Redgram

Disease Management

Introduction

• Pigeonpea can be attacked by more than 100 pathogens.
• These include fungi, bacteria, viruses, nematodes, and mycoplasma-like organisms.
• Fortunately, only a few of them cause economic losses and the distribution of the most important diseases is geographically restricted.
• At present farmers mainly grow pigeonpea landraces and it is possible that they have some degree of tolerance to most of the pathogens.
• This situation could change once the diverse landraces are replaced by a few improved cultivars.
• The diseases of considerable economic importance at present are sterility mosaic (SM), fusarium wilt, phytophthora blight (PB), macrophomina root rot and stem canker, alternaria blight on the Indian subcontinent; wilt and cercospora leaf spot in eastern Africa; and witches' broom.
• Sterility mosaic, the most important disease on the Indian subcontinent. More work has been done on SM, wilt, and PB than on other diseases.

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Alternaria Leaf Spot

• This leaf spot disease is reported only from India where A. alternata has also been reported to cause a similar leaf spot.
• Initially small necrotic spots appear on the leaves, and these gradually increase in size to characteristic lesions with dark and light brown concentric rings with a wary outline and purple margin.
• As infection progresses, the lesions enlarge and coalesce.
• The disease is mostly confined to older leaves in adult plants, but may infect new leaves of young plants, particularly in the postrainy-season crop. Resistant cultivars such as ICPL 366 and DA 2 are now available.

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Bacterial Leaf Spot And Stem Canker

Xanthomonas campestris pv. cajanit

• The disease was first reported from India in 1950 by Kulkarni et al.
• It has also been reported from Australia, India, Panama, Puerto Rico, and Sudan.
• The disease occurs in most years, but appears to cause losses only in certain seasons.
• Symptoms on leaves are characterized by the appearance of minute, brown lesions surrounded by a yellow halo.
• These lesions often coalesce and form larger ones.
• On the main stem and branches, rough, cankerous dark brown lesions of various shapes and size appear.
• In the case of severe infections, branches may dry prematurely or break at the infection site.
• The pathogen is specific to pigeonpea and is seedborne.
• It is possible to control the disease in the field with sprays of Streptocycline @ (streptomycin and tetracycline -100 ppm) repeated at 10-day intervals.
• It may be possible to erradicate the pathogen through seed treatment with appropriate antibiotics.
• Genotypic differences in susceptibility have been reported.

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Collar Rot

• The disease, also called southern blight in the Caribbean region, has been reported to occur in India, Puerto Rico, Trinidad, USA, and Venezuela.
• The disease incidence is usually observed at the seedling stage.
• It causes substantial seedling mortality within 45 DAs in situations where pigeonpea is sown in warm weather soon after a preceding cereal crop; and particularly when the crop stubble remains close to the soil surface.
• The pathogen finds an excellent substrate in undecomposed stubble, and emerging pigeonpea seedlings show mortality due to attack by the pathogen.
• Disease incidence can be reduced if the previous crop stubble is buried deep, and is allowed to decompose well before pigeonpea is sown.
• Seed dressing with fungicides such as tolclofos-methyl should also reduce seedling mortality.
• Genotypic differences in susceptibility have been observed.

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Dry Root Rot

Rhizoctonia bataticola, Macrophomina phaseolina

• The disease has been reported to occur in India, Jamaica, and Trinidad.
• It was first reported from India by Ash (1927). Typical symptoms include root and basal stem rot with a large number of minute, fungal sclerotia visible under the bark.
• Plants dry prematurely, particu- larly when they face drought stress. Infection of seedlings and leaf infection has been reported from India.
• It was observed that disease incidence to be severe in off-season, irrigated, summer crops in several parts of India; however, the disease is usually a minor one in the normal-season crop.
• The pathogen is both soil- and seed borne.
• Seedling infection can be reduced by seed dressing with fungicides such as benomyl, thiram.

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Fusarium Wilt

Fusarium udum

• This is the most important soilborne disease of pigeonpea and was first described in 1906 from Bihar State, India.
• The disease appears in young, June-sown seedlings in August but the highest mortality occurs at flowering and podding time from November onwards.
• Although the disease first appears in patches in a field it can extend to the entire field if pigeonpea is repeatedly cultivated in the same field.
• The fungus can be isolated from apparently healthy 15-day old plants from a wilt-sick plot.
• Even though plants are infected at an early stage, they seem able to "keep fighting" with the fungus until flowering and podding.
• The yield loss depends on the stage at which the plants wilt; it can approach 100% when wilt occurs at the pre-pod stage, about 67% when wilt occurs at maturity, and 30% when it occurs at the pre-harvest stages.
• The loss in plant stand due to early wilt is compensated to some extent by neighbouring plants' ability to produce more biomass.
• Wilt incidence generally increases when the crop is ratooned or retained as a perennial.
• The annual pigeonpea crop loss due to wilt in India alone has been estimated at US $ 36 million.

Symptoms Of Diseases

• Symptoms can appear 4 to 6 weeks after sowing. The initial visible symptoms are loss of turgidity in leaves, and slight interveinal clearing.
• The foliage shows slight chlorosis and sometimes becomes bright yellow before wilting.
• Leaves are retained on wilted plants. The initial characteristic internal symptom of wilt is the browning of the xylem vessels from the root system to the stems.
• The xylem gradually develops black streaks, and brown or dark purple bands appear on the stem surface of partially wilted plants extending upwards from the base.
• When the bark of such bands is peeled off, browning or blackening of the wood beneath can be seen. in wilt-tolerant genotypes these bands are confined to the basal part of the plant.
• Sometimes, especially in the later stages of crop growth, the branches dry from the top downwards, but symptoms are not seen on the lower portions of the main stem or branches.
• Small branches on the lower part of the plant also dry.
• When the main stem of such plants is split open, intensive blackening of the xylem can be seen.
• In humid weather, a pinkish mycelial growth is commonly observed the basal portions of the wilted plants.
• Partial wilting is usually associated with lateral root infection. Tap root infection results in complete wilting.

Epidemiology

• The fungus is soilborne on diseased plant debris and it survives only on the tissues which it colonizes as a parasite.
• Fungus spreads abort 3 m through the soil in one season, apparently along roots.
• The amount of wilt incidence appeared to be influenced by the retentive nature of the soil, but not directly by its water content.
• Low soil temperature and increasing plant maturity favoured wilt.
• Fungal population was highest at 30% soil water-holding capacity and at the soil temperatures between 20° and 30° C.
• The fungus was found to survive in infected plant stubble for 2.5 in Vertisols and 3 years in Alfisols.

Control Measures

Cultural Practices

• Being a soilborne disease, any farming practice or cultural operation that reduces the soil population of f: udum should help to reduce wilt incidence.
• Application of super- phosphate increased wilt incidence and green manuring with Crotolaria juncea decreased it.
• The amount of plant death was somewhat less in plots that received heavy applications of nitrogen as farmyard manure.
• Superphosphate and green manure together increased wilt incidence.
• Zinc retarded colonization of pigeonpea Fusarium, and hastened the disappearance of the fungus from soil (Sarojini, 1950).
• Reported reduction in incidence of wilt from 64 to 38% in a susceptible variety when it was intercropped with sorghum.
• The residual effect of this intercropping was also observed in the second season.
• Mixed cultivation of pigeonpea and sorghum also reduced wilt mixed cropping of sorghum grown for grain rather than for fodder reduced wilt iflcidence in pigeonpea.
• Rotation with sorghum, tobacco, or fallow for 1-2 years; intercropping with sorghum reduced wilt incidence in a sick plot.
• Sowing susceptible cultivars in the postrainy season resulted in much lower disease incidence than when the same cultivars were sown earlier in the year.

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Nematodes

• Sixty-five species in 24 genera of nematodes from 24 countries have been found associated with pigeonpea roots.
• Of these, root-knot, lesion, cyst, reniform, and spiral nematodes are considered important. In India, cyst, reniform, and root-knot nematodes are important.

Disease Characteristics

• Close examination of the roots of 30- to 35-day-old infected plants reveals minute pearly white bodies that are females of H. cajani.
• These females gradually mature and turn brown; most of them are dislodged from the roots when the plants are lifted for examination.

Host Range

• The nematode is mainly confined to plant species in the Leguminosae family.
• Cajanus platycarpus, C. crassus var. crassus, Cicer arietinum, Cyamopsis tetragonoloba, Dolichos lablab, Dunbaria ferruginea, Flemingia strobilifera, Glycine max, Phaseolus aconitifolius, P. atropurpureus, P. aureus, P. calcaratus, P. lathyroides, P. lunatus, P. mungo, P. vulgaris, Pisum sativum, Rhynchosia bracteata, R. cana, R. densifiora, and Vicia sativa have all been reported as hosts.
• Sesamum indicum (family Pedaliaceae) is the only non-legume host.

Disease Cycle

• Infective second-stage juveniles randomly penetrate the tap roots and lateral roots reaching the vascular tissue within 72 h.
• They place their heads adjacent to the stele, and begin to feed and swell.
• Cells near the feeding site become angular with thickend walls, and giant cells are formed containing dense granular cytoplasm with four to five nuclei.
• The nematode gradually passes through its third and fourth stages, and becomes an adult female.
• The female enlarges in size, damages the cortex, and erupts from the epidermis.
• Nematode parasitism results in widespread rupturing and discontinuity of the xylem vessels.
• Juveniles which establish in the cortex develop into males, and those which feed in the stelar region develop into females.
• The adult male matures in 10 days, while swollen, lemon-shaped females are formed after 12 days. Males are encountered in large numbers; but females can reproduce in the absence of males.
• Fifteen days after penetration, infective juveniles can be seen in the soil.
• Eggs are deposited in egg-sacs, and also within the female body which gradually transforms from white to a brown coloured protective cyst.
• A female produces 100 to 300 eggs, depending on the health of the host plant.
• The egg-sac generally contains one-third of the total eggs produced.

Control

• Rotation with cereals such as sorghum, maize, or pearl millet will help to reduce nematode population densities. Chionachne sp, Echinocloa colona, Paspalum scorbiculatum, Setaria italica, Trilobachne sp, Zea mays, and Z. mexicana are reported as non-hosts of H. cajani.
• Solarizing soil by covering it with transparent polythene sheets during the summer months significantly reduces the population densities of H. cajani in Vertisols.
• Irrigation prior to covering soil with polythene significantly improves the effects of solarization.
• The use of a bacterium, Pasteuria penetrans, appears to be promising in cbntrolling H. cajani.
• Aldicarb Carbofuran , fensulfothion, phorate are effective in reducing H. cajani popu- lations in the soil. These chemicals also reduce the populations of R. reniformis and Meloidogyne spp.

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Reniform Nematode

Rotylenchulus reniformis

• This nematode is found in 38 countries, primarily in tropical and subtropical regions of the world.
• It severely affects crop production in Fiji, where pigeonpea is a major subsistence and cash-earning pulse in the drier zones.

Disease Characteristics

• The most common below-ground symptom of nematode infection is the presence of soil-covered egg masses on the roots.
• The root-masses of infected plants are smaller than those of non-infected plants.

Disease Cycle

• This nematode has the unique ability to develop to the pre-adult infective stage through a series of three moults without feeding.
• Egg masses of R. reniformis contain up to 150 eggs.
• The nematode prefers to penetrate roots in the zone of elongation.
• The immature female feeds serni-endoparasit- ically, with the anterior one-third of the body inside the root.
• Heavy infection causes severe damage to the'epidermis and cortex, and females establish feeding sites in the phloem cells.
• The female begins to enlarge on the ventral side around the vulval region, and continues to swell to become reniform in shape.
• Males are usually found close to female feeding sites.
• The reniform nematode is generally considered to be bisexual, with a sex ratio of 1:1, and reproduces by cross fertilization.
• The life cycle is completed in 24-29 days in females, and 16-20 days in males.
• Rotylenchulus reniformis can survive without any host for more than 300 days without losing its infectivity.

Control

• Application of dibromochloropropane (DBCP) (50 L ha-l), metham sodium (250 L ha-1, copper oxychloride (50 kg ha-1, dirnethoate, monocrotophos, aldicarb, thionazin (4 to 16 kg ha-1, phenamiphos (10 kg ai ha-1, and ethoprophos and oxamyl (2500 ppm foliar spray) have all been reported to effectively control R. reniformis.
• Rotation of pigeonpea with rice or maize has been found to effectively check the nematode population build up in Fiji. Pigeonpea genotypes ICP 12744, Basant, PDM 1, Norman, AGS 522, GAUT 82-75, GAUT 83-23, and GAUT 84-22 have been reported as resistant in pot screening tests.
• However, the reaction of these genotypes in field conditions awaits confirmation.

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Root-Knot Nematode

Meloidogyne spp

• Five species of Meloidogyne are known to attack pigeonpea.
• These are M. incognita M. javanica, M. arenaria, M. hapla, and M. acronea.
• The first two species are the most important because of their wider distribution in pigeonpea-growing regions of the world.
• Meloidogyne incognita and M. javanica are reported on pigeonpea in India.
• These are hot-weather organisms, and are important in regions where summers are long and winters are short and mild.
• Pigeonpea yield losses due to the root-knot nematodes are estimated at 8-35%.

Disease Characteristics

• The above-ground symptoms of Meloidogyne spp infection are stunting, suppressed growth, chlorosis, reduction in leaf size and generally reduced plant vigour.
• Production of root-knots (galls) on the root system is the most characteristic symptom of root-knot nematode attack.
• The size and shape of the galls vary.

Disease Cycle

• The one-celled egg passes through embryogenesis, resulting in a first-stage juvenile within the egg.
• The first moult takes place inside the egg, and the infective second-stage juvenile hatches out of the egg shell.
• The juvenile penetrates the roots and migrates through the root cells to reach the vascular system where it starts feeding.
• The feeding cells are called giant cells. The second-stage juvenile begins to swell and moult.
• Third and fourth stage juveniles do not posses a stylet but this reappears when the nematode undergoes its final moult. The nematode remains sedentary during feeding.
• The male is a sedentary parasite only during its juvenile development, and emerges as a slender worm possessing a stylet, oesophagous with a median bulb, spicules, and sperms in the testes.
• The male is generally not involved in reproduction.
• Adult females extrude a gelatinous matrix into which 200 to 500 eggs are deposited. The total duration of the life cycle under optimum conditions (25 to 30° C temperature) is 3 to 4 weeks.

Control

• Pigeonpea - wheat - cropping has been found to check nematode population - Saveral lines a pigeonpea resistant to meloidogyne spp were repeated as available.

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Phoma Stem Canker

Phoma cajani

• This disease, reported from Brazil and India generally occurs in adult plants, and is characterized by the appearance of brown, cankerous lesions on the stem.
• These lesions, that have grey centres and dark brown margins, may coalesce and girdle the stem.
• Lesioned portions often develop swellings. Numerous pycnidia are seen in the lesions.
• Affected branches dry prematurely.
• Leaves are also infected by the fungus.
• The pathogen survives on dead crop debris, but is not seedborne.
• Some degree of host resistance has been reported.
• Sanitary practices should help in managing the disease.

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Phytophthora Blight

Phytophthora drechleri f.sp. cajani

• A recently recognised disease of pigeonpea, phytophthora blight (PB) was first suspected at New Delhi in India in 1966.
• Surveys in India between 1975 and 1980 indicated PB to be widespread with an average incidence of 2.6%, next only to SM and fusarium wilt in diseases occuring on pigeonpea. Its incidence was very high (26.30%) in West Bengal.
• At ICRISAT Center PB incidence was observed to be relatively high in short-duration pigeonpeas compared to that in medium- and long-duration types.
• The close spacing used for short-duration types could favour blight development.
• Phytophthora blight is more important in short-duration types as the loss in stand due to this disease drastically reduces yields, because these types have neither time nor plasticity to compensate for lost plants in the way that medlum- and long-duration types can.

Symptoms

• Pigeonpea seedlings become infected with PB as soon as they emerge.
• Young seedlings are killed within 3 days, and may go unnoticed.
• The seedlings show crown rot symptoms, topple over, and dry. When the seedlings are older; i.e., about 1 month old, symptoms first appear as water-soaked lesions on the primary and trifoliolate leaves which become necrotic within 5 days.
• The leaflet lesions are circular to irregular in shape and can be as large as 1 cm in diameter.
• The whole foliage can become blighted within a week. Stem symptoms usually appear later on the main stem, branches, and petioles as brown to dark brown lesions, distinctly different from the healthy green portions.
• Infected stems break easily in the wind.
• Stem lesions initially have a plane surface which later becomes depressed.
• It is also common to find stems swollen into cankerous structures at the edges of the lesions; this usually happens in plants that are infected but not dried.
• The lesioned areas sometimes develop cracks and shred. Stem lesions are often centred on a leaf scar, and extend in each direction from the apparent invasion site.

Epidemiology

• High humidity helps the rapid development of Phytophthora Blight related high disease incidence was mptoced imder poor soil surface drainage.
• The disease incidence has in high low-lying areas of fields where temporary water stagnation occurs after heavy rains.
• The disease incidence was relatively higher in Alfisols than Vertisols.
• How the pathogen perpetuates from one season to another is not very clear.
• In the absence of potassium (K), high doses of nitrogen (N) increased PB incidence.
• Addition of K decreased disease incidence regardless of the presence of N or phosphorous (P) in the soil.
• ICRISAT Center indicated that disease development was faster when day and night temperatures were more or less the same, i.e., ranging between 20 and 25° C, the weather was cloudy, and relative humidity. was between 70 and 80% .

Control Measures

• Grow resistant/tolerent varieties.

Fungicides

• Studies on the control of PB using fungicides are very limited. Brestan-60, the best when applied before inoculation. Seed treatment with metalaxyl {4 ai./kg-l seed) controlled up to 35% incidence but only in the initial stages.

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Powdery Mildew

Oidiopsis taurica

• The disease has been reported from several countries including; Ethiopia, India, Kenya, Malawi, Tanzania, Uganda, and Zambia.
• Although powdery mildew symptoms appear more often on old leaves, young leaves can also be infected under favourable weather conditions.
• In cases of severe infection, affected leaves turn yellow and show twisting and crinkling.
• The host range of the pathogen is very wide. and the inoculum is always present in pigeonpea-growing, semi-arid regions.
• The disease has rarely been reported to cause severe losses.

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Rust

Uredo cajani

• Rust has been reported from many countries including Bermuda, Colombia, Guatemala, India, Jamaica, Kenya, Nigeria, Puerto Rico, Sierra Leone, Tanzania, Trinidad, Uganda, and Venezuela.
• Even though the disease is observed in many countries, it has rarely been reported to cause severe losses.
• The leaves show characteristic dark brown, uredial pustules, and consequent leaf-drop is common.
• Host resistance is available.

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Sterility Mosaic

• Sterility mosaic (SM) is the most important of pigeonpea in India and Nepal. It was first reported from Pusa in Bihar, India.
• The disease is present in the major pigeonpea-producing states of India.
• It is a serious problem in northeastern (Bihar and Uttar Pradesh), and southern (Tamil Nadu) state.

Losses

• The diseases causes an estimated annual loss of 205,000 tonnes of grains in india alone.A susceptible genotype infected in the early stages(first 45 days)of crop growth shows near complete sterility and yield losses up to 100%.
• As the plants grow older(>45 days),their susceptibility to the SM pathogen decreases;such plants show partial sterility.
• Genotypes such as NP(WR)15,that develop mild mosaic symptoms are partialy sterile;and their yield loss is less (19-64%).
• Disease incidence is usually higher in ratooned and perennial pigeonpea crops.

Symptoms

Symptoms of sterility mosaic

Diseased plants are pale green and bushy in appearance, without of flowers and pods. Diseased plants are usually in groups. Sometimes a plant may not show symptoms in the first flush, but when ratooned the new growth shows clear symptoms tend to disappear as the plants mature.

Disease Cycle in Nature

• The disease cycle of SM is not fully understood.
• Since the pathogen is nor seedbore, the disease is likely to be introduced by the mite vector into rainy-season crops from external sources.
• It is observed that pigeonpea sown late (September) close to the rainy-season sowings develops more disease, indicating the spread of the disease from early infected plants in early sowings to the late-sown crop.
• A large season-to-season variation in the incidence of the disease in farmers' fields is observed in most parts of India.
• At present there is no information available to explain this variation.

Control Measures

• Grow resistant/tollerant varieties viz. ICPL 157, NP(WR)15, Bahar etc.

Chemical Control

• Seed dressing with a high dose of carbofuran as 25% Furadan 3G@ was found to protect the plants from SM infection for up to 45 days after sowing.
• Seed treatment with 10% aldicarb protected the crop till maturity.
• Three acaricides, Tedion, Morestan, and Kelthane, all at 0.1% concentration, were sprayed on SM infected plants harbouring eriophyid mites, all the three acaricides were highly effective, killing more than 90% of the mites.

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Yellow Mosaic

Reported from India, Jamaica, Nepal, Puerto Rico, and Sri Lanka, this disease was probably reported first from Sri Lanka. The disease first appears in the form of yellow, diffused spots scattered on the leaf lamina, not limited by veins and veinlets. Such spots slowly expand and in later stages of disease development, affected leaflets show broad, yellow patches alternating with green colour.

• Sometimes the entire lamina turns yellow.
• Leaf size is conspicuously reduced in early infections.
• In peninsular India, disease incidence is relatively higher in late-sown pigeonpea.
• More than 40% yield loss has been reported.
• The vector is Bemisia tabaci Genn.
• Since disease incidence is rarely severe, no reports on managing the disease have appeared in the literature.

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