This lecture covers the basics of tea production and shows how the industry has changed with the move from seedling tea to clonal tea.
This has impacted on nearly all the recommendations, from propagation to fertilization. The guidelines given are a basis for a general understanding of the practices used.
Each country will have its own clones and management recommendations.
When using chemicals ensure that the chemical is registered and follow all the instructions on the label.
Dr Michael St.J. Clowes andPeterChard April 2013
- 1. INTRODUCTION
Tea is an ideal crop for smallholder production as it is hardy, vegetative and produces its crop over a long harvesting period (at least 6 months and almost a year at the equator).
Tea requires good management for establishment, but is then ideally suited to smallholder production.
It is important to have a factory close by to process the leaf as transport is a major cost. The key to yield is to pluck on time, to standard and to maintain the plucking table.
These aspects are all within the control of the smallholder provided that holidays, etc. do not disrupt plucking.
The 5 largest producers of tea in the world in 2011, with annual production of made tea in tonnes per annum are:
- China 1.26 milliont
- India 805 000t
- Kenya 345 800t
- SriLanka 318 000t
- Turkey 198 000t
The 5 top African producers are:
- Kenya 345 800t
- Uganda 56 000t
- Malawi 41 000t
- Tanzania 35 000t
- Burundi 9 000t
Most African countries produce tea for their own demand and Mozambique, Zambia, Zimbabwe and South Africa all produce some tea for local consumption.
Tea is sold, and sales quoted, in US dollars or cents per kg of made tea. The auction in Mombasa in Kenya is the most advanced in Africa. Indicator prices of its highest quality Pekoe Fannings (PF) was US$ 3.20 per kg in February 2013. In contrast, the average price of teas auctioned at the combined Colombo (Sri Lanka), Kolkata (India) and Mombasa (CKM) auctions in February 2013 was US$ 2.89 per kg.
Prices have fluctuated from a low of US$ 2.17per kg in Mombasa at the end of 2008. Over the last year the average auction price at the CKM auctions varied from a low of 2.42 in March to a high of
Tea is made from the tea shoot comprising stalk, young leaves (normally 2-3) and unopened buds of the tea plant Camellia sinensis. This is an evergreen tropical plant from South East Asia and has been cultivated for so long that the exact origin in unknown.
There are two recognized groups of varieties of tea:
- Assam Camellia sinensis var. assamica;and
- China Camellia sinensis var.sinensis.
Within each group there are many selections called ‘Jats’ which originate from particular areas. The modern tea industry is based on much higher yielding jats (clones) of improved Assam and China hybrids.
The tree normally grows very tall (6 m, China and 15 m, Assam tea) but is kept to below waist height by pruning. This enables the pluckers to be able to pluck the shoots efficiently from the flat surfaced ‘plucking table’. A combination of pruning, tipping and skiffing (using a horizontal straight cut), are used to create the plucking table from which shoots develop.
Unwanted leaves and stems (e.g. hard dormant banjhi shoots) which appear above the plucking table are broken back and discarded before plucking.
These jats have qualities of taste which are characteristic of a particular region, district or even tea estate. Assam and China teas differ in their type of leaf, their quality and manufacturing processes.
Assam (black tea) leaf is withered and passed between rollers which expose the leaf juices to the air which then oxidize or ferment. The leaves are then dried in hot air which results in black fermented tea.
China (green tea) leaf is heated in steam to destroy the fermenting agent in the leaf before rolling and when dried results in unfermented green tea.
Although the strength and flavour of the liquid brewed from tea leaves is important, tea is drunk mainly for the stimulating effect of the caffeine contained in the leaves, which may be as high as 4% of the dry matter in fine shoots.
Tea leaves contain a range of chemicals called catechinstogether with an enzyme called polyphenol oxidase. When leaves are macerated (crushed or cut) there is mixing of the chemicals and biochemical reactions take place.
The products of these reactions are 2 chemicals that impart the quality to the tea.
- Theaflavinswhich produce brightness and briskness;and
- Thearubiginswhich impart strength andcolour.
- 2. GROWTH AND HARVESTING
A tea tree in its natural state will grow into a small tree. Under cultivation the tea tree is pruned down to a bush convenient for plucking off the ‘plucking table’. The plucked shoots comprise leaves (2 – 3) produced singly and alternately from nodes with a terminal bud which is 10 mm long when flushing and only 3 mm long when dormant (banjhi).
Shoots can be individually harvested by both hands simultaneously. Plucking involves placing the shoot between the first 2 fingers to hold the shoot and then use an upward movement to snap off the shoot which is then left in the palm. Plucking continues until both hands are full. Leaf is then placed into the plucking basket suspended around one’s waist or forehead, dependent on which system the pluckers have become accustomed to and use.
Plucking is a skill that is time consuming but produces the best combination of high quality and yield. Labour has become costly which has prompted a move to using hand held shears (holding trays are attached to collect the shoots) and mechanical harvesting with machines. Machines can be hand held or onwheels.
With tea the leaf growth is the most important single factor because the leaves provide the yield. The traditional Assam jats have large, horizontal leaves, poor light penetration into the bush and a poor response to nitrogen fertilize and produces lowyields.
China tea has narrow, slender leaves, allows for better light penetration and is more responsive to fertilizer.
Tea leaves are produced in flushes at regular intervals with the quantity of leaf produced at each flush being influenced by rainfall, temperature and fertilization. Each flush that is harvested removes apical dominance. This stimulates a new flush of buds to develop into new shoots. Plucking results in a cyclical production of shoots of plucked origin. Shoots also develop as a result of buds of non- plucked origin developing into shoots but are often out of step with the plucking cycle.
When the tea is ‘flushing’, the shoot grows to produce shoots that are harvested once they reach the 2 and 3 leaves and a bud stage. The leaves below these are called maintenance leaves and should not be picked because they are hard, of poor quality and provide the nutrient reserves for theplant:
Figure 1: The leaves of the tea bush that are harvested
During a flush, a series of leaves suitable for harvesting are produced and flushing can be prolonged by good fertilization, particularly with nitrogen. After a flush the plant may become dormant and produce a small terminal bud no longer than 2 – 3 mm in length. This is termed a banjhishoot and gives rise to 2-3 coarse or brittle leaves of poor tea making quality. During this stage of the plant’s growth, no harvesting takes place. The skill in managing a tea plantation consists of making sure that shoots are harvested at the correct stage of growth. If shoots are too young there is a loss in yield and if too old there is a loss in quality. Harvesting at the correct stage optimises both yield and quality. It is extremely important to determine your quality potential and match it with market requirements.
GENERATIONS OF SHOOTS ON THE BUSH AT ONE TIME
This concept is important to understand. This concept was researched in great detail and illustrated by work in Malawi.
The onset of rainfall in early November in Malawi resulted in a large crop peak 42 days later over the Christmas period. The rains release buds from dormancy to produce a large flush of shoots all at one time (a single generation all at once). They grow at different rates and are ready to pluck 42 days later. Plucking these shoots sets up new generations of shoots, now of plucked origin which are cyclically replaced at each plucking. The rate of shoot growth is very dependent on temperature and influenced by vapour pressure deficit (VPD), measured by the difference between wet and dry bulb temperature at 2pm.
The interval between successive pluckings has to be divisible into shoot age. The number of times it divides into shoot age gives the number of generations of shoots on the bush at any one time.
Shoots taking 42 days to mature allows one to pluck on a 7 day cycle (6 generations), a 10/11 day cycle (4 generations) or 14 day cycle (2 generations). Quality is highest with the short 7 day cycle, as selection can be more precise but productivity is low. In contrast the longest plucking round of 14 days produces poorer quality and requires more breaking back of unproductive shoots. It is most productive with the most shoots per m2 and is higher yielding. However the best compromise for yield, quality and production is the 10/11 day round. Economics, the market and the ability to deal with a coarser leaf, by the use of an LTP hammer mill, rather than CTC (cut, tear, crush) rollers, determine the best strategy to use.
REGENERATION OF SHOOTS BY PLUCKING
Plucking removes apical dominance and results in a replacement of more than one replacement shoot per shoot plucked. Shoots build up after pruning and become smaller as more shoots are produced over the pruning cycle. Number of shoots and shoot mass are the main components of field yield.
Shoots (pieces) per kg provides a useful measure of potential plucker productivity and yield potential. Shorter plucking rounds produce fewer and smaller shoots, whilst longer rounds produce more and largershoots.
Originally all plantations were grown from seed but this practice has changed with the use of Vegetative Propagation (VP) methods to produce clones.
Seed gardens used to provide seed for seedling tea. They needed to be isolated by 20 – 30 m from other seed gardens or fields in plucking. The objective is to ensure that insects are not able to come in from elsewhere to pollinate the trees in the seed garden. Trees in a seed garden are grown at 5 – 6 m spacing between trees. Trees should be mulched, fertilized and weeded but notpruned.
The tea bush produces white flowers about 25 mm across which give rise to large green seeds which ripen to a dark brown colour about 15 – 20 mm in diameter. One kg of polyclonal seed will contain 400 seeds.
Seed is harvested daily from the ground and then graded over a 15 mm sieve. Seeds for export are soaked for only 1 hour before the sinkers are removed for germination testing and packing.
Seed for local use is soaked for 24 hours and sinkers used for testing and packing. Testing involves cracking open 100 seeds determining how many are damaged, scarred or cheesy.
Above 97% good (viable) seed is considered acceptable for export. A phytosanitary certificate is also required. Seed must be used as fresh as possible and should be stored in bins or pits with a sand or subsoil medium having a moisture content of 10 – 12%.
Seed germination is tested by placing the seed in a single layer on top of a level bed of sand, 150 mm deep with the ‘eye’ to the side. Sand is sprinkled to just cover the seed which is kept moist by daily watering. Polythene can be used to cover the seed. Overheating or an excessive buildup of humidity must never be allowed. When 75% of the seeds have germinated with 40 mm long roots they may be removed. Germinated seed is graded according to root length, into 3 grades. Cracked seeds should be replaced into theseedbed.
Place seedlings in their separate grades into containers of water for transport to the nursery where pots should be already prepared for planting.
Seedlings are now seldom produced from seed, as vegetative propagation (VP) of selected clones is quicker and produces more uniformmaterial.
Clones are made from cuttings from mother bushes selected for desirable qualities. A number of clones were selected for their performance in drought years. Clones are categorised according to their performance as one of the following:
- Field Clones are good rooters with high yield potential and arehardy;
- Rootstock Clones are very vigorous, produce poor quality tea or are susceptible topests;
- Scion Clones have good quality and plucking characteristics but are poor on their own;and
- Generative Clonesfor production of polyclonal seed have good compatibility at flowering, high yields and goodquality.
Any combination of 5 of the following cultivars was recommend in the past (in Malawi) to establish a polyclonal seed garden: MT12; MT7; M9; S11; M4; C5; C15; S13 and S17.
Clones can be categorised according to these categories within each production area. The Tea Research Foundation (Central Africa) in Malawi provides material to countries in Central and Southern Africa. The clones in Malawi are divided into two groups:
- Field Selections (prefix FS with an additional letter in front for the site e.g. S Swazi Now Nsuwadzi) to produce SFS 150; SFS 180; SFS 204; SFS 210;and
- Progeny Clones (prefix PC) e.g. PC 1; PC 81; PC 87; PC 105; PC 108; PC 110; PC 113; PC 114; PC 117; PC 118; PC 119, etc. as newer clones becomeavailable.
Kenya and Tanzania are countries with their own breeding programs and clones to suit their specific conditions.
SOILS AND CLIMATE
Tea is grown over a wide range of climates, but in the tropics the crop is grown mainly at higher altitudes. Temperature is important with the optimum being from 25°C to 30o C. Above 30oC and with high VPD, leaf function is reduced. Flushing is reduced by lower Winter temperatures and the shorter hours of daylight. Some tea closer to the equator is productive throughout the year. The main plucking season is from December to March inMalawi.
Tea requires 1 000 mm – 1 500 mm of evenly distributed rainfall per year. Tea thrives on humid conditions within the mist belt. Natural rainfall is supplemented by irrigation in the hotter, lower
lying areas and Zimbabwe was the first country in Africa to grow tea under irrigation. While mature tea is not killed by frost, growth and flushing are reduced. The planting of windbreaks helps to shelter the crop from high winds, which can damage the plants and reduce yields. However, when planted close to the tea, they do compete for water, light andnutrients.
The crop grows best on slightly acid, free-draining soils which are moderately fertile. The optimum pH is 5.0 – 5.5, although good yields are obtained on soils as low as pH 4.0. Liming is seldom necessary in tea growing areas unless fertilization acidifies the soil. Once planted, the bushes are left undisturbed for many years. Some tea estates have bushes which have been producing for over 100 years. However, normally the economic lifespan is considered to be from 50 – 80 years, or earlier if beneficial for the planting of superior clones.
Mature tea bushes develop an extensive root-system which accumulates large food reserves which are important to supply re-growth after the bush has been pruned. A deep, well-drained soil is needed to ensure that the roots can develop to their maximum extent.
- 3. PRUNING
Pruning is done for threereasons:
- To form a good framework and ensure the survival of the plant under drylandconditions;
- To maintain an efficient plucking surface (plucking table) by regularly cutting back the canopy; and
- To decentre and encourage lateral growth when the bushes are too spindly and erect after planting.
In Central and Southern Africa the recommendations for irrigated and dryland tea are as follows:
1. First prune to 30 cm at the end of July (6 – 7 months after planting) and then pluck in at 40 cm. Delay a year if the plants are weak or planting was late;and
- If pruning is delayed to the following July, skiff (horizontal straight cut) plants to 55 cm in December/January and pluck in at 60cm.
Decentering is an option for spindly plants which are cut back to 10 – 25 cm. Technical advice should be sought before doing this and skilled operators must be used. Decentering removes apical dominance to force young plants to grow laterally (i.e. outwards).
Table 1: Under rain fed conditions with 2 pruning programs (See 1 and 2 above):
The height rise is 2 cm per annum (i.e. prune back to 2 cm above previous prune) to reach a maximum height of 40 cm.
Pruning time is brought forward by 10 – 14 days each year.
Tea in the 7th to 10th year is on a 2 year pruning cycle. Tea after the 10th year can be pruned on a 3 year cycle.
Table 2: Under irrigated conditions also with 2 programs (see 1 and 2 above):
The height rise is 5 cm every 2 years (i.e. prune back to 5 cm above previous prune) to a reach a maximum height of 40 cm.
Pruning time is brought forward by 14 – 21 days at each prune.
Tea from the 7th year onwards is put onto the normal pruning cycle for mature tea.
There is the option of a 2 or 3 year pruning cycle for tea that is mature. Advantages of each are given below:
- Quicker recovery topruning;
- Easier to control plucking table creep (rise);and
- Even shoot size over pruningcycle.
- Pruning costslower;
- Smaller area to prune which makes it easier to keep to schedules;and
- More crop in September/October; crop distributionimproved.
An adaptation is a 4 year cycle with a skiff cut after 2 years Time of pruning:
Two year cycle, complete pruning by the middle of June.
Three year cycle, complete pruning by the end of May.
Four year cycle, complete pruning by the end of May and skiff mid to end of June in year 2.
Pruning is always 2 cm above the last prune to reach a maximum table height of 66 cm on a 2 year pruning cycle and 60 cm on a 3 year pruning cycle.
Once a maximum pruning height of 60 – 66 cm has been reached, tea is down-pruned, in the first week in April to 35 – 45 cm (not lower than 30 cm). 5 – 10 cm of wood (growth) is left above the down prune. The level of nitrogen can be reduced in the year of down pruning. There is a slow recovery from a down prune.
Tea pruning knifes are custom made and have a hook at the end to help cut off knots from larger diameter wood. Knifes must be kept razor sharp and are sharpened regularly. It is important to have a clean cut with no snags to limit disease and promote good bud development. A bush cutter can also beused.
Prunings are placed on top of the frame immediately and only removed once bud break occurs. This protects the bush from sun scorch and limits disease (e.g.Hypoxylon)
Prunings should remain in the field as a source of much. See mulching section.
- 4. THENURSERY
Selected mother trees are planted close to the nursery. These are often from cuttings selected from the original bush. The clone (variety) is first tested in trials against standards for yield, quality, agronomic characteristics which include pest and disease tolerance. The VP plants are classified as suited to being used as clonal stock for (a) field clones; (b) scion clones; (c) rootstocks or; (d) generative clones. This VP material is used to produce nursery plants for planting in the field. A field is planted with one clone for ease of management.
CUTTINGS FROM MOTHER BUSHES
Tea bushes which have been selected to provide cuttings are not plucked but allowed to grow to produce long branches. The mature hard wood at the base of the branch and soft wood at the growing end are discarded. The cuttings are taken from the green wood with hard leaves in the middle of the branch.
Each cutting comprises:
- A piece of stem with 3 – 6 cm below the last leafwith
- One leaf (Single node cutting) cm,or
- Two leaves (Binodal cutting) and an axillarybud.
The actual cut at the top and bottom of each piece of stem should be done with a very sharp blade
or knife. The angled cut should be parallel to the angle of the leaf. The diagram below shows how cuttings are taken from a growing branch of the motherbush.
Figure 2: LeafCuttings
The cuttings are dipped in fungicide (e.g. Captan at 1 g/litre water) and immediately rooted in the nursery into pots. Soil analysis will indicate the need for phosphate and the necessity for pH correction.
Dappled shade over the nursery is important to moderate temperature. At midday, shade should allow 25 – 45% sunlight onto the beds. Woven fibrous grass (e.g. Imperatacylindrica) can be used, with the weave in a North – South direction and must be 2.5 – 3 m above ground, supported on straining wire and poles. Shade must last 18 months with little attention, except to be thinned out for the plants to harden. Shade netting can be used but cannot be thinnedout.
Ensure there is a good fire break around the nursery.
Mini pots measure 125 mm x 85 mm lay flat and are used to establish cuttings using misting with a knapsack sprayer. They are filled with sieved dry subsoil. After 5 months the rooted cuttings are transplanted into standard pots.
Standard pots are 225 mm high with a diameter of 85 mm. They are used for propagation under polythene misting and the rooted cuttings, after 5 months of misting in mini pots.
The standard pots for rooting cuttings under polythene are filled in three stages:
- First fill the bottom with topsoil to a depth of 100mm;
- Then add to the next 30 mm (middle) a mix of topsoil and subsoil in equal volumes;and
- The remaining upper 80 mm is filled withsubsoil.
The cuttings need low organic matter for callusing. The roots that develop then grow into the more fertile soil in the lower part of the pot.
The pots are placed in 1 m wide beds with the length of bed determined by the terrain and management.
The pots should be well watered 2 days before planting. The cuttings are planted into the centre of the pot leaving 12 mm of lower stem above soil level and firmed in before lightly watering.
PROPAGATION: ROOTING CUTTINGS
VP material is produced either by using plastic tents or producing a regular fine spray mist (every 20
- 30 minutes) using a knapsacksprayer.
A frame using BRC wire mesh of 20 x 20 cm and 180 cm wide is erected over each bed. The frame supports a clear polythene sheet that can be tucked in at the sides and ends (edges).
Once standard pots are planted and watered the polythene sheet is placed over the frame and the edges with soil to seal the tent. Condensation should be seen in the tent within 2 days of planting. If there is no condensation the tent has not been sealed properly, is torn or the nursery too shaded. After about 5 months when the majority of the cuttings have rooted, the hardening-off process begins with an interval of 10 – 14 days between the four stages:
- Unseal polythene tent;
- Open one side by 15cm;
- Open both sides by 15 cm;and
- Remove the polythenesheet.
Removing the sheet increases watering. Heavy watering at longer intervals is preferred to light watering and overwatering must be avoided. Pots should be checked by hand for moisture content before watering and a day after.
Fungus attack can be treated with chemicals (Captan 1 g/litre or copper oxychloride 2 g/litre) as a full cover spray below the polythene or after itsremoval.
MIST SPRAYING WITH KNAPSACK
The cuttings are planted into mini pots filled with subsoil.
The frequency of spraying depends on weather conditions and the age of the cuttings.
During the first 3 months, water should be sprayed at 20 – 30 minute intervals from 7h30 up to 16h30. The blue or green polijet or equivalent nozzle is ideal. A 15 litre knapsack of water will cover 100 000 single node cuttings in 15 minutes.
In months 3 – 5 the frequency of spraying should be reduced to suit the conditions. After 5 months the plants are transplanted to standard pots.
TRANSPLANTING TO STANDARD POTS
After 5 months the cuttings are transplanted into standard pots, with drainage holes in the lower half of the pot. The pots are half-filled with top soil; the rooted cuttings are carefully removed from the mini-pot and planted without disturbing the core of roots. The pot must be filled with topsoil and shaken to settle the soil, followed by a good soaking with water. The plants are graded by size for ease of planting out later with best plants being plantedfirst.
MANAGEMENT IN THE NURSERY
If rooted cuttings appear yellow, sulphate of ammonia 500 g/200 litres (equivalent to 50 g per 20 litre knapsack) should be applied. (5 litres solution per square metre of nursery. 20 litre knapsack covers 4m2).
Any flower buds that develop should be removed to prevent heavy callus formation and encourage vegetative growth.
Planted pots must be moved up 1 row at 8, 12 and 16 months to prevent roots growing into the ground. This also stimulates a fibrous root system to develop in the pot instead ofbelow.
Watering is by trial and error and guided by the plant, weather and pot moisture content. The farmer will benefit from the experience gained by the powers of sight and feel overtime.
Plants must be hardened off a few weeks before transplanting to the field. This is done by thinning out the overhead shade.
Note: Use standard pots to propagate binodal cuttings by misting. Fill soil in 3 separate layers as for standard pots.
Cleft grafting (50-70% success) is used to convert mother trees/bushes (hardwood rootstock) to produce clonal material from the cleft grafted clonal scion.
Chip budding uses young vigorous rootstock from one clone to join onto a high performance scion clone. The combination makes it as good or better than any comparable field clone. The young budded plants are raised in nurseries as described for VP material above.
Grafting is both an art and science and should only be attempted by experienced personnel.
5. PLANTING, WEEDING and FERTILISATION
After 18 months in the nursery the young hardened off plants are planted into the land. They are planted at a convenient spacing, varying from 120 x 90 cm (9259 plants/ha) for the most extensive dryland situation, to 120 x 60 cm (13 889 plants/ha) for intensive irrigation.
Seedlings grown as ‘stumps’ were common initially, but VP clones have taken over, due to uniformity of plant characteristics and higher yieldpotential.
The pots are planted into holes made deeper and wider than the pots on the day of planting which is usually during the rains. Phosphate (60 g Single Super Phosphate (SSP) or 30 g Double Super Phosphate (DSP)) should be sprinkled on the soil dug out and mixed back into the hole.
The soil must have a well wetted profile at planting, to at least twice the planting depth. The land must already have been prepared, weed free and marked out ready for planting.
Each pot is split across the base and up one side and placed in the hole. Soil is added or removed to ensure that the pot is level with soil level in the field.
The polythene is lifted away without damaging the plant, the hole filled and the soil around the plant firmed in with open extended fingers.
Shade is recommended (shady branches or bracken) to protect young plants on hot sunny days. It is preferable to complete planting and shading by 9h30 ensuring good establishment in hot conditions. In times of serious drought it might be necessary to hand water to ensure the survival of the plants.
The ground around and between the young tea plants should be kept free of weeds by hand weeding or the use of herbicides. Weed control is critical whilst the plant is developing a full canopy. Once the branches and leaves form a complete cover over the ground, weeds are seldom aproblem.
Fertilization has been very well researched using field trials in Malawi and the recommendations from Malawi form the basis of this lecture.
The major elements are NPK and minor ones are copper (for fermentation in very high rainfall leached areas low in copper), magnesium, manganese, molybdenum, zinc (foliar applied to stimulate crop), boron and iron. Each country growing tea will have detailed recommendations based on age and stage in the pruning cycle
The levels of fertilizer applied vary with soil fertility, seedling or clonal tea, age, stage in the pruning cycle, clone and fertilizer use. Leaf and soil analyses should be used as a guide to the amounts of fertilizer applied.
Seedling tea in Malawi is likely to receive 165 kg N; 33 kg P2O5 and 33 kg K2O %: equivalent to 5:1:1 ratio of NPK, with N level increasing progressively from 20 kg N/ha in year 2 by 10 kg N/annum to year 10 then by 5 kg N/ha/annum to year 25 and by 5 kg/ha/annum to 70 years.
Clonal tea in Malawi receives higher levels of NPK and a higher proportion of P and K in a ratio of 15:3:5. Starting with;
Table 1: Levels of Nitrogen over 5 years
Irrigation systems must be properly designed, commissioned and operated. It is of importance that weeds are controlled, young tea mulched and the prunings kept in the field.
Pests are controlled on the basis of good scouting and careful use of chemicals. Only use products registered for use in tea and strictly follow all the recommendations on the label.
Processing helps to maintain the quality which is determined by the plucking standard. Processing must ensure a uniform flow of material with systems in place to see that the operation is carried out efficiently.
Processing is both an art learned by experience and a science. The drying process is a science with many formulae based on leaf and air and the psychometric charts to fine tune the process. There is still a lot to be learned about monitoring and controlling the process on the basis of biochemistry.
Dr Michael St.J. Clowes andPeter Chard April 2013
- 1. IRRIGATION
Tea was first irrigated in Zimbabwe in 1927 on the New Year’s tea estate on the Tanganda River near Chipinge. In order for the crop to survive the dry winter months, water was led through ducts from the river and run between alternate rows of tea bushes. The irrigation cycle for the whole estate covered 3 weeks. Overhead irrigation on other estates and in other tea growing areas is now most common.
The response to irrigation is dependent on temperature and humidity. In the hottest months low humidity measured as Vapour Pressure Deficit (VPD) at 2 pm limits response to irrigation. In cooler months temperature limits response.
Irrigation reduces termite attack and the incidence of Phomopsis stem rot.
All other aspects of management must be to the highest standard before consideration is given to introducing irrigation to tea.
The current recommendations for Malawi, based on intensive irrigation research are:
- Replace 50% of an 80 mm deficit or irrigate once a fortnight – whichever islonger;
- Do not commence irrigation until the first week in August, except in very dryyears;
- Do not irrigate in the cooler months after the rainsstop;
- Do not irrigate in the year of pruning maturetea;
- Ensure young tea is mulched in the first 2 years;and
- Irrigation requires intensive management, is time consuming, costly and not always profitable.
Climate change could result in higher rewards from irrigation in drought years.
It is important to ensure good soil conservation techniques and prevent run-off and leaks from the system.
Nozzles and pumps are subject to wear warranting regular checks.
Ensure that any irrigation system installed is properly commissioned and there are regular checks on output, pressures and efficiency.
Zimbabwe has applied intensive irrigation on clonal tea and has produced very high yields of tea commercially (4 000 – 6 000 kg green leaf per ha).
2. PESTS ANDDISEASES
Pests and diseases are seldom of economic importance in cooler areas of production but become significant in areas with warm winters. In the longer term climate change may impact on pest and disease incidence and significance. It is important to scout regularly in the field and to always examine plucked leaf for signs of pests and disease. A magnifying glass is needed to identify the smaller insect pests. A brief description of pests and diseases found in tea in Malawi is given below. Plucking helps to reduce pest and disease levels by regularly removing young shoots. Changes in weather impact on pest and disease incidence and economic levels for damage.
The main pests that reach levels often requiring control are:
- Mosquito Bug;
- Tea Thrips*; and
- Red Spider Mite (RSM) and other mites. Sporadic pests that might need to be controlledare:
- Scale insects;
- Grasshoppers, crickets andlocusts;
- Faggot worm andbagworms;
- Stinging caterpillar or nettlegrubs*;
- Cockchafer or whitegrub;
- Black tea thrips;and
- Tea leafweevil.
Note* Chemical control is similar for each of thesepests
MAIN PESTS REQUIRING SCOUTING ANDCONTROL
Mosquito Bug (Helopeltisschoutedeni)
The adults can cause damage at any time of the year. The adult is colourful resembling a cross between a large orange ant and giant mosquito with long black antennae. The body is 7 – 10 mm long with a 1.5 – 2 mm wide thorax. The bug attacks young leaves and green stem. Bug damage is diagnosed initially by the many (90 – 160) circular pale green spots from young nymphs or by angular 1 – 3 mm diameter spots caused by older nymphs. These spots soon blacken from the toxins produced.
In severe cases it looks as if the bush has been blackened by fire.
Damage to young branches by the Mosquito Bug results in infected branches becoming cankerous and dying back for up to 10 months later.
Spraying is based on scouting before plucking, a damage index and spraying with one of the pyrethroids or another recommended chemical.
This became a major pest in Malawi in the 1960’s and coincided with the removal of shade trees. Damage is severe in the pre-rains, hot dry season from September to December. The adult is only 1 – 2 mm long and yellowish white in colour. Thrips are identified by the narrow straw-like wings with fringed edges around the margins and long bristles. Thrips feed on young plant material and the first sign of damage is a pair of dotted parallel lines on young leaves as they unfurl. Attacked new leaf margins turn brown and cup shaped. Stunting of shoots and defoliation occurs to produce leafless twigs. Infestation is related to time of prune and the later the prune, the greater the damage.
Timing of pruning can help to limit damage:
- Down pruning (major ratoon) should be by the first week inApril;
- Cut across prune (normal prune) by the last week in May;and
- Young tea should be pruned mid-June – mid-July, pruning the youngest tealast.
If these pruning recommendations are adhered to, thrips should only be a problem in dry years.
Tea tipped or pruned before September is most prone to attack and scouting is necessary. Once 1 – 2thrips per shoot are noticed, a chemical spray should be applied asrecommended.
Pyrethroids and a range of other chemicals are registered for use.
Red Spider Mite (Oligonychuscoffeae) (RSM) and other mites
They are generally sporadic but are included for ease of reference and comparison. Serious outbreaks of RSM have occurred since 1930 in Malawi and it is a common pest of tea. Outbreaks can occur any time of the year. The legs and front portion of the body are bright crimson and the abdomen is dark purplish and range in length from 0.25 – 0.4 mm (the male is narrower and smaller than the female but within this range). Levels build up first in unpruned tea, initially feeding on the top side of mature leaves which become reddish, coppery or bronze before turning black, dying and falling off. This leaves the bush with very little maintenance foliage resulting in a poor flush (growth of new shoots). China tea and some clones are more susceptible to attack. Weak and unhealthy tea predisposes plants to infestation by RSM. Cultural control (good management) is preferred to chemical treatment, as there has been no increase in yield in experiments that were carried out. However, chemicals are necessary to control outbreaks in young tea. Infestation can be sporadic on mature tea starting from the roadside and moving haphazardly into the field. Spot spraying has been used with mixedresults.
Good cover of the maintenance foliage (older leaves) is essential with two sprays in a season being sufficient.
Yellow Tea Mite (also known as the citrus silver mite)
They are common under shade and in nurseries and more serious in cooler months. It is smaller than the RSM (0.2 – 0.25 mm). Yellow Tea Mites are found on the underside of young leaves.
The three species are very similar, attacking young and mature tea at the end of the dry season and extending into the rains. Build up is slower than for the mites mentioned above. Scarlet mite is only found on the underside of mature leaves and near the base.
Intensive feeding causes darkening, if the tissue and the petiole splits and dries up causing premature defoliation and in severe cases stunting or death of plants. The adult female (males are rare) are 0.3 mm in length, elongated oval and bright red. It is important to spray the underside of mature leaves to achieve effectivecontrol.
This pest has been identified in South Africa. The adult female is minute 0.15 – 0.2 mm and the males smaller. The adult is dull purple in colour with 5 white waxy ridges on the upper side of the abdomen. This pest attacks young tea before the rains and feeds mainly on the upper surface of the leaf but occurs on the underside as well. The infested leaves turn purplish brown and resemble leaves under acute waterstress.
Chemicals for the control of RSM and other mites are basically similar but expert advice on chemical use is recommended. Only apply registered chemicals and follow label recommendations. Chemicals are specific for mites (e.g. Tetradifon) and some are no longerrecommended.
Sporadic Pests sometimes needing to be controlled Carpenter Moth, a lepidoptera
Has been present in Malawi since 1978 and is found on Eastern Highlands Tea Estate in Zimbabwe. The caterpillar ring barks stems and branches within a web that it has spun over itself. It then retreats into a hole that it makes into the bark. When fully grown, in the middle of the rains, the caterpillar is 20 – 21 mm long with a dark brown head and cream body. On young tea the damage is to the main stem and on older confined mainly to the branches. It is important to spray the pest early (August-September) before it is protected by its web. Control is mainly to protect young tea and chemicals such as Carbaryl are available for use before the pest becomes established. There is only one generation peryear.
Termites (white ants)
A number of species attack tea. Tea that is damaged by sun scorch, cankers or pruning snags or suffering from a combination of heat and moisture stress, is prone to termite attack. Damage tends to be most severe in the first 3 years after planting. Termites sever the plant at or below ground level and the plant dies several weeks later. Damage is most extensive in the dry season. Mulch must be properly applied to reduce the impact of termite attack and to protect the plant from moisture stress. It is essential to have an area of 15 to 20 cm free of mulch around each plant. Irrigated tea is less susceptible to attack. Termites were controlled using chlorinated hydrocarbons but their use is no longer permitted due to their persistence in the soil. New chemicals are expensive and expert advice isrequired.
This is the oldest recorded pest in Malawi and has been around a long time, but seldom builds up to needing control. There are times when a serious outbreak occurs and control measures are necessary. When fully grown the grub is 10 – 15 mm in length and bluish green in colour with two longitudinal wavy lines on its back. Pupation takes place in a web within a cocoon after turning white. Severe attack results in defoliation leaving only the mid-ribs uneaten. Attacks start in the inside of the bush on the older leaves. The threshold for spraying is 20 grubs on 5 sample bushes or when the population is buildingup.
Are characherised by legless, immobile females covered by wax or scales. Scales can cause severe defoliation on young tea but are not important on mature tea. It is important to ensure cuttings are
free of scale as they are very difficult to control. Infected nursery plants must be isolated and destroyed. ‘Armoured’ scales prefer the upper side of the leaves and soft scales the underside. Sooty mould and ants are often associated with scales. Scale insects are difficult to control with spraying, but chemicals are available, such as Pirimiphos-methyl.
Citrus Aphid (Taxopteraaurantii)
Has a world-wide distribution and attacks many other crops, such as soft citrus, mango and orange. Aphids commonly occur feeding with piercing mouth parts on young shoots in August – September and then again in December – January. The affected leaves curl downwards and become distorted and growth is retarded. Aphids are brown, soft bodied, pear shaped and small (up to 0.3 mm) with two cornicles (projections) at the rear. Aphids can secrete honeydew which encourages the growth of sooty mould and the presence ofants.
Chemical control with Dimethoate, Fenitrothion and Malathion is sometimes necessary in seedlings (particularly when grown under polythene tents), tea before tipping and source bushes left for cuttings.
In mature tea plucking and predation tends to keep aphids under control.
Grasshoppers (Elegant red, black and yellow), Crickets and Locusts
These pests seldom cause economic damage. Hand picking and use of chemicals (pyrethroids) have been used to control these pests.
Faggot and Bagworms
The caterpillar coats itself with fragments of leaves and grows to feed and add more twigs, branches or leaves and is suspended from a leaf or twig by silk. It changes into a pupa within the case and finally into an adult moth. There is one generation per year. It is preferable to collect by hand as chemical control is not easily achieved.
Stinging Caterpillars (nettle grubs)
Are easily identified by their attractively coloured (1 – 3 cm long) and tufted bodies (white, green, yellow with blue or brown markings). They feed mainly on the underside of mature leaves and can severely defoliate young plants. Chemical control using pyrethroids is effective.
Cockchafer or White Grub (C shaped) is common
The grubs feed on grass and dead material but occasionally feed on the roots of tea plants. Damage can only be serious within the first 2 years. Signs of damage are roots being eaten away and ring barking of stems, to leave an extensive callus above the damage. Affected young plants turn yellow, wilt, defoliate and often die. The grubs live in the soil at 5 – 20 cm depth.
Black Tea Thrips(Heliothripshaemorrhoidalis)
They are black and 1.5 mm long. These thrips are common in pine plantations and other crops. Infestation is during the dry season with heavy rains and cold weather reducing the population. Thrips feed on the undersurface of leaves and damage appears silvery, with large black spots which are excreta. Defoliation can occur. Control is the same as for the Tea Thrips and Jelly Grub.
Tea Leaf Weevil (Systatessmeei)
Is widespread and occurs on many crops.
The weevil can cause extensive damage to young tea in the field and nursery from January to April. The adult feeds at night eating out the edges of the leaf (punch hole indentations). The adult is black, 1 cm in length and has a swollenabdomen.
The main economic diseases of tea attack the following parts:
- Root – Armillaria RootRot;
- Stem – Phomopsis Stem Rot and Hypoxylon Wood Rot;and
- Leaf – Grey and BrownBlights.
Root rot is a fungus that attacks a wide range of trees. It is important before bush clearing to ring- bark trees well in advance of uprooting preventing the spread of this disease.
During land preparation it is important to remove all old roots off the land and burn them. Diseased bushes occur in patches and the bark must be examined which is often cracked above the collar at soil level and can, in some rainy seasons, produce Armillaria toadstools in Malawi. The root must also be examined for the presence of a white mycelium (fungal mass of thread) below the bark that smells ofmushroom.
Control before establishing tea is essential in areas where this disease is present. A 20 – 30 cm ring of bark and soft tissue is removed from hardwoods (broad leaved trees) in Winter at 1 – 1.5 m above ground level. Coppice growth from below the ring needs to be removed regularly.
The roots of infected hardwood trees that are not ring barked are a source of infection for many years as they rot slowly. This stresses the importance of also removing roots and burning them as part of land preparation.
Control in the tea land consists of removing all infected trees showing signs of chlorosis, wilting of leaves, cracks on the collar and mushroom-like smell. As a precautionary measure a ring of healthy trees from around the infected ones should also be removed. A trench can be dug round this area to isolate the remaining healthy tress from infection.
The area should be planted to weeping lovegrass or Guatemala grass and only replanted to tea after 2 – 3 years.
Pseudophaeolus Root Rot (Pale Brown Rot).
This disease is not common and occurs in patches but can spread rapidly. A fungal mat of white or cream, to pale or bright yellow occurs on the outside of the bark. The bark is soft and brittle and reddish brown when scraped away.
This disease has been controlled by trenching (1 m deep 0.5 m wide) or digging up roots of 2 rows of bushes surrounding the infected patch. The soil from the trench is thrown inwards and not outwards, to limit the possible spread ofdisease.
Violet Root Rot
This is confined to clayey, waterlogged, over irrigated or flooded soils.
Leaves turn yellow, droop and often drop off while still green. Roots appear roughened due to enlargement of the lenticels and may smell of vinegar. Infected roots generally contain fructifications (fruiting bodies) of short pink or orange stalks with white heads ofconidia.
Control is based on removing the infected bushes and correcting the irrigation and drainage.
Purple Root Rot
This is not common. Infected plants have a purple brown branched mycelium covering the root surface. The thick fruiting body which is velvety to the feel and purple in colour is several
centimeters wide. It occurs just above the soil surface. Control is similar to Armillaria root rot.
Charcoal Stump Rot
This is a root fungus that occasionally invades dead or dying plants killed by lightning or under severe stress. There is no cracking of the stem and no mycelium and the wood is marked with a sharp angular or wavy black double line. Old fructifications are charcoal black and give the fungus its name. Stumps of Grevillea and other trees may act as hosts.
Control is as for Armillaria root rot.
Grey and BrownBlight
Are weak pathogens that do not need to be controlled with fungicides.Captan and copper oxychloride areeffective.
Growers should determine why bushes are stressed and remedy the situation.
The characteristic lesion on a mature leaf is circular to oval from 1 cm diameter and can cover the whole leaf. The black fruiting bodies are present in concentric rings on the upper surface. On young leaves the lesion is irregular and dark brown to black with concentric rings.
Spots are yellowish at first, turning to reddish brown as the size increases. The spots frequently spread inwards from the margin of the leaf and form a semi oval patch. The whole leaf may be affected. The fruiting bodies appear as minute black dots scattered on both surfaces of the leaf. It can also invade young stems.
Eye Spot of tea
Is common on semi mature leaves and occasionally on young leaves and stems. It is only of economic importance to young tea and source bushes. The fungus causes minute (up to 3 mm wide) more or less circular brown to dark brown spots on leaves. The spots have swollen purplish brown margins surrounded by a light coloured halo. Spots can coalesce and resemble mosquito bug damage.
The fruiting bodies are white to grey tufts mainly on the upper surface of the leaf. Captan and copper oxychloride are effective as full cover sprays. Cuttings can be dipped in a 0.1% solution of Captan for 2 – 3 minutes. The fungicide should be shaken off theleaves.
Is a fungal disease which affects the leaves of the plant, usually during wet and humid weather, causing small, translucent spots which expand to form depressions like blisters on the under-side of the leaves.
The blisters turn white-and release spores which infect other leaves. Only young leaves are affected. Control of the disease is by using resistant varieties, aided by spraying the bushes with a mixture containing copper. Spraying should be done immediately after plucking to give the maximum period before the next plucking. This is to allow the spray to kill the fungus and reduce copper levels on newly formed shoots.
Are superficial and do not harm the plant. Aphids or scales associated with sooty mould may need to be controlled.
Algal Leaf Spot or Red Rust
Appears as an orange yellow to reddish brown encrustation on the upper surface of the mature leaf and no control is necessary.
Tea in Africa is manufactured as black fermented tea as opposed to unfermented green tea.
Tea produced in Central and Southern Africa has a low-medium quality. Teas are sold on the basis of briskness, brightness, colour and strength. This is determined after manufacture by expert tea tasters, tasting tea without milk and then milk is added to assess colour.
Processing can only maintain the quality based on the standard and type of leaf delivered to the factory.
The variables of temperature, humidity and moisture density of air all interact with the leaf during processing to produce the final product. Tea making is a mix of art and science, but efforts are being made to make it less of an art and more of a science, by introducing more sophisticated monitoring and control systems.
Green leaf must be delivered fresh to the factory, free of any contaminants or objects that can damage the processing machinery.
This chapter briefly summarises the five processes involved in manufacture:
Physical withering involves the removal of moisture from the leaf before it is processed. This is so the material can be handled by the processes that follow. Green leaf is delivered in custom made trailers, trucks or hessian bags. It is important not to damage the tea in transit as damage will start the fermentation processprematurely.
The tea is placed in withering troughs which force ambient air up through the layer of green leaf. The drying power of the air is determined by the difference between the wet and dry bulb temperature of the air(dryness).
The following facts help explain the process:
- Water is removed from the leaf as watervapour;
- 1 kg of water needs 1450 kJ of heat to convert it tovapour;
- The air used for withering picks up the watervapour;
- The air provides heat to vaporise thewater;
- Air at 25oC holds 0.01 kgwater;
- This air can pick up a maximum of 0.0028 kg of watervapour;
- When air picks up water it cools due to vaporisation;and
- A lot of air is required to pick up a little water. In practice the following considerations areimportant:
sides or ends from poor leaf distribution over the trough area;
- Leaf must be turned as leaf at the bottom tends to gets drier than at thetop;
- Very wet leaf needs heat to remove the surfacewater;
- The leaf must be loose when spreading. Compacted leaf starts to chemically wither prematurely, heats up and can result in ‘redleaf’;
- The wet air must be exhausted and notrecycled;
- Leaf must be turned well to ensure that there is no build-up of heat;and
- The percentage wither required is worked out on a dry matter (dm)basis.
- For example, leaf with 79% moisture has 21% drymatter.
- 1 000 kg of green leaf would contain 210 kg ofdm.
- If 1 000 kg is withered to 72%, then 210/1 – 0.72 = 750 kg and is equivalent to 75% of the original weight and is therefore referred to as a 75%wither.
Withering appears to be a simple process but has many variables that have to be considered. The science of drying is complex but well understood.
It is the accommodation of the changes occurring over time that is difficult to account for in practice.
Chemical withering starts as soon as the shoot is plucked and involves an increase in;
- Amino acids that occur as a result of a break down in proteins. Acid peaks at 20 – 30 hours after plucking and thendecreases.
- Caffeine increases during withering and is favoured by high temperatures (30oC). The increase is 0.2% over 16hours.
- Polyphenol oxidase increases beforedecreasing.
All of the above chemicals have an impact on the final product but further biochemical study is needed to determine the real effects on quality.
Maceration is the mechanical processes used to break down the cells. Withering must prepare the leaf for this process.
It is important for maceration to produce porous tea particles with a large surface area of an even size. Particles must have sufficient mechanical strength to prevent them from becoming fine dust in handling and storage.
The maceration process has evolved from orthodox rolling to produce large particles for flavour and aroma but not for theaflavin production.
The rotovanehas an outer cylindrical jacket housing a central conveying rotor with opposed vanes. It is continuous and can deal with large volumes and provides considerable cell distortion and mixing of juices. It is not good for particle formation as the particles are too big. It can be used before aCTC.
CTC (cut tear curl) takes in a fast but steady stream of withered leaf and processes it quickly, between special contra-rotating rollers, designed to cut, tear and curl the leaf. Despite high quality engineering precision the leaf needs to go through a pre-conditioner (rotovane) first, then followed by 3 stages of CTC to produce the almost ideal particles. The CTC can handle drier leaf than theLTP.
The LTP (Lawrie tea processor) is equivalent to a hammer mill and uses high speed impact. It cannot produce top class teas. It is simple to operate but must not be over fed. The CTC process canfollow.
Fermentation is a chemical process better described as ‘oxygenation,’ which starts with plucking but is accelerated by maceration. Low temperatures cause minimal damage and might well improve the product. There needs to be good contact between the ‘dhool’ (macerated leaf) and the air. Temperature is important as it determines the ratio between theaflavinand thearubigin. Temperatures can increase during maceration to 30 – 35oC and dhool must be moist, loose and cooled quickly after maceration.
Lower temperatures are believed to produce brighter and brisker teas. A set of CTC rollers are often used to provide a mid-fermentation stage agitation to increase exposure to the air. A low pH of below 5 is best for a high TF/TR (Theaflavin to Thearubigin) ratio, and would favour storage as the products of fermentation are more stable when produced at low pH.
Fermentation takes place on a moving bed and can be in two stages with a ball-breaker mid-way. Fermentation is a chemical reaction that imparts quality to the cup.
Six catechins are oxidized by the enzyme polyphenol oxidase firstly to form intermediate compounds called orthoquinones. The theaflavins (TF) are produced between catechins and galocatechins and thearubigins (TR) in a similar manner with peroxidase. In practice the TF is most closely correlated to quality both based on the bush and the made tea. Large TR molecules have a detrimental effect on taste producing harsh tastes and after tastes. The simple TR’s have a positive effect giving tea its body and colour. As a broad generalisation high TF’s are associated with well-made tea and high TR’s with poorly-made tea.
Drying is based on the use of hot air and the principles are similar to withering, but are simpler, as hot air is used with fairly low relative humidity. The tea has to lose a large amount of water still (70- 75% water). The temperatures of the outlet air above the drier do not rise until drying reaches 40% on a wet basis. Moisture loss is in 3 phases. 1) The initial phase (wet) is at a constant rate. 2) When dhool is at about 40% mc on wet basis it reaches the first fast falling rate. This is when water migrates freely out of the particle before coming into contact with air. 3) The second falling rate is when all the water must be forced out from within theparticle.
It is important to feed the drier consistently and evenly and not overload it. The output of a drier should be related to the moisture content of the tea it has to dry. A faster moving bed with loose dhool over a ballbreaker is ideal to keep the material loose for drying and preferably ‘alive’ on a vibration bedplate. Vertical vibration is preferred to horizontal. The use of weirs should be limited to one at the end.
Driers in common use are Conventional or ECP driers and the more modern Fluid Bed Driers (FBDs). The tea in a FBD is ‘suspended’ on a bed of air which carries the tea along the drier. It is the most progressive drier and is in widespread use.
Each drier has its own characteristics and each operation is influenced by the vagaries of climate which impact on the leaf. The most challenging times are at peak flush when all capacities are tested and when the conditions are very wet and humid.
Removes stalk and fibre and grades the tea by size and density, for sale. Care must be taken to prevent the tea from absorbing too much moisture. It is essential that the moist air from drying and withering does not enter the sorting room. The tea is passed over a series of vibrating or shaking screens.
Tea Grades and standards vary as in addition to size, the density and appearance are also important. Cup quality is an additional factor. Different estates and countries have particular preferences.
There can be up to 8 grades based on size from over 2.0 mm to below 0.5 mm, although it is more common to have 6 basic grades in auctions, as in Mombasa in Kenya.
BP1 Broken Pekoe 1 (12 – 14%) light in body with good flavour.
PF1 Pekoe Fannings 1 (58 – 60%) black grainy particles smaller than BP1 above.
Dust 1 (4 – 6%) dust.
PD Pekoe Dust (10 – 12%).
Fannings1 often sifted out of primary grades having strong flavour and good colour.
Dust Strong tea also used in tea bags.
There are additional terms such as Clonal (Cl) which further describe the tea on offer.