Pests and diseases are classified by their nature and cause. Understanding the three main categories — metabolic, metazoal, and microbial — is essential for selecting appropriate prevention and control strategies. Each type interacts with host organisms in different ways and requires distinct management approaches.
VCAA FOCUS: You must be able to define each category, give examples, and describe the characteristics (signs, symptoms, transmission, impact) of each type affecting Victorian plants and/or animals.
Metabolic diseases result from disruptions to normal metabolic processes within an organism — they are not caused by an external pathogen. They typically arise from:
- Nutritional deficiencies or imbalances
- Hormonal disruptions
- Organ dysfunction
- Environmental stress (e.g., heat, cold)
| Disease | Animal | Cause | Key Symptoms |
|---|---|---|---|
| Milk fever (hypocalcaemia) | Dairy cows | Low blood calcium at calving | Muscle tremors, inability to stand, coma |
| Grass tetany (hypomagnesaemia) | Cattle/sheep | Low blood magnesium; lush spring grass | Tremors, convulsions, sudden death |
| Bloat | Cattle/sheep | Excess gas in rumen; lush pasture | Swollen left side of abdomen |
| Pregnancy toxaemia (twin lamb disease) | Sheep | Energy deficit in late pregnancy | Weakness, head-pressing, death |
| Iron deficiency anaemia | Pigs | Insufficient iron; confined piglets | Pale skin, reduced growth |
| Disorder | Cause | Symptoms |
|---|---|---|
| Chlorosis | Iron or nitrogen deficiency | Yellow leaves, reduced growth |
| Blossom end rot (tomato, capsicum) | Calcium deficiency (often linked to irregular watering) | Dark, sunken lesion at base of fruit |
| Tip burn (lettuce) | Calcium deficiency in rapidly growing tissue | Brown, necrotic leaf margins |
KEY TAKEAWAY: Metabolic disorders are managed through nutrition management, soil/ration testing, and adjustments to feeding and environment — not through pesticides or antibiotics.
Metazoal pests are multicellular animal organisms (metazoa) that cause harm to agricultural plants and/or animals. They include insects, mites, nematodes, and larger parasites. They are visible to the naked eye (though some are small), and can act as:
- External parasites (ectoparasites) — live on the outside of the host
- Internal parasites (endoparasites) — live inside the host
- Plant-feeding pests — feed on plant tissue
| Pest | Animal Host | Classification | Damage/Symptoms |
|---|---|---|---|
| Intestinal worms (e.g., Haemonchus contortus — Barber’s Pole worm) | Sheep, cattle, goats | Endoparasite | Anaemia, bottle jaw, weight loss, death in heavy infestations |
| Sheep lice (Bovicola ovis) | Sheep | Ectoparasite | Intense itching, fleece damage (wool break), reduced wool value |
| Cattle tick (Rhipicephalus microplus) | Cattle | Ectoparasite | Weight loss, anaemia; vector for tick fever (Babesia) |
| Blowfly strike | Sheep | Ectoparasite (larval stage) | Maggots feeding on skin; severe tissue damage |
| Mites (mange) | Pigs, poultry, cattle | Ectoparasite | Skin irritation, reduced production |
| Pest | Host Plant | Classification | Damage |
|---|---|---|---|
| Aphids | Wide range (vegetables, cereals, ornamentals) | Sap-sucking insect | Sap removal, honeydew, sooty mould, virus vectors |
| Western flower thrips | Vegetables, ornamentals | Sap-sucking insect | Silvery scarring, petal damage, TSWV virus vector |
| Root-knot nematodes | Vegetables, fruit trees | Endoparasite (nematode) | Gall formation on roots; stunted growth |
| Codling moth | Apples, pears | Insect (larval stage) | Larval tunnelling into fruit |
| Two-spotted mite | Vegetables, grapes | Arachnid | Stippling of leaves; defoliation under heavy pressure |
EXAM TIP: Aphids and western flower thrips are explicitly named in the VCAA study design — know their life cycles, damage patterns, and both chemical and non-chemical control options.
Microbial diseases are caused by microscopic organisms (microorganisms) including:
- Bacteria — prokaryotic, single-celled
- Fungi — eukaryotic; can form spores
- Viruses — non-cellular; require a host to replicate
- Protozoa — unicellular eukaryotes
- Phytoplasmas — bacteria-like organisms in plant phloem
| Disease | Pathogen Type | Animal | Signs | Transmission |
|---|---|---|---|---|
| Milk fever | metabolic — see above | — | — | — |
| Footrot | Dichelobacter nodosus (bacteria) | Sheep, cattle, goats | Lameness, foul-smelling interdigital lesion | Soil, direct contact |
| Fungal rusts | Puccinia spp. (fungi) | Plants | See below | Airborne spores |
| Bovine Respiratory Disease (BRD) | Multiple bacteria/viruses | Cattle | Fever, nasal discharge, coughing | Stress, crowding |
| Newcastle Disease | Virus (Paramyxovirus) | Poultry | Respiratory, neurological signs | Highly contagious; notifiable |
| Disease | Pathogen | Host | Symptoms | Spread |
|---|---|---|---|---|
| Fungal rusts (e.g., wheat stem rust, coffee rust) | Puccinia spp. (fungi) | Cereals, ornamentals | Orange/brown powdery pustules on leaves and stems | Airborne urediniospores |
| Phytophthora root rot | Phytophthora spp. (oomycete) | Avocado, fruit trees | Root decay, yellowing, dieback | Water movement in soil |
| Bacterial wilt | Bacteria (Ralstonia solanacearum) | Tomatoes, potatoes | Sudden wilting; brown vascular tissue | Soil, water |
| Botrytis (grey mould) | Botrytis cinerea (fungus) | Grapes, strawberries, vegetables | Grey fluffy mould on fruit/leaves | Airborne spores, humidity |
| Tomato Spotted Wilt Virus (TSWV) | Virus | Vegetables, ornamentals | Bronze/spotted leaves; necrosis | Western flower thrips vector |
REMEMBER: Fungal rusts and footrot are explicitly listed in the VCAA study design — know both their biological nature (pathogen, lifecycle) and management strategies (see the control KK).
| Feature | Metabolic | Metazoal | Microbial |
|---|---|---|---|
| Cause | Nutritional/physiological | Multicellular animal organisms | Microorganisms (bacteria, fungi, viruses, protozoa) |
| Contagious? | No | Some (e.g., external parasites) | Generally yes |
| Visible? | Visible symptoms | Often visible (organism) | Not visible; symptoms visible |
| Control approach | Nutrition, management | Pesticides, biologicals, vaccination | Fungicides, antibiotics, vaccines, cultural methods |
| VCAA examples | Milk fever | Aphids, western flower thrips, intestinal worms | Footrot, fungal rusts, milk fever (actually metabolic — note this!) |
COMMON MISTAKE: Milk fever is a metabolic disease (low blood calcium) — not a microbial disease. Despite its name, it is not caused by a pathogen. Similarly, footrot is bacterial (microbial), not metabolic.
Understanding whether a pest or disease is metabolic, metazoal, or microbial determines the correct diagnosis pathway and management approach. Metabolic conditions require nutritional and management interventions; metazoal pests are controlled by targeting the organism directly; microbial pathogens require hygiene, chemical, biological, or vaccination strategies. All three categories threaten Victorian agricultural and horticultural productivity and must be managed within integrated, sustainable frameworks.
