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.
Causes of Diseases
Diseases are caused by the following agents:
(i) Viruses (ii) Bacteria (iii) Fungi (iv) Nematode (v) Nutrient deficiency
SELECTED DISEASES OF CROPS
| Name of Disease | Causal Organism | Method of Transmission | Symptoms and Economic Importance | Prevention and Control Measures |
| Maize Snut | Fungus (ustilago maydis) | Air borne Fungus spores deposited on fruits | i. Reduced yieldii. Galls on ears, leaves and tarsels which later turn black | i. Destroy diseased plant.ii. Use resistant varietiesiii. Seed treatment. |
| Rice Blight | Fungus (piricularia oryzae) | Airbone spores on leaves | i. Small longitudinal red spots on leaves which turn | i. Use clean seedsii. Avoids heavy use of nitrogen fertilizersiii. Use resistant varieties |
| Maize Rust | Fungus (puccinia polysora) | Airborne spores deposited on leaves | i. Red spots on leavesii. Reduced yieldiii. Death of the crop | i. Early plantingii. Crop rotationiii. Use resistant varieties |
| Cercopora a leaf spot of cowpea | Fungus | Thr20ough wild | i. Reddish brown spots on leavesii. Lesions on leavesiii. Chlorosisiv. Dropping or falling of leaves | i. Spray with fungicidesii. Crop rotationiii. Plant resistant varieties |
| Rosette disease of groundnut | Virus | By piercing and sucking insect (Aphid). | i. Yellow leaves with mosaic mottling.ii. Stunted plant with curled leaves.iii. Wilting and death of plantiv. Shortening of the internodes | i. Early plantingii. Crop rotationiii. Use insecticidesiv. Uproot and burn infected plantsv. Use resistant variety. |
| Cassava Mosaic | Virus | i. Through piercing and sucking insect (whitefly) (Bemisia nigerensis)ii. Infected plant cutting | i. Mottling of leavesii. Mosaic pattern on leavesiii. Stem/leaf distortioniv. Stunted plantv. Reduction in yield | i. Use resistant varietiesii. Uproot and burn infected plantiii. Spray with insecticides to kill vectoriv. Use disease free stem cuttingv. Farm sanitation. |
| Leaf blight of cassava | Bacterium xanthomonas manihotis | i. Infected cuttingii. Rain splashingiii. Insectsiv. Tools | i. Blighting of leavesii. Wilting of plantiii. Falling off lf leavesiv. Reduced yieldv. Canker of stemvi. Die-back of stem | i. Use resistant varietiesii. Use disease free cuttingiii. Early plantingiv. Practise crop rotation |
| Cocoa black pod disease | Fungus phytophthora palmivora | i. Rain splashii. Insects | i. Brown spots on podii. Rottening of pods.iii. Entire pod turns blackiv. Low yield | i. Remove and destroy infected pods.ii. Regular weedingiii. Spray with fungicides e.g. Bordeaux mixture.iv. Avoid overcrowding of cocoa plant. |
| Coffee Leaf rust | Fungus | i. By windii. By rain splash | i. Yellow or brown spot on leavesii. Orange powdery mass on the leafiii. Reduction in yieldiv. Dropping of leaves | i. Plant seeds from healthy plants.ii. Use resistant varietiesiii. Spray with copper fungicides |
| Black arm (bacterial blight of cotton) | Bacterium | i. Through leavesii. Stems near the ground | i. Angular spots on leavesii. Boll rotiii. Exudates from affected leavesiv. Retarted growth and death of plant | i. Seed dressingii. Uproot and burn infected plantsiii. Use resistant varietiesiv. Crop rotation |
| Root knot of tomato/okro Damping off disease of okra | Nematode Fungus | Nematodes in soil Infected soil | i. Knotting or galling or rootsii. Retarded growthiii. Early death of plantiv. Reduction in yieldv. Retarted growthvi. Cells becomes water loggedvii. Gradual wilting of plantsviii. Death of plant | i. Soil sterilizationii. Crop rotationiii. Use resistant varietiesiv. Uproot and burn infected plantsv. Spray with copper fungicidevi. Use resistant varietiesvii. Sterilization of soil |
| Onion twister disease | Fungus | i. Infected soilii. Water splashiii. Infected bulb. | i. Twisting of leavesii. Grey patches on leavesiii. Reduction in yieldiv. Death of plant | i. Crop rotationii. Use resistant varietiesiii. Spray with fungicidesiv. Early planting |
| Stored produce mould | Fungus | i. Infected seed or fruitsii. High humidityiii. By soil | i. Black mould on seeds and fruitsii. Pungent smelliii. Sour tasteiv. Decay of seeds and fruits in store | i. Proper drying of seed before storageii. Spray with fungicidesiii. Maintain low humidity in storeiv. Remove contaminated seeds before storage. |
GENERAL EFFECTS OF DISEASES ON CROP PRODUCTION
Disease cause lots of damage of crop and their effects are as follows:
- Disease generally reduce the yield or productivity of crops
- They also reduce the quality of crops
- They cause the malformation of plants or the whole plants
- They can kill or cause the death of a whole palnt
- 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
- The render vegetables and fruits unattractive and unmarketable
- Their activities cause retarded growth in crop plants
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 predator during feeding
- Through insect vector
- Through visitors to the farm
- Through weeds which may harbor pathogen
- Through irrigation water.
GENERAL CONTROL OF CROP PLANT DISEASES
Diseases of crop plans can be controlled by the following methods:
(1) Cultural control; (2) biological control: (3) chemical control.
- Cultural Control: This involves the use of crop rotation, resistant varieties, tillage practices, regular weeding, fallowing, timelines of planting, pruning, uprooting and burning of infected plant, soil treatment or sterilization, rouging 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 chemical such as fungicides, nematicides, insecticides to dust or spray plants and plant materials in order to prevent or control plant disease.
Summary of General Control Measure 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 Fermassan 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 crops’ residues after harvesting to prevent the build-up of disease pathogen or practice good farm sanitation or hygiene
- Sterilize 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
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