Agronomist notebook: Potato growing in times of extended rains

Chris Gasperi who uses uses an apical root cutting technology to produce seed potato in his farm in Nyandarua. Harvested potatoes should not be left in the sun as this causes scalding, which lowers the quality, as well as making them turn green and become inedible. FILE PHOTO | NATION MEDIA GTOUP

As unexpected rains continue to pound different parts of the country, among those who are counting losses are potato farmers.

A visit to a potato farm in Nyandarua County last week brought me face-to-face with the challenges the rains have unleashed on potato growers, thanks to the flooding and the cold weather.

On one of the farms, the potatoes had brown lesions on the leaves, a symptom of potato blight caused by the fungus Phytophthora infestans.

Peter, the farmer, said the disease had spread fast when the rains started, adding the leaves started to rot, then collapsed, shrivelled and turned brown.

As the leaves shrivelled, on the stems, dark brown lesions developed. The rapid spread of the disease causes total leaf defoliation. From the stem, the disease normally spreads to the tubers as the rainwater washes the spores into the soil, creating a film of water around the tubers that facilitates the spread of the diseases.

Infected tubers can easily be identified by slightly sunken water-soaked areas on the surfaces. Upon cutting the tubers, brown rusty colour is seen.

The best practice is that one should sort and remove affected tubers since they normally develop bacterial soft or dry rot as a secondary infection during storage. Potato blight normally leads to growth of small tubers or total crop failure if not well-managed.

With the erratic rains expected to continue, it is, therefore, important prevent the disease. The first preventive measure is planting tubers that are resistant to the disease.


Second, enhance farm hygiene so that the disease doesn’t spread when the rains start.

Third, affected tubers should be properly disposed of after grading is done and all the volunteer crops removed so that they don’t act as a source of secondary infection.

Normally, the fungus can remain in the soil once the previous crop is harvested, thus it is advisable to practise crop rotation with crops such as spinach or sukuma wiki.

Avoid excess use of nitrogenous fertilisers as this leads to extra foliage, which creates favourable conditions for the spread of the disease. Excess nitrogen may also result in lodging, which makes the potatoes more susceptible to blight.

Proper weeding to avoid mechanical damage prevents the spread of the disease. Preventive fungicides can be used to control the disease.

Away from the disease, with the heavy rains, harvesting of potatoes normally becomes a tough task. Given that potatoes are a heavy produce, the wet soil makes harvesting impossible, leading to rotting of the tubers.

When harvesting, care should be taken to ensure the tubers do not get mechanical injuries, which serve as an entry point for pathogens.

Harvested potatoes should not be left in the sun as this causes scalding, which lowers the quality. Also, exposing the tubers to sunlight makes them turn green and become inedible.

When harvesting during the rainy season, avoid washing off the mud as this decreases the shelf life of the potatoes. Only wash them when they are ready for use.


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