INTEGRATED DISEASE MANAGEMENT IN GROUNDNUT

Nearly 90 percent of the groundnut grown in the country is kharif sown from May - June to September – October. India has the distinction of being the largest producer of groundnut in the world. But average yield is very low i.e., 7.5 qt/ha. Groundnut crop is prone attack by numerous diseases to a much larger extent than any other crops. One of the most important factors contributing to low yield is disease attack. More than 55 pathogens including viruses have been reported to affect groundnut. Some diseases are widely distributed and cause economic crop losses while others are restricted in distribution and are not considered to be economically important at present. The diseases which are of minor magnitude today may become major in a while. Among fungal foliar diseases, only a few are economically important in India such as leaf spots (early and late) and rust which are widely distributed can cause losses in susceptible genotypes to the extent of 70-80 percent when both of them occur together. Belatedly, Alternaria alternata leaf spot is becoming increasingly important on rabi/summer crop and also on kharif groundnut crop. Other fungal foliar diseases like anthracnose, leaf scorch, Phomosis leaf spot, Phyllosticta leaf spot, Pestalotiopsis leaf spot, Phoma leaf diseases, Drechslera leaf blight, Zonate leaf spot and Cylindrocladium leaf spot are not economically important at present time. Similarly, seed and soil-borne diseases collar rot; stem rot and dry root rot have been realized as major limitations in crop production. These diseases cause severe seedling mortality resulting in patchy crop stands in sandy loam soils and reduce the yields by 30-40 percent. Several economically important virus diseases like bud necrosis, peanut (groundnut) mottle and peanut clump disease have also started assuming importance in the recent years in India. bud necrosis and Peanut mottle virus (PMV) is a serious  disease of groundnut, wide spread with wide host range and can cause severe yield losses ranging up to 60 percent. Reports on diseases caused by nematodes in groundnut are very few in India. the root knot nematodes have been reported to cause damage in various parts of the country.

Early and late leaf spot:

The leaf spots are also known as tikka disease incited by two species of fungus Cercospora arachidicola and Cercopora (Phaeoisariopsis) personata pathogen. Small dark brown circular spots appear on the leaves. In case of severe attacks, defoliation occurs and only the stem remains. The yield of susceptible varieties is substantially reduced. The fungi survive for a long period in the infected plant debris, pods and seeds as conidia, dormant mycelium and perthecia in soil. The volunteer groundnut plants also harbour the pathogens.

Symptom: The disease occurs on all above ground parts of the plant, more severely on the leaves. Both the fungi produce oval to elongate lesions also on petiole, stem and pegs. The lesions caused by both species coalesce as infection widens and severely spotted leaves shed prematurely. Lesions are sub circular in shape and from 1 to 10 mm in diameter. They are dark brown in colour on the upper leaflet surface where most sporulation occurs and a light brown on the lower leaflet surface. The leaf symptoms produced by the pathogens can be easily distinguished as the early leaf spot are circular to irregular, light to dark brown centre with distinct yellowish halo. These spots are bigger in size compared to late leaf spots. Infection starts about one month after sowing. Small chlorotic spots appear on leaflets, with time they enlarge turn brown to black and assume sub circular shape on upper leaf surface. On lower surface of leaves, light brown colouration is seen. Lesions also appear on petioles, stems, stipules and in severe cases several lesions coalesce and results in premature senescence. In late leaf spot infection starts around 55-60 days after sowing in kharif and 40-45 days after sowing in rabi. These spots are dark brown to black which are nearly circular and appears on the both leaf surfaces. Lesions rough in appearance and halo is not so distinct. In severe cases, disease leaflets become chlorotic, then necrotic lesions coalesce resulting in premature senescence and shedding of the leaflets. Prolonged high relative humidity for 3 days, low temperature (20 degrees) with dew on the leaf surface, heavy doses of nitrogen and phosphorous fertilizers and deficiency of magnesium in soil favours the disease.

Management:

• Removal of volunteer groundnut plants and destruction of infected crop debris is important in reducing the primary source of infection.
• Crop rotation is of primary importance in avoiding early season infection.
• Keep weeds under control and grow groundnut and pearl millet in 7:1 ratio.
• Time of sowing and plant spacing are important considerations.
• Seed treatment with Carbendazim or Thiram @22g/kg of seed.
• The early and late leaf spots are effectively controlled by the 2-3 spray application of Carbendazim 1gm or Mancozeb 2gm or Chlorothalanil 2g, hexaconazole 2 ml/litre of water at 15 days interval starting from 4-5 weeks after planting.

Groundnut Rust

Rust:

The casual organism of rust disease is puccinia arachidis. The disease attacks all aerial parts of the plant. High relative humidity (above 85 percent), heavy rainfall and low temperature (20-25 degrees) favours disease.

Symptoms: Rust can be readily recognized as orange red coloured pustules (uredinia) usually circular and ranges from 0.5 to 1.4 mm in diameter and appears on the lower leaflet surface. The epidermis ruptures and exposes powdery masses of reddish brown uredospores. Pustules first appear on the lower surface and in highly susceptible cultivars; the original pustules may be surrounded by colonies of secondary pustules. The rust pustules may also appear as small, necrotic, brown spots appear on the surface of the leaflet. They may be formed on all aerial plant parts apart from flowers and pegs. Late in the season, brown teliosori, as dark pustules appear among the necrotic patches. Severely infected leaves turn necrotic and desiccate but are attached to the plant. The disease is usually found when the plants are about 6 weeks old. In severe infection, lower leaves dry and drop prematurely which also leads to production of small shriveled seeds.

Management:

• Crop rotation and field sanitation.
• Early sowing in the first fort night of June to avoid disease incidence.
• Strict plant quarantine regulations should be enforced to avoid the spread of rust on the pods or seeds to disease free areas.
• Destroy volunteer, self sown groundnut plants, reservoir hosts and debris to reduce the primary source of inoculums.
• Intercropping of pearl millet or sorghum with groundnut (1:3) is useful in reducing the intensity of rust.
• Foliar application of aqueous neem extract @2-5 percent is useful and economical for the control of rust.
• 3-4 Spray of Tridemorph 2gm/lit of water at 15-20 days interval gives good management of rust.
• Spray application of Chlorothalanil 2g/lit or Mancozeb 2g/lit or Wettable sulphur 2g/lit of water on 35 and 50 days after sowing.

Alternaria leaf spot

Leaf spot disease is caused by Alternaria arachidis, A. alternate and A. tenuissima pathogen.

Symptoms: Lesions produced by A. arachidis are brown in colour and irregular in shape surrounded by yellowish halos. Symptoms produced by A. tenuissima are characterized by blighting of apical portions of leaflets which turn light to dark brown colour. In the later stages of infection, blighted leaves curl inward and become brittle. Lesions produced by A. alternata are small, chlorotic, water soaked and spread over the surface of the leaf. The lesions become necrotic and brown, found irregular in shape. Veins and vein lets adjacent to the lesions become necrotic. Affected leaves show chlorosis and in severe attacks become prematurely and senescent. Lesions coalesce and their central portions become pale, rapidly dry out followed by ragged and blighted appearance.

Management:

• Use healthy certified seeds.
• Deep summer ploughing and removal plant debris is helpful in reducing disease incidence.
• Foliar spray of Copper oxychloride and Mancozeb 2.0 g/litre water are effective in controlling the disease.

Stem rot/Sclerotium wilt:

The disease is incited by Sclerotium rolfsii

Symptoms: The first symptoms are development of white fungal threads over affected plant tissue particularly on stem which results in sudden wilting of a branch which is completely or partially in contact with the soil. The leaves turn brown and wilt but remain attached to the plant. As the disease advances, a white mycelium web spreads over soil and basal canopy of the plant. The white sclerotia of mustard seed size produced in the infected tissues which later turn to brown colour as the disease develops and spreads. The entire plant may be killed or only two or three branches may be affected. Infections of pegs can take place independent together with stem. Lesions on the developing pegs can retard pod development and seeds in the infected pods show a characteristic bluish-grey discolouration.

Management:

• Cultural practices such as deep burial of organic matter, plant debris before sowing is particularly useful in reducing the sclerotium wilt.
• Deep ploughing and cultivation of groundnut in flat or slightly raised beds is helpful.
• Crop rotation with wheat, corn and soya bean may minimize the incidence of stem rot.
• Soil application of castor cake or neem cake @ 10q/ha.
• Seed treatment with Carbendizim/Thiram/Capton @2-3 g/kg seed is effective.
• Seed treatment with 4gm Trichoderma viride formulation followed by application of 2.5 kg Trichoderma viride formulation mixed with 100kg farm yard manure before sowing is recommended.

 Collar rot or seedling blight or crown rot:

The disease is caused by Aspergillus niger and A. pulverulentum. The fungus is both seed-borne and soil-borne and so the infection can be seen at any stage from sowing onwards. Deep sowing of seeds, high soil temperature (30-35 degrees) and low soil moisture is congenial for disease development.

Symptoms: The disease usually cause damage within one month of sowing and appears in three phases. The seed can be attacked at any time after its sowing.

Pre-emergence rot: Seeds may be killed in pre-emergence rotting attacked by soil borne conidia and rotting of seeds prevent the seeds to germinate. Seed are covered with black masses of spores and internal tissues of seed become soft and watery.

Post-emergence rot: Post-emergence infection causes the death and rapid decay of seedlings. The pathogen attacks the emerging young seedling and cause circular brown spots on the cotyledons. The symptoms spread later to the hypocotyls and stem. Brown discoloured spots appear on collar region. The affected portion become soft and rotten, resulting in the collapse of the seedling. The collar region is covered by profuse growth of the fungus and conidia affected stem also show shedding symptom.

Crown rot: The infection when occurs in adult plants show crown rot symptoms. Large lesions develop on the stem below the soil and spread upwards along the branches causing drooping of leaves, wilting and death of the plants.

Management

• Deep sowing of seed should be avoided as etiolated hypocotyls are prone to infection
• Avoiding mechanical damage, destroying plant debris, deep ploughing and rotation of groundnut with gram and wheat is useful in reducing the collar rot disease incidence.
• Seed treatment with Trichoderma viride/T. harizanum @ 4g/kg seed and soil application of Trichoderma @25kg/ha preferably in combination with organic amendments such as castor cake or neem cake or mustard cake @ 500kg/ha.
• Treat the seeds before sowing with Thiram 75% WP @5g/kg seed or Captan 80% WP 3g/kg seed or Mancozeb 75% @ 3g/kg or Carbendazim 50% WP @ 2g/kg of kernels control the seed borne infections.
• Spray Mancozeb 3gm/lit of water immediately when disease is noticed.

 Dry root rot/dry wilt:

The causal organisms of the disease are Macrophomina phaseolina and Rhizoctonia bataticola. Fungus remains dormant as sclerotia for a long period in the soil and in infected plant debris. The primary infection is through soil borne and seed borne sclerotia. The secondary spread of sclerotia is aided by irrigation water, human agency, implements, cattle etc. prolonged rainy season at seedling stage and low lying areas have more disease.

Symptoms: The disease may appear at any stage of the crop growth. Water soaked necrotic spots appear on the stem just above the ground level. In the early stages of infection reddish brown lesions appears on the stem just above the soil level. The lesions darken as the infection spreads upwards to the aerial parts and down into the roots. Roots, pegs and pods also rot and become covered with sclerotia. When infection spreads to underground roots, the sclerotia are formed externally as well as internally in the rotten tissue. Roots commonly attacked in the association with stem rots and wilt. The symptoms of the leaf infection are characterized by marginal zonate and irregular spots. The leaves and branches show drooping, leading to death of the whole plant. Pod infection leads to blackening of the shells and sclerotia can be seen inside the shells. The kernels turn black with abundant sclerotia internally and externally on the testas and shells.

Management:

• Good Agricultural Practices (GAP) such as balanced fertilization, timely irrigation and pest management encourage good crop growth which may help in reducing the disease.
• Seed treatment with spores and mycellial fragments of Trichoderma viride has been shown to prevent invasion by M. phaseolina.
• Seed treatment with Carbendazim 2g/kg seed or Captan 3g/kg seed or Thiram @ 4g/kg seed is most useful.
• Spot drench with Carbendazim at 0.5 g/lit effective in managing the disease.
• PCNB (Brassicol & 75% WP) 0.5% can also be applied or in the form of soil dust 25 kg/ha in two split applications, 12.5 kg/ha before sowing and the rest 12.5 kg/ha at 15 days after first application.

Peanut stem and bud necrosis or bud rot or bud blight

The disease is caused by tomato spotted wilt virus, Peanut spotted wilt or groundnut ring mosaic virus. The virus perpetuates in the weed hosts viz., Parthenium hysterophorus, Erigon bonariensis, Commelina bengalensis, Tagetes minuta and Trifolium subterraneum. It is transmitted by thrips viz., Thrips tabaci and Frankliniella sp. And survives in the hosts of insects which act as vector. The population of vectors increases rapidly from January – March and August – September and hence the crop suffers a heavy loss in both the seasons.

Symptoms: First symptoms are visible 2-6 weeks after infection as ring spots on leaves. The newly emerging leaves are small, rounded or pinched inwards and rugose with varying patterns of mottling and minute ring spots. Necrotic spots and irregularly shaped lesions develop on leaves and petioles. Stem also exhibit necrotic streaks. As the plant mature, it becomes generally stunted with short internodes and short auxiliary shoots. Leaf lets formed on these auxiliary shoots show a wide range of symptoms including reduction in size, distortion of the lamina, mosaic mottling and general chlorosis. In advanced conditions, the necrosis of bud occurs. Drastic reduction in flowering is noticed and seeds are produced are abnormally small and wrinkled with the dark black lesions on the testas.

Management:

• Early sown crop during kharif and rabi/summer seasons is less infected due to checked movement of migrant thrips.
• Optimum plant population should be maintained.
• Rogue out bud necrosis affected plant s up to 6 weeks after sowing.
• Tolerant varieties like ICGS 11, ICGS 44 and R8808 should be used.
• Seed treatment with Chloropyriphos @6ml per kg seed followed by Mancozeb @ 3grams or Carbendazim @ 1gram per kg seed.
• Sowing should be done at optimum soil moisture level.
• Intercropping of groundnut with castor or bajra or jowar in 7:1 ratio.
• Spray Imidachloprid 0.33 ml or Dimethiate 1 ml and neem oil 2 ml/ litre water mixed with surfactant at 20 days after sowing.

 Root-knot nematodes

The root knot nematode Meloidogyne arenaria, M.hapla, M.javanica is a minute round worm which cause typical gall on the roots of groundnut.

Symptom: Groundnut plants infected with the groundnut root knot nematode commonly develop enlarged roots and pegs which widen into galls of various sizes. It damages the plants by devitalizing root tip and causing formation of swelling in roots. The above ground symptoms are reduced growth and small pale green or yellowish leaves, pretending to wilt and generally appears as clearly defined patches in the field. Symptoms also associated with the infection are premature leaf fall, wilting, decline in production and loss in field. Pods also become infected and develop knobs, protuberances, or small warts. Plants infected with root knot nematodes may show various degrees of stunting and chlorosis and usually linger throughout the growing season, seldom killing the plant prematurely.

Management:

• Deep ploughing during hot weather, soil solarization and flooding can significantly decrease levels of infestation of root-knot nematodes in soils.
• Rotation of groundnut with non host crop like pearl millet or sorghum, wheat, corn etc provide satisfactory level of control.
• Non fumigant systemic nematicides like Carbofuran 3G @ 3g/m, aldicarb snd phenamiphos (Nemacur) are most effective when applied in furrows during sowing, at the rates of 2-3 kg of active ingredient per hectare.

References:

Ashish kumar and C.R. Prajapati Krishi Vignan Kendra, Nagina (Bijnor) and Krishi Vignan Kendra, Baghpat.

S. V. Patel University of Agriculture and Technology, Modipuram, Meerut (UP).