Damping Off: Pythium aphanidermatum, pythium debaryanum

• Damping off is a serious problem in tobacco nurseries both in heavy clay soils and light sandy soils.
• It is caused by several soil inhabiting fungi predominant being Pythium aphanidermatum, pythium debaryanum, Phytophthora sp. and some times Rhizoctonia solani are also involved.
• The disease may appear at any stage of the seedlings but maximum damage is observed 5-6 weeks after sowing.

Symptoms

Disease is noticed in two phases, viz.

• Pre-emergence damping-off
• Post-emergence damping-off

Pre-emergence damping-off

• Seedlings are infected which die before emergence from the soil resulting in poor and uneven stand of the seedlings

Post-emergence damping-off

• Post emergence damping- off is the most destructive phase.
• The tiny two leaved seedlings may disappear due to wet rotting of stems and the seedlings stand diminishes daily.

Generally damping-off occurs in patches which spread quickly if unchecked.

• The older seedlings show shriveling and brown discoloration of the stems near the soils and start rotting under wet soil conditions.

Pythium Oospores

The decayed stems collapse and seedlings topple over leaving parchment like brown crest over the soil.

• The wet rotting and sudden collapse of seedlings starts in circular patches and under wet weather conditions may extend over the whole seedbed causing near total loss.

Life cycle

• Oospores and chlamydospores in soil act as initial source of infection.
• Under favourable wet conditions oospores germinate liberating zoospores which infect the seedlings at soil level or just below.

Epidemiology

• High humidity, high soil moisture, cloudiness and low temperatures below 24° C for few days are ideal for infection and development of disease.
• Crowded seedlings, dampness due to high rainfall, poor drainage and excess of soil solutes hamper plant growth and increase the pathogenic damping-off.

Control measures:

Cultural and Preventive measures

• Deep ploughing in summer destroys the fungus
• Preparation of raised seed beds 15 cm high with channels around to provide drainage
• Rabbing the seed bed before sowing with slow burn farm waste materials likely paddy husk, tobacco stubbles, waste grass and palmyrah leaves etc.,
• Use seed rate 3 Kg/ha only to avoid over crowding of seedlings.
• Regulate waterings to avoid excessive dampness on bed surface

Chemical Control

• Bordeaux mixture 0.4% @ (40 gm of copper sulphate +40 gm of lime in 10 liftes of water) or Fytolan or Blitox @ 0.2% (20 gm in 10 litres of water) is to be applied with rose can to a bed of 10 sq.m area 2 weeks after sowing.
• Two to three spray drenchings of beds with 0.2% Redomil72 WP commencing three weeks after seed sowing (20 g of Ridomil in 10 l of water)
• This has to be repeated once in 4 days under normal weather conditions and once in 2 days under wet and cloudy conditions. In case of heavy rain, the application has to be repeated.

Schedule of application

• Application of fungicide should be given one week after completion of germination.
• Subsequent application, depending upon weather conditions, should be given at 4 days interval if it is bright, sunny and dry, and 2 days when it is cloudy and wet weather. Application should be repeated after every rain.
• Approximately 40 litres of spray fluid for 40 m2 of seedbeds, applied through a rose-can or 20 litres through knapsack sprayed is required for each application.

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Anthracnose : Colletotrichum tabacum

• Anthracnose is caused by Colletotrichum tabacum
• The disease causes serious damage to the seedlings in nurseries only and it occurs rarely in the fields.
• It occurs in all the tobacco-growing tracts of India.

Symptoms

• Initially, infection starts on lower leaves as pale-brown circular spots of 0.5 mm diameter with papery depressed centres outlined by slightly raised brown margin.
• The leaf-spots may remain small with white areas in the centre or coalesce to form large necrotic lesions.
• Under continuous humid weather, dark brown or black, elongated, sunken necrotic lesion appears on midrib, petiole and stem resulting in petiole and stem rot. Such seedlings do not establish in the field if planted. Primary infection starts from affected bits of aerial parts left in the soil in the previous season.
• The pathogen is not seed-borne but persists in the soil on dried plant debris.

Management

• Raised seed beds and rabbing with farm wastes help in reducing the initial infection
• Removal and destruction of all diseased debris minimises the pathogen in the soil.
• Discarding the diseased seedlings especially with necrotic lesions on stem will save gap filling in the field.
• Protective spraying with Bordeaux mixture at 1.0% (2-2-500) or 0.2% (2 g/lit) Zineb or Dithane Z 78 at 0.1% or (1 g/lit) Cuman during wet weather especially in nurseries where anthracnose appeared in previous season, is helpful.

Chemical application

• Apply fungicide one week after completion of germination.
• Subsequent application, depending upon weather conditions, should be given at 4 days interval if it is bright, sunny and dry and once in 2 days when it is cloudy and wet weather.
• Application should be repeated after every rain.

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Black shank: Phytophthora parasitica var. nicotianae

• Black shank occurs sporadically in every type of tobacco and causes more damage, to tobacco grown under high rainfall or irrigated conditions in light soils of Karnataka, Andhra Pradesh, Telangana and Gujarat.
• It is caused by soils inhabiting fungus Phytophthora parasitica var. nicotianae and appears both in nursery as well as in the field.

Symptoms

In nursery the affected seedlings show blackening of roots and stem near soil. Under wet weather conditions the conidial infection spreads to leaves, which show water soaked lesions resulting in leaf blight and collapse. The leaf blight phase of the disease, though rare in occurrence spreads very rapidly during cloudy and continuous cyclonic weather resulting in heavy damage.

Under such conditions large circular, black or brown water-soaked lesions appear on the basal leaves. In the field blackening starts at the collar region and spreads both downwards and upwards causing rotting and necrosis of the whole stem including basal leaves. First sign of infection seen in the vigorously growing plants is yellowing of leaves, sudden wilting and ultimately death of plants under warm weather.

The disease appears in scattered patches in the field. The stems of such plants when split open show brown to black dried pith in disc like plates characteristic of black shank.

Life cycle

• One of the principal means of spread of the pathogen is the infected plants and plant parts in the soil.
• Generally the fungus is spread by rain or irrigation water.
• The fungus can remain viable in the soil for more than 5 years. Initial infection of the plants is by soil-borne spores and mycelium.
• Secondary spread of the disease is through wind-borne sporangia. Black shank develops when the moisture-holding capacity of the soil is from 19 to 100 per cent.

Epidemiology

• The disease develops faster when moisture is high and the temperature is 16° to 30" C. Black shank incidence is high in heavier soils than lighter soils but soil organic matter content has little effect on disease incidence.

Control

• It can be checked in the nursery by rabbing the seedbeds or drenching the seedbeds with 0.4% Bordeaux Mixture 2 days before sowing to minimise the initial inoculum potential.
• Subsequent application of fungicides like 0.2% Blitox, Fytolan or Cuman L is essential for checking leaf blight and seedling blight.
• At the time of transplanting, seedlings with blackened stem should be discarded to prevent spread of disease to field.
• Sanitary measures like removal and destruction of the affected plants and disinfecting the spots with 0.4% Bordeaux Mixture drench will prevent spread of the disease in the main field.
• Leaf blight phase of the disease can be checked by spraying 0.2% Dithane Z. 78 or Cuman or Blitox or Fytolan.
• Grow resistant varieties like 'Beinhart 1000- l' and 'NC 2326' which are moderately resistant can be used. A bidi variety, 'K 20' has good tolerance to black shank under Gujarat conditions.

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Powdery Mildew: Erisiphe cichoracearum var nicotiane

• Powdery mildew, also known as white mold or ash disease, occurs in all types of tobacco.
• Generally the damage caused is slight, but it is severe in flue-cured tobacco grown in low-lying areas of Andhra Pradesh and in tracts where tobacco is grown as rainfed crop from spring to monsoon season as in Karnataka.
• The crop becomes unprofitable in case of severe incidence.

Symptoms

• Initially greyish white spots (about 0.5-lcm in diameter) appear at the base of the lower leaves of the maturing plant.
• Under favourable conditions of low temperatures (16° -23° C) and high humidity these white patches spread to upper leaves, enlarge and cover the entire surface of the leaf.
• Such leaves on curing get scorched and show brown patches rendering them unfit for marketing.
• Sometimes leaves with incipient infection result in blemished on curing which reduce the commercial value of leaves.

Epidemiology

• Excessive nitrogen fertiliser causing vigorous vegetative growth and close planting which increase the humidity around the plants and shade are predisposing factors for infection and development of the disease.
• The fungus normally attacks maturing plants in the field. The optimum condition for infection is 16° to 23°C and relative humidity of 60- 75%.

Control

• Avoiding over fertilising, overcrowding of plants and removal of affected basal leaves will help in minimising primary infection and spread of the disease.
• Early planting to escape cool temperature prevalent at the maturing stage will also reduce the incidence.
• Apply sulphur powder (200 mesh) @ 40 kg/ha to soil between plant row 6-8 weeks after planting. Mix ash or sand to sulphur for easy application.
• See that sulphur does not fall on tobacco leaves. Recommended for black soils only.
• Spray 0.2% Karathane or Thiovit or 0.05% Bavistin just before the disease sets in Repeat at 10-12 days interval if necessary
• Use resistant varieties like Swarna or Line 2359 developed at Central Tobacco Research Institute, in disease endemic area.

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Frog Eye Leaf Spot : Cercospora nicotianae

• Frog-eye leaf spot disease occurs in the seed-beds as well as in main fields. It damages the marketing quality of leaf especially of flue-cured and wrapper tobaccos.
• The disease is severe on tobacco grown under monsoon conditions.

Symptom

• Frog eye spots appear mainly on the lower leaves.
• They are usually circular, brown or tan with dark-brown and pale ashy or white parchment like centre resembling frog's eye at early stages During wet weather they may not be typical as above, as the outline may be angular and the white centre may be absent and they may enlarge from 0.25 cm to 1.5 cm in diameter.
• During frequent rains large necrotic areas appear which destroy much of the leaf. During hot dry weather, frog eye-spot may be only pinpoint in size.
• In flue-cured tobacco the same pathogen causing infection at the harvesting time when cured results in innumerable, small, uniform, dark-brown to black spots popularly known as 'Barnspots' which affect the market value of the leaf.

Management

Cultural Methods

• The nurseries should be located away from barns and all the diseased debris removed promptly and destroyed.
• Avoiding excess nitrogen fertilisation and picking over ripened leaves also helps in minimising leaf spotting.

Chemical methods

• Fungicidal sprays with Bordeaux mixture 0.4% (4g/lit), Zineb 0.2% (2 g/lit) or 0.1% (1g/lit) Benomyl as per the schedule suggested under damping-off has checked leaf spotting in the seedbeds.
• In the field, removal of sand leaves immediately after the transplants have established, and weekly application of 0.4% (4g/lit) Bordeaux Mixture or 0.1% (1g/lit) Benomyl or Thiophanate methyl or 0.03% (0.3g/lit) Bavistin 5-6 weeks after planting has given adequate control of frog-eye spot.
  

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Brown Spot: Alterniaria longipes

Alternaria tenuis

Alternaria alternata

• Brown spot in contrast to frog-eye spot is not normally observed in the nursery but is very much prevalent in the field.
• In India a decade back it was considered to be of minor importance but gradually it is gaining importance specially in tobacco grown in monsoon season.

Symptoms

Initially it appears on lower and older leaves as small brown, circular lesions, which spread, to upper leaves, petioles, stalks and capsules even. In warm weather (300C) under high humidity, the leaf spots enlarge, 1-3 cm in diameter, centres are necroses and turn brown with characteristic marking giving target board appearance with a definite outline. In severe infection spots enlarge, coalesce and damage large areas making leaf dark-brown, ragged and worthless.

• On leaves nearing maturity, leaf spots are surrounded by bright yellow halo, due to production of toxin 'alternin' by the fungus.
• Extensive toxin production can make the apparently undamaged areas of the leaf cure out dead and black.

Life cycle

• The fungus over summers in the soil as mycelium in the diseased plant debris such as stems of tobacco, weeds and other hosts.
• Under favourable weather in the next season conidial production starts which infect the lowermost leaves.
• As the season progresses, repeated infection cycles of the fungus attack healthy tissues of all aerial parts of tobacco of any age under high humidity.
• There is enormous spore density in the air near the end of the harvesting.
• Fungus persists as a mycelium in dead tissue for several months.

Control

Cultural

• Removal and destruction of diseased plant debris can check the primary infection promptly.
• Continuous growing of tobacco after tobacco must be avoided in the heavily infected fields.

Chemical

• Weekly, spraying of fungicides such as Maneb, Dyrene (2, 4-dichloro-6-0- Chloranilenotriazine) at 450 g per acre or 0.12.5% Benomyl and Thiophanate methyl or 0.2% Dithane Z 78 do give reduction in leaf spotting.

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Minor Leaf Spot Diseases

Curvularia leaf spot: Curvularia verrucosa

• Reported a serious leaf spot disease on Nicotiana tabacum in and around Muzaffarpur (Bihar) during 1962 and onwards.
• Symptoms consisted of round or oval spots with concentric zones. The lesions enlarge, coalesce and affect the whole lamina.
• Infected leaves first became pale yellow and then turned brown to dark brown.

Phyllosticta leaf spot Phyllosticta nicotianae

• Brown, irregular, zonate spots, 1-10 mm in diameter, dark-brown in the centre and light brown towards the margin were seen.
• Pycnidia were embedded in the necrotic tissue. A shot-hole disease on tobacco seedlings.
• Large white blotches up to 15 mm in diameter were seen on the leaves, the centres of which fall out to give the shot-hole effect.
• The fungus was identified as Phyllosticta nicotianae.
• In India a leaf spot disease on tobacco caused by P. tabaci was reported from Maharashtra.

Control

• Spraying with (Carbendazim 50%), Bavistin 50 W.p @ 0.025% (2.5 gm in 10 litres of water) at 10 days interval or Dithane Z-78 @ 0.2% (20 gm in 10 litres of water) at weekly intervals. Bavistin may be sprayed at 12-15 days interval.

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Angular Leaf Spot (Pseudomonas angular)

• Among several bacterial diseases of tobacco, Angular leaf spot is the only one commonly prevalent in India.
• It is confined mostly to Karnataka where tobacco is grown in spring and monsoon season. Its incidence in other tracts is negligible.
• It occurs both in nursery and in the main field.
• Recently 'wildfire' caused by Pseudomonas tabaci was reported on tobacco from Coimbatore district in Tamil Nadu.

Symptoms

• Spots appear at any stage of seedlings or plants in field.
• In seed beds 2-3 small, angular, dark-brown to black-coloured spots appear on individual leaves. When held against light, a yellow halo is seen around the spots.
• In the field, the lesions are dark tan or black, much bigger and coalesce. Lesions are restricted between veins, irregular and on rapidly growing tobacco, leaves become puckered and torn, and the centre of the lesion fall out leaving little besides the veins.

Epidemiology

• Cloudiness, stormy wind with rain causing water splashing are favourable factors for infection and rapid spread of the disease.
• In the absence of storm with rains, excess soil moisture accompanied with high humidity, excess nitrogen and low potassium nutrition enhance the incidence.

Management

• Phytosanitary measures of prompt removal and destruction of diseased debris and susceptible weeds from the seedbeds and discarding affected seedlings.
• Use of Bordeaux mixture 0.4% checks the disease adequately in the nurseries. However, in the field the disease was effectively, checked by spraying antibiotics like Streptomycin or Streptocycline at 200 ppm (200 mg in 1 lit).
• Spraying Thiophanate methyl at 750 g/ha or carbendazim at 500 g/ha controls this disease.
  

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Hollow Stalk: Erwinia carotovora

Stray incidence of hollow stalk has been observed on FCV tobacco in recent years.

Symptoms

• Diseased plants were identified by the sudden wilting and rotting of the plants.
• Leaves wilt, droop and hang down. Water-soaked large lesions were generally seen on the wilted leaves.
• Symptoms on the stem consist of elongated black or dull brown patches girdling the stalk.
• Such stems when break open show browning of the pith followed by soft rotting and death of the tissue. In general, hollow stalk appears at topping and suckering time.

Epidemiology

• The bacterium is a wound parasite and belongs to soft-rot group of bacteria. They are the natural inhabitants in soil.
• Development of the disease is favoured by damp, cloudy weather with high humidity. It was also reported that use of mineral oils as suckericides favour the reproduction of the bacterium and development of hollow stalk.

Control

• Sanitary measures are more important than chemical spraying.
• Topping and de suckering should be avoided during damp and cloudy weather, which are ideal for the spread of the bacterium.

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Mosaic

Tobacco Mosaic Virus (TMV) (Mormor tabaci)

• Mosaic is very widely distributed and is the most common virus disease on tobacco. This is the plant virus discovered first, on which great deal of classical work has been done for basic virus and hereditary studies.
• Mosaic appears sporadically in nurseries as well as in fields. While it is easy to recognise a mosaic-affected plant in the field it is difficult to differentiate diseased seedlings in the nursery.
• In some fields 90-100% of the plants show mosaic by harvesting time, while the adjacent fields are free from it. The damage is related to age of the plants.
• The maximum loss in yield and quality of leaf occurs when crop is infected at young stage.
• It was found that if the plants are infected at the transplanting time, the loss in yield would be between 32 and 60%, 20 and 30% a month after planting and 7 and 17% at topping time.
• Crops infected near topping time may yield as much as healthy crops but the quality is usually impaired.
• Mosaic-affected leaf shows more of nicotine, total nitrogen and albumen and less of soluble sugars and carbohydrate content.

Host range

• Mosaic has very wide host range among 'herbaceous dicots, about 116 plant species belonging to 29 different families susceptible to common TMV.

Symptoms

• Affected plants show leaves with molting or mosaic pattern of light green and dark-green areas.
• Primary symptoms appear on newly formed young leaves as vein clearing, greenish yellow motling.
• Infection on young plants results in stunted growth, malformation, distortion and puckering of leaves. Dark-green blisters and some time enations (leafy growth) appear on the dorsal side of the leaf.
• Immature leaves show varying degree of yellow motling and chlorosis. In severe infection due to a highly virulent strain of TMV, various necrotic dark-brown spots also develop resulting into 'mosaic scorch' or 'mosaic burn' under hot sunny dry spell, damaging large areas of lamina.
• There are many strains of TMV and symptom expression depends upon the strain and the prevailing environmental factors. Mosaic-affected leaves after curing show green spots which lower the market value of the leaf.

Transmission

• TMV is highly contagious and transmitted by sap. It is easily transmitted by mere contact of a diseased plant with a healthy one.
  Air-dried tobacco is a common source of new infection. Workers who chew or smoke natural leaf tobacco during nursery operations may spread the virus into the seedlings.
• Old stems and leaf trash of affected plants buried in the soil are the other sources of infection and spread.
• In the nurseries, seedlings may get affected due to the presence of susceptible weed hosts.
• The virus is not insect transmitted. The aphid (Myzus persicae Sulz) which occurs commonly on the tobacco plant is unable to transmit the virus.
• Occasionally chewing insects like Grasshoppers, flea beetles and caterpillars transmit TMV but this type of spread is insignificant under field conditions.
• The virus is not transmitted through tobacco seed although it is transmitted through tomato seed.
• Experimentally the virus is graft transmissible.

Host-parasite relationship

• TMV is highly infectious. Entry of TMV is direct through the epidermis and rarely through stomata. Entry depends upon the type of wound, toughness of epidermis, presence of inhibitors of virus and virus concentration.

Environmental influence

• Increase in temperature and light reduces the time required for symptom expression.
• Ultimate effect depends upon age of the plant, virus concentration and growth conditions for the host.
• Infection is inhibited above 38 – 48° C, above 27° C or below 10° C the symptoms may disappear.
• Long days and relatively high light intensity, which favour host growth generally favours initial virus synthesis.
• Young plants in general are more susceptible.

Management

• Best effective way is to keep the crop mosaic free.
• Constant vigilance is required right from the seedling stage till harvest involving phytosanitary measures, and nothing can be done once TMV becomes systemic.
• Infected seedlings should be removed promptly and destroyed.
• Workers should disinfect their hands with soap and running water before handling seedlings, weeding or doing other cultural operations.
  Use of tobacco (smoke, snuff, or chewing) should be prohibited strictly while working in seedbeds or field.
• Weeds (Solanum nigrum) and plant (Brinjal, Tomato, Chillies) susceptible to the virus should be destroyed.
• In fields showing high incidence of mosaic, rotation for 2 years should be followed.
• Rouging of the affected plants before first interculturing considerably reduces the spread of the disease.
• However, in areas where TMV is an over-riding problem, resistant lines developed at CTRI, Rajahmundry, viz. TMVRR-2 and TMVRR-2a, TMVRR-3 can be used to overcome this problem.
  Spray leaf extracts of Basella alba, Bougainvillea (one litre of extract dissolved in 100-150 litre of water) on 30th, 40th and 50th days after transplanting.

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Leaf Curl

• Leaf curl, is fairly widespread in all tobacco tracts in India. Generally the disease appears 3-5 weeks after planting and sometimes late in the nursery.
• The virus is transmitted by an insect vector, white fly Bemisia tabaci. It is also graft transmissible but not by sap or seed. There are a large number of hosts of this virus as well as of its vector.

Symptoms

The most characteristic symptom of tobacco leaf-curl disease is the production of leafy outgrowths known as enations from the veins on the lower surface of the leaves.

	Combined with this, is a stunting of the whole plant and twisting and curling of the leaves. The manifestation of the disease varies greatly according to environmental conditions and the variety of tobacco concerned.

Environmental influence

• White flies become more active in dry periods after monsoon showers. Leaf-curl is therefore, noticed more during this period.
• Seedlings of Flue-cured tobacco are easily infected and not the older plants which have plenty of epidermal hair and stick gum which perhaps prevents proper activity of white flies in feeding, acquiring and transmitting the virus.
• Tobacco plants become more resistant to Tobacco leaf curl virus as they grow older.
• The susceptibility of the plant was directly correlated with nitrogen supply and inversely correlated with phosphorus supply.
• Potash in increasing doses reduced susceptibility.

Control

• Remove and destroy the diseased tobacco seedlings before and after planting whenever they are seen in the field. Fill the gaps with healthy seedlings if it is not too late.
• Alternate weed hosts for whitefly should be removed and destroyed.
• In endemic areas Sesbania sp. may be grown as a barrier crop around tobacco fields.
• Install 12 yellow sticky traps (castor oil coated) per hectare to monitor the whitefly population. If 100-200 whiteflies stick to the trap the following insecticide schedule has to be adopted.

1st spray	Chlorpyrifos 20% E.C. @ 25 ml. In 10 litres of water
2nd spray	Monocrotophos 36%, W.S.C. @ 15 ml in 10 litres of water
3rd spray	Acephate 75% S.P. @ 10 gm in 10 litres of water
4th spray	Demeton-s-methyl 25% E.C. @ 15 ml in 10 litres of water

• Spraying has to be done preferably in the evening hours (4-6 P.M) with high volume spraying and it should be ensured that the under side of the leaves also is covered.
• Further the spraying schedule has to be adopted on community basis.

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Tobacco Distorting Virus

• In 1968, incidence of a severe type of mosaic virus was noticed in Natu tobacco (sun-cured cigarette tobacco) on a fairly wide scale in Warangal and Karimnagar districts of Telangana

Symptoms

• Affected plants show highly stunted growth, abnormal suckering and the leaf tips are modified into rat-tails.
• The leaves show typical mosaic mottling, puckering, distortion and suppression of lamina into various shapes leaving only long midrib in extreme cases.

Control

• Screening the germ plasm for Tobacco distorting virus indicated that besides TMV resistant line TMVRR-l, developed at CTRI, Rajahmundry, there are 23 genetic types (15 exotic and indigenous air-cured types) found resistant to TDV.
• Growing D.G. 3 (Desi Guntur) is an improved Natu tobacco variety which is resistant to TDV virus

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Root-Knot : Meloidogyne species

• Root-knot, a disease caused by nematodes, is prevalent in all types of tobacco grown in lighter soils.
• Its incidence is reported from Gujarat, Karnataka and light sandy soils of Telangana and Andhra Pradesh where tobacco nurseries are raised on commercial scale, although stray incidence of root-knot was observed in other tobacco-growing tracts of Bihar and Tamil Nadu.
• Root-knot menace is increasing every year especially in tobacco nurseries, which are abandoned under severe infestation and new sites are taken up for raising seedlings.
• Infection at early stage results in heavy loss in yield and quality of leaf.
  So far no systematic survey of root-knot disease has been undertaken in India but damage to the extent of 60% has been noticed in individual plots.
• Losses from root-knot are generally heavy and in combination with other pathogens the disease has become disastrous.

Symptoms

• In nurseries, the disease can be identified by the sickly appearance of the seedlings. Affected seedlings are pale and stunted. When such seedlings are pulled out, characteristic symptoms of galls are seen on the roots.
• These galls vary in size from a pinhead to many times the thickness of the root on which they grow. They are round to irregular in shape.
  In the field, the disease can be identified by the stunting and yellowing of plants which in severe cases of infection wilt and die.
• Such plants when pulled out show enormous galls on the roots. In severe infection, individual galls coalesce to form multiple elongated, thick galls.
• In the case of light infection, minute pinhead sized galls are seen on the roots, such plants do not show any external symptoms and are able to make normal growth.

Life Cycle

• Eggs of the nematodes remain in soil and under favourable soil moisture and temperature, these hatch into pre-parasitic larvae, which migrate till they reach host roots. The larvae penetrate the roots and stimulate the gall formation. The females lay about 400-500 eggs in mass which hatch into larvae within the knots.
• Often the cortical tissue of the root is broken and eggs are pressed out. After the crop is harvested, the knots containing eggs and larvae are left behind which can remain dormant for a longer time in cultivated soil under favourable conditions.
• Meloidogyne species has a very wide host range and infect crop plants as well as weeds.
• Under favourable soil temperature conditions (22 – 31° C) the nematodes complete their life cycle in less than 20 days, thus producing more generations in a season resulting in heavy gall formation.
• At temperature below 8 or above 32° C the females do not mature. Soil moisture influence on nematodes is relatively less but the nematodes are noticed to be more active after irrigation. With 40-80% moisture-holding capacity of soil, little difference is noticed on root-knot development.

Control

• Basis of root-knot control is to reduce the nematode populations to a minimum and to prevent the spread and development of existing nematode species to a damaging level.
• Nematodes are obligate parasites, hence keeping the tobacco nurseries and fields free from weeds and other vegetation specially during off-season will starve them.
• Immediate uprooting of harvested plants for the roots to dessiccate thereby killing the nematodes inside them, and deep ploughing and turning over the soil during hot summer months, will result in dessication and death of the egg masses and larvae.
• Use of gall-free seedlings will prevent spread of nematodes to the planted fields.
• Half the battle against nematodes can be won if they are checked in the nurseries which are the source of infection.
• Rabbing the seedbeds by burning paddy husk or dried material before sowing effectively reduced the disease.
• Root knot nematode can be controlled by application of carbofuran 3 G (or) Aldicarb 10 G in planting holes at 0.4 kg a.i/acre.
• Avoid irrigation water from infested fields to the non-infested ones.
  Use of non-host crops in rotation with tobacco is next best solution against nematodes. But rotation for at least 3 years is essential for effective reduction of nematode populations.
• Six crop plants, viz. Arachis hypogaea , Sesamum indicum , Cajanus cajan , Gossypium hirsutum, Capsicum annuum and Cuminum cyminum were found non-host to M. javanica and these can be used as rotation crop against root-knot.
  

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Sore Shin: Rhizoctonia solani

• Sore shin is considered to be a minor disease, though in certain individual plots damage caused is fairly high.
• The sore shin fungus Rhizoctonia solani attacks many plant species of economic importance and can remain as a saprophyte on decaying plants or as sclerotia in the soil.
• It appears both in tobacco nursery as well as in the field.

Symptoms

• In nursery, symptoms appear as a dark brown lesion on the stem near soil line, which enlarges, girdles the stem, causing decay and toppling over of the seedlings under favourable wet conditions similar to damping-off.
• In poorly drained field the dark brown lesion on the stalk, due to infection near the soil, extends upwards as a dark-band and the roots remain unaffected.
• Under cool wet weather the infection spreads rapidly round the stem and deep in the cortical region.
• Entire stem shows black lesion, dry rot of the cortex and pith with minute black sclerotia visible in the grey mycelium; leaves become yellow, wilt and die.
• Affected plants have weak stem which cannot withstand heavy wind.

Control

• The disease being of minor importance no suitable control measures have been reported.
• However, the fungus being a soil inhibitor, usual phytosanitary measures are suggested.
• In seedbeds soil disinfection with a suitable soil fumigant or dry heat (rabbing) before sowing will control primary infection.
• In the field no satisfactory control is known, neither crop rotation helps, nor any resistant types have been developed so far.

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Barn Diseases Of Tobacco

• Tobacco leaf, being the valuable end-product of the plant is invariably invaded by several fungal and bacterial pathogens during curing and storage.
• Such invasion generally reduces the quality and marketability of the cured leaf. In India, so far no work has been initiated on the barn and storage diseases of tobacco.

Barn rot

• This is also known as 'House burn' or 'Pole rot, which is common on air-cured tobacco.
• Flue-cured tobacco is also susceptible. There are four phases of leaf decay leading to barn rot. They are:

Stalk rot

• It is the earliest to appear.
• Lesions or spots develop on the stalks and gradually extend to the leaves.

Cankers are formed on the stalk which girdle the stalk leading to yellowing of leaves above the girdle. Such type of leaves when cured, are pale or off-coloured, thin, papery and of poor quality.

Web rot

• This phase is the most destructive of the house burn as the leaf affected by web-rot is worthless.
• Web rot affects the leaf during wet weather at the time of harvest. Affected leaves are brown, soft and watery but become brittle during curing.
• When handled, such leaves shatter and do not come to condition.

Vein rot

• Vein rot generally starts from the stalk rot and extends into the, midrib and lateral veins.
• All the vein tissues disintegrate except the fibres.
• Affected leaves turn white or straw coloured and decay. Such leaves have no commercial value.

Freckle rot

• Numerous small dark spots ranging from pinhead to 6 mm in diameter appear on the leaf blade. They are reddish-brown to black in colour.
• Different phases of 'barn rots mentioned above are caused by Botrytis cinerea, Rhizopus arrhizus, Aspergillus spp., Alternaria tenuis and Sclerotinia sclerotiarum.
• Barn rot is not influenced by weather. Prolonged moist weather during growth period is favourable for barn rot development.
• In the barn ideal conditions for spread appear as a result of high humidity produced by over-crowding the tobacco and failure to ventilate properly.
• When the humidity reaches 85-95% danger from barn rots is imminent.

Control of barn rots

• Barn rots can be prevented by proper manipulation of ventilators and humidity during curing.
• Increasing the temperature inside the barn 5°-15°C above that of the outside air and careful ventilation for 3 or 4 days will usually prevent damage.
• Curing barns should be built in the open on high ground and adequately ventilated. Usually serious barn rot damage occurs in barns, which are poorly located, tightly constructed and inadequately ventilated.

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Orobanche

Introduction

• Orobanche is popularly known as broomrape in English speaking countries, 'Tokra' in north India, 'vakumba' in Gujarat, 'bambaku' in Maharashtra, 'pokayilaikalan' in Tamil Nadu, 'bodu' or 'malle' in Telangana and Andhra Pradesh.
• It is a flowering parasite on tobacco roots and occurs in all the tobacco tracts in India. It is a holoparasite and draws its nourishment from tobacco by means of haustoria attached to the roots of tobacco.
• The loss in the yield and quality of leaf is very much variable from 30-70% in Tamil Nadu and 10-50% in other states.
• Experiments conducted at CTRI, Rajahmundry, indicated that loss in yield varied from 20-50% depending on the time of infection and availability of soil moisture.
• Symptoms In the early stages of infection, symptoms of wilting, drooping and ribbing of leaves are observed in the early hours.
• These symptoms are the first indication of underground infection of tobacco roots by the parasite.

Five to six weeks after planting, young orobanche sprouts emerge from the soil at the base of tobacco plants. Presence of numerous orobanche shoots around the plants in field is the most important visible symptom. Plants attacked early in the season are generally stunted showing typical wilting of leaves. Plants attacked late in the season do not show visible symptoms of infection but the yield and quality of leaves are reduced.

• Orobanche emerge in clusters, they are 15-45 cm tall, pale brown or purple in colour. Often 10-15 shoots are found attached to the roots of a single host plant.

• Of the two species reported in India, viz. Orobanche cernua and O.indica, the former is more a serious parasite on tobacco and it is restricted to solanaceous plants.
• some more hosts in other families, e.g. Cannabinaceae, Compositae, Euphorbiaccae and Tiliaceae. Further he noticed that Capsicum annuum (chillies) and Tridax procumbens stimulated germination of orobanche seed without getting parasitised.
• The seed is reported to be viable for 2 years though and remained dormant in the soil for at least 13 years.

Etiology

• The causal organism Orobanche cernua Loefl. Var. desertorum Beck. is an annual, fleshy flowering plant, erect, 15-45 cm high.
• Stem more or less round, pale-brown, solitary (or in bunches) thickened at the base, covered with scaly leaves ending in spike, calyx with 2 narrow lobes.
• Corolla a white tube with bluish or violet lobes stamens 4 epipetalous with lobed anthers; ovary superior, one-celled with numerous ovules; stigma rather big and 4-lobed, fruit is 2-valved, capsule containing numerous seeds.
• Seeds are very minute 369x206 , very light, approximately 1,90,000 weigh 1 gram, ovoid and reticulate in black soil under normal growing conditions Orobanche completes its life cycle in 12-14 weeks.
• Orobanche seed germinates during the second week after planting tobacco and infects the root during third week.
• Till the end of fifth week, underground infection of Orobanche develops into various sizes. On the sixth week, Orobanche shoots emerge above ground.
• Flowering is completed by seventh week, seed formation by eighth week and capsule drying by ninth week end.
• Drying off commences by tenth week, withering of stem and dehiscence of capsules by eleventh and twelfth weeks. Studies on the germination of Orobanche seed all round the year indicated that November to February was favourable for maximum germination when the minimum soil temperature varied from 190 to 220C.
• Germination was minimum during April to June when the soil temperature was 250 to 31.50C. O.cernua is a holoparasite and the seeds germinate in presence of roots of tobacco and other host plants to the extent of 0 to 4.6%.
• Host root exudates stimulate germination 21 days at 210C in the presence of vitamin PP (nicotinamide) and vitamin B-6 (Pyridoxine hydrochloride) to the extent of 45.5% and 90% respectively.
• Orobanche seeds being very minute in size are produced in enormous numbers in each capsule. When the capsule is dry, it splits open at the top and the small dust-like seeds are easily scattered by wind.
• They mix up with the soil and remain viable for long periods. Irrigation and drainage water carry the seeds from one field to another. It is also spread through the animals, man or cultivation equipment.
• Seeds present in the soil germinate in presence of susceptible hosts like tobacco, tomato, brinjal and safflower.

Control

• Prevention of formation of the inoculum potential form the basis of orobanche control. Regular weekly hand pulling of tender orobanche shoots before they set seed, has reduced the original stand by 85% after 2 years and by 96% after 4 years.
• Periodical hand pulling carried out meticulously by every grower in a large block for at least 4 years will give adequate control of this menace.
• An alternative and equally effective method is spray drenching the emerged Orobanche shoots at tender stage (7-10 cm height) with Allyl alcohol at 0.1% spray which scorches the Orobanche shoots and they wilt and die within 48 to 72 hours after application, whereas the host plants remain unaffected.
• However, for orobanche shoots in advanced stage or those that appear at later stage, concentration of Allyl alcohol has to be increased up to 0.2% beyond which in becomes phytotoxic to the host.
• Direct application of kerosine oil 3-4 drops per shoot has given better knock down effect. Since kerosene oil is highly toxic to host, attempts are being made to develop a safe, easy and economic device for application.
• Combination of both the methods, viz. removal by manual labour up to first priming of leaves followed by chemical spray is suggested.
• This would be more economical depending upon the intensity of infection. soil application of Ethylene dibromide (EDB) @ 2-5 ml/m and DBCP @ 2 g/m in heavy clay or light medium soil significantly reduced broomrape infestation.
• Incidence of orobanche was much reduced in tobacco following jowar or gingelly in kharif but chillies though reported to be a trap crop failed to give the same effect that Sinapisalba as a spring gap crop preceding tobacco considerably reduced orobanche emergence.
• Working on the effect of dates of planting on the incidence of orobanche, that late planting reduced the incidence of orobanche but the crop growth is also affected mainly due to soil moisture stress.

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Drought Spot

• During prolonged dry weather, tobacco leaves sometimes develop numerous large red-brown spots between the veins. Each spot is surrounded by a yellow zone.
  
• The spot coalesce to form large irregular patches. The margins of the leaves curve downward and dry up.

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Leaf Scald and other Injuries

• This is also known as 'sun-burn' and it occurs during hot weather. Large irregular dead areas are formed on the lower surface of the leaf. Affected leaves wilt and dry up.
• Damage due to lightening, hailstorm, excessive moisture, herbicide injury and others were also reported on tobacco leaves. However, their recurrence and damage is very rare and negligible.
  
  

 

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