How to grow French beans for higher returns

French beans, also known as snap or green beans 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. 

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 is advisable to carry out a soil test before planting. 

Planting 
Planting should be scheduled so that most of the crop is ready between October to mid-December and from mid-February 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.

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.

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 poding 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.
• Planting to 10 days (post-emergence), apply 35mm of water per week per crop.
• 10 days post-emergence to flowering blooms, apply 50mm per week.
• At poding 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.
Control 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

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 kilogramme of seeds may produce 200-600kg of produce depending on the variety and crop management.

KEY FACTS
• French beans contain protein, fat, calcium, iron, phosphorus, vitamins A, B, D and starch.
• 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.
• Use of herbicides may be economically feasible for the commercial French beans grower.
• The first weeding should be done two to three weeks after they sprout, followed by a second weeding about two weeks later.