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CONTROL OF DISEASES
A. Diseases in Seed Beds
1. Damping off : Pythium Aphanidermatum
This is a fungal disease caused by the soil borne pathogen. It is the most common and serious disease in tobacco nurseries causing death of seedlings. Conspicuous symptom of this disease is the sudden collapse of young seedlings in patches leading to uneven stand. Brown watery soft rot of young seedlings, girdling of hypocotyls and finally toppling and death of seedlings leading to wet rot are the characteristic symptoms. The pathogen spreads quickly and affect the seed bed causing enormous loss of seedlings.
High humidity, high soil moisture, cloudiness, temperature below 24oC, continuous wet weather, location of nursery in low lying areas are the favourable factors for high incidence of damping off disease.
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 burning farm waste materials like paddy husk, tobacco stubbles, waste grass etc.
- Seed rate @ 3.5 kg/ha is to be used to avoid over crowding of seedlings.
- Waterings are regulated to avoid excessive dampness on bed surface.
- Bordeaux mixture @ 0.4% (40 gm of copper sulphate + 40 gm of lime in 10 litres of water) or Fytolan or Blitox @ 0.2% (20 gm in 10 litres of water) is to be applied with rose can 2 weeks after sowing. 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.
- Spraying ridomyl MZ 72 W.P. @ 20 g in 10 litres of water twice at 20 and 30 days after germination. Ridomyl should not be sprayed before 20 days after germination or more than two times or more than the recommended dose. (500 g of ridomyl is sufficient for one acre of nursery).
2. Black shank and leaf blight : Phytophthora parasitica var. nicotianae
This is a soil borne disease. It frequently occurs during mid nursery period causing leaf blight and blackening of roots and stems leading to death of seedlings. Water soaked brown to black lesions appear on the leaf. These patches enlarge and coalesce leading to wet rot of leaf tissue and midribs. Cloudy weather, prolonged dampness and temperature below 22oC are the favourable factors for sudden outbreak of this disease.
Cultural and preventive measures
Black shank can be controlled by following the same schedule suggested for `Damping-off'. Mc.Nair-12 or CM-12(KA) or K-326 which are black shank tolerant varieties are to be used in irrigated northern light soils.
Spraying Fytolan or Foltaf or Blitox @ 0.2% concentration when the seedlings are 50-60 days old controls leaf blight. Ridomyl MZ 72 W.P. @ 0.2% can also be used to control this disease.
3. Collar rot : Sclerotium rolfsii
Just like damping off, sudden death of seedlings in patches is noticed in seed beds. Blackening of the collar region, wilting and rotting of leaves are the symptoms.
Spraying 0.4% Bordeaux mixture or 0.2% Ziram or 0.2% Blitox, 3 weeks after seed germination. This is to be sprayed twice or thrice at weekly intervals depending upon weather conditions.
4. Leaf spot diseases
a. Anthracnose : Colletotrichum tabacum
This disease can be seen in any stage of the nursery. It will cause heavy loss to the nursery when favourable conditions to the fungus prevail. Small, light green to white water soaked lesions develop on young leaves. These lesions enlarge in wet weather to form oily, circular spots of 3 mm diameter. The spots dry up, become papery, thin, grey white surrounded by brown border. Effected leaves become wrinkled and distorted. Elongated, black or brown lesions on the midrib and petiole are seen in severe cases. On the stem, several elongated or oblong sunken brown lesions develop making the seedlings unfit for transplanting.
Optimum temperature for the occurrence of this disease is 18oC. High relative humidity, reduced light and overcast weather are favourable for the outbreak of this disease.
b. Frog-eye spot : Cercospora nicotianae
Generally this disease is seen 4-5 weeks after germination. Brown, round spots resembling frog eye form appear on the lower leaves of the seedlings. Spots appear first on basal leaves and gradually spread to upper leaves. Spots are small, circular, brown or tan with grey centres. In severe cases coalesce to become bigger spots leading to drying up of leaves.
Frequent waterings and wet weather leading to high humidity and temperature around 27oC are favourable for the development of the disease.
Spraying with Carbendazim 50% (Bavistin, Dhanustin, Jekestin 50 W.P. etc.) 3 gm in 10 litres of water at 30 days and 40 days after germination controls this disease. If necessary another spray has to be given after 10 days.
5. Root-knot nematode: Meloidogyne javanica/ Meloidogyne incognita
The light soil nurseries are prone to this disease. Root knot is caused by nematodes and prevalent in most of the light soil nurseries. Sandy soils and red sandy loams with adequate soil moisture favour the disease development. Several weeds growing in and around nurseries are alternate hosts of root knot nematodes. They are responsible for build-up of the nematodes in the soil.
Characteristic symptoms of root knot disease are yellowing of leaves, stunted growth of seedlings, wilting of plants, premature death of seedlings leading to bald patches in seed beds. Affected seedlings when pulled out show several galls on the roots. Galls vary in size, coalesce and produce multiple galls which give the seedlings a sickly appearance. Root knot affected seedlings do not establish well in the field resulting in heavy gaps.
- Soon after the nursery season, ploughing of the nursery area thoroughly destroys the nematode population by exposing them to the hot sun. The area should be ploughed subsequently, 3 or 4 times during summer months also.
- Before raising the nursery, the soil must be tested for nematodes.
- Rabbing the seed beds before sowing with paddy husk @ 6 kg/m2 or slow burning farm waste material to destroy nematode larvae and egg masses gives initial protection to seedlings for about a month.
- Soil solarisation by covering the nursery soil with 100 gauge white alkathene sheet during summer for about 6-8 weeks controls nematodes effectively.
- Nursery should not be grown where brinjal or tomato were grown previously.
- Rotation of the nursery area with crops resistant to nematodes like groundnut, redgram, marigold, cotton, gingelly and chillies is recommended. If such rotation crops are grown for more than three years the nematodes can be controlled.
- The site of nursery should be changed every year. If one year the nursery is infected with nematodes, nursery should not be grown in that site next year.
- Application of Basamid @ 40 gm/m2 10 days before sowing effectively reduces the nematode.
6. Tobacco Mosaic Virus : Mormor tabaci
This is a contagious disease caused by virus. The infested leaves show characteristic mottling with light and dark green patches and seedlings are stunted.
- Workers should disinfect their hands with soap and running water before weeding or handling seedlings.
- Workers should not chew or smoke while at work in nursery.
- Diseased seedlings should be removed and destroyed.
B. Diseases in field crop
Black shank : Phytophthora parasitica f. nicotianae
This is a soil borne disease and it occurs when there is heavy and continuous rainfall, high soil moisture and relative humidity. The blackening of the stem of the diseased plants is noticed upto 30 cm height from the base. At the blackened portion inside the stem, the pith transforms into black round plate like discs. Roots also will be blackened, becomes soft and finally rottening is seen. The leaves of the effected plants becomes yellow, wilt and finally plant dies.
- In fields known for severe infection with Black shank, Mc.Nair-12 or CM-12 (KA) or K-326 varieties which are tolerant to black shank are to be used in northern light soils.
- Any one of the following fungicides @ 100 ml per plant is to be applied in planting hole and then planting is done.
- Bordeaux mixture 0.4% (40 gm of Copper sulphate + 40 gm of lime in 10 litres of water).
- Copper oxychloride (Blitox or Fytolan or Foltaf) @ 0.2% (20 gm in 10 litres of water).
LEAF SPOT DISEASES
a) Frog eye spot : Cercospora nicotianae
The causal agents, symptoms and favourable weather conditions are the same as described under nursery diseases. In addition, this disease also appears as minute dark brown or black spots on stems, midribs, flower stalk and capsules.
Spraying Carbendazim (Bavistin/Dhanustin/Jekestin etc.) @ 0.03% (3 gm in 10 litres of water) at 15 days interval, commencing from 30 days after planting, effectively controls frog eye spot disease.
b) Brown spot : Alternaria alternata
Brown spot is a major disease affecting both FCV and non-FCV tobacco's in India. It is prevalent in most parts of India. This disease generally occurs when there is high relative humidity and atmospheric temperatures around 20-25oC. Especially when dew fall is more, more number of spots will be seen on the leaves. Dark brown or burnt spots separately or coalesced, appear on the diseased leaves. The spots will be like small water soaked spots first, later gradually grow to one cm size in 5-6 days and turns to brown colour. Sometimes yellow rings are seen around the spots just like frog eye spot. When the spots are closely observed, there will be circular rings on the spots. The disease will first appear on the lower leaves and gradually spreads to the upper leaves. Affected leaves dry up and do not cure properly. Dark brown, sunken, elongated spots appear on stems, petioles, seed capsules and stalks also.
Dithane M 45 or Indofil M 45 @ 20 g in 10 litres of water in combination with plantamycin or streptomycin or streptocyclin 3 gm is to be sprayed on the crop such that all the leaves are covered. For this high volume sprayers like sikar or hi-tech or knapsack sprayers should be used. Depending on the intensity of the disease, two or three sprays should be given at weekly or 10 days interval. Use of excess doses of nitrogen fertilisers should be avoided.
Broomrape (Orobanche) : Orobanche cernua
|It is a complete root parasite affecting the yield and quality of tobacco. The shoots emerge in clusters and their basal portion is attached to tobacco roots through which it draws nourishment and depletes the host resulting in yield loss of 24 to 52%. Affected plants become stunted, leaves turn pale and wilt. Initially leaf tips droop and as the attack intensifies, all the leaves wilt. High soil moisture due to irrigation or rain after planting, low soil temperature during winter months encourage heavy incidence of Orobanche.|
- Deep ploughing twice or thrice in summer buries Orobanche seed to deeper depth and there by helps in reducing the emergence of Orobanche.
- In Orobanche sick fields, growing tobacco for one or two seasons is to be skipped off.
- Avoiding growing of brinjal, tomato and bhendi crops in the sick fields.
- Periodical removal of Orobanche shoots before flowering and setting of seeds reduces the menace. The Orobanche shoots should be destroyed by burning.
- Growing trap crops such as jowar, gingelly, blackgram and greengram in kharif facilitates orobanche germination but will not allow it to grow. This reduces the Orobanche seed load in the soil.
- CTRI developed a spear with a 2 m stick at the end of which 18 cm length, 8 cm breadth and 0.5 cm thick sharp iron blade is arranged. Cutting the Orobanche shoots within 3-4 days after emergence out of the soil, before flowering of Orobanche either upto the soil level or 2-3 cm below the soil level with spear effectively controls orobanche.
Leaf curl virus: Ruga tabaci
|This virus is transmitted through whitefly. Different types of viruses cause different symptoms. With mild types the leaves show slight wrinkles and drooping. In case of severe types the leaves and stem curls and leaves become thick and shrinkled and unfit for curing. Sometimes enations are also seen on under side of the upper leaves.|
|Diseased leaves show vein clearing, puckering of leaves, downward curling of leaf margins; leaves become brittle; thickening of veins. Since whiteflies spread disease, conditions such as too much bushy vegetation around tobacco fields serve as breeding places for whiteflies and thus spread the disease. Growing of brinjal, sunflower nearby tobacco fields also encourage build-up of whiteflies.|
- Alternate weed hosts around nursery area are removed and destroyed.
- Yellow-sticky traps (20 cm x 15 cm size galvanized iron sheet painted with yellow colour coated with castor oil) are installed @ 5 per acre.
- If the population of whitefly is 100 per each sticky trap, the following insecticides may be sprayed at 10 days interval commencing from 4 weeks after germination.
- Imidacloprid 200 S.L. @ 2.5 ml in 10 litres of water (or)
- Thiamethoxam 25 WG @ 2gm in 10 lit of water
|Spraying has to be done preferably in the evening hours (4-6 pm) with high volume sprayers and it should be ensured that the both sides of the leaves also are covered. Further the spraying schedule has to be adopted on community basis.|
Tobacco Mosaic Virus: Mormor tabaci
The leaves infested with tobacco mosaic virus show dark or light green patches. This is a contagious disease. This disease spreads through contact by labour and implements used for interculture. If the plants are infected in the early stages they show stunted growth reducing the yield and quality considerably. If the disease occurs when the crop is full grown, the tobacco yields are not affected.
Unclean cultivation, indiscriminately touching diseased and healthy plants, use of tobacco products by workers while working in field, presence of susceptible weeds and crop plants near fields are some of the factors responsible for disease development.
- In areas where TMV is a serious problem resistant varieties like Jayasree (MR), VT 1158, are recommended.
- Since it is a contagious disease, the following sanitary measures are suggested to prevent the disease spread.
- Washing hands in soap water before and after field operations, roguing the diseased plants early in the season and avoiding smoking during field operations.
- Prophylactic sprays with virus inhibitors of plant origin like Basella alba and Bougainvillea spectabilis and neem leaf extracts @ 1% dilution on 30th, 40th and 50th day of planting tobacco are useful.
- Mosaic affected plants are to be removed within 3 weeks after planting and replaced with healthy seedlings.
Root-Knot Nematode: Meloidogyne javanica
This is most prevalent in tobacco grown in light soils. Nematode infested plants do not grow vigorously and show stunted growth. The leaves become yellow in colour. In severe cases the plants die.
In root-knot affected fields, crops like groundnut, marigold, gingelly, chillies, cotton, redgram are to be grown for two seasons. Deep summer ploughing of the field also helps in reducing the incidence of root-knot.
Root Rot: Fusarium Wilt
In FCV tobacco grown in Northern light soils and in burley tobacco grown in agency areas now a days this disease is seen. The leaves on one side of the plants lose vigour and wilt like paralysis on the affected plants with this disease. Gradually complete plant will be affected and finally the plant dies. Initially the root tips show rottening and gradually all the roots rot. The leaves become yellow in colour. The plant will finally die due to lack of supply of water and nutrients.
This disease can be controlled by pouring 75 to 100 ml of calyxin 0.05% solution (calyxin 10 ml in 20 litres of water) at the plant bases of infected plants and the surrounding plants. This solution should not be sprayed on the plants.
By pouring a mixture of Dithane M45 20 g and carbendazim (bavistin) 5 g in 10 litres of water at the plant bases of infected plants and surrounding plants @ 75 to 100 ml. This solution can also be sprayed on the plants.
Powdery mildew : Erysiphe cichoracearum
- Apply sulphur powder (200 mesh) @ 40 kg/ha to soil between plant rows 6-8 weeks after planting. Ash or sand is to be mixed with sulphur for easy application. Care to be taken that sulphur does not fall on tobacco leaves. Recommended for black soils only.
- Spraying 0.2% Karathane or Thiovit or 0.05% Bavistin just before the disease sets in and repeating the same at 10-12 days interval if necessary.
- Using resistant varieties like Swarna or Line 2359 developed at Central Tobacco Research Institute, in disease endemic areas.