Prota 2: Vegetables/Légumes
Sp. pl. 2: 723 (1753).
Papilionaceae (Leguminosae - Papilionoideae, Fabaceae)
2n = 22
French bean, snap bean, navy bean, common bean (En). Haricot vert, haricot mangetout (Fr). Vagem (Po). Mharagwe (Sw).
Origin and geographic distribution
Phaseolus vulgaris was domesticated in Central and South America more than 6000 years ago. Domestication occurred independently in Mexico and Guatemala, and Peru and neighbouring countries. Small-seeded ecotypes occur in the wild in northern Argentina and Central America. Dry seeds were introduced and planted in Spain during the 16th century and from there Phaseolus vulgaris was taken to France. The unripe pods soon became popular as a vegetable in Europe. The use of the green pods was made possible by a considerable reduction of the ‘parchment’ consisting of cross -intersecting fibres in the fruit wall. This character, polygenic in heredity, was improved by breeding in Europe during the 18th and 19th centuries. The hard fibres along both sutures (‘strings’) were first eliminated by a dominant mutation, which appeared spontaneously during the 19th century. However, several genes control the presence of strings and this character is somewhat linked with pod straightness, and it is only recently that French bean with long and straight, consistently stringless pods has been bred successfully. At present French bean is produced all over the world and can be found in all countries of tropical Africa. It is more popular in Francophone than in Anglophone countries, more in urban than in rural areas, more in highland than in lowland regions, and more during the cool season than during the hot season.
Young pods of French bean are boiled after cutting both ends and carefully removing the strings, if present; they may then be cooked or fried with sliced onion and garlic, or used in salads. They can be boiled together with rice. They require a shorter cooking time than dry bean seeds. The full-grown but immature seeds (larger than dry ones) are sometimes also eaten, and sold in their pods on vegetable markets (shell beans). They can be cooked without previous soaking in water and in a shorter time than dry seeds. Leaves are occasionally used as a vegetable, but few cultivars have leaves of sufficient tenderness. Crop residues are often used as fodder.
In East and southern Africa the use of ripe seeds of Phaseolus vulgaris as a pulse is more important than the use of unripe pods or seeds as vegetables; a separate article in PROTA 1: Cereals and pulses deals with pulse aspects of Phaseolus vulgaris.
Production and international trade
World production of French bean (combined with yard-long bean) in 2002 was estimated by FAO at 5.8 million t from 855,000 ha. China produced 2.0 million t, Turkey 515,000 t, the European Union 664,000 t, tropical Africa about 75,000 t and northern Africa 312,000 t. An important part of the tropical African production is exported to Europe: nearly 40,000 t, the most important exporters being Senegal, Burkina Faso, Kenya and Zimbabwe. French bean is the third most important agricultural export product of Kenya, after only tea and pineapple.
The production of French bean pods for canning is important in Europe. An important amount of French bean seeds is produced in East African highlands (e.g. Kenya, Tanzania) for European seed companies.
The nutritional composition of French bean pods, raw and ends trimmed (edible proportion 83%) is: water 90.7 g, energy 99 kJ (24 kcal), protein 1.9 g, fat 0.5 g, carbohydrate 3.2 g, dietary fibre 3.0 g, Ca 36 mg, Mg 17 mg, P 38 mg, Fe 1.2 mg, Zn 0.2 mg, carotene 330 μg, thiamin 0.05 mg, riboflavin 0.07 mg, niacin 0.9 mg, folate 80 μg, ascorbic acid 12 mg (Holland, B., Unwin, I.D. & Buss, D.H., 1991).
Adulterations and substitutes
French bean pods can be replaced by pods of several other leguminous crops. Yard -long bean (Vigna unguiculata (L.) Walp.) is the most appropriate substitute in tropical lowland, but in Africa its use is mostly restricted to consumers from Asian origin. Young pods of runner bean (Phaseolus coccineus L.) are also used as a substitute; the large pods are sliced before cooking. The young pods of lablab (Lablab purpureus (L.) Sweet) are also used in the same way as those of French bean.
Climbing, trailing or more or less erect and bushy annual herb, slightly pubescent; taproot well developed, with many lateral and adventitious roots; stem up to 3 m long, angular or nearly cylindrical. Leaves alternate, 3-foliolate; stipules triangular, small; petiole up to 15(–30) cm long, grooved above, distinctly thickened at base, rachis (1.5–)2.5–3.5(–6) cm long; stipels small; leaflets ovate, (5–)7.5–14(–20) cm × 5–10(–15) cm, basal ones asymmetrical, apical one symmetrical, entire, slightly pubescent, 3-veined from the base. Inflorescence an axillary or terminal raceme up to 15(–35) cm long, with flowers arranged along the rachis in pairs or solitary. Flowers bisexual, papilionaceous; pedicel up to 1 cm long, slender, with ovate bracteoles; calyx campanulate, tube c. 3 mm long, lobes triangular, 2–3 mm long; corolla white to pale purple or red-purple, standard very broadly obovate, hood-shaped, c. 1.5 cm long, wings obovate, c. 2 cm long, keel sharply upturned, c. 1 cm long; stamens 10, 9 fused and 1 free; ovary superior, c. 0.5 cm long, laterally compressed, style upturned and spiralled, with collar of fine hairs below the ellipsoid stigma. Fruit a linear pod up to 20 cm long, straight or more commonly curved with a prominent beak, fleshy when immature, green or yellow, sometimes red, purple or with purplish stripes, (2–)5–7(–9)-seeded. Seeds globose to kidney-shaped, ellipsoid or oblong, 0.5–1.5(–2) cm long, black, brown, yellow, red or white, sometimes with speckled, flecked or saddled patterns; hilum oblong to elliptical. Seedling with epigeal germination; cotyledons oblong, thick; first two leaves simple and opposite, subsequent leaves alternate, 3-foliolate.
Other botanical information
Phaseolus comprises about 50 species, most of them in the Americas. Most French bean cultivars belong either to the climbing, unbranched ‘pole’ type, or to the dwarf ‘bush’ type. The pole cultivars have indeterminate growth up to 3 m high and are normally supported. The bush cultivars are early maturing, 20–60 cm tall, and have determinate growth with short internodes. Stringless cultivars now predominate. The so-called ‘wax beans’ are yellow because chlorophyll is absent from the pods, petioles and young stems. Green French bean pods may be uniformly green, or purple-striped due to the presence of anthocyanins. Some cultivars produce uniformly purple pods (e.g. ‘Mangetout à cosse violette’). Purple striped and yellow pods are easier to pick by hand amongst the green foliage. Most French bean cultivars have cylindrical pods, but large flat stringless ones, called ‘slicing bean’, are popular especially for home gardening in Europe. Local African cultivars – genetic mixtures resulting from earlier introductions and often heavily infected with seedborne diseases – can be found where small -scale farmers keep their own seeds.
Growth and development
For good seed germination, the soil must be warmer than 12°C, optimal growth occurring at 22–25°C. The plant can withstand occasional day temperatures of 35°C, which can, however, induce flower abortion. The roots form nodules containing nitrogen-fixing Rhizobium bacteria. Several Rhizobium species fix nitrogen with Phaseolus vulgaris, e.g. Rhizobium phaseoli. Flowering starts 28–35 days after sowing. Self fertilization is common and the rate of cross pollination by insects is generally small. Such hybrids are often easily recognized by differences in seed colour. Young pods can be harvested 15–20 days later. The harvest of the immature pods induces prolonged flowering. The harvest period can last 10–15 days to 20–30 days for bush and pole cultivars, respectively.
Phaseolus vulgaris is well adapted to elevations of 1500–2000 m in East Africa. It can, however, be grown under lowland conditions provided maximum daily temperatures are not higher than 30°C, as occurs during the dry winter season in the Sahel. Most French bean cultivars are sensitive to soil acidity and aluminium toxicity, the optimum pH is 6.1–7.6. In African regions where there is no Phaseolus vulgaris growing tradition, nitrogen fixation by Rhizobium spp. may be not efficient enough to ensure normal growth, also because modern cultivars are bred on European soils very rich in nitrates. French bean is sensitive to salinity and to excessive soil boron. In heavy clay soils germination problems may occur if irrigation is applied between sowing and emergence.
An average fertilizer recommendation for French bean is about 40 kg N, 90 kg P and 90 kg K per ha for bush cultivars, and 60 kg N, 120 kg P and 120 kg K for pole cultivars, depending on the soil fertility situation. As the growth of the root system is slow, it is recommended that some nitrogen fertilizer be applied even where effective nodulation can be expected. However, excess nitrogen will predispose the crop to bacterial blight and aphid infestation. In acid soils tricalcium phosphate may be applied in the furrow before sowing, in neutral or alkaline soils triple superphosphate. Direct contact between seed and fertilizer should be avoided. During the vegetative stage weeds must be rigorously controlled, but damage to the roots and stem base should be avoided. During this stage irrigation should be applied twice weekly if it does not rain. Overhead irrigation is recommended if certified disease-free seeds have been sown and if aphids and thrips are a problem. However, overhead irrigation should be avoided for French bean grown for seed production, since it may induce bacterial blight.
Propagation and planting
Seed weight of French bean varies from 0.2–1 g. For bush cultivars, seeds are sown either following traditional triangular patterns, or more often nowadays in lines, with 20 seeds/m sown solitary or 4–5 together at 20–25 cm intervals, with 60–80 cm between lines. This requires about 100 kg seed/ha. Seeds of pole cultivars are sown 5–6 together in lines at intervals of 40–50 cm, with 100–120 cm between lines. The seeds are buried at 3–4 cm depth, or up to 7 cm if the soil surface is dry and the soil not too heavy. For climbing types, 2 m high poles (straight branches, bamboos or bars) are placed after emergence of the seedlings. In humid tropical conditions, it is recommended that they be placed vertically (rather than 2–4 tied together at the top) to avoid the development of web blight. In Kenya some farmers grow French bean for export in hydroponics in screen-houses.
Diseases and pests
All parts of the French bean plant may be attacked by one or more diseases and pests, the severity and incidence depending on location and season. Several seedborne diseases are widespread. In Africa Phaseolus vulgaris is affected by essentially the same diseases and pests that are found on other legumes such as Cajanus cajan (L.) Millsp., Vigna subterranea (L.) Verdc. and Vigna unguiculata (L.) Walp.
In seedlings damping off caused by Pythium aphanidermatum or Rhizoctonia solani can be serious in excessively wet soils and particularly if seeds are not treated with appropriate fungicides. Older plants can be killed by several wilt-causing pathogens (Sclerotium rolfsii, Macrophomina phaseolina, Fusarium solani f.sp. phaseoli and by nematodes). Infestation with these diseases is generally severe in cases of repeated cultivation without crop rotation. Management of these soilborne diseases is very difficult. In Western countries pesticides for the control of some soilborne pathogens are commercially available. In Africa the control is by the use of soil amendments (including neem extracts for nematode control), long crop rotations, or leaving the land fallow for several seasons.
The seedborne diseases angular leaf spot (Phaeoisariopsis griseola), anthracnose (Colletotrichum lindemuthianum), common bacterial blight (Xanthomonas campestris pv. phaseoli) and halo blight (Pseudomonas syringae pv. phaseolicola) can cause serious losses in French bean crops. It is not possible to control bacterial blight once established. Copper sprays can only minimize further spread and provide limited protection to apparently healthy plants. The use of certified disease-free seed is critical in averting the introduction of these diseases into new fields. These diseases are aggravated by overhead irrigation and excessive nitrogen fertilization. Most modern French bean cultivars are resistant to common races of Colletotrichum. It is possible to effectively control angular leaf spot and anthracnose by seed dressing and by fungicide sprays.
Other diseases that can cause serious crop losses are bean rust (Uromyces appendiculatus var. appendiculatus), powdery mildew (Erysiphe polygoni) and web blight (Rhizoctonia solani). Some cultivars are relatively resistant to bean rust. Application of fungicides may be necessary if bean rust or powdery mildew infections occur during the early stages of growth.
Most French bean cultivars on the market nowadays are resistant to bean common mosaic virus (BCMV, aphid-transmitted and seedborne). In tropical lowlands where French bean is grown together with cowpea, legume strains of cucumber mosaic virus (CMV) and several cowpea viruses transmitted by beetles have been observed on French bean. The control of BCMV is based on the use of certified disease-free seed, resistant cultivars and vector management.
The most important insect pests during the first 4 weeks of growth are bean flies (Ophiomyia spp.), which tend to be severe during dry spells after a rainy season. Aphids (Aphis fabae and Aphis craccivora) can also be a problem at early growth stages. Both bean flies and aphids can be effectively controlled by seed treatment with systemic insecticides such as imidacloprid. In Africa cutworms (Agrotis spp.) and caterpillars (Spodoptera spp.) may be a problem especially in soils amended with farmyard manure. Important pests of older plants include thrips (Frankliniella occidentalis,Frankliniella schultzei and Megalurothrips sjostedti), pod borers (Helicoverpa armigera and Maruca testulalis) and spider mites (Tetranychus urticae). Thrips, particularly Frankliniella occidentalis, are very difficult to control as they are resistant to many commonly used pesticides. Pod borers are easily controlled by Bacillus thuringiensis products. Spider mites are generally severe during the hot, dry season and in many cases are a result of excessive application of foliar pesticides early in the season.
Integrated pest management (IPM) strategies for French bean production are centred on the concept of avoidance of foliar pesticide applications for as long and as much as possible. This should give natural control agents a chance to keep pest populations at low levels. The second principle is not to apply any pesticides after flowering to avoid the contamination of pods. IPM includes crop rotation with non-leguminous crops, the use of certified disease-free seed, seed treatment with a systemic fungicide and insecticide, e.g. carboxin and imidacloprid for control of damping-off diseases and bean flies respectively, and treatment of late-season pests such as thrips, pod borers and spidermites by insecticides.
In Africa French beans are picked by hand, usually twice a week, resulting in 4 harvests for bush cultivars and 7–9 for pole cultivars. This gives higher yields and better quality than mechanical harvesting, which is predominant in Europe and requires not only costly machinery, but also the use of special cultivars that produce all pods at the same time and large plots of flat land.
Under the best growing conditions, yields of 7–8 t/ha of French bean pods can be obtained with bush cultivars and 14–16 t/ha with pole cultivars. In case of seed production up to 1000 kg/ha dried seeds can be produced under average growing conditions.
Handling after harvest
Sorting of young pods is needed to remove broken, malformed and overripe pods. For the French market grading is done in two main grades – extra fine and fine pods. Extra fine ones should be very tender, seedless, with no strings and free from any defects; the width should be less than 6 mm and the minimum length 10 cm. The fine pods may have small seeds and be short with soft strings; the width should be 6–9 mm. Specifications of grades in other countries may differ somewhat. French bean pods are packed in corrugated cardboard boxes of 3 kg gross weight or in plastic pre-packs weighing 250, 500 or 1000 g.
Pre-cooling is done using forced air coolers at 7–8°C. At this temperature and a relative humidity of 95–100%, the pods can be stored for one to two weeks.
Most French bean cultivars are of European or North-American origin. Germplasm collections are kept by research institutes in Europe and North America, and catalogues of seed companies offer a considerable variability. The collection preserved by the Centro Internacional de Agricultura Tropical (CIAT), Cali, Colombia holds mostly pulse cultivars; it includes interesting potential parents for disease resistance, general tolerance or adaptability to adverse soil conditions, which could be introduced into French bean. Cultivars traditionally recommended for the tropics are the bush cultivars ‘Contender’ and ‘Tendergreen’, and the pole cultivars ‘Kentucky Wonder’ or ‘Phénomène à rames’. These cultivars may also be replaced nowadays by more recent cultivars chosen after local experimentation and better adapted to tropical conditions (e.g. the bush cultivar ‘Délinel’). In Indonesia ‘Perkasa’, selected by East-West Seed Company from local material, has good fruit setting under hot, humid lowland conditions, yielding up to 10 t/ha.
Phaseolus vulgaris is predominantly autogamous and cultivars are inbred lines. Cross-fertilization can occur when flowers are visited by Xylocopa (big, dark blue Hymenoptera) and can be detected in seedlots of the following generation by off-coloured seeds. Controlled hybridization is a delicate operation taking about 2 minutes per flower. New homogeneous lines can be obtained after 7–8 generations (2.5–3 years when 3 generations per year). When breeders make crosses between pulse cultivars and French bean cultivars, the polygenic nature of the absence of parchment makes 2 or 3 backcrosses necessary, while the dominant heredity of the stringless character may cause reappearance of strings in the progeny of the best F2 or F3 plants.
Interspecific crosses can be made with Phaseolus coccineus L. and Phaseolus acutifolius A.Gray (a drought resistant species from Central America), and resistance to Xanthomonas phaseoli has been introduced from these species. Powdery mildew resistance was found in Phaseolus vulgaris germplasm from Haiti.
French bean is an important vegetable with high nutritional value and economic potential. It is of interest for family gardens (pole cultivars), the domestic market and the export market. It would be interesting to improve local cultivars with disease resistances (in addition to resistance to anthracnose and bacterial blight already obtained in Europe and the United States), proper fruit setting at high temperatures for adaptation to tropical lowland conditions, improved nitrogen fixation and adaptation to adverse soil conditions, using genes available from tropical pulse cultivars.
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Sources of illustration
• Smartt, J., 1989. Phaseolus vulgaris L. In: van der Maesen, L.J.G. & Somaatmadja, S. (Editors). Plant Resources of South-East Asia No 1. Pulses. Pudoc, Wageningen, Netherlands. pp. 60–63.
Correct citation of this article:
Messiaen, C.-M. & Seif, A.A., 2004. Phaseolus vulgaris L. (French bean) In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/Légumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, inflorescence; 2, fruiting branch; 3, seeds.
field with young plants
French beans for export
open pods and beans