Agronomist's notebook: The problems in greenhouse tomato farming

For good harvest, always ensure the structure is in good shape and watch out for diseases like tuta aboluta and blossom end rot

Among the farmers I visit most of the time in my work are those growing tomatoes in greenhouses.

Their number has been rising over the years as people embrace the structures and seek to produce quality tomatoes.

The other week, I visited one of the tomato greenhouse farmers on the outskirts of Nairobi and her crop seemed to be doing well from a cursory glance.

A walk inside the greenhouse, however, revealed some of the crops were diseased, a common problem many greenhouse farmers face.

To begin with, some of the fruits appeared water-soaked and leathery and the leaves had irregular mines and looked translucent.

I identified the problem as blossom-end rot, a physiological disorder which appears as a water-soaked spot on the tomato fruits.

The spot normally enlarges, becomes sunken and turns black or dark leathery brown. Pepper, eggplant and squash are also affected by this disorder that arises from calcium imbalance in the plant.

There are several factors that inhibit calcium uptake for proper crop development. These include irregular or inadequate watering and second, excessive nitrogen fertiliser application during the growing stage of the fruit formation.

Third, high pH inhibits calcium uptake from the soil and lastly, improper cultivation results to roots damage affecting uptake of the mineral.

Normally, irregular watering results to fluctuations in soil moisture since the soil is either too wet or too cold. This fluctuation reduces the uptake and movement of calcium into the plant from the soil.

Calcium deficiency appears on younger leaves and the fruits (blossom-end rot) because of the low transpiration rate. This, therefore, means that mineral can be available in the soil but the plant is unable to take it.

It is, therefore, vital for farmers to monitor the soil moisture level to avoid fluctuations.

PURCHASING CERTIFIED SEEDS

To curb blossom-end rot, farmers should always check the watering of the plant, and the soil pH should be slightly acidic. Lastly, during weeding, ensure you do not damage the roots.

With blossom end rot, once it has attacked the crop, the only option is to remove the affected fruits and discard them.

However, once detected, you can apply calcium foliar feed on the plants to correct the disorder in the subsequent fruits.

Tuta absoluta attack is another common greenhouse problem. The pest known as tomato leaf miner causes 50-100 per cent loss and the larva is the destructive stage.

It mines between the leaves, tunnels the stems and burrows the fruits.

The crops on the farm appear to have translucent irregular mines on the leaves, while the fruits seem burrowed.

Many farmers complain that the pest is stubborn since despite chemical application, it persists leading to losses. Tuta absoluta is known to develop resistance to chemicals hence one must use chemicals with alternate active ingredients.

Mechanical methods such as sticky traps that contain a pheromone or a lure help in monitoring the presence of the pest. Usually, the pheromone attracts the male hence controlling breeding, an effective way of curbing the pest in the greenhouse.

Biological methods can also be used to eliminate the pest and this involves predatory bugs such as Macrolophus pygmaeus and extracts from neem seeds, among others.

A torn or worn out greenhouse serves as an entry point for the Tuta absoluta pest. Further, the greenhouse door should always be locked to prevent the direct entry of flying pests.

Overcome the effects of soil-borne diseases in tomatoes by purchasing certified seeds that are resistant to fusarium wilt and maintain high greenhouse hygiene by cutting weeds.

If well-managed, tomatoes are high-yielders, which tempts many farmers to ignore crop rotation.

Do not grow crops that are members of the Solanaceae family (potatoes, eggplant, chili peppers or tobacco), where tomato belongs, in greenhouse where you have harvested the crop.