Crop diseases and their control measures

29 June 2021 | Agriculture

Windhoek • Hanks Saisai

Crop production has been carried out since time immemorial and 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 practiced 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.
Plant diseases are conventionally classified into two groups namely, parasitic, and non-parasitic diseases. To this end, a farmer must understand that parasitic diseases are normally caused by the activities of microscopic organisms such as fungi, bacteria, and viruses, whereas non-parasitic diseases are caused by unfavourable soil and climatic conditions resulting in the abnormal developments of the foliage, fruits, tubers, and roots.

Parasitic diseases are classified into the following categories:
• 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 is 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. Another fungal disease is Septoria Leaf Spot, which is also caused by prolonged overcast conditions, warm weather & 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 causing 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 4 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 sterilize the entire crop field or garden. Complete sterilization may be a good way to neutralize bacterial disease causing organisms, however it may also harm useful microorganisms.
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. To ensure no occurrence of these diseases in future, farmers must carefully remove all infected plant material (leaves, roots and fallen leaves) and destroy them by burning.

• 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 & 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.
Overwatering, dry spells during critical stages of growth and poor soil preparation (under fertilization, overfertilization and incorrect application of fertilizers) can cause abnormal growth of crops.
Overall, for one to successfully grow crops, a disease and pest control program should be in place and Good Agricultural Practices (GAPs) such as early planting and crop rotation must be incorporated into a farmer’s operations. Lastly, when using remedies that are aimed at controlling diseases by spraying or dusting, farmers should follow the directions of use for effective and efficient results.
*Hanks Saisai is Agribank’s Technical Advisor – Crops & Poultry