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Yes you did soil test, but did you look for diseases?

By George Mbakaya

It is the planting season and farmers across the country are busy planting all manner of crops. So you have done the soil testing and are expecting blooming crops on your farm. But there’s more to soil testing than just establishing the missing nutrients.

Last week I got an email from a farmer in Molo who wanted to know why his tomato crop was wilting yet he had done a comprehensive soil analysis before planting.

Besides, he had adhered to routine spraying of the crop to control pests and diseases. On analysis of the plant specimens, it showed symptoms of root rots, a common soil-borne fungi disease. Upon further inquiry on what entailed the comprehensive soil test, I learnt the tests were done to establish the nutrient levels and the soil pH. What the farmer did not know is there is a pathology test which determines common fungi and bacteria that exist in the soil that might adversely affect the crop.

Of all diseases plants are susceptible to, soil-borne ones are the most frustrating. Farmers may think they are doing everything right yet their plants become sickly, stunted and near death.

Soil-borne diseases are caused by microorganisms that survive and move about in the soil. Most cannot be seen by the naked eye and go undetected until the plant becomes affected.

Effective soil management

Effective soil management begins with the accurate identification of pathogens in the farm. Soil test is the most accurate and comprehensive way of monitoring soil borne disease inoculum.

The test can be used before planting to identify areas most at risk to soil diseases; this will allow for better decision before planting. One advantage of soil testing is that it eliminates guesswork in soil-borne disease management.  Many of these plant diseases caused by soil borne pathogens can be difficult to predict, detect and diagnose. Investigations into these pathogens are further limited by the nature of the soil environment, which is complex. This makes gaining an understanding of soil-borne plant pathogens, and the diseases they cause, a challenging aspect of plant biosecurity.

Also, many soil borne pathogens persist in soil for years. When optimal conditions (right moisture, temperature, and presence of a host plant) arise, these soil-borne pathogen structures often serve as source of inoculum for disease development.

Whether the aim is to determine the causal agent of a disease problem or enumeration of the levels of soil-pathogen population before planting, soil testing is all you need.

Fungal pathogens

Fungal pathogens are perhaps the most common type of soil-borne plant pest. Clubroot, caused by the fungus, affects crops such as kales and results in distortion of roots, wilting and stunted growth.

Damping off is a serious plant disease that commonly results in seedling death and is often caused by soil borne fungal pathogens. Symptoms range from rotting within the seed coat before germination, to decay of the taproot or rootlets following germination. Surviving plants are stunted, and affected areas often show uneven growth.

Bacterial soft rot can result when wound sites of plant roots are infected with bacteria. This results in a slimy rot that can affect any part of the plant.

Pathogenic nematode species can be particularly pervasive in soil. For this reason they can pose a serious threat to vegetable crops. Cysts from these nematode pathogen contain hundreds of eggs that can remain dormant in the soil for up to 30 years. These cysts are commonly dispersed when infested soil is carried by machinery, footwear or on plant roots to other growing areas. Due to the invasive nature of soil borne pathogens, biosecurity best practice is important for controlling the risks presented by them.

There are several cost effective biosecurity measures that can be implemented to reduce the risks presented by soil borne pathogens, but effective control plan starts with soil testing. [George Mbakaya]

The writer is an expert on agricultural solutions and sustainable agriculture

[email protected]

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