FRENCH BEANS BASICS

French beans, also known as snap or green beans, and locally as mishiri, are a major export crop. Interest in the crop is fast-growing for both fresh consumption and processing (mainly canning and freezing).French beans contain protein, fat, calcium, iron, phosphorus, vitamins A, B, D and starch. French beans grow well in lower midland to lower highland zones of altitudes ranging from 1500-2100 metres above sea level. Rain-fed cultivation is possible in areas with well-distributed, medium to high annual rainfall of 900-1,200mm per annum. However, to maintain continuous supply especially during the off-season, irrigation is essential. Up to 50mm of water per week is required. French beans can grow in different soil types, ranging from sandy, loam to clay. The optimum temperature for production is 20-250C. However, the beans survive in temperature ranging from 14-320C depending on the variety. Extreme temperatures result in poor flower development and poor pod set. Seedlings will not tolerate temperature lower than 100C. They, however, grow well on friable (easily crumbled), silty loam to heavy clay soils, which are well-drained and high in organic matter.

SOIL TEST

The optimum soil pH is 6.5 to 7.5, but the beans can tolerate a low pH of up to 4.5.Below a pH of 4.5, plant growth is impaired through limitation of development of the rhizobium bacteria that are responsible for the nitrogen fixation in the galls formed on the bean roots.

It’s advisable to carry out a soil test before planting.  Various varieties are grown mainly for export, and they are determined by the market preference. They include Amy, Teresa, Samantha, Julia, Paulista, Vernando, Serengeti, Cupvert, Tokai and Bakara, Monel, Gloria, Claudia, Morgan, Amy coby, Espada, Maasai and Nerina.

Fresh market varieties are Amy, Pekara, Teresa, Paulista, Rexas, Samantha and Cupvert. Varieties for processing include Julia, Vernandon and Sasa.Planting should be scheduled so that most of the crop is ready between October to mid-December and from mid-January to end of May.In warm areas, beans take 55-60 days from planting to first picking, hence, plant from mid­-August to mid-October, then plant again early December.

French beans are sown directly into the seed bed. The land should be ploughed and harrowed properly just before planting. With irrigation, French beans can be grown all-year round but the main export season is from October to May. Spacing should be single rows of 30x15cm (a seed per hole) or double rows of 60x30cm. The spacing will depend on the variety, soil fertility, water availability as well as climate. It is advisable to plant in blocks of about four rows separated by a path of about 50cm.

FERTILISER AND MANURE APPLICATION

The seed rate required is 25-60kg/ha (10-24kg/acre) of certified seeds depending on the variety.Apply 200kg/ha (80kg/acre) DAP along the rows before planting. Contact between fertiliser and seed should be avoided by mixing the former thoroughly with the soil in the planting furrow. Apply 150kg/ha (60kg/acre) Calcium Ammonium Nitrate (CAN) for top-dressing twice. First when two to three leaves appear and the second at the beginning of flowering. 

Avoid use of excess nitrogen as it may promote vigorous vegetative growth at the expense of pod production. Foliar feeds are recommended to boost crop development and production. The choice of the fertiliser depends on the fertility of the soil and variety requirements. Farmyard manure is also recommended especially where soils are low in organic matter, for example, on the heavy clay and sandy soils. It should be applied in planting furrow and worked into the soil at the rate of 10 tonnes/ha.A kilo of seeds requires 4-8kg of fertiliser depending on the variety and soil conditions.

WEED CONTROL
The first weeding should be done two to three weeks after they sprout, followed by a second weeding about two weeks later. Care should be taken to avoid damaging the shallow roots, especially during the first weeding. The crop should not be weeded at flowering time and when the field is wet to avoid the shedding of flowers, spread of diseases and soil compaction.

Use of herbicides may be economically feasible for the commercial French beans grower. Pre-emergence herbicides such as: Lasso 4 EC (Alachlor) and Stomp (Pendimethalin) can be used. Basagran (Bentazon) can be applied post-emergence for control of broad leaved weeds.

IRRIGATION
Constant water supply is essential because soil moisture affects yield, uniformity and quality of French beans. Lack of water during flowering and podding causes flower abortion and curved pods leading to reduced yields. French beans, however, are very sensitive to waterlogged conditions.

The irrigation regime below is based on crop water requirement at various stages of growth as well as the soil and weather conditions.

i) Planting to 10 days (post-emergence), apply 35mm of water per week per crop.

ii)10 days post-emergence to flowering blooms, apply 50mm per week.

iii) At podding stage, apply 35mm per week.

Supporting French beans
Climbing varieties that grow to about 1.8m (6ft) high need to be supported. This is done by use of trellises, poles, or other means at least 200cm (8ft) high.

DISEASES
RUST
The disease is caused by the fungus Uromycesappendiculatus. This is a very serious disease to French Beans and other beans. It is favoured by high humidity conditions.

Symptoms include presence of slightly raised, small white spots, on the surface of the lower leaf. The spots turn red to dark brown after a few days. Control is by crop rotation, use of tolerant varieties and chemical recommended fungicides.

ANGULAR LEAF SPOT

It is a fungal disease caused by Phaeoisariopsisgriseola. Leaves, stalks, and pods have angular brown or red spots with purple edges and grey to brown centres. The leaves may then fall prematurely. Control by use of healthy, certified seeds and treat seeds using recommended fungicides.

ROOT ROTS
Affected plants show yellowing and drying of stem at soil level. Stunting also occurs. The crop may further show poor seedling establishment, uneven growth, chlorosis and premature defoliation of severely infected plants. Control through seed dressing and drenching with recommended fungicides during the vegetative stage.

BACTERIAL BLIGHTS
It is caused by Pseudomonas phaseolicola and Xanthomonaphaseoli. This is a serious disease for beans, especially in cool and wet areas. The disease is spread through splashing from exuding lesions and plant debris. Plants show ring-like spots on the leaves, drying of leaf margins, yellowing and water-soaked pods. It is controlled by use of certified seeds, rouging and destruction of affected plants, crop rotation and chemical sprays using copper-based fungicide.

ANTHRACNOSE

The disease is caused by a seed-borne fungus called Colletotrichumlindemuthiamum. The fungus is and affects all aerial plant parts. It is spread by rain splash, wind or mechanical contact. The disease usually occurs in cool, damp weather. It is characterised by appearance of sunken, brown spots with black edges on pods; angular brown spots on leaves; and oblong stripes on stems.

Controlled by use of certified seeds, field sanitation, crop rotation, use of resistant varieties and recommended fungicides.

BEAN COMMON MOSAIC VIRUS
The disease is seed-borne and it is transmitted by aphids. The symptoms vary with variety, stage of growth, and environmental factors. They include a mosaic (that is mottling, curling and stunting of leaves,) systemic necrosis and malformations. The plant produces excessive number of lateral shoots. Control: Use of certified seeds, plant resistant varieties, rouging of infected plants and control of aphid vectors using insecticides.

POWDERY MILDEW

It is caused by a fungus known as Erysiphe spp.It attacks stems, leaves, flowers, and pods, which appear covered with white powdery growth that later turns black. In severe cases, the leaves turn yellow and drop off.

Control: Field hygiene, crop rotation and use of recommended fungicides
Downy mildew.The underside of crop leaves exhibit white to greyish growth, which later cover whole leaf surface.

Control: Field hygiene, crop rotation and use chemical fungicides.

HARVESTING
Picking of pods begins six to eight weeks after planting, depending on the area and variety, and continues for about one to two months. The pods are carefully picked, and not pulled from the plants, and should have the stalk attached to them.

Picking should be done at regular intervals depending on buyer specifications. Harvesting may be done twice a week for the fine beans and three times a week for the extra fine beans. This continues for around three weeks.
One gets yields of nine to 15 tonnes/ha and six tonnes/acre in 2½ months. A kilo of seeds may produce 200-600kg of produce depending on the variety and crop management.

CATEGORIES
1) Extra fine pods: Are very tender, turgid, seedless, with no strings, and free from any defects. The width of the pods (maximum diameter) should be less than 6mm and the minimum length of 10cm.

2) Fine pods may have small seeds and be short with soft strings, be turgid and tender. The width of the pods should be between 6-9mm while the length of 12-l4cm is recommended.

3) Bobby beans comprise those which do not qualify for inclusion in the higher classes but satisfy the minimum requirements specified above. Beans should be reasonably tender and seeds should not be too large.

CHALLENGES
(i) Rejection of the produce if they do not meet the set quality standards

(ii)Poor disease and pest management can lead to poor quality produce.