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Crop diseases and their control measures

2021-07-06  Staff Reporter

Crop diseases and their control measures
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Hanks Saisai


Crop production is a form of agriculture that has been carried out since time immemorial, and it mainly involves the tilling or cultivating of the soil to sow seeds that later mature into plants. 

These plants produce cereal grains, vegetables or medicinal plants that are consumed or used by people globally.

However, where crop production is practised and/or grown, there is a likelihood of disease outbreaks that can cause serious damage to all crops, resulting in severe financial losses. A disease simply defined is an abnormal condition that can affect a living organism, such as a plant (crops). 

As a farmer, it is important to know the different types of diseases and how to effectively control them to ensure success in your crop production enterprise.


Fungal diseases

Fungal diseases are parasitic organisms that belong to the group of Eukaryotic organisms, and they usually reproduce by producing spores that are easily distributed by wind and water. Late blight, early blight, and mildew are some of the most common fungal diseases that occur in tomatoes or potatoes. 

These diseases are caused by prolonged overcast weather conditions that are associated with warm weather and high relative humidity. These are favourable conditions for fungal micro-organisms to thrive and rapidly reproduce spores on the leaves and branches of crops.

This in return has an effect on the growth of crops and usually results in retarded growth and lower yield potential. Another fungal disease is Septoria Leaf Spot, which is also caused by prolonged overcast conditions, warm weather and high relative humidity. 

To control the occurrence of fungal diseases such as early or late blight, there are certain practices that a crop farmer can incorporate in primary production.

When planting tomatoes, potatoes, carrots, and beans that are usually affected by fungal diseases, it is essential to avoid overcrowding as it promotes the spread of the spores among the plants (allowing rapid spread of the disease).

 Correctly spaced crops easily dry off quickly after rain or irrigation reducing the risk of fungal disease occurrence. Fusarium wilt of tomatoes cannot be controlled by simply spraying or dusting using fungicides. To this end, the affected crops must be removed with their root systems and destroyed by burning them.

Some common Fungicides such as Bayleton, Dithane M45, Fungisprey and Copper oxychloride can be used as spraying or dusting remedies to control fungal diseases.

Bacterial diseases

Bacteria are microscopic single-celled organisms that reproduce by simple fission (or simply breakage). Bacterial diseases in plants usually take on many forms and cause wilting, spotting on the leaves, cankers on the stems and the rotting of roots and tubers. 

In most cases, bacterial diseases attack the vascular system and cause serious problems. Black rot is a very common bacterial disease that affects cabbages and is introduced into gardens through seed coats.

Bacterial wilt disease of solanaceous crops (tomatoes and potatoes) can be in the soil for many years (at least four years) and once it has been allowed to enter the vascular system (Xylem vessel) it can cause the wilting of crops. For most bacterial diseases, spraying or dusting with a chemical is normally impractical as it may require the farmer to completely sterilise the entire crop field or garden.

To break the lifecycle of bacterial diseases, farmers are encouraged to practice crop rotation and ensure that crops such as tomatoes and potatoes are not grown in close proximity, as they are susceptible to the same diseases. Another way is to buy certified seeds that are disease resistant. Furthermore, farmers can implement early planting that will ensure that crops that are grown during warm months are grown when the activities of micro-organisms such as bacteria and fungi are at their least.

 Viral Diseases

Viruses are infectious agents of minute proportions that usually have a significantly negative effect on crops causing mottling, stunting, crinkling and distortions of the foliage. Usually, viral diseases are spread by gardeners who touch diseased plants and then proceed to touch healthy plants. Pests are some of the agents that spread viral diseases (aphids, mites, white flies and leaf hoppers) by sucking & chewing on diseased plants and transferring microorganisms to healthy plants.

Crop rotation can assist to address the issue of viral diseases and in more severe cases it is usually wise to simply destroy the crop field by burning all crops. Non-parasitic diseases conventionally result from unfavourable soil and climatic conditions, as well as nutritional deficiencies. 

Potatoes and tomatoes are usually the most affected crops as well as beetroot and carrots. Over watering, dry spells during critical stages of growth and poor soil preparation (under fertilisation, fertilisation and incorrect application of fertilisers) can cause abnormal growth of crops.


- Hanks Saisai is AgriBank’s technical advisor – crops and poultry.

2021-07-06  Staff Reporter

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