Nathan Mala spraying tomato seedlings in 2016. Steaming soil has helped him control bacterial wilt and other tomato soil-borne diseases. Photo by Laban Robert.

After a two-season sustained decline in the greenhouse tomato produce, Nathan Mala is recovering in yields after steaming the soil to kill pathogens like bacteria wilt.      
The Kiambu County farmer steamed the soil to kill the bacterial wilt before packing it into 400 polythene bags for his 8m by 30m greenhouse in Nyambari.
The soil was also mixed with farm yard manure, but he used folia and other granular fertilisers to boost the nutrients content.
The farmer spent more than Sh10,000 in 2015 in controlling soil borne diseases such as fusarium wilt, bacterial wilt and other pathogens, which cause death of the tomatoes before flowering.
But after learning the steaming trick from Amiran Kenya, the farmer resolved to try it after taking soil samples for testing at the Kenya Agricultural and Livestock Organisation (KALRO) laboratory in Nairobi.
“It is tedious to steam the soil because of the net loss of the nutrients and killing of beneficial organisms. I did not have the complicated equipment for the same. I improvised; I have seen the fruits of the toil,” he said.
He repeatedly filled a 200kg capacity drum with soil and 20 litres of water before heating it in an open fire.
Every day, the farmer filled 20 polythene bags with the treated soil until he reached 400.
In preventing recontamination of the soil he covered the floor with a polythene liner.
Although he did not have an instrument of confirming if the heat had killed the pathogens, the drying or limited steam from the soil was a sign of certainty.
The door path into the greenhouse had a disinfectant, jik, for cleaning the feet before entry.
For effective control of the pathogens he germinated the seedlings in a soilless culture, which is a rich nutrient medium.
The seedlings are clean and the roots are least disturbed during transplanting, which enables fast establishment and a vibrant growth vigour.

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In 2015, Mala harvested about 60 crates of about 32kg for four months starting October. Bacteria wilt was “so intense and devastating”.
The market was poor because of the many people, who targeted the December festive season. He never recovered his investment.
But after the steaming of the soil in May 2016, the harvest from August the same year shot to 12 crates of 32kgs per two weeks.
The harvest continued until January 2017.
Bacteria wilt is caused by Pseuclomonas solanacearum. The soil-borne pathogen invades the root system of the tomatoes and other crops in the solanaceae family.
The clogging prevents movement of water and nutrients to the upper parts of the plant. Lack of water after the blockage of the xylem vessel causes wilting.
This happens in less than a week and once it has set in, there is no recovery of the crop.
Some crops in this family include chillies, capsicums, cucumber, among others.
Available chemical application in Kenya cannot completely suppress the bacteria.
The disease differs from fusarium wilt because there is no yellowing of the leaves.
Apart from the drying, a farmer can notice bacteria wilt attack by cutting the stem of the affected plant and analyzing it. If the interior stem in discoloured, place the piece in a glass of clear water.
If a white, milky substance runs out of the stem end, then the plant is affected by bacteria wilt.
It costs Sh1,000 for one to test soil at KALRO.