Crop production

• Potato plants prefer temperatures between 10 and 22 ⁰C with an average of 15 ⁰C. This is why in Cameroon potatoes are cultivated in altitudes above 800 meters.
• Potato has a rather high water demand. The crop requires well-distributed rainfall of 500 to 750 mm through all growth stages during a period of 3 to 4.5 months. Potato is especially sensitive to long wet or dry periods during flowering and tuber formation. Waterlogging as well as dry soil are not favourable to tuber production. If potato is cultivated in dry periods or areas with low or irregular rainfall, irrigation is usually necessary.
• Light to medium, loose soils rich in organic matter, and with appropriate depth and a pH of 5.5 to 7 are ideal for potato. Soils for potato production should drain well to avoid waterlogging and at the same time have a good water retention capacity to ensure steady water supply. Too stony, shallow, compacted and poorly drained soils are not suited to potato production.
• On lighter soils, tubers usually develop a nicer shape and colour as well as flatter eyes. On heavy soils the tubers turn out more smooth-skinned and occurrence of scab is lower. Sandy, quick-drying soils lead to rougher skins and russeting as well as raised scab infection. Soils with a pH below 5.0 will produce poor quality tubers and abnormal growth, while a high soil pH will cause problems with common scab. Due to their low nutrient contents and low base-exchanging capacity, widespread acidic laterite soils are naturally low in fertility and have a low production potential. However, appropriate soil fertility management measures such as supply of organic matter to the soil, increasing soil pH by liming, application of compost, and irrigation make them suitable for growing crops.

• Potatoes should be cultivated in open fields, and not under the shade of trees. Well-ventilated locations allow the crop to dry quickly after rain or dew, thus reducing the risk of infection by fungal diseases.
• Slightly sloping fields dry and drain quicker than fields in the valley-basin. Potato fields should not be too steep to avoid soil erosion. If grown on slopes, the ridges should follow the contour lines.
• Soils for potato cultivation should be free from bacterial wilt and nematodes (find information on symptoms here). Choosing a field where potato or other plants of the family of Solanaceae such as tomato and peppers have not been planted in the last 3 years ensures pest and disease-free soils in most cases.
• Access to water for irrigation allows for off-season production and is a big advantage in potato and vegetable production.
• Nearby trafficable roads simplify transport of the harvest.

• The average production cycle of commonly grown potato varieties in Sub-Saharan Africa ranges from 90 to 120 days. The shorter the production cycle, the less is the risk of losses by late blight infestation and pest attack.
• If potato is cultivated without irrigation, the ideal season in Cameroon is from March to June, when moderate rainfalls ensure good growing conditions. In July and August, when heavy rains occur almost every day, disease pressure may be too high for organic potato production.
• It is also possible to cultivate potatoes from mid/end August until November.
• When irrigation is available, potatoes are best cultivated in the dry period from November to March, when pressure of late blight is low.

In situations of heavy nutrient depletion or unfavourable growing conditions or specific nutrient deficiencies, organic farmers apply supplementary measures that are necessary to speed up the improvement of growing conditions for plants, such as:

• Using commercial organic fertilisers that do not contain chemical residues, if accessible and affordable, organic residues such as seed oil cakes, pelleted chicken manure, brewery by-products, fruit peels, coffee husks and plant ashes can be applied to the soil while wood shavings and dust, rice husks, etc. can be mixed with other materials for compost production.
• Amendments such as lime are used to correct low soil pH. Microbial fertilisers such as rhizobia, and potentially also mycorrhizal fungi, can enhance nitrogen fixation in the soil and nutrient mineralisation and thus improve nutrient availability of crops.
• Application of micro- or trace elements such as magnesium, boron and manganese can be important particularly on lighter soils that may not supply enough of these elements.
• Last but not least: water. Without water plants do not grow. Insufficient water supply limits mineralisation and transportation of nutrients in the soil and from the soil to the plant and thus plant growth and yield development are negatively affected. On the other side, too much water encourages nutrient losses through leaching, and may also cause root rots. In dry conditions appropriate use of irrigation to supplement soil water requirements can be essential to achieve good yields.

Compared to composting, another method that is generally recommended in organic agriculture, green manures have some advantages:

• Green manures can produce over 40 tons of plant biomass per hectare. They bring large quantities of nitrogen into the production cycle and make other nutrients available to the following crops. Composting, in contrast, is about recycling available plant and animal (waste) material, and making a highly valuable fertilizer with lots of phosphorus and other nutrients, but with little amounts of nitrogen.
• Green manures protect the soil from erosion by wind and water, preserving soil moisture and soil organic matter. Thus, they contribute decisively to soil conservation.
• Some green manures effectively suppress weeds.
• Sowing and, where necessary, slashing of green manures requires labour, but saves on fertilizer costs and can save on labour for weeding.
• Green manures do not require capital or inputs, if seeds are available.
• Green manures, in general, do not need to be irrigated. They take advantage of available rainwater or remaining soil moisture.
• Green manures do require no transportation, as they grow, where they are needed.
• Some green manures have edible plant parts, some are highly valuable animal feed.

The most obvious use of green manures in the common cropping systems of potato farmers in Cameroon would be to use the green manures instead of the traditional fallow. The higher fertilisation value of leguminous green manures may allow shortening the fallow period, as soil fertility restoration is speeded up. Alternatively, green manures can be sown among traditional row crops or relay intercropped towards the harvesting time of the main crop. Competition to food crops is reduced, if green manures primarily grow during the dry season. Alternatively, green manures such as jack bean or velvet bean can be grown in alleys. Evaluation of traditional cropping systems in Africa showed that rotation of legumes with other crops is more productive than intercropping.

Legumes do not significantly contribute to higher soil nitrogen contents when their grains and residues are removed for human and/or animal nutrition. If the legume biomass or residue is burnt or fully removed from the fields, negative nutrient balances arise. It is therefore important to ensure that all or at least part of the legume residues are retained in the field, if soil organic matter content is to be maintained.

If green manures are left as mulch on the soil surface, they effectively contribute to erosion and weed control and retain moisture in the soil. However, nutrients from mulches are released only slowly. If green manures are incorporated into the soil, a relevant share of the nutrients are mineralised in one season. Thus, the fertilizing effect on the subsequent crop is greater after incorporation. At the end, the total amount of nutrients that is made available to plants is about the same whether green manure residues are left as mulch or incorporated.

The dense plant covers of green manures not only protect the soil from erosion by wind and water, but also prevent propagation of weeds saving time for weed control. If green manures leave a thick dry mulch cover, they can provide favourable conditions for planting of the following crop without any need for weeding or soil preparation.

Some green manure species provide generous amounts of high protein fodder for livestock. This may encourage cooperation between animal farmers and plant production farmers: animal feed may be produced on parts of the farmland in exchange for manure. But despite all the benefits, green manures may, as a sole soil fertility management measure, not be sufficient to maintain or even improve soil fertility.

Leguminous green manure species depend on the availability of sufficient phosphorus, the presence of the right rhizobium for nitrogen fixation, healthy seeds and sufficient soil moisture.
 

One reason why farmers do not grow green manures is that they do not know which species to plant and how to integrate them into their cropping system. It is important to know where, when and how to plant which species in order to obtain satisfying results. Other bottlenecks relate to ready availability of seed for the green manure crops, labour intensiveness, and unreliable performance of the green manures.

There are several ways of integrating green manures into the farming system:

i.            Legumes are grown as short-term, improved rotational fallow.

ii.           Long-term green manures are grown for more than one season.

iii.          Food and non-food legumes can be intercropped with regular crops.

iv.          Short duration non-food legumes can be grown towards the end of the cereal growing season using residual moisture (relay cropping).

v.           Or legume trees are grown in an agroforestry system to provide nutrient rich biomass.

• Before large scale planting, it is advisable that farmers try out, on smaller pieces of land, the different types of green manure plants and observe, how they grow in the local conditions, and that they practice on managing them. The selection of appropriate green manures is essential to maximize their potential and minimize possible inconveniences. Green manures must suit the local climate, soil, and pest and disease situation, and fit into the cropping system. Annual green manures must be fast growing, have vigorous growth and be non-woody. They should grow well in the poorest soils and not need any fertilizer, nor irrigation, nor pesticide, and they should not be closely related to the incoming crop to avoid promotion of pests and diseases that may affect the following crop.
• To keep farmland productive, green manures must produce at least 25 tons of fresh organic matter per hectare and year. Under favourable conditions, common green manure species may produce up to double the amount of biomass and collect at least 80 kg of nitrogen per hectare and year. However, non-legumes can also be grown, as long as they produce enough biomass and develop a good root system. Non-legumes may also survive better in some conditions, may grow faster and sometimes tolerate extreme weather conditions or poor soils. Hence, it is always good to try and see what best suits the local conditions.
• The most obvious application of green manures in potato farming systems in Cameroon is replacing the traditional fallow in the dry season by an improved leguminous fallow. If potato is grown during the short rainy season from (March to June) and harvested in July, and the crop is followed by maize during the long rains until November, a leguminous green manure may be sown after the maize crop has been harvested in November, or may be relay-intercropped into the maize in September or October and remain in the field through the dry season.
• So far, little experience has been made integrating green manures or cover crops with low-growing vegetables such as potato. Trials have been made with Vicia intercropped with cabbages. In irrigated vegetable and potato fields, animal manure or compost may be economically more advantageous because the land under irrigation is too valuable for growing green manures.

Niébé / Cowpea (Vigna unguiculata)

Cowpea is an herbaceous legume native to West Africa crawling, climbing, or growing bushy, depending on the type. It is one of the most important tropical dual-purpose legumes. It is widely used as vegetable (leaves and flowers) or grain for human consumption, and it makes excellent fresh forage, silage or hay (ideal­ly in combination with a cereal/grass crop). Its rapid growth makes it an effective plant to control erosion.

Cowpea has a high fertilizing potential fixing about 80 kg N per hectare (compared to Phaseolus bean species that only fix about 30 kg N/ha). The legume can double the yield of a following maize crop compared to no fertilization and produces higher yields than a comparable amount of nitrogen provided to maize with inorganic N fertilizer.

Cowpea grows in a wide range of soils including very acid soils with pH 4 and soils with low fertility, with a preference for lighter soils that allow good rooting. Cowpea is better adapted to strongly acid soils than Lablab bean or Velvet bean. However, the legume does not tolerate extended flooding or salinity. It grows best at 25 to 35 °C and is very susceptible to frost. Cowpea is moderately tolerant of drought.

Depending on the variety, flowering can start as early as 30 days after sowing and seeds can be ready for harvest at 55 days, or it can take more than 90 days to flower, and 210 to 240 days to mature. While determinate, bush types are better for machine harvesting, in-determinate types will supply fresh leaves and flowers over an extended period of time for the household.

If cowpeas are new to a field, the seeds should be dusted with rhizobia, nitrogen fixing bacteria, to enable the legume to fixing nitrogen from the air. The seeds are either dusted with purchased bacteria or with soil from a field where cowpea has been grown. The legume is either broadcast with up to 90 kg of seed per hectare or sown with 30 to 60 cm between rows and 10 to 15 cm between plants with 20 to 50 kg per ha and a sowing depth of 3 to 5 cm. 

Cowpea is susceptible to a wide range of diseases and pests, including post-harvest diseases attacking the grain. It also acts as host to pests of Phaseolus beans and is sensitive to root-knot nematodes, which thrive in the same soils where cowpeas are grown.

Cowpea is widely grown as intercrop in cereals such as maize, either as climbing types that require strong companion plants, or as bush-type. When planted at the same time as maize within the maize row they will save on labour for weeding of maize.

Alternatively, short-cycle cowpeas can be grown into the dry season protecting the soil from the heat. They will die and then can be incorporated into the soil before sowing the next crop.

Tithonia, Mexican or wild sunflower (Tithonia diversifolia)

Tithonia, a shrub relative of the sunflower and native to Central America, but today found throughout the tropics, is often considered a weed. However, it can be used for animal fodder, compost, fuelwood, and for insect control, or to improve soil fertility. Tithonia was found to increase maize yields to up to 80 %. Nutrients are most concentrated in the tender green leaves and stems. Green leaves contain up to 6 % of nitrogen and 0.5 % of phosphorus, comparable to velvet bean. When worked into the soil, these nutrients quickly become available for the following crop.

Tithonia does not fix nitrogen as do leguminous plants. The species makes woody stems. For use as green manure, the leaves and stems are cut in a young stage of growth, chopped and carried to fields, where they are used as a mulch or are worked into the soil. Due to the amount of work that is needed for “harvesting” Tithonia, the use of Tithonia as a nutrient source is more profitable with high-value crops such as vegetables than with relatively low-valued maize. The transfer of Tithonia biomass to fields constitutes the redistribution of nutrients within the farm eventually requiring provision of nutrients to the places where Tithonia is grown to sustain production.

Tithonia is mostly grown in hedges along roadsides or farm boundaries, or on contours. When mixed to the soil the fresh plant material is spread over the half-made ridges at a rate of 2 kg per m2 two weeks before sowing a crop and then covered with 5 to 10 cm of soil to finish the ridges.

When applied as mulch, the material is spread below the crop 6 to 8 weeks after planting of the crop. Covering the mulch with a little soil facilitates nutrient release. Mulching and green manuring can be combined, which will provide more nutrients. Interplanting of Tithonia in the field is not recommended as it would compete with crops for nutrients.

Tithonia grows on most soils to a size of 1.5 m to 4.0 m in height and width if left uncut. To prevent Tithonia from becoming a weed, Tithonia hedgerows must be cut back regularly. Tithonia can be harvested as often as every four months. Tithonia reproduces from stem and root cuttings, as well as through numerous, small seeds that are light enough to be dispersed by wind.

Sunnhemp (Crotalaria juncea)

Crotalaria green manure can contribute significantly as an organic fertilizer in potato production. The erect, high-growing, non-vining, annual legume is best grown in rotation. However, it may also be intercropped with vegetables like kales by cutting it back regularly. It grows up to altitudes of 2000 m and has some drought resistance. Innoculation of Crotalaria seed with rhizobia is not necessary. The green manure is sown by broadcasting. The plants rapidly produce a thick ground cover. For green manuring the plants are ideally slashed and worked into the soil about 60 days after planting at bud or early-bloom stage, when they are about 1 m tall. For seed production the plants need about 5 months with sufficient water until flowering followed by dry weather for seed development. The plant material can be used as feed and the beans are edible.

Crotolaria may be grown as a short-term fallow before maize or after maize into the dry season.

Intercropping of green manure species in general and of Crotalaria grahamiana in particular with potato as a component crop was advantageous in research trials in Cameroon. Similar results with other green manure species were found.

Relay intercropping of Crotalaria into a one-month-old potato crop may not be recommended, as the green manure may not develop well in the dense potato crop.

If Sunnhemp is left until its cycle ends and seeds are ripe, the seedpods need to be harvested to avoid that the mature seeds germinate subsequently and become weeds.

Velvet bean (Mucuna pruriens)

Velvet bean is probably the most common green manure crop in the tropics. The species prefers a humid environment with up to 2,500 mm of rain, but will grow in drier areas, too. It is drought resistant, but will die during the dry season. In cool climates it will grow 3 to 4 months into the dry season.

The typical Mucuna type is vining, but there is also a bush or dwarf type (Mucuna enana). Both, the vining and the bush type produce a dense canopy and suppress weeds well. The seeds are not edible due to L-dopa and need to be boiled well before consumption. Mucuna leaves can be mixed with 4 times of Napier grass for fodder, and the residues can be used as forage, silage or hay.

The bush type flowers in 80 to 90 days, whereas most velvet beans will take up to a year to produce seeds. The bush velvet bean grows best in temperatures of 20 to 30 °C and to elevations of 2,000 m. It is sensitive to frost, and very insect resistant. It can fix as much as 200 kg/ha of nitrogen and produce about 4 tons/ha of dry matter in fertile soils. It also contributes to nematode control.

Mucuna can be planted at the onset of the rainy season as an improved fallow on severely degraded soils. It can also be sown as a relay crop in cereal crops with repeated pruning and subsequently used as an improved fallow. The following crop is then sown into the dead mucuna mulch.

The bush type may also be intercropped to annual crops.

Lablab bean (Lablab purpureus)

Lablab is a fast growing, climbing legume. Bush varieties are also available. It has a high drought tolerance when it is established, and may stay green throughout the dry season. Its long cycle provides cover for an extended time if there are no hard frosts, being able to perform as a biennial plant.

Lablab beans grow well to up to about 1,500 meters. It requires more fertile soils than jackbean. Lablab starts flowering after 3 months and continues producing seed as well as remaining green. In contrast to velvet bean or jackbean the lablab bean can be cut off near the ground and will grow again with reduced vigour.

As animals like lablab very much, the beans cannot be grown where animals run free. On the other hand, lablab beans can be a high value dry season pasture for animals. Young lablab pods, and immature and dry beans can be eaten.

Jackbean (Canavalia ensiformis)

Jackbean is resistant to drought, poor soils, insects and diseases. A climbing and a bush type exist. The species will grow 5 to 6 months through the dry sea-son and can protect the soil during this time preventing erosion. The species is very useful to regenerate wasteland or as a green manure after the late monsoon crop is harvested. Jackbeans grow up to about 1,800 meters. The plants begin flowering after 4 to 5 months, then continuously produce seedpods for at least an entire year.

Jackbean can be used in grain fields, under orchard trees or to shorten fallow periods. As it is not as vigorous as the velvet bean, mixtures of the two species may do best.

Pigeon pea (Cajanus cajan)

Pigeon pea originates from India, but is widespread in Eastern and Central Africa. It has a shrubby growth habit and is usually perennial, but new varieties have been developed, which produce seeds within 3 to 4 months. The plant can be used for human consumption as well as fodder for animals and green manure or cover crop. For human consumption pigean peas are eaten fresh or are dried and cooked or milled or sprouted.

Nitrogen fixation of pigeon pea is reported to be around 90 kg per ha and year. It grows well under dry conditions. 650 mm of rain per year are sufficient for the crop to grow. Intercropping with maize or rice is widespread in America and Asia. The plant is also known in Cameroon and might be used in the crop rotation as a cash crop adding nutrients to the soil.

Fish poison bean (Tephrosia vogelii)

The fish poison bean has a shrubby growth habit, reaches up to 4 m of height and is perennial. It is very adaptive, tolerates different climates, poor soils, droughts and strong wind. However, the plant requires 850 mm of annual rainfall. It produces first seeds after 3 months. As it is a legume, it fixes nitrogen in its nodules. The fish poison bean has been used for decades as improved fallow in some areas of the Cameroon North-East District. As its name suggests, it is poisonous to fish, if the extract has contact with water, and its leaves are toxic to livestock. Therefore, livestock will not eat it. Fish poison bean extract is used as an acaricide on livestock. Some effects against insect pests have been reported, but have not been tested scientifically so far.

Different other plant species may be suited as green manure/cover crops, especially leguminous plants. In Cameroon, many leguminous trees are abundant, as well as different kind of beans. Organic farmers are encouraged to experiment with those plants and find the best solution for their specific farm.

Compost is a highly valuable organic fertilizer, especially in tropical conditions, as it will contribute stable organic matter and help to build the soil in the long term. Compost is a common name used for decomposed organic materials. Compared with uncontrolled decomposition, as it naturally occurs, decomposition in the controlled composting process is faster, reaches higher temperatures and results in a product of higher quality. Nutrients provided by compost are more balanced from a plant perspective than nutrients derived directly from animal manure. Compost improves soil structure and increases water retention capacity of soils. It also can suppress soil-borne diseases in some cases.

Compost making relies on materials that are available on the farm and does not require any special equipment (maybe with exception of a shredder and watering cans or some kind of sprinkler, or if the operation is done on a large scale), making it a cheap method. On the other hand, compost making requires a lot of work for collecting and preparing the materials, and to subsequently turn the compost heap regularly during the composting period. Therefore, compost production may be economical mainly in cropping systems with high value crops such as vegetables and potatoes.

Ideally, compost is made from equal amounts of animal manure and fresh plant materials, and dry woody materials. Farm-own plant material includes collected crop residues, weeds, or plants that are cultivated specifically for their biomass. Wood ashes and some old compost can be added as well. Compost production requires humid conditions. Therefore, regular watering is required in dry weather to ensure a proper decomposition process.

The crops can be grouped based on their nitrogen (or nutrient) demand, distinguishing heavy feeders, moderate feeders and fertility builders. Heavy feeders include crops such as maize, cabbage or Leek. These crops depend on high amounts of nitrogen to produce good yields. Moderate feeders include root and tuber crops, and fruit as well as leaf vegetables. Fertility builders include legume crops such as beans, peas, and (leguminous) green manures that are cultivated for improvement of soil fertility mainly.

The nitrogen that is provided by fertility builders is best exploited, if a legume crop is followed by a heavy feeder. Heavy feeders should then be followed by a moderate feeder. After heavy feeders such as corn or cabbages only few nutrients remain in the soil. Growing two heavy feeders after each other requires a high nutrient supply with fertilizers.

Organic farmers grow green manures or leguminous crops in the rotation to restore the nitrogen level of the soil. Leguminous green manures that are grown for soil improvement mainly will leave more nitrogen in the soil than legume crops, of which the grains are harvested. One or two year traditional fallows consisting of spontaneous plant growth (most of which are weed plants) is ideally replaced by an intensive (or improved) fallow to restore the nitrogen level of the soil. Introduction of an improved fallow will furthermore allow shortening the fallow period resulting in an intensification of the cropping system.

As an alternative to green manure crops, nitrogen fixing herbaceous feed crops can be grown in the rotation. The produced feed can be offered to animal farmers for grazing in exchange for the animal manure. The challenge may be though to ensure that the animal manure remains in the field and is not deposited outside the field.

Crop rotation design not only plays a key role for soil fertility management, but it is also important for control of soil-borne pests and diseases such as nematodes. For this reason, crops of different botanical plant families should be rotated. To prevent build-up of soil borne pests and diseases most crops, including potato, should not be grown on the same field more than every third or fourth season. As long as no soil-borne pests or diseases are noticed, potatoes may be grown every third season in the same field. During the cultivation break, also other crops of the nightshades family related to potato such as tomato, capsicums (peppers), eggplant, black nightshade and other should not be cultivated in this field. In addition, volunteer potato plants need to be rogued out. As a result, not more than a third or a fourth of the farmland is cultivated with potato or other nightshades at a time. The same rule applies in case of intercropping of potato with other crops.

The potato crop has relatively a high nitrogen demand in the first half of its growth period. Therefore, it does especially well after crops that leave a loose soil and a high amount of easily degradable organic material behind. Thus, suitable preceding crops include grain legumes such as beans and other vegetables. Leguminous green manure or feed crops are an option, too. Brassicas such as broccoli, cabbage and mustard plants preceding the potato crop reduce bacterial wilt and root knot nematodes in the soil. When harvesting the preceding crop or mulching the green manure crop, attention must be paid not to compact the soil.

After the crop, high amounts of soluble nitrogen remain in the soil. To avoid leaching of the nutrients in case of rain or erosion by wind if the crop is followed by a dry season, either a succeeding crop with elevated nitrogen demand such as maize, cabbages, leeks or a cereal or a fodder crop should be cultivated soon. Alternatively, the soil is covered with a cover crop to catch the nitrogen and prevent erosion.

The potato crop generally leaves a clean field for the succeeding crop. This allows reduced or minimal soil tillage for the preservation of the soil structure.

Intercropping is quite common in Cameroon. This practice improves food security by minimizing the consequence of failure of an individual crop, takes advantage of complementary characteristics of crops, reduces risks due to pests and diseases, and contributes to erosion control.

Some farmers traditionally grow mixed crops such as maize and beans or potato and beans, either by inter- or by alley-cropping. In the early rain season, potato is often intercropped with maize, cocoyam, cabbage, or other annual crops. Beans are a common choice of intercropping with potatoes in the late season. Unsuitable crops for association with potato are celery and nightshades such as peppers, aubergine and tomato. Commonly, intercrops are planted at the bottom or at the edge of the furrows and the potatoes on the ridge.

Although intercropping with a short season legume such as beans may increase total crop yield and help prevent spread of diseases, potatoes may rather be cultivated in rotation with other crops in order to produce high potato yields, suppress weeds efficiently, and simplify management. Consociated crops grown on the potato-ridge must be harvested at the same time as the potatoes.

Organic farmers look for varieties with fast early growth building a canopy quickly for good weed suppression, early tuber formation to reach good yields before appearance of late blight, and low susceptibility to diseases as well as low nitrogen requirements. Short growth duration can be very important in Cameroon to avoid losses by late blight, which is generally a more serious threat during the months of June to September when rains are consistent.

Some varieties show reduced risk of scab, hollow hearts, dry core, late blight, silver scurf, blackleg etc. However, no variety is immune to all these conditions. Therefore, regional recommendations may be helpful in variety selection to choose the varieties with the best resistances.

Before selecting a new variety or when producing for a new buyer, farmer cooperatives and larger farms need to discuss the varietal requirements with the retailer, as he/she will request varieties that are demanded more by the market. Farmers producing for local markets have more freedom in variety selection and can gradually introduce new varieties with different characteristics to their customers. Ideally, new varieties are best cultivated on a small scale in the first year allowing comparison with established varieties.

Criteria for variety selection:

• Yield: Varieties that produce many larger size tubers yield more and are preferred by the customers (and make a higher price).
• Varieties with tolerance or resistance to diseases such as potato late blight and viruses reduce the dependence from plant protection measures and limit the risk of crop failure.
• Earliness: Fast growing varieties with a short production cycle of 90 days can be cultivated in the (short) inter-season.
• Market preferences: certain customers or retailers prefer white skin potatoes while others prefer red skin potatoes. In addition, the size of the tubers and the depth of the eyes can be relevant criteria.

The quality of seed is highly relevant for successful potato production. The planting material should be free of seed-borne diseases. The use of certified seeds reduces the risk of infection with tuber diseases. Therefore, certified seed potatoes should be used whenever possible. However, such planting material may not be available or may be too expensive. Instead, high quality seeds with known history e.g. from a known seed multiplier may be used.

• Any purchased planting material should be controlled for quality defects such as black scurf (Rhizoctonia) marks or wet rot. Defects should be reported to the supplier immediately (keep the labels of the seed).
• Basically, the same quality criteria apply for saved, non-certified planting material as for certified seed. In practice, non-certified seed bears a higher risk of carrying seed-borne diseases such as viruses than certified seed. Therefore, in case of farm-own seed selection farmers should mark healthy plants on their production fields and check the external quality of the saved seed to reduce the risk of transmission of diseases to the next potato crop.
• Dressing of planting material with antagonists such as Bacillus subtilis and Pseudomonas species may reduce infection with Rhizoctonia and dry core. However, such potato seed dressings may not be common in Cameroon and effectiveness has not been proved yet.
• Calibrating planting material facilitates cultivation, as calibrated plants will build more even stands.
• For certified organic potato production, the planting material must be derived from organic propagation. If such seed material is not available for certain varieties, an exceptional permission from the inspection body may be necessary before purchasing seed tubers from conventional propagation.

Before planting, the potato seed should be pre-germinated (or chitted). Pre-germination significantly increases yield safety, as pre-germinated tubers emerge faster and are ready for harvesting 10 to 14 days sooner than not pre-germinated tubers. This reduces the likelihood of infection of the sensitive sprouts with Rhizoctonia (Black scurf) or Erwinia (Blackleg), and reduces the risk of late blight. 

Moreover, pre-germination leads to fewer sprouts and as a result to fewer stems per unit area of foliage, reducing the number of tubers, but increasing their size. The additional workload for pre-sprouting is reclaimed through higher yields and higher yield security.

• Temperature has the largest influence on the number of later sprouts in the pre-germination process. New potato varieties should have fewer sprouts in order to reach the required size quickly. Tubers for seed potato production should have more sprouts resulting in many tubers of rather small size.
• Short and strong multiple sprouts are extremely important for a high yield. Furthermore, seed tubers with short, firm sprouts can be transported to the field with lower risk of sprout injury than tubers with longer sprouts.
• Development of the sprouts is primarily affected by light intensity. If the tubers are well-exposed to diffuse light (but not direct sunlight), the sprouts will be slow in growth, coloured and sturdy. Seed tubers held in the dark will develop pale and long sprouts that easily break off. Long sprouts deplete the seed tuber and make it shrivel.
• Thus, for pre-germination the seed tubers are placed in a cool and bright place, avoiding direct sunlight. In case of artificial light lamps with warm tones, more than 100 W per t of planting material are used for 8 to 10 hours per day. To ensure exposure to light from all sides no more than two layers of potatoes are laid out in the chitting crates or onto flat trays.
• Seed tubers should have medium size, and no damages or signs of pests or diseases. Small tubers will produce less potatoes than medium size tubers.
• Pre-germination is started 4 to 10 weeks before the desired planting date, depending on the variety and pre-germination temperature.
• The ideal temperature for pre-germination is 10 to 12 °C. The higher the temperature during pre-germination is, the faster germination takes place. A slow germination is preferred to a fast germination. If necessary, the seed potatoes can be exposed to 18 to 20 °C over 2–3 days to induce germination. When the first sprouts emerge, the temperature is ideally lowered again to 10 to 12 °C (seed potatoes: 8 to 10 °C).
• Ideal air humidity is 70–80. If the tubers form too many roots, humidity is too high and needs to be lowered, or the pre-germination box is placed in the sun for a short period of time.

The decision of the way to work the soil is mainly an economic one. If labour is cheap and well available, it is efficient to do land preparation by hand in a traditional way using hand-held hoes. This is hard physical work and a strong strain for the back. As long as many workers in rural areas are available, this work ensures employment. As soon as labour costs rise, it might be economic to use mechanical soil tillage, for example in form of single-axle tractors, motor cultivators or even large tractors. The decision should always be based on cost-benefit calculations.

After primary soil preparation, the soil is finely worked to minimize clods. For mechanical cultivation on light soils, the seedbed can be prepared with a spring tine cultivator including a cage roller / packer. On heavier soils, a rotary hoe is commonly used in mechanical soil cultivation. Using manual labour only, the soil can be refined by a second pass with the hand hoe, breaking all bigger clods.

The more intensely a soil is cultivated though, the more the soil aggregates are broken up. This exposes soil organic matter and speeds up its breakdown and loss. Regular intensive cultivation degrades soil structure. Therefore, reducing tillage depth and intensity of cultivation contributes to better soil structure and reduces risk of soil erosion. In addition, the less the soil is compacted before planting, the better for the potato crop.

Potatoes need three weeks to emerge from the soil and another three weeks to cover the ground adequately. Therefore, on slopes, potatoes should be planted in rows along the contour lines to reduce the risk of erosion.

The distance between ridges can range from 75 to 90 cm. In case of mechanical cultivation row distance is usually 75 cm, since most machinery is designed for a track width of 1.5 m. Larger row spacing results in larger ridges with better water storage, better aeration of the plants, better nutrient supply and fewer green tubers. Larger row distance may help reduce the risk of late blight spread to some extent. In some areas where the soils are not deep enough to get enough soil for appropriate hilling, larger row spacing is useful.

The recommended planting distance in the row is of 25 to 40 cm depending on the row distance, the size of the seed, the variety and the desired size oh harvested tubers. Larger planting distance leads to less, but larger tubers. The larger the row distance, the smaller the planting distance within the row to achieve a favourable plant density. Planting at wide spacings can result in gappy crops particularly where planting is irregular or emergence poor. This may result in reduced yield per surface. Therefore, planting at higher densities should be considered, although increasing plant density will reduce the average tuber size. Plant populations below 25’000 plants per hectare are generally not recommended.

In Cameroon, potatoes are usually planted by hand at a depth of 10 to 15 cm to the top of the tuber. Planting depth is greater under warm, dry conditions than under cool, wet conditions. Shallow planting should be avoided, because the lower nodes of the stem must remain covered by soil to encourage tuber initiation, and to avoid greening of tubers and damage by the tuber moth. The sprouts of seed potatoes should face upwards. Attention is to be paid to prevent breaking of the sprouts. Seed of the same size should be planted together in one area to ensure a uniform crop for better management.

Until emergence, potato plants do not need any nitrogen, as they take it off the mother tuber’s reserves. The majority of the nitrogen is required during the short period between emergence and tuber development for the production of green plant material. Appropriate nitrogen supply within the first 35 to 50 days after emergence results in strong leaf growth resulting in good tuber growth. The more nitrogen is stored in the leaves, the more tubers develop every day, and the longer yield production lasts (unless leaf blight occurs).

During tuber formation, the plant continues to extract nitrogen from the soil, but the majority of the nitrogen is transferred from the leaves. When the nitrogen supply in the leaves is exhausted, the tubers mature. In some cases, high nitrogen supply may lead to an increase in hollow hearts in large tubers, secondary growth, and growth tears.

If nitrogen supply is too high, the constant development of new leaves and stems will result in a large, dense foliage. This can delay tuber formation and growth. Early infestation of leaf blight can lead to losses of yield. In addition, too high and too late nitrogen supply has a negative impact on dry matter and starch content of the tubers and on the flavour. Furthermore, it increases susceptibility to damage and to discolouration in the raw state and after cooking, and reduces storability. A too high release of nitrogen with a low potassium supply impairs maturation due to re-sprouting. This, in turn, complicates haulm destruction.

Organic farmers only use organically bound nitrogen sources including farmyard manures, leguminous green manures and grain legumes. The release of nitrogen from these sources depends on the nitrogen content of the source, its C:N-ratio and the conditions for mineralisation in the soil. The higher biological activity of a soil is, the higher its organic matter content is, and the better a soil is aerated and moist, the higher nitrogen mineralisation is (besides considering temperature).

The nitrogen demand of potatoes depends on different factors such as yield expectations, the variety, and growth conditions. It can range from 80 to 200 kg per hectare. Biologically active soils deliver about 20 kg of N per hectare (the higher the fertilisation or the amount of pre-crop residues, the higher mineralisation of nitrogen in the soil) under favourable mineralisation conditions during the vegetation period. With every ridging and hoeing, additional 10 to 20 kg of nitrogen per hectare are mineralised.

Leguminous green manures as pre-crop can provide 50 to 150 kg of available nitrogen per hectare, if the conditions for mineralisation and timing are good. Grain legumes leave, depending on the species, between 30 and 100 kg of available nitrogen per hectare to the succeeding crop (soybean 30 kg, grain peas: 50 to 80 kg, field beans: up to 100 kg).

Traditionally ridging is done with hand-hoes in Western Africa. Mechanically equipped farmers will use a tine weeder, disc harrow, ridge harrow, rotary hoe or rotary hiller for hilling and weed management. The rotary hoe makes well-covered ridges, but is not applicable on very stony soils, and is less suitable for ridging at the end of the rows. The rotary hiller makes well-covering ridges, but causes greater soil disturbance with risk of capping. It is only recommended for difficult, cohesive soils. Ridge-shaping boards may be recommended for the last tilling round. Hoeing equipment with rigidly mounted duckfoot-blades should be employed only when there is no risk of damaging potato roots. Multi-purpose devices exist that complete harrowing, earthing up and possibly hoeing in one operation.

From a weed management perspective, the ideal time for weed control is before the weeds become visible (in the white-thread stage /pre-emergence); at the latest when the weeds reach the two-leaf stage. Going through the rows with a tine harrow before emergence of the crop (blind harrowing) provides early first control of weeds.

Newly emerged potato plants are sensitive and should not be hilled or harrowed. Potato crops are usually hilled twice: a first time when the plants are hand-high, and a second time 3-4 weeks later. If the hilling is carried-out properly, weeds are effectively controlled. In the context of Cameroon, the measure of ridging is usually sufficient to control weeds. At the first hilling, the soil is pulled-up gently around the potato plants building homogeneous ridges. Plants with leaves of more than the size of a fist should not be covered anymore. After the first hilling, the height of the ridges should be around 30 cm. At the second hilling, soil is moved from the furrows to the top of the ridges.

In case of mechanical weed management, only light tractors with narrow tyres should be used for driving though the crop, to avoid compaction of the ridges.

Organic farming orients itself at a three-step approach for pest and disease management, where each step builds the foundation for the next one: step 1: soil and crop management; step 2: habitat management; step 3: direct control. The aim is to optimize the first and second step practices that encourage natural self-control of pest and disease pathogens, and to minimize the direct control measures.

As for pest and disease management in general, an integrated systems approach should be taken for control of pests and diseases in potato production, too. Such an approach makes use of (i) resistant/tolerant varieties, (ii) available agronomic control strategies such as crop rotation with consideration of required cultivation breaks, and an optimized cultivation system, (iii) alternative treatments (e. g. organically-based pesticides, plant ‘strengtheners’ and bio-control agents which can replace synthetic and copper-based fungicides) and (iv) optimisation treatments utilising forecasting systems (if available) with the aim of maximising synergistic interactions between (i), (ii), (iii) and (iv).

Measures in case of infections:

As soon as late blight occurs in the own or neighbouring fields, the dosage of the treatments must be increased to 800 to 1000 g of pure copper per hectare.

In addition, the diseased plants must be rogued by removing the stems and foliage within a 3 m radius of the infection, leaving the tubers in the soil. Remove the leaves and stems from the field and bury them in a hole.

If at least 30 mm of precipitation has fallen since the last application, or if many new leaves have been formed since then, the coating must be renewed. The treatment should be repeated after one week at the latest.

Harvest the tubers of the rogued plants after 2 to 3 weeks. This allows the skin of the tubers to set, reducing the risk of infection from sporulating leaves during harvest.

The maximum permitted copper application rate per hectare and year in certified organic farming schemes is between 2 and 6 kg of pure copper per hectare and year.

Management

To the day, no organic plant protection product for controlling bacterial wilt is available. Therefore, the disease needs to be managed through cultural practices. An essential element is prevention of disease development on a farm. This includes (i) using healthy (certified) seed only, (ii) using resistant varieties where available, (iii) maintaining a proper crop rotation with non-nightshades, (iv) regularly applying compost to the soil.

If bacterial wilt is observed on a field, further development and spread of the disease in the field must be prevented by: (i) careful removal and burning of infested plants and tubers to reduce spreading of the disease from plant to plant, (ii) spreading dolomitic lime around infected plants to increase soil pH; (iii) no cultivation of potatoes or other nightshades on these fields for several years (in serious cases for more than 10 years), (iii) as bacterial wilt spreads rapidly in flooded fields, avoid watering the field or try to make sure water does not flow over the infested surface of the field by digging channels that allow water to flow around it, (iv) not using water contaminated with bacterial wilt to irrigate the crop, (v) disinfecting farming tools after use.

Although crop rotation is not sufficient to control bacterial wilt, the disease incidence can be reduced, if cultivation of non-susceptible crops such as cereals is combined with other control measures.

The earlier bacterial wilt infections are detected, the better. Therefore, farmers are recommended to observe their potato (and other nightshade crops) for symptoms of bacterial wilts from 35 days onwards.

Reduced potato yields can be due to low soil fertility and insufficient plant nutrition. However, it can also be due to virus infection. Potato is affected by different viruses, one being the Potato Leaf Roll Virus (PLRV). The symptoms on the plants after infection vary depending on the virus, ranging from leaf mottling to mosaic appearance of the leaves and crinkling to dwarf growth. Heavily infected plants will give a poor crop of small tubers. In mild infections, plants often show no signs of disease at all. Therefore, virus infections are underestimated by most farmers because the viruses are not visible by eye, and the plants rarely die.

The viruses are transmitted by aphids and other sucking insects such as thrips, mites and whiteflies, and with infested tubers. Aphids spread the disease within the field and from one field to another. Mechanical transmission through tools is also possible, but secondary compared to transmission by aphids. Some viruses like PLRV also infect other nightshade crops and weeds such as tomato, tobacco and jimson weed.

Often virus infection is due to continuous re-use of farm-own planting material, transferring infection from one crop to the next with infested seed tubers. When farmers select the smaller tubers for seed, and the chosen tubers are those already infected with viral diseases (leading to smaller tubers), the risk of transmission of the pathogens is increased.

As symptoms appear during the early stages of potato growth, observations should begin when the plants rise, walking along the raised seedbeds and looking for plants showing symptoms of the disease.

Management principles are nearly the same for all viral diseases. Viruses can be controlled by (i) Using virus-free seed tubers; (ii) Using resistant varieties where available; (iii) Uprooting infected plants to reduce spreading of the disease within the field, and destroying them. Maximum effectiveness is obtained when removing all the plants in a radius of 1 m around the diseased plant. This is particularly important in seed fields. Controlling nightshades and volunteer potatoes on own and neighbour fields, because these plants can be reservoirs for the pathogen; (iv) Avoiding overlapping of potato crops; and (v) Controlling sucking insects as the main vectors of the viruses spraying.

The Potato Tuber Moth (Phthorimea opercullella) is the most serious pest of potatoes in Africa. The small butterfly occurs wherever potatoes are grown. Besides potato, the insect also attacks tomatoes and other nightshades. The damage is caused by the larvae of the moth (caterpillars) that make tunnels in the potato tubers.

The female moths are active after sunset and at dusk and lay eggs onto sheltered places of leaves and stems of the potato plants and near the eye buds of exposed tubers through cracks in the soil or in the store. The moths also lay egg on potato buds during pre-sprouting and on bags with potato tubers in the store. One female lays up to 200 eggs.

When the whitish to later pale greenish caterpillars hatch from the eggs after 8 days, they feed below the upper skin layer of the leaves and stems and bore their way to the tubers, where they make long, irregular black tunnels filled with excreta. Caterpillars hatching from egg that were laid onto tubers begin feeding on the tubers immediately upon hatching. In the feeding tunnels disease-causing fungi, bacteria and mites develop. The tubers start to rot and develop an unpleasant smell. Infested tubers cannot be used for human consumption anymore.

When the 15 mm large caterpillars have fed enough after about 2 weeks, they pupate in a cocoon covered with soil particles and debris among dead potato leaves, soil litter, eyes of tubers. In storage the pupae are found between tubers, in the entrance to the larval tunnels, and on the walls and floors of the storage room.

At harvest, the pest is transferred with the tubers from the field to the store, where it can reproduce and infest other tubers. Stored infested tubers can result in destruction of the entire crop at storage. Between potato crops, the pest survives as pupae in the soil, or in discarded potatoes left in the field at harvest. 

The tubers are attacked when they grow larger and come nearer the soil surface. At this time, the soil cracks and the larvae move from the foliage to attack the tubers, and to lay eggs on or near them. Infestations are worse in heavy soils that crack when dry.

The adult moth is 8 to 10 mm long. The males have fringed brown wings with dark spots, the females have an X pattern when the wings are together. The moths live for up to 10 days.

Protection

Intercropping potatoes with hot pepper, onions or peas keeps the moths away to some extent. Mulching the ridges with neem leaves during the last 4 weeks before harvest can significantly reduce insect damage.

Treatment of the potato plants with neem during cultivation limits the proliferation of the pest.

At storage, storing potatoes in layers with branches of lantana (Lantana aculeate) or Eucalyptus globulus provide some protection of the tubers. Tuber infestation can be reduced by bedding the potatoes in the leaves of the Peruvian pepper tree (Schinus molle). The tubers can also be sprayed with neem seed extract with 1 kg of neem extract in 40 l of water before they are packed in bags. Alternatively, Bt (Bacillus thuringiensis) powder mixed with fine sand (1:25) dusted controls the pest in the store too. Application of plenty of wood ash or diatomite earth onto the plants prevents rapid build-up of tuber moth. Pheromone traps can be placed in the storage room to confuse the moths.

The moment for dehauling has come, when the tubers have reached their preferred size for marketing or if no further growth of marketable yield is expected. For example, if a heavy infestation of late blight took place and more than half of the leaf area is destroyed, it is better to dehaulm and preserve the tubers from tuber blight. At dehaulming the tops of the plants are cut manually or mechanically at the base of their stems. If the foliage is healthy, it can be collected and composted. Leaves with disease symptoms must be burnt or composted adequately with frequent turning and thorough heat periods in the pile to kill the disease. After dehaulming the tubers are then given 14 to 21 days to develop matured and hardened skins. Well-matured (skin-stable) tubers are less sensitive to damage and storage rot. The tubers are mature, when the skins are firm and cannot be removed by lightly rubbing the tubers with the fingers.