations may be restricted to the terminal portion. In early infection, pods are seldom produced. In late infections, pod size is reduced, shriveled, and mottled with discolored testa. The virus is not transmitted by seed; it is transmitted by thrips (Fig. 8.6).

Management:

i. Use resistant/tolerant cultivars: ICGS 11, ICGS 44, ICGV 87141, ICGV 87187, ICGV 87119, ICGV 87121, ICGV 87160, ICGV 87157, or ICGV 86590. The genotypes R- 2001-3 and R 8808 showed moderate resistance to Bud necrosis virus disease.

ii. Spraying of 4% coconut leaf, sorghum leaf and 5% neem seed kernel extract 20 and 35 days after transplanting, alone or in combination with 1.25 ml monocrotophos/litre significantly reduce the incidence of bud necrosis disease (BND) and increased the pod yield by 60%.

iii. Control of (thrips) vector. Spray crop with Imidacloprid 17.8 SL at the rate 0.25 ml/ liter or monocrotophos 36 SL at the rate 1.0 ml/liter or Oxydemeton methyl at the rate 1.5 ml/liter at 30 and 40 days after sowing.

iv. Adjust date of sowing to avoid the peak disease incidence.

v. Sow groundnut at a high plant density and maintain a good plant stand.

vi. Intercropping of groundnut with cereals, i.e., groundnut and bajra in 4 : 1 ratio will restrict spread of the virus.

vii. Avoid groundnut cultivation adjacent to the crops that are susceptible to BNV, such as green gram or black gram.

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Disease # 10. Groundnut Rosette:

Three rosette diseases have been recognized. They are “groundnut chlorotic rosette” (GCR), “groundnut green rosette” (GGR), and “groundnut mosaic rosette” (GMR). GCR and GMR are predominant in eastern and southern Africa, whereas GGR appears to be restricted to western Africa (Reddy 1984b). The disease trans ted by aphid vector Aphis craccivora in a semi persistent or circulative manner.

Symptoms:

Groundnut chlorotic rosette (GCR) is characterized by general chlorosis, with a few green islands on young leaflets. Early infected plants are stunted, progressively producing small chlorotic, curled, and puckered leaflets. Older leaflets are bright-yellow with dark- green patches.

Plants infected late, show typical leaf symptoms without the marked stunting and bushy appearance. Groundnut green rosette (GGR) infected plants show mild and narrow chlorotic streaks on young leaflets. The older leaflets are dark green and reduced in size with their margins rolled outward. Early infected plants are stunted and bushy, whereas on late infected plants a proliferation of axillary shoots may be observed.

Management:

i. Several long-duration cultivars with resistance to rosette are currently available. These include RG 1, RMP 12, RMP 91, KH 14-9A, M 25-M 68, and M 69 – M 101. Short duration rosette resistant cultivars are being developed.

ii. Aphis craccivora is mainly responsible for the spread of rosette disease. Spray of endosulfan 4% dust with 1 kg a.i./ha or demeton-s-methyl 72-96 ml a.i/ha provide effective control for aphids. It is essential to know the peak period of aphid migration before application of insecticides.

iii. The eradication of volunteer groundnut plants is helpful to prevent perpetuation of virus inoculum during the off-season.

iv. Early sowing and maintenance of a good plant stand are helpful in reducing the disease incidence.

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Disease # 11. Peanut Clump:

Peanut clump is caused by peanut clump virus (PCV). Early-infected plants do not produce pods. Even in late-infected plants, yield losses of up to 60% have been observed.

Symptoms:

Affected plants are severely stunted, and the new quadrifoliates exhibit mosaic mottling and chlorotic rings. Subsequently produced leaflets are dark green with faint mottling. Infected plants become bushy and produce several flowers. Very few pods are produced on infected plants and the size of pod is reduced.

Management:

i. Avoid sowing virus-infected seed.

ii. Use soil solarization for at least 70 days during summer months.

iii. Use a soil biocide such as carbofuran.

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Disease # 12. Root-Knot Nematode:

The root-knot nematodes (Meloidogyne spp.) are the most important nematode species causing damage ranging from 20% to 90% in infested fields of groundnut. Root galls contain white swollen adult females.

The body tapers anteriorly to a narrow neck and mobile head with stylet, massive median bulb and large esophageal glands. An egg sac often protrudes posteriorly from the female to the exterior of the gall.

It contains several hundred eggs. Often one or more elongate males are present in an egg sac. The females are 0.5 mm to 0.8 mm long. At the center of its posterior region, the female cuticle has a pattern of cuticular markings surrounding the anus and vulva. The second stage of juveniles invade roots at or close to the tip and migrate to the site of differentiating vascular tissues.

Consequently several giant cells formed around the nematodes head. The complete life cycle takes 3 weeks or more, depending on host and temperature. Males average about 1.1 mm in length. The posterior is characteristically twisted through 90 degrees or more. Larvae are about 400 mm long and have a delicate stylet.

Disease Symptoms:

The symptoms of damage caused by Meloidogyne hapla are similar to those caused by M. arenaria. Root-knot nematodes enter and damage groundnut roots, pegs, and pods. Infected plants develop enlarged roots and pegs. Galls develop into various sizes resulting from an internal swelling from the root tissue.

Infected pods develop knobs, protuberances, or small warts. Infected plants with root-knot nematodes may show various degrees of stunting and chlorosis. Root development is reduced, and vascular systems of infected tissues are disrupted, resulting in the poor flow of water and nutrients from the roots (or pegs) to the shoot. Infected plants tend to wilt under drought conditions.

Management:

i. A crop rotation of cereal-cereal-groundnut can significantly decrease the level of root-knot nematode infestation in soils.

ii. Nematicides used in groundnut are fumigant and non-fumigant types with contact or systemic properties. Application of a fumigant nematicide like ethylene dibromide (EDB) is made 18 cm deep at a soil temperature between 15-21°C at the rate 18 or 19 1/ha.

iii. Non fumigant nematicides are aldicarb, carbofuran, and phenamiphos. These nematicides are effective when applied at sowing at the rate 2-3 kg a.i./ha.

iv. The best results are obtained when applications of nematicides are made in a band 17-25 cm wide and incorporated 2-4 cm into the soil.

v. Soil solarization during the hot dry season, also helps to control nematodes.

vi. Grow resistant cultivars: NC 343, NC 3033, NCAC 17090, or ICGS 2.

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Disease # 13. Root-Lesion Nematode:

Root-lesion nematodes (Pratylenchus brachyurus) are small vermiform nematodes. The adults are generally less than 0.5 mm in length. Females have a long slender Stylet with rounded knobs, whereas males usually have less developed or no stylet. Body annulations are fine. The vulva is in the posterior portion of the body. There is a single ovary.

Disease Symptoms:

Lesion nematodes are migratory endoparasites that attack groundnut roots, pegs, and pods. They feed within parenchymatous tissues. Both mechanical and chemical damage result from the nematodes feeding within the tissues. Root lesions develop and with large nematode populations these lesions coalesce, causing extensive discoloration and damage that results in reduced growth and pod production.

The pod lesions begin as tiny, tan to brown, pin-point areas on the surface. As the nematodes feed and reproduce the affected area becomes larger and darker. Old lesions are characterized by their blotchy appearance and indistinct margins.

This is caused by the darker necrotic parenchyma, the outer cells of the pod and the necrotic areas become diffused. Sometime nematodes are established in the roots without visual symptoms above the ground, and cause yield reduction.

Management:

i. Apply carbofuran at the rate a.i. 4-8 kg/ha in the infected rows.

ii. Grow resistant genotypes such as PI 390606, PI 395233, or PI 365553.

iii. Solarize the soil during the hot dry season to control root lesion nematodes. The soil temperature during solarization should rise above 60°C to kill the nematodes as well as soil borne fungi.

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Disease # 14. Ring Nematode:

Disease Symptoms:

Ring nematodes (Macroposthonia xenoplax) are short and thick bodied nematodes. They move slowly and cannot be collected from the soil in Baermann funnels, but may be recovered after centrifugal flotation or by migration from soil in shallow layers. The females are fusiform and have less than 200 annuluses. These are broad, often overlapping, and have smooth, irregular posterior margins of the annulations.

They have a single outstretched ovary. The vulva is located posteriorly, close to the anus. Males have longitudinal incisures in lateral fields and candal alae. These nematodes are ectoparasites, partly embedded in root tissues with a long stylet, reaching well into the root.

Disease Symptoms:

Usually only a large population of nematodes produce symptoms of a chlorotic appearance that have been called “peanut yellows”. Many root primordia and young roots were killed, resulting in a few lateral roots. Pod yields from nematode-infested plants were only 50% of the healthy plant yields.

Management:

i. Follow a crop rotation such as tobacco-maize-groundnut to reduce the population of nematodes.

ii. Fumigants and organophosphate nematicides are effective against ring nematodes.

iii. Solarize the soil during the hot dry season.