Getting healthy beans Introduction French beans (Phaseolus valgaris L.) is a major vegetable crop grown in Kenya for export. It is m<strong>ai</strong>nly grown by small-scale farmers and virtually all of it is exported to Europe. The m<strong>ai</strong>n French bean production areas in Kenya are N<strong>ai</strong>vasha, Kirinyaga, Meru and Athi River. They use a bean variety m<strong>ai</strong>nly producing extra fine and fine grades. The picking of French beans begins in N<strong>ai</strong>vasha after 60 days from the crop emergence and <strong>co</strong>ntinues for about three weeks ,depending on the prev<strong>ai</strong>ling weather <strong>co</strong>nditions. The French bean is extensively grown as a green vegetable and nearly 6,000 hectares of land is under its cultivation in Kenya. The production of the beans is estimated to be about 1,250,000 tonnes per year. It is grown for the tender pods, shelled green beans and dry beans. Planting requirements French beans require an optimum temperature range of 16 – 25°C. Soils must be well dr<strong>ai</strong>ned and with high level of organic matter and a pH range of 6.5 – 7.5. Experience has <strong>co</strong>nfirmed that higher production is achieved in <strong>co</strong>oler weather. Long r<strong>ai</strong>ns and foggy periods are, however, harmful to the crop due to heavy disease presence. Nutrients management During the planting ensure you use DAP at 200kg/ha and when the plant is 2 – 4 weeks, use Urea/ CAN 200kg/ha depending on soil type. The bean is a leguminous crop. It has a capacity of fixing atmospheric nitrogen into the roots with the help of rhizobia bacteria which reside in the roots. This inherent capacity of the plant to fix atmospheric nitrogen leads to the poor response to applied nitrogen. As such, only phosphatic and potassic fertilisers are re<strong>co</strong>mmended. A starter dose of nitrogen should also be given. Recent experience from farmers who are growing French beans is that Boom flower, which is a unique crop nutrient, has been shown to increase yield by over 33 per cent, with low cases of disease arising and also early maturity when applied after every 15 days after emergence. Diseases i. Anthracnose A fungus known as Colletotrichum lindemuthianum causes this disease. Yellowish to brown sun<strong>ke</strong>n can<strong>ke</strong>rs are produced and <strong>co</strong>ol and humid weather is the predisposing factor. Control Seed treatment is essential, use of resistant varieties li<strong>ke</strong> tweed wonder is also re<strong>co</strong>mmended. Use may be made of any other re<strong>co</strong>mmended fungicides. ii. Powdery mildew It is caused by Erysiphe polygoni, a fungus that attacks the leaves first and later spreads to the stem and pods. White powdery spots that enlarge and <strong>co</strong>mbine with others to eventually form an almost <strong>co</strong>mplete <strong>co</strong>ating of a whitish powder is the important symptom. Severe infection leads to defoliation. Control Use of re<strong>co</strong>mmended fungicides. iii. Rust High relative humidity is responsible, along with the fungus, Uromyces phaseoli typica. Minute, white r<strong>ai</strong>sed spots appear on the lower surface of the leaves, which gradually be<strong>co</strong>me dark brown to black in <strong>co</strong>lour. Control Use any other re<strong>co</strong>mmended fungicides. iv. Common blight It is a bacterial disease that is caused by Xanthomonas phaseoli. The symptoms of the disease are the water soa<strong>ke</strong>d spots that appear on the lower side of the leaf. These spots enlarge and <strong>co</strong>alesce and turn brown, giving a burnt appearance. Insects li<strong>ke</strong> the grasshopper and the Mexican bean beetle transmit this disease. Control Use of disease free seed, long rotation and use of resistant varieties are the <strong>co</strong>ntrol measures. v. Mos<strong>ai</strong>c disease This is a viral disease that causes stunting of the plants, mottling and leaf malformation. The m<strong>ai</strong>n insect vectors are species of Aphis. The only satisfactory <strong>co</strong>ntrol method is the use of resistant varieties. B. Pests The important pests that attack French beans are bean aphid’s beetles, weevils and thrips. i. Bean aphids Aphis craccivora infestation causes characteristic damage to tender aerial parts of the plants and produces symptoms li<strong>ke</strong> curling of leaves, twisting of twigs and shedding of flowers. Control Spraying with any re<strong>co</strong>mmended insecticides. ii. Beetles Epilachna varivestis is the causal organism that is <strong>co</strong>mmonly known as Mexican bean beetle. Both larvae and adults feed on the leaves, usually on the lower side. Control Spraying with any re<strong>co</strong>mmended insecticides. iii. Thrips Infested leaves turn pale and drop. Infested pods exhibit silvery white <strong>co</strong>lour. Control Spraying with any re<strong>co</strong>mmended insecticides. iv. Stem fly The maggot of Ophiomyia phaseoli causes drying of the plants when it enters into the stem. Control Spraying with any re<strong>co</strong>mmended insecticides. NB: With the ban on some organophosphates on basis PHI among others, farmers are using products with low PHI to <strong>co</strong>ntrol pests and mostly pyrethroids li<strong>ke</strong> Albaz 10Ec. By Nicholas Mutune Technical and Development Assistant Manager, Highchem Essentials Ltd 40 HORTICULTURAL NEWS I <strong>FEB</strong> / MAR <strong>2013</strong>
2nd edition www.iftex.org HORTICULTURAL NEWS I <strong>FEB</strong> / MAR <strong>2013</strong> 41
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