Assess the causes and effects of diseases on agricultural production, including but not limited to:
- plant diseases
- animal diseases
Causes of plant diseases:
- Abiotic factors
- They mainly occur due to the deficiency or excess of nutrients, light, moisture, aeration, abnormality in soil condition, atmospheric impunities etc.
- Examples are: Black tip of mango (due to SO2 toxicity), khaira disease of rice (due to Zn deficiency), whiptail of cauliflower (Mo deficiency), hollow and black heart of potato (due to excessive accumulations of CO2 in storage), bitter pit of apple (due to Ca deficiency).
- Mesobiotic causes
- These disease-causing entities lie between a state of living and non-living and thus are mostly viruses and virus like substances. Some example includes:
- Viruses: They are infections agents made up of one type of nucleic acid (RNA or DNA) enclosed in a protein coat. Examples of viral diseases of plants are: potato leaf roll, leaf curl of tomato and chillies, and mosaic disease of many plants.
- Viroids: They lack the protein coating present in viruses and are only infection causing nucleic acids. They cause diseases like potato spindle tuber, citrus exocortis, chrysanthemum stunt, cadang cadang of coconut palm, star crack of apple, etc.
- Biotic (Animate) causes
- This category includes the pathogens which are living or cellular organisms and mostly include a range of eukaryotic and prokaryotic organisms.
- Prokaryotes like bacteria which are unicellular microorganisms lacking true nucleus cause diseases like brown rot or wilt of potato, soft rot of potato and vegetables, citrus canker, etc.
- Disease causing eukaryotes include:
- Fungi: Potato wart, powdery mildew, rust, smuts, red rot of sugarcane (nearly 80% of plant diseases are caused by fungi).
- Straminopiles (Oomycetes): Downy mildews, late blight of potato, white rust of crucifers, damping off etc.
- Protozoa: Hart rot of coconut palm and phloem necrosis of coffee.
- Algae: Red rust of mango or papaya or litchi
- Metazoan animals (Nematodes): Root knot of vegetables, ear cockle of wheat, citrus decline etc.
- Parasitic flowering plants (Phanerogamic plant parasites): Dodder, Striga, Orobranche, Loranthus, Phoradendron, etc.
Effects of plant diseases:
- Reduces production and harvest of consumable, biofuel and fibre crops.
- Post-harvest diseases can be catastrophic because some disease spread at a speed of light from one plant to another and thus at times, a whole field of crops get destroyed leaving the farmers at a huge loss.
- Many post-harvest pathogens may not show any symptoms rather produce toxins that can cause harm to human health once consumed.
- Plant disease can also devastate natural ecosystems and cause imbalance in the environment.
- When a large amount of crops are attacked by plant pathogens, food production reduces drastically and in severe cases cause famine.
Causes of animal diseases:
- Nutritional defects –
- An imbalance of required food nutrients in dietary intake is the cause of nutritional defects.
- Animals receiving inadequate amounts of vitamins, minerals, fats, carbohydrates, and protein cannot produce necessary defence efficiently. Therefore, their levels of resistance to disease are lowered.
- Physiological defects –
- These defects cause an improper functioning of glands, organs, or body systems and has a direct relationship with diet.
- For example, the thyroid gland regulates the rate of body metabolism and depends upon an adequate supply of iodine to function properly. An improperly functioning thyroid gland may increase the nutritive requirements of animals to the point that very few nutrients are available for growth or production.
- Morphological defects (physical defects) –
- An accident or negligence is responsible for physical defects and can include cuts, scrapes, scratches, bruises, and broken bones are examples of morphological defects.
- Any one of these can temporarily or permanently reduce the efficiency of an animal.
- Pathogenic defects -
- Cause by organisms that produce toxins or poisons that upset the normal metabolic activity of the animal. Some of them are discussed below:
- Viral diseases include zoonosis, foot and mouth disease etc
- Bacterial diseases include anthrax, bumblefoot, botulism etc.
- Fungal diseases include athletes’ foot, candidiasis, Basidiobolomycosis etc.
Effects of animal diseases:
- Decrease in animal protein can cause food scarcity and thus increase in the cost of food resources.
- At times proper carcass management becomes difficult if number of deaths due to diseases are more.
- There are risks of diseases being spread to human that come in contact with these animals and can cause human death as well.
Extract from HSC Biology Stage 6 Syllabus. © 2017 Board of Studies NSW.