1. VARIETIES OF TOBACCO
Tobacco varieties can only be grown if they have been approved by the controlling body of the tobacco industry in that specific country. Leaf tobacco can be refused by co‐operatives if the cultivars have not been approved. All varieties have undergone a trial and test period to establish yield, chemical constituents, physical properties, smoke characteristics and adaptability. Only after this process can the cultivar be approved for cultivation. It then further undergoes a trial period of 1 or 2 seasons before it is accepted as a commercial variety. The available varieties and their characteristics are:
TL 33: This variety of flue‐cured tobacco is resistant to powdery mildew, tobacco mosaic virus and strains 1 and 2 of black shank. It has adapted to a wide range of areas and climatic conditions. It is a good looking, open grained tobacco that has a bright lemon colour with a pleasant aroma. Under the correct conditions it has the ability to produce an orange coloured tobacco.
TL 38: This variety is resistant to powdery mildew and strain 2 of black shank disease. It yieldstobacco with an open grained leaf and has good aromatic properties. It is well adapted for production in the various tobacco producing areas with different climatic conditions. TL 38 tends to produce leaf with more body and a higher nitrogen and alkaloid content than TL 33, which makes it better adapted to lighter, sandy soils.
CDL 28: A dark air‐cured variety that is resistant to powdery mildew, tobacco mosaic virus and has a slight tolerance to red rust. It is the first variety grown in South Africa that has a wide tolerance to disease. It was released as a dark air‐cured tobacco but under the correct conditions and practices it can be light air‐cured.
TN 86: This variety has fairly upright leaves on the stem which can aid in the handling of the tobaccoduring harvesting. It is resistant to black shank and to some strains of the potato virus. TN 86 must be grown in areas that have the correct climatic conditions. The conditions in the shed must be carefully monitored. It produces tobacco of high quality and has a good yield.
The variety chosen should be based on soil type, curing facilities, the type of leaf the farmer is aiming to produce and, to some extent, the farmer’s personal preference.
2. PREPARATION OF SEEDBEDS
Seedbeds should be prepared by removing all grass, weeds, stones and rocks from the area and mowing all the surrounding grass to stop weed seeds being blown onto the seedbeds.
The whole site should be ploughed as early as possible in February to a depth of 30 cm, to allow all organic matter to decompose before the seed is sown. The surface should be broken down to a fine, even tilth by harrowing or by crumbling the clods by hand, using hoes. Water can be used to facilitate this process. All types of grass and stones should be removed. Irrigate the soil to field capacity to a depth of 0,5 metres. Once the site has been ploughed and worked down to a fine tilth, the actual beds can be laid out. These are normally 1 metre wide and 30 metres long, with a path of 0.5 metres between each bed.
The soil from the path can be thrown onto the beds and leveled off to give each bed a slight rise, to allow water to run off and prevent it from collecting in hollows and forming pools on the bed.
Figure 1: Dimensions and shape of beds
The whole seedbed site should be near the farmhouse to allow for easy supervision, and the site large enough to allow for a four‐year rotation using a grass such as Rhodes grass. The rotation would consist of one year seedbeds and the three following years under grass, so that each year the seedbeds are in an area that has had grass growing on it for the previous three years. This is to avoid a build up of nematodes in the soil. There should be a good supply of clean water, preferably from a borehole, and the whole site should be securely fenced. 90 ଶ of seedbed will supply enough plants to plant out 1 hectare of tobacco land.
The idea of fumigating the soil of the tobacco seedbeds is to kill weed seeds, grass seeds, nematodes and fungi and so provide healthy growing conditions for the young plants. All seedbeds must be fumigated before fertiliser and seeds are sown. There are a number of chemicals which can be used for fumigation, but the most popular is a liquid called methyl bromide, which rapidly becomes a gas when it comes into contact with the air. This gas is highly poisonous and should not be inhaled; a gas mask should be worn when handling the containers. Methyl bromide is sold in 680g containers and is applied at the rate of 50g per square metre of seedbed. A seedbed 30 metres long and 1 metre wide gives an area of 30 square metres and would require 1 500g of methyl bromide. The practice is to cover each seedbed and path with a purpose made plastic sheet. These sheets are 30 metres long by 3 metres wide, to allow them to form a tent over the beds. Approximately 20 hessian bags of mulch should be put under each tent so that the mulching material can also be fumigated. The soil should be watered 1 week before fumigating, and damped down again just before the tents are put into position. The mulch in the bags should be watered, the tents put over each seedbed, and the edges buried and watered to make sure that no gas can escape. The gas is released into the tent by means of tubes which are then sealed off and the tent is left in place for 48 hours. The normal practice is to fumigate the beds in succession using the same tents over again. The plastic sheets which form the tents should be carefully checked for holes so that the gas does not escape.
The ideal fertiliser for use on tobacco seedbeds is a mixture containing 6N 17P 6K and 0.04% of Boron. The potash (K) should be in the form of Sulphate of Potash, as Muriate of Potash produces chlorine, which affects the young plant. The mixture is applied at the rate of 1 kg per 10m² of seedbed, and this is done by hand, as evenly as possible. A good plan is to divide the bed into 4 and apply each quarter separately. The fertilisers should be mixed into the top 50 mm of soil. While the seedlings are growing, it may be necessary to top dress with nitrogen if the leaves start turning yellow. Nitrate of Soda should be used at the rate of 10 g per m².
Once the beds have been marked out, cultivated to a fine tilth, fumigated and fertilized, they are ready for sowing. Sowing is done from June to late July, to give the plants time to grow enough for transplanting in October to November, which is usually at the beginning of the rainy season. Tobacco seed is very fine and should be mixed with water and watered onto the seedbeds with a watering can.
A gram of seed contains between 8 000 and 18 000 individual seeds and is sown at the rate of 0,5 to 0.75 g per 10m² or 80 g for 100m². A gram of seed occupies a volume of 3,6 ml. It is a good idea to have a measuring cylinder calibrated to 3.6 ml so that the correct amount of seed can be easily measured out.
The seed is mixed with water in a watering can and watered onto the seedbed, stirring and walking at the same time. Any dregs should be sown by adding fresh water and repeating the watering on the same bed. The ideal is to have 500 plants per m² at germination and about 200 plants per m² fit to transplant.
The seedbeds and young seedlings are protected by applying mulch to the seedbed immediately after the seed has been sown. The material commonly used is the fine stems of grasses such as lovegrass and some of the vlei grasses, which are cut into lengths of 25 cm. This chopped grass is broadcast evenly over the seedbeds and paths so that the soil surface is just covered. Remember that the mulch has to be fumigated before use and is chopped up, bagged in hessian or jute bags (not plastic bags) and put inside the tents at fumigation. As the seedlings grow, the mulch is gradually thinned out by removing some of the chopped grass.
Other materials used as mulches are fumigated and then spread 6 to 8 mm deep. Perforated plastic sheets in the form of a tent are laid over the beds after sowing. Watering is done through the plastic sheet, and the plastic forms a small greenhouse and speeds up germination and the early growth of the seedlings. It is also a protection against frost and is particularly suitable for seedbeds sown in early June. The plastic sheet is removed 20 to 30 days after the seed has been sown and can be used again the following year.
3. CARE OF SEEDLINGS
This is a very important factor in obtaining good seedlings. The usual mistake is to water too much in the early stages of germination and growth. This causes the seedlings to be chilled and also leaches nutrients out of the soil. A general guide is 3 liters of water per square metre of seedbed early on, increasing to 8 ℓ/m2 later on. Remember that the seedbeds are watered up to field capacity before the seed is sown, and the mulch which is applied after sowing helps to prevent water loss by evaporation from the soil surface. The frequency of watering should be as follows:
|Table 1: Recommended Frequency of Watering of Tobacco Seedlings
|Stage of Growth
|Frequency / Day
|3 – 5 Times
|Keep surface always moist but not wet.
|Roots can draw water from top 1 – 2 mm of soil.
|Seedlings 2 cm in Diametre
|Roots can draw water from top 3 – 5 mm of soil
While the seedlings are growing, it is advantageous to bore holes in the seedbed with a soil auger to assess the moisture level below the soil surface, and to make sure there is no drying out of the soil at any depth. During watering, both the beds and the paths should be moistened.
It is important to harden off the young plants in the seedbed before planting out, so that they can withstand the shock of being transplanted and any temporary drought after they have been planted out. Once they have been transplanted and have resumed growth, hardened off plants grow faster, attain a larger size, and produce larger and more leaves and a greater yield.
Hardening off is done by withholding water from the plants in the seedbed for periods so that they become wilted. The procedure is as follows:
Commence when the plants are about 15 cm tall. About 2 ‐ 3 weeks before they are due to be planted out, water is completely withheld from the crop;
By 10 o’clock the following morning the plants will be wilted with the leaves and stem looking very limp through lack of water ; and
At this stage, apply a double dose of water to the plants; if they were getting 8 ℓ/m2 give them 16ℓ/m2 and then leave alone until they are again wilted by 10am. Repeat this process until the day before they are due to be pulled and transplanted. Immediately prior to transplanting, give the beds a good watering to help with the pulling of the young plants.
Obviously a good shower of rain during the hardening off period interferes with the hardening off process. On heavy soils, a longer period is allowed for hardening, because these soils hold more water than light, sandy soils.
This is not a hardening technique, but is used to hold back growth by clipping back the leaves of the plants. Clipping is used to delay the growth of the plants and to even up a bed by slowing the growth of the larger plants and giving the smaller plants time to catch up. It is most important to disinfect the shears every few minutes while clipping the plants in the beds, to prevent the spread of mosaic virus disease. This can easily be spread by the hands of the workers and by the shears of the clipper.
PESTS AND DISEASES
The following pests and diseases are likely to be encountered in the seedbeds, and their control methods are shown in the table as follows:
Table 2: Tobacco Seedling Pest and Disease Control Measures