Nematodes are worm-shaped pests in the soil that are invisible to the naked eye. They can cause significant damage rangingfrom trivial injury to total damage of plant material.
Nyamira politician Victor Swanya has found a unique way of controlling the pests, also called root worms, in his farm that has 18 greenhouses.
Swanya’s love for farming is unparalleled. He has been practising large-scale greenhouse farming for three years but his great concern is the intrusion of nematodes.
He plants cabbages, potatoes, tomatoes and pepper (both green and red) in his farm. He supplies the produce to various supermarkets and other vegetable stores in Nairobi.
He says plant-parasitic nematodes can reduce crop harvest by up to 70 per cent. Commonly referred to as root-knot nematodes, these pests are introduced into new areas by infested soil or plants. They can also be in the soil that is attached to tools and equipment that have been used elsewhere. The green leafy cabbage in his farm is no longer sold to the market for consumption but shredded and buried in the farm to control the nematodes.
“Most visitors to my farm have always wondered why I waste the cabbage that I have spent money and time on. Farmers must be ready to use available means and knowledge to kill pests and control other diseases in the farm,” Swanya says.
“We wish we could sell the cabbage head and maybe bury the stem and leaves but we are interested in killing the nematodes, which are rampant in our gardens and in greenhouses.”
Judy Niikumi, an agronomist in the farm says that when cabbage decomposes, it produces a toxic substance that kills nematodes. She says cabbage is more reliable than other crops such as spinach and sukuma wiki.
Other than being cheaper and readily available than chemicals, cabbage produces the highest amount of toxic substances that kill the nematodes.
“Ingredients in different crops change with time or maturity. The unique mechanism that produces the toxic substance is very high in cabbages,” Niikumi explains.
To speed up the decomposition process, water is poured on the area where the cabbages are buried.
Mrs Niikumi says nematodes are not evenly distributed across a field but appear in patches of poor growth intermixed with healthy looking plants. Typically, they are spread by the movement of infested soil or infected plants.
She says they cause major losses in commercial farms, greenhouses and home gardens. Once the nematodes are in the roots, she says, effective treatment is impossible.
Niikumi says symptoms of infested crop include stunted plants, lost leaves, poor fruit set, small fruit size and wilting during midday.
These symptoms occur because the damaged roots cannot take up water and nutrients from the soil properly. She says crop rotation is one of the oldest and most economical methods of controlling nematodes.
Rotation is the practice of not growing a certain vegetable crop in the same site for more than one year.
Another way of killing nematodes is by the use of chemicals – nematicides. The chemicals help reduce the pests’ population for a limited period because the nematodes that escape treatment resume feeding when the chemical’s strength reduces.
“Nematicides can be effective but they have to be used repeatedly, and most farmers cannot afford that,” she says. Chemicals used include phenamiphos, dazomet, carbofuran, 1,3-dichloropropene, methyl bromide and ethaprop.
To control them, Niikumi advises farmers not to leave stalks stand after harvesting. Crop rotation is also advisable with non-solanaceous crops that are not affected by nematodes, but this may take three to five years to kill the nematodes before the same type of crops are replanted.