Crop diseases and their control measures

Disease outbreaks can cause serious damage to all crops, resulting in severe financial losses.

14 July 2021 | Agriculture



To successfully grow crops, a disease and pest control programme should be in place and good agricultural practices (GAPs) such as early planting and crop rotation must be incorporated into a farmer’s operations.

Agribank’s technical advisor for crops and poultry, Hanks Saisai, says where crops are grown, there is a likelihood of disease outbreaks that can cause serious damage to all crops, resulting in severe financial losses.

“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,” says Saisai.

Plant diseases are conventionally classified into two groups namely parasitic and non-parasitic diseases.

Parasitic diseases are classified into fungal diseases, bacterial diseases and viral diseases.

• Fungal diseases

Fungal diseases are parasitic organisms that 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 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 turn, has an effect on the growth of crops and usually results in retarded growth and lower yield potential,” Saisai says.

Another fungal disease is septoria leaf spot, which is also caused by prolonged overcast conditions, warm weather and high relative humidity.

Saisai says 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.

“Correctly spaced crops easily dry off quickly after rain or irrigation, reducing the risk of fungal disease occurrence.”

He adds that fusarium wilt of tomatoes cannot be controlled by simply spraying or dusting using fungicides. The affected crops must be removed with their root systems and destroyed by burning them.

• Bacterial diseases

Bacterial diseases in plants can 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 plant’s vascular system and causes 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 such as tomatoes and peppers can remain in the soil for many years and once it has been allowed to enter the vascular system it can cause the wilting of crops.

“Complete sterilisation may be a good way to neutralise bacterial disease-causing organisms; however, it may also harm useful microorganisms.

“To break the lifecycle of bacterial diseases, farmers are encouraged to practise 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,” Saisai advises.

Another way is to buy certified disease-resistant seeds. Furthermore, farmers can implement early planting to ensure that crops grown during warm months are grown when the activities of micro-organisms such as bacteria and fungi are at their least.

• Viral Diseases

Usually, viral diseases are spread by gardeners who touch diseased plants and then proceed to touch healthy plants, says Saisai.

Pests are some of the agents that spread viral diseases by sucking and chewing on diseased plants and transferring microorganisms to healthy plants.

Saisai says crop rotation can help to address the issue of viral diseases. In more severe cases, it is usually wise to simply burn 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.

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