Agronomist's notebook: What you need to know about soil diseases

Saturday March 23 2019

A researcher collects soil samples for testing.

A researcher collects soil samples for testing. While water issues are easier to correct, soil diseases are much harder to detect and control. FILE PHOTO | NATION MEDIA GROUP 

ANN MACHARIA
By ANN MACHARIA
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It is always disappointing when a farmer plants seeds and they fail to germinate or they sprout but do not take off after transplanting.

Among things that cause this failure is inadequate water, too-much watering and soil-borne diseases such as fusarium wilt and bacterial wilt.

While water issues are easier to correct, soil disease are much harder to detect and control. Weeks ago, I visited a distressed open field tomato farmer who called me saying he urgently needed my help.

The leaves of the crops were yellowing on one side and some were wilting. I uprooted one crop, and after cutting the stem, I observed brownish vascular tissues or discolouration at the base of the stem.

This is usually an indication of fusarium wilt attack, which is a soil-borne disease caused by a fungus that can survive for a longer period waiting for the host plant. Affected plants usually have stunted growth or die before they mature.

The disease is favoured by warm soil temperatures and high relative humidity. The fungus enters the plant through the root system and spreads. It develops more quickly in soils that are high in nitrogen and low in potassium content.

Other than tomatoes, the disease also affects capsicum, black nightshade, cucurbits, and legumes
Fusarium wilt attack is commonly confused with bacterial wilt, which is also a common disease in sandy soils.

Bacterial wilt develops in high temperatures and moist soils. More often, the pathogens enter through microscopic wounds that occur during the management practices or injuries caused by insects.

The bacterium clogs the stem and prevents water and nutrients from moving throughout the plant and eventually kills the crop.

PLANT CERTIFIED SEEDS

Initially, the disease manifests itself by wilting of the younger leaves, which can quickly go unnoticed. Sometimes the base of the plant may show brown cankers; a symptom of an injury associated with an open wound, root rot, and the plant eventually wilts to death.

To check the presence of bacterial wilt, cut a fresh stem at the base of the plant and place it in water for some time. A stream of white slimy substance is usually an indicator of the bacteria present in the vascular tissue.

Planting varieties that are resistant to fusarium wilt and bacterial wilt are the best remedy to this disease since there are no effective chemicals to control them.

However, copper-based chemicals can be used to suppress the diseases. To minimise the diseases, follow good agricultural practices, which include conducting a soil test to ensure that the soil is free of the disease.

Also, ensure that the soil pH is ideal for the crop especially for bacterial wilt since it thrives best when the pH is high.

Farmers should also plant certified seeds and disease-free transplants to prevent the introduction of the pathogen on their farms.

Practising crop rotation with non-solanaceae family crops that are not affected by fusarium wilt and bacterial wilt also helps to control the diseases.

Affected plants should be uprooted and burnt. Apply wood ash in the holes left after uprooting the crop to prevent the spread of the disease. However, farmers should avoid using infected plants as compost.

All the farm implements should be disinfected before moving them from one field to the next.

In case the disease persists, soil solarisation should be done before planting. This is achieved by placing a clean plastic paper on the soil surface for four to six weeks during the sunny period.

This method of soil treatment destroys both the beneficial and harmful micro-organisms in the soil such as fungi and bacteria.