A plant disease may be defined as a departure or deviation of the plant from the normal state of health, presenting marked symptoms or outward visible signs. In other words, disease is an unfavourable condition caused either by the pathogens present within a living organism or by nutritional deficiency. It usually results in physiological and anatomical abnormalities expressed in characteristic symptoms. The harmful effects of a disease to the plant or to any of its parts and products generally result in the reduction of the economic value of the plant.
Causes of Diseases
The primary causes of disease can be classified into two groups – pathogens and physiological factors.
a) Pathogens: Pathogens are disease-inducing living organisms or agents which pass through a regular cycle of development and reproduction. Examples are:
b) Physiological Factors: The physiological factors may be physical, chemical or environmental. Many of them are essential for normal growth of plants, but when they are deficient or present in excess, they cause diseases.Examples of physiological factors are:
- Nutrient deficiency.
- Inorganic salts.
GENERAL EFFECTS OF DISEASES ON CROP PRODUCTION
Diseases cause lots of damage to crop and their effects include the following:
- Diseases generally reduce the yield or productivity of crops.
- They also reduce the quality of crops.
- They cause the malformation of parts of plants or the whole plants.
- They can kill or cause the death of a whole plant.
- They cause reduction in the income of the farmer.
- They increase the cost of production through the expenses incurred in the course of controlling them.
- They render vegetables and fruits unattractive and unmarketable.
- Their activities cause retarded growth in crop plants.
- It leads to delayed maturity.
- Damage to crop parts leads to reduction in photosynthesis.
Ways by which Diseases Spread on a Crop Farm
Diseases can spread on a crop farm through any of the following ways:
- By rain splash.
- Through the use of contaminated tools and equipment.
- The use of infected planting materials.
- Wind blowing pathogens to other crops.
- Through animals, especially predators during feeding.
- Through insect vectors.
- Through visitors to the farm.
- Through weeds which may harbour pathogens.
- Through irrigation water.
GENERAL CONTROL OF CROP PLANT DISEASES
Diseases of crop plants can be controlled by the following methods:
- cultural control;
- biological control;
- chemical control.
- Cultural Control: This involves the use of crop rotation, resistant varieties, tillage practices, regular weeding, fallowing, timeliness of planting, pruning, uprooting and burning of infected crops, soil treatment or sterilization, roguing or uprooting of infected crops, maintenance of farm hygiene, timeliness of harvesting, etc to control or prevent diseases.
- Biological Control: This involves the use of natural enemies of the disease to reduce or totally eliminate the disease.
- Chemical Control: This involves the use of chemicals such as fungicides, nematicides, and insecticides to dust or spray plants and plant materials in order to prevent or control plant diseases.
Summary of General Control Measures of Diseases of Crops
- Weed the farm regularly to prevent the disease pathogens from being haboured by weeds.
- Practise crop rotation.
- Remove and burn infected plants.
- Plant disease-resistant varieties of crops.
- Use healthy seeds or stems for propagation.
- Practise seed dressing with plant protection chemicals such as Fernasan D before planting.
- Spray plant protection chemicals such as fungicides and nematicides.
- Early planting helps the crops to escape the period of disease occurrence.
- Spray insecticides to control the insect vectors.
- Destroy crop residues after harvesting to prevent the build up of disease pathogens or practise good farm sanitation or hygiene.
- Sterilise soil to control soil-borne diseases.
- Avoid close planting to reduce the rate of spread of diseases.
- Imported seeds and plants should be quarantined before their introduction into the country.
- Timely harvesting reduces the period of exposure.