MEANING AND CAUSES OF DISEASE
A crop is a plant cultivated by man for a specific purpose. A plant disease is a deviation of the plant from the normal state of health, presenting outward visible signs. Diseases are caused by pathogens and enhanced by some physiological factors.
CAUSES OF DISEASE
Plant diseases are caused by pathogens. Pathogens are disease causing organism which passes through a regular cycle of development and reproduction. Examples of pathogens that cause plant disease are viruses, bacteria, fungi, parasitic worms and rarely protozoa. Some of these pathogens are carried by vectors and other agents.
Physiological factors such as nutrient deficiency in the soil, heat, presence of inorganic salts in the soil and soil moisture content has a major role to play in influencing plant susceptibility to diseases.
SELECTED DISEASE OF CROPS
| Name | Casual organism | Method of transmission | Symptoms and economic importance | Prevention and control measure |
| Maize Smut | Fungus (Ustilago maydis) | Fungus spores deposited on fruits | ||
| Rice Blight | Fungus (Piricularia oryzae) | Airborne spores on leaves | ||
| Maize Rust | Fungus (Puccinis polysora) | Airborne spores deposited on leaves | ||
| Cercopora a Leaf spot of Cowpea | Fungus | Through Wind | ||
| Rosette disease of Groundnut | Virus | By piercing and sucking insect (Aphid) | ||
| Cassava Mosaic | Virus | (Bemisia nigerensis) | ||
| Leaf blight of Cassava | Bacterium Xanthomonas manihotis | |||
| Cocoa black pod disease | Fungus Phytophthora palmivora | |||
| Coffee Leaf rust | Fungus | |||
| Black arm (bacterial Blight of cotton) | Bacterium | |||
| Root Knot of Tomatoes/ Okra | Nematodes | Nematodes in soil | ||
| Damping off Disease of Okra | Fungus | Infected soil | ||
| Onion Twister Disease | Fungus | |||
| Stored produce mould fungicides | Fungus |
EVALUATION
- What is plant disease?
- List four common disease causing organisms.
GENERAL EFFECTS OF DISEASE ON CROP PRODUCTION
- Disease generally reduce the yield of crops
- They reduce the quality of crops
- They can cause malformation of parts of plants
- They can kill or cause death of a whole plant
- They cause reduction in the income of a farmer
WAYS BY WHICH DISEASE SPREADS ON CROP FARM
- By rain splash
- Through the use of contaminated equipment
- The use of infected planting materials
- Wind blowing pathogens to other crops
- Through animals
- Through insect vectors
- Through visitors to the farm
- Through weeds
GENERAL CONTROL OF CROP PLANT DISEASES
- Cultural method: the use of crop rotation, planting of resistant varieties, tillage practice, regular weeding, pruning etc. to control or prevent disease
- Biological control: this is the use of the natural enemy of the disease to reduce or totally eliminate the disease
- Chemical control: this involves the use of pesticides such as fungicides, nematicides, insecticides to prevent disease or control disease.
GENERAL EVALUATION
- What are pathogens?
- List two examples of pathogens.
- List three physiological factors that makes plants susceptible to diseases.
- List five air borne disease of crops.
READING ASSIGNMENT
Revision question 6 and 9 in Essential Agricultural Science for Senior Secondary Schools by O. A Iwena chapter 26, page 247.
WEEKEND ASSIGNMENT
SECTION A
- The following are pathogens except _____. A. viruses B. bacteria C. fungi D. algae
- Maize rust is caused by ______. A. bacteria B. fungi C. virus D. nematodes
- Rosette disease of groundnut is caused by ____. A. fungus B. virus C. bacteria D. nematodes
- Cocoa blackpod disease is caused by ______. A. virus B. bacteria C. fungi D. nematodes
- Root knoll of tomato is caused by ______. A. nematode B. fungus C. bacteria D.virus
SECTION B
B. List disease pathogens
2 Discuss briefly the cassava mosaic disease under the following headings
A. Casual organism B. Transmission C. Symptoms D. Control
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