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Farmers rescue sukuma wiki from yield-slashing, 20-year soil disease

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Farmers in Tigoni have broken out of continuous pesticide use to curb 80 per cent clubroot yield losses with a single pesticide shot followed by calcium and lime soil treatment.

Clubroot can eliminate entire vegetable harvests. With no known pesticide capable of completely eradicating the fungi that causes it and being capable of staying in unused soils for more than 20 years, it is one of the most costly diseases for sukumawiki, cabbage, cauliflower, broccoli, and collard greens farmers.

“Between 2022 and February this year, we experienced a continuous decline in our sukuma wiki and broccoli harvests,” said John Njenga a farmer in Tigoni, Kiambu County. “This was despite buying a pickup full of goat manure thinking our soils had lost nutrients.”

After reaching out for expert help from a Yarra agriculture extension agent, Njenga– a first-time farmer on two acres of leased land– was surprised to learn that most of his vegetables had developed swollen, club-shaped roots instead of the usual fine fibrous root system.

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This reduced the ability of the crops’ roots to take and transport water and nutrients to the rest of the plant. This makes the crop unable to feed itself and causes wilting, stunting, and yellowing.

“We were first advised to dilute 50 millilitres of Acoster pesticide with 50 millilitres of  Copagel– a liquid soluble copper– in 20 liters of water and pour this mixture generously where at the base of our crops, he said.

One liter of Algreen Copagel-50 costs Sh800 and also helps prevent bacterial spots. A liter of Acoster 50EC costs Sh4,600.

This mix fires the first shot, killing many of the fungi that cause clubroot. However, the disease can not be eliminated by fungi but needs to be controlled by treating the soil with Calcium and Lime.

Calcium nitrate which improves the strength of roots is recommended for use because its ‘calcium’ is more easily taken in by the destroyed plant roots than other calcium fertilisers such as CAN. An example of a calcium nitrate fertiliser is YaraTera Calcinit. A 25-kilogram bag of which costs Sh3,500 with instructions on application given by the manufacturer.

Alongside calcium nitrate, applying lime helps to maintain a pH above 7.2 which is ideal for growing vegetables but makes it difficult for the fungi to survive because clubroot prefers acidic soils (soils with a pH of less than less than 6.5). 50 kilograms of agriculture lime costs Sh1,300.

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0.4 to two tonnes of lime can be applied per hectare depending on the soil type and its level of acidity. Lime can be applied at the base of the plant to reduce the amount used. However, the instructions on the amount of lime to be applied are given by the manufacturer.

“We have other instances of disease and pests on our sukuma wiki and broccoli which we are managing but having taken care of clubroot, all the other issues have been a cakewalk for us,” John said.


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