Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Diseases of Crop
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Diseases of Crop
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Subject: Agricultural Science
Class: Senior Secondary 2
Term: 3rd Term
Week: 1
Theme: Crop Production
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State the meaning of disease State the meaning of Disease. List the effects of disease on crop production Name the important disease of major crops. State the causal or ganism of each disease. State the preventive and control measures.
A disease generally refers to any condition that impairs the normal functioning of an organism (human, animal, or plant). It is a deviation from the normal healthy state, characterized by specific symptoms. Diseases can be caused by living organisms (pathogens) or non-living factors (environmental stress). Unlike an injury, which is a physical disruption from an external force, a disease is a progressive, internal dysfunction. A crop disease is any abnormal condition in a plant that interferes with its normal physiological processes, leading to reduced growth, yield, or quality of the harvested product. These conditions are typically caused by living organisms known as pathogens (such as fungi, bacteria, viruses, nematodes) or adverse environmental factors (nutrient deficiencies, extreme temperatures, waterlogging, drought). When caused by pathogens, a crop disease is infectious and can spread from one plant to another. Examples of symptoms of crop diseases include: Leaf spots: Discoloured patches on leaves.
Wilting: Drooping of leaves and stems due to loss of turgor.
Rotting: Decay of plant tissues (roots, stems, fruits).
Stunting: Reduced overall plant growth.
Chlorosis: Yellowing of leaves due to lack of chlorophyll.
Galls/Tumours: Abnormal swellings on plant parts.
Necrosis: Death of plant tissues.
Mosaic patterns: Irregular light and dark green patterns on leaves. Crop diseases have significant negative impacts on agricultural production, particularly in Nigeria where smallholder farmers often lack resources for effective control.
These effects include: Reduction in Crop Yield: This is the most direct and common effect. Diseases can destroy plant parts (leaves, stems, roots, fruits), interfere with photosynthesis, nutrient uptake, or water transport, leading to fewer or smaller harvested products. For instance, Black pod disease in cocoa can destroy up to 80% of pods if uncontrolled.
Reduction in Crop Quality: Even if yield is not severely affected, diseases can compromise the quality of the harvested produce. For example, diseased fruits may have spots, discolouration, or rot, making them unmarketable or unsuitable for consumption. Yams affected by rot lose their market value.
Increased Cost of Production: Farmers may incur additional costs for purchasing and applying fungicides, bactericides, or other control agents. Labour costs also increase for activities like rogueing (removing infected plants) or sanitation.
Food Shortages and Famine: Widespread disease outbreaks, especially in staple food crops, can lead to significant food losses, contributing to food insecurity and even famine in affected regions. The impact of Cassava Mosaic Disease on cassava production in Nigeria highlights this.
Economic Losses to Farmers and the Nation: Farmers suffer financial losses due to reduced yields and unmarketable produce. At a national level, reduced agricultural output impacts Gross Domestic Product (GDP), foreign exchange earnings (for export crops like cocoa), and can necessitate food imports.
Reduced Market Value: Diseased produce often sells at a lower price, or may not be accepted in the market at all, further reducing farmer income.
Increased Environmental Pollution: Improper or excessive use of chemical pesticides for disease control can lead to soil, water, and air pollution, affecting human health and biodiversity.
Disruption of International Trade: Certain diseases can lead to trade restrictions or embargoes on agricultural products from affected regions, impacting export potential. This section outlines common diseases affecting key crops grown in Nigeria, along with their causal organisms and common symptoms. | Crop | Disease Name | Causal Organism | Common Symptoms | | :----------- | :---------------------------- | :---------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Maize |
1. Maize Streak Virus (MSV) | Virus (Genus: Mastrevirus) | Characteristic yellow or whitish streaks along the veins of leaves, starting from the leaf base. Plants are stunted, and cobs are small and deformed. Transmitted by leafhoppers. | | |
2. Downy Mildew | Fungus (Peronosclerospora sorghi) | Pale yellow to white stripes on leaves, typically along the midrib. White powdery growth (spores) on the underside of leaves in humid conditions. Stunted growth, malformed cobs, premature death. | | Cassava |
1. Cassava Mosaic Disease (CMD) | Virus (African Cassava Mosaic Virus) | Characteristic mosaic pattern (yellow and green patches) on leaves, leaf distortion, reduced leaf size, and severe stunting. Can lead to up to 80% yield loss. Transmitted by whiteflies and infected stem cuttings. | | |
2. Cassava Bacterial Blight | Bacterium (Xanthomonas axonopodis pv. manihotis) | Angular, water-soaked spots on leaves that enlarge and become brown, leading to blight. Wilting, stem dieback, gum exudation on stems, and vascular discolouration. | | Yam |
1. Yam Anthracnose | Fungus (Colletotrichum gloeosporioides) | Dark brown to black spots on leaves, petioles, and stems. Lesions can coalesce, causing blighting and defoliation, leading to dieback of vines. Can also affect tubers, causing dry rot. | | |
2. Yam Mosaic Virus (YMV) | Virus (e.g., Dioscorea alata Potyvirus) | Light green to yellow mosaic patterns on leaves, leaf distortion, blistering, and stunting of vines. Tubers may be smaller. Transmitted by aphids. | | Cowpea |
1. Cowpea Mosaic Virus (CPMV) | Virus (Comovirus) | Severe mosaic patterns, blistering, leaf distortion, stunting, and reduced podding. Seeds may be discoloured. Transmitted by beetles. | | |
2. Cercospora Leaf Spot | Fungus (Cercospora canescens) | Circular to angular brown spots with reddish-brown borders on leaves. Severely infected leaves turn yellow and drop prematurely, leading to defoliation and reduced yield. | | Oil Palm |
1. Ganoderma Butt Rot | Fungus (Ganoderma boninense) | Basal stem rot, external fungal growth (bracket fungi/conks) at the base of the trunk. Yellowing and wilting of older fronds, eventual collapse and death of the palm. Can affect palms of all ages, especially older ones. | | |
2. Blast Disease | Fungus (Rhizoctonia lamellifera, Phytophthora palmivora) | Affects young oil palm seedlings in the nursery. Yellowing and browning of older leaves, followed by rot of the spear (youngest) leaf and eventual death of the seedling. | | Cocoa |
1. Black Pod Disease | Fungus (Phytophthora palmivora) | Dark brown or black lesions on cocoa pods, rapidly expanding and rotting the entire pod. White mycelial growth may be visible under humid conditions. Can also infect chupons and flowers. Leads to severe yield losses. | | |
2. Cocoa Swollen Shoot Virus (CSSV) | Virus (Badnavirus) | Swellings on young stems and chupons, red vein banding, leaf discolouration (mosaic, mottling), and malformation of leaves. Pods are smaller and rounder, with fewer seeds. Transmitted by mealybugs. | | Tomato |
1. Early Blight | Fungus (Alternaria solani) | Dark brown to black circular spots on older leaves, often with concentric rings (target-like pattern). Can also affect stems (collar rot) and fruits (dark, leathery spots near the stem end). Leads to defoliation and reduced fruit yield. | | |
2. Bacterial Wilt | Bacterium (Ralstonia solanacearum) | Sudden wilting of entire plants during the day, with partial recovery at night, eventually leading to permanent wilt and death. Vascular tissues (xylem) in the stem turn brown when cut. No leaf spots. Primarily soil-borne. |
Food Security and Livelihoods: This topic directly links to food security in Nigeria. Students understand that effective disease management ensures higher yields of staple crops (like cassava, yam, maize), leading to more food availability, stable prices, and improved nutrition for families. It also directly impacts the livelihoods of millions of smallholder farmers whose income depends on healthy crops, reducing rural poverty. For example, a student from a farming community can relate how their family's income is affected by a bumper harvest versus a disease-ridden one. Role of Agricultural Extension and Research: The knowledge gained about specific diseases, causal organisms, and control measures prepares students to appreciate the work of agricultural extension agents and researchers. They learn that plant pathologists and agronomists develop resistant varieties and integrated pest management (IPM) strategies to combat diseases, providing vital information to farmers. Students can consider careers in these fields, helping to solve real-world agricultural problems in Nigeria. Environmental Stewardship and Sustainable Agriculture: Understanding chemical control methods alongside cultural and biological methods allows students to grasp the concept of Integrated Pest and Disease Management (IPM). They learn that while chemicals offer quick fixes, their overuse can harm the environment and human health. This promotes a mindset of sustainable agriculture, encouraging environmentally friendly practices like crop rotation, use of resistant varieties, and proper sanitation, which are crucial for long-term food production and ecological balance in Nigeria.