Causes of crop diseases

Causes of crop diseases

Hello, students! A healthy crop grows well and gives a good harvest. A diseased crop may have yellow leaves, rotten roots, weak stems or poor fruits. Farmers in Nigeria need to know what causes these problems.

Meaning of crop disease

A crop has a disease when something keeps disturbing its normal growth or the way its parts work.

Crop disease is a harmful deviation from the normal functioning of the physiological processes of a plant.

Simply put, a diseased crop cannot carry out its normal life activities properly. For example, damaged roots may not absorb enough water for the plant.

Two main groups of causes

Infectious causes are living organisms or agents that cause diseases which can spread between plants. • Non-infectious causes are unsuitable growing conditions. These diseases do not spread from one plant to another.

Infectious causes

A disease-causing organism or agent is called a pathogen. Major crop pathogens include fungi, bacteria, viruses and nematodes.

Fungi cause many leaf spots, rots and rusts. For example, fungi cause maize rust and yam anthracnose. Many fungal diseases spread easily when conditions are warm and wet.

Bacteria are very tiny organisms. Some enter crops through wounds or natural openings. They cause diseases such as bacterial wilt of tomato and bacterial blight of cassava.

Viruses multiply inside living cells. They may cause patchy leaf colour, curled leaves and poor growth. Cassava mosaic disease is caused by viruses. It can spread through infected stem cuttings and whiteflies.

Nematodes are tiny roundworms. Harmful types attack crop roots and reduce water and nutrient uptake. Root-knot nematodes cause swellings on the roots of crops such as tomato and okra.

Non-infectious causes

Nutrient shortage can cause poor growth. For example, nitrogen shortage may make older maize leaves turn yellow. • Too little water can cause wilting, while waterlogged soil reduces the air available to roots. • Extreme temperatures, harmful chemicals and excess salts can damage crops.

Cause, carrier and favourable condition

A pathogen causes an infectious disease. A vector carries a pathogen from an infected plant to a healthy plant. For example, a whitefly carries cassava mosaic viruses; the whitefly is not the virus.

Crowded planting, poor drainage and high humidity can favour disease development or spread. They are not pathogens. Infected seeds, cuttings, soil, water and dirty tools can also help pathogens reach healthy crops.

Exam reminder

Do not assume that every yellow leaf is caused by an infection. On a maize farm in Kaduna, yellow leaves may result from nutrient shortage, root damage or disease. Always separate the cause, the carrier and the conditions that favour disease.