1. WEED CONTROL
Experience has shown that lack of weed control during the five weeks immediately after planting out can reduce the yield of the crop by as much as 50%. This is the period when the plant is becoming established and starting to grow, and weed competition at this stage will deprive it of vital water and nutrients. After 5 weeks, the tobacco plant begins to grow rapidly and the crop will grow strongly and smother out any weed competition.
It is of great importance to keep the young crop clean. Weeds are dealt with in two ways; through mechanical cultivation and with herbicides.
Once the crop has been planted and the gaps caused by plants that have died have been re‐filled, labour should be available for weed control.
The first mechanical operation is to put a ridger through the crop to repair the ridges which may have been damaged during the planting out of the crop. This should be followed immediately by hand weeding to kill the weeds on the top of the ridges, and to improve the shape of the ridges. These operations should be repeated 2 weeks later and again after 4 weeks if necessary.
Tobacco is a broad leaved plant, and broad spectrum herbicides, such as those used on maize, will kill the tobacco plants. The herbicides which are registered for use on tobacco will only target the weed grasses:
Oxadiazon ‐ This is a pre‐emergence herbicide that is sprayed onto a clean soil surfacebetween the ridges. The seedlings should be strong and actively growing. The chemical is applied at 4 to 5 litres/ha in a spray mixture of 500 litres/ha to ensure an effective cover. When applied to dry soil one should irrigate as soon as possible afterwards, as damp soil keeps the chemical active. Dry soil will render the chemical inactive.
Haloxyfop‐R‐methylester ‐ This is a post‐emergence herbicide that is sprayed onto the landson actively growing weeds. It is sprayed at a rate of 0,4 to 1,5 litres/ha on normal grasses and at a rate of 4 litres/ha on bush buffalo grass in a minimum of 50 litres of spray mixture per hectare. At irregular growth stages one should spray at the correct stage and then again a second time when they re‐appear at the right stage.
- DISEASE CONTROL
Disease can affect tobacco at three vulnerable stages:
in the seedbeds;
In the lands while the plant is growing; and In the barns while the crop is being cured.
Diseases of the tobacco seedbeds have already been dealt with. The following diseases can affect the crop while growing in the lands.
Wildfire and Angular Leaf Spot: Bacterial diseases which cause spotting of the leaves and the spots
become dead tissue. The result of this is to reduce the amount of carbohydrate in the leaf and this in turn reduces the amount of sugar in the leaf after curing. Nitrogen is concentrated in the diseased areas, and the amount of nicotine is reduced. The ratios of sugar to nicotine and nitrogen are altered, giving an undesirable smoke. Once over 20% of the leaf is diseased, there is a rapid fall in quality. These two diseases must be controlled in the seedbeds as there is no control for them in the lands.
AnthracPCIe: A fungal disease causing brown spots on the leaves of the plant. This disease must becontrolled in the seedbeds as there is no control for it when it occurs in the lands.
Frogeye: A fungal disease which causes round spots, white or pale brown in the centre with anarrow dark brown margin. High levels of infection reduce the sugar levels after curing, upsetting the sugar‐nicotine‐nitrogen ratios and reducing the smoking quality of the leaf. There is no cure for this disease in the lands.
Mosaic: Caused by a virus and gives the leaves a mottled appearance. Again, the sugar in the leaf isreduced, but the main affect is a reduction in yield which can be as much as 70%. The only control for mosaic is strict hygiene in the seedbeds and in the early stages of the crop in the lands. No smoking or snuff taking should be allowed. Workers must wash their hands before handling the tobacco plants.
Alternaria: A fungal disease causing large irregular brown areas on the leaves and stems. Prevalentat the top of the plant and develops under humid, hot conditions. The disease reduces the sugar in the cured leaf and increases the nitrogen, giving a harsh acid smoke. There appears to be an increase in potassium in the infected leaf, giving a high burn rate, and the leaves are more prone to shattering.
White Mould: A fungal disease causing circular white powdery patches on the lower leaves. The IETvarieties of tobacco are resistant to the disease.
Rosette and Bushy Top: Virus diseases which affect the young leaves at the top of the plant, andthey are transmitted by aphids. The aphids can be killed by applying Mercaptothoin to the soil, to be taken up by the plant, or by spraying the plants with Dimethoate or Imidacloprid solution.
3. PEST CONTROL
Cutworms, false wireworms, white grub: Attacks from these pests can be prevented by addingChlorpyriphos to the water at planting.
Budworm, Leaf miner, Laceworm: The crop should be sprayed with Acephate at the first sign ofinfection, but protective clothing must be used when handling this chemical.
Aphids: Important because they transmit Rosette and Bushy Top. See above for control.
Eelworm This is the most important pest of tobacco. It is controlled by using sound rotations, byfumigating the soils in the seedbeds, and by applying EDB to the planting holes before the plants is put into the lands.
4. TOPPING AND SUCKERING
This is the term used to describe the removal of the flowers from the top of the plant. If this is not done, the plant will flower and then produce seed, causing nutrients to be taken from the leaves. The effect of topping is to increase the yield of leaf by increasing its size, and to alter the chemical composition of the leaf. Vigour is transferred from the flower to the leaves. Failure to top each plant results in low yields of poor quality tobacco.
The young leaves on the plant respond more readily to topping than do the older leaves at the base of the plant. Therefore, the earlier the plant is topped, the more leaves are protected. Early topping is done when the flower bud is just forming, and no colour of the flower is showing. Topping is done 8 to 10 weeks after growth has started in the lands. The effects of early topping are as follows:
A heavy‐bodied leaf with a dark colour is produced; The leaf has high nicotine content;
Possibility of infection by white mould is reduced; and
The leaves of the upper and middle part of the plant are more fully expanded.
Late topping is carried out when the plant is in full flower, 12 weeks after growth has started in the lands. Late topping has the following effects:
The leaf is light‐bodied and lighter in colour; The leaf has lower nicotine content;
The susceptibility of the middle and upper leaves to white mould infection is increased; The middle and upper leaves are less expanded.
The yield of the crop is reduced by about 15 kg per hectare for each day that topping is delayed after the extended bud stage (the time when the flower bud is just beginning to open). As topping affects the quality or style of the tobacco as well as the yield, the farmer has to decide on the exact time of topping. He is guided by his experience and the type of tobacco that he requires. In general, it is better to top higher with a crop that is growing vigorously than one that has been planted out early and top lower with a crop that is growing less well. 18 Cured leaves per plant may yield the same return as 24 leaves, and handling costs may be lower.
The number of times the crop has to be topped depends on how the flower buds develop. Ideally the crop should all flower at the same time, but this never happens in practice. Usually topping is carried out twice and sometimes three times. When topping, the labourers walk through the crop and break off the flower buds by hand close to the top leaf. Thick stems should be cut with a knife, which makes a clean cut and prevents bacterial diseases getting into the stem of the plant. The employment of three labourers per day for each hectare of land which is planted to tobacco will be required for topping three times.
The tobacco plant has three potential suckers in each axil, and these develop at intervals. A sucker is the shoot which grows out from the base of the leaf, the point where the leaf joins the stem. If the suckers are allowed to grow, the yield of the crop is reduced and the size and quality of the leaf is affected. Good suckering can increase the yields of the crop by 250 kg/ha.
Full suckering, removing the suckers on the plant, together with early topping, produces the best quality leaf; having a heavy body, dark colour, high nicotine content and good leaf expansion. Late topping and partial suckering gives a light‐bodied leaf which ripens rapidly.
Suckering can be done in two ways, by hand or by using a chemical, Suckeride. When suckering is done by hand, the suckers should not be allowed to grow longer than 100 mm, and a gang of labourers go through the crop once each week removing the suckers. Suckeride is a chemical substance which burns the suckers on contact and prevents them from growing. It is applied by taking a small amount in a cup and pouring this down the stem of the plant just above the leaf joint. This is done twice while the crop is growing in order to kill any suckers that may have been missed the first time. American tobacco growers use systemic suckerides which are sprayed on and taken up by the plant. The systemic suckeride moves inside the plant to the points where the suckers grow and then kills them off. Suckering a crop 5 times by hand would require 26 labour days per hectare.