Greater yam is predominantly a tropical plant. The crop cannot withstand frost and excessively high temperatures. Temperature around 30ºC and rainfall of 120-200 cm distributed throughout the growth period are ideal. Day length greater than 12 hours during initial stages and shorter day length during the later part of the growing season favour satisfactory tuber formation. Yam requires loose, deep, well-drained, fertile soil. The crop does not come up well in waterlogged conditions.
 

Season

Sweet potato requires a warm humid tropical climate with a mean temperature of about 22 ºC. Though sensitive to frost, it can also be grown in the hills up to an altitude of 1500-1800 m as a summer crop. Under rainfed conditions the crop requires a fairly well distributed annual rainfall of 75-150 cm. Being a photosensitive crop, sunny days and cool nights are favourable for better tuber development. The crop can be grown on a variety of soils having good drainage, but grows best in fertile sandy loam soils. Heavy clayey and very light sandy soils are not suitable for proper tuber development.

Season

Rainfed crop: June-July, September- October

Irrigated crops: October-November (for uplands) and January- February (for low lands)

Varieties

Improved varieties: H-41, H-42, Sree Nandini, Sree Vardhini, Sree Rethna, Sree Bhadra, Kanjanghad, Sree Arun, Sree Varun and Sree Kanaka.

H-41 _ Variety with excellent cooking quality, sweet tubers and duration of 120 days.

H-42 _ Variety with excellent cooking quality, sweet tubers and duration of 120 days.

Sree Nandini _ Early maturing, drought tolerant variety with 100 - 105 days duration and suited as catch crop in paddy fallows.

Sree Vardhini _ Early maturing, carotene rich variety for food and feed with a duration of 100 - 105 days.
Sree Rethna _ Early maturing, carotene rich orange fleshed variety with 90 - 105 days duration.
Sree Bhadra _ Early maturing, (90 days), trap crop for nematodes.
Sree Arun Early maturing, (90 days),

Sree Varun highly palatable varieties.

Sree Kanaka _ Short duration (75 - 85 days) variety with very high carotene (8.8 - 10 mg/100 g).

Kanjanghad _ KAU variety obtained through selection and duration of 105 - 120 days

Local varieties: Badrakali Chuvala, Kottayam Chuvala, Chinavella, Chakaravalli, Anakomban.

Seeds and sowing

Sweet potato is propagated by means of vine cuttings. To obtain vine cutting, raise nurseries from selected tubers using the following method. Eighty kg of medium sized weevil free tubers (each of 125-150 g) are required for planting in the primary nursery area (100 m² to plant one hectare).

Plant the tubers at a spacing of 30-45 cm on ridges formed at 60 cm apart and replant in secondary nursery of about 500 m² area at a spacing of 25 cm. Apply urea 15 days after planting at 1.5 kg / 100 m² in the
primary nursery. To ensure better plant growth in the secondary nursery, 5 kg of urea has to be applied in two split doses on 15^th and 30^th day after planting. Vines obtained from the freshly harvested crop are also planted in similar nursery area to obtain sufficient planting material. Cuttings obtained from the apical and near apical portions of the vines are preferable for planting in the main field. Storing of cut sweet potato vines with intact leaves, in bundles covered with banana leaves (dipped in water) and kept under shade for two days prior to planting is recommended. Irrigate the nursery every alternate day during the first 10 days and once in 10 days, thereafter. Vines will be ready for planting on the 45th day.

In the main field, plant vine cutting of 20-25 cm length on ridges 60 cm apart and at a spacing of 15-20 cm between the vines. The cuttings can also be planted on mounds taken at a spacing of 75 cm x 75 cm. On the top of each mound, 3-6 cuttings can be planted. Plant the vine cuttings with the middle portion buried deep in the soil and the two cut ends exposed to the surface. Ensure sufficient moisture in the soil for early establishment of the cutting. Provide adequate drainage and prevent water logging.

Land Preparation

Make the soil to a fine tilth by ploughing or digging to a depth of 15-25 cm. Make ridges 25-35 cm high, 60 cm apart for planting vines.
 

Manuring

Apply cattle manure or compost at 10 t ha-1 at the time of preparation of ridges. The recommended N:P₂O₅:K₂O dosage for sweet potato is 75:50:75 kg ha-1. For the reclaimed alluvial soils of Kuttanad, the recommendation is 50:25:50 kg/ha. Apply N in two equal split doses, the first at the time of planting and the second 4-5 weeks after planting. Apply full dose of P₂O₅ and K₂O at the planting time.

Irrigation

When grown as irrigated crop, provide irrigation once in 2 days for a period of 10 days after planting and thereafter once in 7-10 days. Stop irrigation 3 weeks before harvest. But one more irrigation may be given 2 days before harvest. IW / CPE for higher tuber yield in non-rainy periods is 1:2 (approximate interval of 11 days). The application of N and K₂O at the rate of 50 kg/ha is recommended for the crop grown under irrigation.

Aftercultivation

Conduct two weeding and earthing up operations about 2 weeks and 5 weeks after planting. The top dressing of fertilizers may be done along with the second aftercultivation. Prevent development of small slender tubers at the nodes by turning the vines occasionally during active growth phase.

Rotation and mixed cropping

Under irrigated conditions, sweet potato can be rotated with rice and planted during December-January after harvest of the second crop of rice. As a mixed crop, it can be grown along with colocasia, elephant foot yam etc. Under rainfed conditions, green manure crops such as kozhinjil and sunnhemp can be grown after harvest of the sweet potato and later incorporated into the soil at the time of land preparation for the succeeding crop.

Plant protection

Integrated control of sweet potato weevil

a) Remove and destroy the crop residues of the previous crop.

b) Use healthy and weevil-free planting materials.
c) Apply Chromolaena odorata leaves as mulch @ 3 t ha-1 at 30 DAP.

d) Trap adult weevils using sweet potato pieces (of about 6 cm diameter) of 100 g size, kept at 5 m apart during 50 to 80 DAP at 10 days interval. Tubers may be cut and kept inside wire cages to avoid rat damage.

e) Use pheromone traps (3Z dodecenyl 2E butenoate).

Harvesting

The duration of the crop depends on the variety; but in general, the crop can be harvested in about 3.5 _ 4 months after planting. Harvest the crop when leaves begin to turn yellow and the tubers mature. The maturity of tuber can be ascertained by cutting fresh tubers. The cut surface will dry clear if the tuber is mature and becomes dark green if immature. Harvest the crop by digging out the tubers without causing injury.

Season

The main planting seasons are April-May with the onset of southwest monsoon and September-October with the onset of north-east monsoon. Planting can also be done during February-April, provided sufficient moisture is made available through irrigation. For maximum tuber production, April-May planting is preferred because the crop can effectively utilize both the monsoons. The second best season is September-October.
 

Varieties
H-97: This is a semi-branching variety, tolerant to mosaic disease with duration of 10 months. But the harvest can be prolonged even up to 16 months. The starch content is 30 per cent.

H-165: This is a non-branching type with poor cooking quality having eight months duration. It is tolerant to mosaic but susceptible to wilt disease. The starch content is 24.5 per cent.

H-226: This is a semi-branching type with medium cooking quality having 10 months duration. It is moderately susceptible to mosaic. The starch content is 29 per cent.

M-4: This is an erect type with excellent cooking quality having 10 months duration. The starch content is 29 per cent.

Sree Visakham: This is a semi-branching type with yellow coloured flesh having 10 months duration. It shows high tolerance to mosaic and low susceptibility to pests like red mites, scale insects, thrips etc. The starch content is 26 per cent and vitamin A 466 IU.

Sree Sahya: This is a predominently semi-branching type with 10 months duration. It shows high tolerance to mosaic and low susceptibility to pests like red mites, scale insects, thrips etc. The starch content is

30 per cent.

Sree Prakash: This has seven months duration and the yield potential is 30-40 t ha-1.

Kalpaka: This is a non-branching type with six months duration and is suited as an intercrop of coconut in reclaimed alluvial soils of Kuttanad.

Sree Jaya: This is an early variety with seven months duration and excellent cooking quality. Tuber contains 24-27 per cent starch and is low in cyanogens.
 

Sree Vijaya: This is an early variety with 6-7 months duration and excellent cooking quality. Tuber contains 27-30 per cent starch and is low in cyanogens.
 

Sree Harsha: This has 10 months duration and good cooking quality. Tuber contains 34-36 per cent starch. They are non-bitter and ideal for culinary purposes and the high starch content makes it suitable for preparing dried chips.

Nidhi: This is a high yielding early variety with 5.5-6 months duration. It is tolerant to mosaic and moisture stress. Tuber contains 26.8 per cent starch and 20 ppm HCN.

Vellayani Hraswa: High yielding early variety with 5-6 months duration. It cannot tolerate drought. The cooking quality is very good. Tubers contain 27.8 per cent starch and 53 ppm cyanogen.

Sree Rekha: It is a top cross hybrid with 10 months duration. Tubers contain 28.2 per cent starch with excellent cooking quality.

Sree Prabha: It is a top cross hybrid with 10 months duration. Tubers contain 26.8 per cent starch with good cooking quality.

Seeds and sowing

Tapioca is propagated from cuttings. Select mature healthy stems free from diseases or pests. Discard about 10 cm from the lower mature and about 30 cm from the upper immature end. Stems should be cut into setts of 15-20 cm length using a sharp knife. About 2000 stems are required for planting one hectare. Harvested stems are to be stored vertically in shaded and well-aerated places. Spraying dimethoate (0.05 per cent) on the stem will control scale insects.

Pit, flat, ridge or mound method of planting can be adopted depending upon soil type, topography of land and elevation so that waterlogging is avoided. Pit followed by mound is found to be the best method of planting. Plant the cuttings vertically after smoothening the lower cut end, at a depth not exceeding 4-6 cm. Adopt square method of planting at a spacing of 90 cm x 90 cm @ one cutting per pit. It is preferable to adopt 75 cm x 75 cm spacing for non-branching varieties like M-4.

Gap filling should be done within 15 days after planting preferably with longer setts of 40 cm length. Sree Visakham is a choice variety recommended as an intercrop in coconut gardens. Optimum plant population is 8000 plants per ha with 90 cm x 90 cm spacing.

Land Preparation

Before planting, plough the field 2-3 times or dig to a depth 25-30 cm depending upon soil type to establish a deep porous field in which the setts are to be planted.

Minisett planting technique for quality planting material
For producing minisetts, mature, disease free stems preferably those obtained from indexed meristem culture should be selected. Two node cuttings are taken from these stems using a sharp hack-saw. Top one-third portion is usually discarded in the traditional system, however in the minisett technique, it is fully utilized. The tip of the stem (about 5 to 6 cm long) is carefully cut without causing damage. For preventing dehydration, it is advisable to place the tip cuttings in water. The stem just below the growing tip is very tender with prominent axillary buds. Hence, from this portion, cuttings with four nodes are taken instead of two as the latter may easily get dried up.

Preparation of nursery:

Select an area with well drained soil and irrigation facility. Shade net house of 35 per cent shade is ideal for the germination and growth of minisetts. Mark out the length and breadth and make raised beds of soil : sand mixture in equal proportion. The beds could be of convenient length and width not exceeding 1 m. An area of 220 m² nursery is required for producing minisetts for planting one hectare of land. Two node cuttings are planted end to end horizontally, about 5 cm deep inside the soil, with the buds facing either sides. Tip cuttings and four node top setts should be planted erect at 5 cm x 5 cm spacing to prevent decay due to excess moisture in these tender parts. Minisetts would sprout in a week's time. Mosaic virus infected plants, if any found, should be rogued off as soon as such symptoms are expressed, to keep the nursery disease free. The minisetts will be ready for transplanting in about three to four weeks time. After the basal application of recommended manure in the main field, ridges of 30 cm height are taken with a spacing of 45cm between the ridges and planting is done on the ridges at a spacing of 45 cm. Multiplication ratio by this process is enhanced to 1: 60 as against the traditional method 1:10.

Manuring

Cattle manure or compost may be applied at 12.5 t ha-1 during the preparation of land or while filling up the pits so as to provide about 1 kg of organic manure per plant. Apply fertilizers N:P₂O₅:K₂O at the rates (kg ha-1) shown below:

H-97 and H 226 :              75:75:75
H-165, Sree Visakham,    100:100:100

Sree Sahya:             

M-4 and local:                 50 :50 : 50

N and K₂O may be applied in three split doses, i.e., 1/3 basal, 1/3 two months after planting and 1/3 three months after planting. Dose of P₂O₅ can be reduced to half where the crop is grown for more than 3 years under full dose of recommended fertilizers, since under such situation there would be build up of soil P.

For August-September planted tapioca, apply half N, full P₂O₅ and half K₂O basally with first digging and weeding. The remaining quantity of N and K₂O may be applied 45 days after planting at the time of intercultivation.

In the acid soils of Kerala, 50 per cent of K requirement can be substituted by NaCl.

Note: N:P₂O₅:K₂O at 50:50:100 kg ha-1 is recommended for Sree Visakham when grown as an intercrop in coconut garden. Higher levels of N tend to increase HCN content of the tubers.

Soil application of Mg as MgSO₄ @ 20 kg ha-1 (1.62 g/plant) and Zn as ZnSO₄ @12.5 kg ha-1 (1g/plant) in small channels around the mounds within 2 months of planting cassava providing an interval of 2 weeks between the application of these fertilizer enhances tuber yield and quality. When they are used continuously, their application rates can be fixed based on their status in the soil following the table given below:

Aftercultivation

Keep the field free of weeds and maintain soil loose by 2-3 shallow diggings or hoeing upto 90 days after planting followed by light earthing up. Retain two shoots on each plant in opposite directions and remove excess shoots about 30 days after planting.

Irrigation

Under conditions of well-distributed rainfall, tapioca grows well as a rainfed crop and irrigation is not necessary. However, the crop has to be irrigated to provide sufficient moisture under conditions of prolonged dry periods after planting. When the crop is grown under irrigation, yield increase of 150-200 per cent over the rainfed crop has been observed.

Furrow irrigation with 25 mm water at 100 mm CPE and alternate furrow irrigation with 50 mm water at 75 mm CPE require only less water and labour for optimum yield. Approximate irrigation interval schedules will be 27 and 20 days respectively in summer months.

Intercropping in tapioca

Tapioca is planted at a spacing of 90 cm x 90 cm and it takes about 3-3.5 months time to have enough canopy to cover the land. So it is possible to have an intercrop of groundnut during the early stages of tapioca crop. Bunch varieties like TMV-2, TMV-7, TG-3, TG-14 and Spanish improved are preferred for intercropping in tapioca. The best season for sowing groundnut is May-June. Immediately after planting of tapioca setts, groundnut seeds are sown at a spacing of 30 cm between rows and 20 cm within rows, so that two rows of groundnut can be accommodated in between two rows of cassava. A seed rate of 40-50 kg ha-1 is recommended for dibbling one seed per hill. Only well-matured and bold seeds are to be selected for sowing. In acid laterite soils of Kerala, apply 1000 kg ha-1 of lime as basal dressing. A basal dose of 50:100:50 kg N:P₂O₅:K₂O per ha should be given uniformly to both the crops. One month after sowing of the seed, 20 kg each P₂O₅ and K₂O and 10 kg N / ha-1 may be given to the intercrop along with earthing up. Once pod formation has started (i.e., 40-45 days after sowing) the soil should not be disturbed, as it will affect the pod development adversely. The groundnut crop matures in 105 to 110 days. After the harvest of pods, the haulms are incorporated in the soil along with a top dressing of 50 kg each of N and K₂O per ha for the main crop. By adopting this practice, 20-25 per cent additional income can be obtained.

In sandy areas intercropping tapioca with cowpea / groundnut / black gram / green gram may be recommended giving a spacing of 20 cm on both sides of the ridges. The non-trailing grain cowpea variety V-26 is recommended as a companion crop along with tapioca. For a pure crop of tapioca or for a cropping system involving tapioca as the main crop and the pulse crop suggested above, the field may be irrigated once in 36 days to a depth of 5 cm. This recommen-dation is for shallow water table situations. For deep water table situations, the crop may be irrigated once in 24 days to a depth of 5 cm.

Plant protection

Cassava mosaic disease (CMD)
The disease is transmitted by a white fly Bemisia sp. As a rule, only stem cuttings from disease free plants should be used for planting to minimize the spread of the virus disease. For this purpose, tagging of disease free healthy plants for selection as planting materials must be practised from September to December. All plants showing even very mild symptoms must be rejected. Mosaic tolerant varieties such as H-97 may be used to minimize economic loss of tubers.

Production of disease free planting material of tapioca through nursery techniques

Setts of 3 to 4 node cuttings from apparently disease free plants are collected and planted in the nursery at a very close spacing of 4 x 4 cm so that about 500 setts can be accommodated in one square metre land. Daily watering of the setts has to be done for the first 10 days and on alternate days afterwards. Screening of CMD symptoms may be started 10 days after planting. Setts showing even mild symptoms are to be removed and burnt. This must be continued up to 20-25 days, by that time healthy seedlings can be transplanted to the main field. Supplementary irrigation may be given in the transplanted field till they get established. Screening for disease symptoms and roguing of infested plants may be continued in field at weekly intervals upto harvest. The selected healthy stems are again cut into minisetts and subjected to nursery and field screening. By adopting this technique it is possible to produce healthy plants.

Leaf spot

Spray 0.2 per cent zineb or 1 per cent Bordeaux mixture for control of leaf spot.

Bacterial blight

Bacterial blight is a disease noted in severe proportion in certain parts of Kerala. Chemical control is not effective. Use of resistant or tolerant varieties is the only method of control. Among improved varieties, H-97, H-226, H-1687 and H-2304 are tolerant to the disease while H-165 is highly susceptible. Among the local varieties, M-4, Paluvella, Pichivella, Parappilppan, Anamaravan etc. are tolerant to the disease.

Red spider mites and scale insects

Red spider mites in the field and scale insects under storage are important pests of tapioca. Under field conditions light infestation of mites can be controlled effectively by spraying the crop with water at 10 days interval from the onset of mite infestation. In the case of very severe infestation, the crop can be protected by spraying 0.05 per cent dimethoate at monthly intervals from the time of appearance of mites. The stem may be sprayed with 0.05 per cent dimethoate before storing as a prophylactic measure against the scales.
 

Termites

To control termites infesting planted setts, sprinkle a little of carbaryl 10 per cent or chlorpyrifos in the mounds prior to planting.

Management of storage pests of cassava

Treating chips with granular salt (3 per cent), sun drying thoroughly and storing in gunny bags in godown are very effective against Araecerus fasciculatus and Sitophilus oryzae.

Harvesting

Tapioca becomes ready for harvest 9-10 months after planting. Hybrid varieties like H-226, H-97 and H-165, when grown under recommended management practices have recorded yields up to 40-50 t ha-1 of raw tuber.