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

Women harvest french beans in a farm in Kirinyaga County. French beans are grown in areas that receive adequate, well-distributed medium to high rainfall of about 900-1,200mm. FILE PHOTO | NMG

What you need to know:

  • Rust is a fungal disease caused by Uromyces appendiculatus, which thrives in prolonged periods of warm and moist weather.
  • Planting a border crop of maize helps control the spread of diseases to the adjacent blocks since the cereal prevents the movement of spores by the wind.
  • Planting resistant varieties further plays a significant role in the control of the disease.
  • 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.

Normally, rust attack develops on leaves, pods or stems as pustules of brownish-red, powdery spores

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.

In Kenya, there are different varieties that include Serengeti, Vanilla, Teresa and Samantha. The choice of the variety depends on prevailing ecological conditions, maturity period and market preferences

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.

These are among the mistakes that farmers do unknowingly while producing the French beans.

Rust is a fungal disease caused by Uromyces appendiculatus, which thrives in prolonged periods of warm and moist weather.

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.

Planting a border crop of maize helps control the spread of diseases to the adjacent blocks since the cereal prevents the movement of spores by the wind.

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.

Planting resistant varieties further plays a significant role in the control of the disease.

Other diseases that affect French beans include leaf blight, and bacterial leaf spot while whiteflies, caterpillars, aphids, and thrips are among the common pests.

FORMATION OF PODS

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.

One should also avoid microbial contamination and maintain good farm hygiene by weeding and removing any unwanted plant residues to prevent the spread of pathogens to plants.

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.

Packaging should be appropriate to ensure the product is not affected by rotting, leading to quality deterioration.

Vehicles transporting French beans should also have fans or refrigerators to maintain the quality by preventing loss of moisture.