Laikipia County

Agronomist’s notebook: Watch out for this enemy of French beans

French beans are fast-maturing. They normally take two to three months to be ready. The crop is mainly grown for export, processing and fresh consumption.

I visited several farmers in Laikipia County last week and realised one of the threats to this important crop is rust disease. But what farmers do not know is that they are the main culprits spreading the disease.

On one of the farms, the farmer was harvesting his produce, having placed the crates on the soil dirtying the French beans. The harvested beans were further exposed to direct sunlight, leading to moisture loss.

The symptoms of attack develop on leaves, pods or stems as pustules of brownish-red, powdery spores. Severely infected leaves turn chlorotic lowering the photosynthetic area which results in loss of vigour and sometimes leaf defoliation that leads to decline in yields.

Also, while carrying out management practices, it is essential to ensure that one starts with non-infested blocks towards infested and not the vice versa to prevent spread of the disease.

French beans are grown in areas that receive adequate, well-distributed medium to high rainfall of about 900-1,200mm. In areas where irrigation is done, the crop can be grown throughout the year.

One should have an irrigation schedule and avoid long periods of leaf wetness when the temperatures are warm. In this case, drip irrigation is the most appropriate. Leaf wetness increases rust infestation. However, adequate watering facilitates the formation of pods increasing yields.

Infected plant residues should be deeply ploughed or allowed to wither and fed to animals such as dairy cows. Other preventive measures include spraying sulphur-based chemicals, though sulphur should not be used during flowering since it leads to flower abortion.

Since it is an export crop, it is paramount to observe the correct rate of chemical usage and observe the pre-harvest interval to have the minimum residue on the produce.

The crop can grow well in a wide range of soils that are well-drained and fertile. The pH should be appropriate since low pH affects the development of rhizobium bacteria that are responsible for fixing nitrogen in the soil.

While top-dressing, one should consider the soil fertility as too much nitrogen results in the plant being too vegetative and with little or no pods, hence decreased yields.

While harvesting, ensure the beans are clean by placing crates on top of each other or use buckets. This prevents the beans from getting dirty. Farms should have a shade where the beans are kept waiting to be weighed to reduce moisture loss.

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