HSC Exam Practice

Sample response. A plant pathogen is a microorganism, virus, nematode or other agent that causes disease in plants.

Marking notes. 1 mark for a definition that identifies the causal agent and its role in causing disease in plants. Accept any wording that captures: organism/agent → causes disease → in plants.

Sample response. Example 1: Wheat stem rust, caused by Puccinia graminis, affects wheat and barley. Example 2: Myrtle rust, caused by Austropuccinia psidii, affects Myrtaceae including eucalypts, paperbarks and tea trees. Other acceptable examples: downy mildew in grapevines (Plasmopara viticola).

Marking notes. 1 mark per correctly identified example: a mark requires both the pathogen name (species or common name) and the crop/plant affected. A named pathogen without the host, or a host without the pathogen, scores 0 for that entry.

Sample response. Fungal plant diseases can be managed after infection using fungicides, which target fungal cell biology without harming plant cells; for example, fungicide applications can slow or stop the spread of wheat stem rust in a crop. Management may also include planting resistant varieties and removing heavily infected material. Viral plant diseases cannot be treated once infection is established, because plant viruses replicate inside plant cells using the cell’s own machinery — there is no selective antiviral spray that targets the virus without damaging the plant. Management is therefore entirely preventive: controlling insect vectors (such as aphids transmitting BBTV), using certified virus-free planting material, and removing and destroying infected plants to prevent further spread. The strategies differ because fungal cell biology is distinct from plant cell biology, allowing selective fungicide activity; viral replication exploits the plant’s own cellular machinery, leaving no treatment target.

Marking notes. 1 mark: fungal management includes post-infection chemical (fungicide) treatment; 1 mark: viral management is entirely preventive because no cure exists once infected (accept “no antiviral spray”); 1 mark: explains why strategies differ with reference to biology (fungal vs plant cell distinction / viral replication uses plant’s own machinery).

Sample response. Root-knot nematodes are parasitic roundworms that enter plant roots as second-stage juveniles and migrate to the vascular tissue. They inject secretions that cause plant cells to enlarge abnormally, forming galls or root knots. These galls disrupt the plant’s xylem and phloem, blocking the uptake and transport of water and nutrients from roots to shoots. The result is wilting, yellowing, stunted growth, reduced fruit or seed production, and in severe infections, plant death. Because galls persist in the root system and nematode eggs remain viable in soil for years, repeated infection cycles are common and soil contamination is long-lasting.

Marking notes. 1 mark: mechanism — nematode juveniles penetrate roots and inject secretions causing cell enlargement forming galls; 1 mark: effect on plant function — galls disrupt vascular transport (xylem/phloem), blocking water and nutrient uptake; 1 mark: consequence at the plant/crop level — wilting, stunted growth, reduced yield, or plant death.

Sample response. Indirect economic effects (those arising from the presence of disease but not from direct damage to individual plants) include: (1) export market loss — importing countries may ban produce from regions where a notifiable disease is detected, causing revenue loss for the entire district even if most plants are unaffected; (2) increased control costs — farmers incur ongoing expenditure on fungicides, bactericides, monitoring programs, quarantine compliance, and replanting, which reduce profitability. Other acceptable indirect effects: fall in land values in affected districts; government biosecurity response costs; loss of consumer confidence.

Marking notes. 1 mark per correctly described indirect economic effect. Must be clearly indirect (not “yield reduction” or “plant death”, which are direct). Accept any two from: export ban, increased control costs, land value reduction, government/biosecurity response costs, loss of consumer market confidence.

Sample response. Biosecurity refers to the measures used to prevent the introduction and spread of pests and diseases, including strict border controls, inspection and quarantine of imported plant material, restrictions on the movement of soil and plant material between regions, and surveillance programs to detect new threats early. In Australia, biosecurity for plant disease is enforced by the Department of Agriculture, Fisheries and Forestry (DAFF). Prevention is preferred over eradication because many plant pathogens — once established in Australian environments — spread rapidly via wind, water, insects or human movement and cannot be eliminated; myrtle rust is an example where despite detection in 2010, eradication was impossible once the pathogen became established across multiple states. The financial and environmental cost of managing an established disease over decades vastly exceeds the cost of maintaining border controls that prevent entry in the first place.

Marking notes. 1 mark: describes biosecurity as measures to prevent entry and spread of disease (not just “keeping pests out”); 1 mark: explains why prevention is preferred over eradication (pathogens spread rapidly once established / eradication of established disease is often impossible); 1 mark: uses a named example to support the preference for prevention (myrtle rust, fire blight, or any other lesson example).

Sample response. BBTV-positive property numbers increased sharply from 48 in 2010, reaching a peak of 312 in 2013, before declining consistently following the introduction of intensified vector control measures in 2014, falling to 71 confirmed properties by 2019 — a reduction of approximately 77% from peak. The overall 10-year trend is therefore an initial period of rapid increase followed by a sustained decline following the management intervention.

Marking notes. 1 mark: identifies the two phases (increase then decrease) and notes the peak year (2013); 1 mark: includes at least two specific data values supporting the trend (e.g. 48 in 2010, peak 312 in 2013, or 312 in 2013 declining to 71 in 2019).

Sample response. BBTV is a virus transmitted exclusively by the banana aphid (Pentalonia nigronervosa). The aphid acquires the virus by feeding on the phloem of infected plants and then transmits it to healthy plants during subsequent feeding. The intensified vector control program introduced in 2014 targeted the aphid population using insecticides and surveillance; by reducing aphid numbers, fewer aphids carried and transmitted the virus between plants and properties. Additionally, mandatory destruction of all confirmed BBTV-infected plants removed the virus reservoir that aphids use to acquire new infections, progressively reducing the pool of infected source material available for vector acquisition. Together, these actions broke the transmission chain: fewer vectors carrying the virus, and fewer source plants to infect them, reduced new confirmed cases year on year.

Marking notes. 1 mark: identifies banana aphid as the vector and explains how it transmits BBTV (feeds on infected plant → acquires virus → transmits during feeding on healthy plants); 1 mark: links vector control measures to reduced aphid-mediated transmission (reduced aphid populations = fewer virus transfers between properties); 1 mark: explains that destruction of infected plants removes the virus source, further limiting vector acquisition and breaking the cycle.

Sample response. Complete eradication is difficult because BBTV persists in infected plant material (including ratoon shoots that regrow after cutting), and because wild banana relatives and ornamental banana plants around properties can act as asymptomatic reservoirs for both the virus and its aphid vector. As long as any infected plants or aphid populations survive in the region, there is potential for the virus to re-enter commercial plantations. Accept also: aphid populations are difficult to eliminate completely / wild host reservoirs maintain virus in the region.

Marking notes. 1 mark for any biologically valid and specific reason why complete eradication is difficult: must go beyond “it keeps spreading” — must identify a specific biological reason (reservoir hosts, persistence in plant material, difficulty of 100% aphid control, etc.).

Best-answer example: myrtle rust. Myrtle rust is caused by the fungal pathogen Austropuccinia psidii (a basidiomycete rust fungus). It infects plants by landing wind-dispersed urediniospores on the young growing tissue of Myrtaceae hosts. The spores germinate in the presence of moisture, and hyphae penetrate the leaf surface, grow through host tissue causing cell death, and produce orange-yellow pustules. Each pustule releases millions of new urediniospores, completing the cycle in as little as 7–10 days — explaining the speed of continental spread documented from 2010 to 2023. The economic effects on agricultural production are substantial. The nursery industry, valued at over $900 million annually, has been directly impacted: popular Myrtaceae ornamental lines (lilly pilly, tea tree, bottlebrush) are now subject to visible infection reducing their commercial value and consumer demand. The bush food industry, which relies on native Myrtaceae fruits including riberry (Syzygium luehmannii), faces reduced yield and quality affecting a growing export market. Control costs — including fungicide programs, research investment, and regulatory compliance — represent further ongoing indirect economic impacts. The ecological consequences are equally severe but distinct in kind: they cannot be offset by financial compensation. The decline of paperbark (Melaleuca quinquenervia) in coastal NSW wetlands threatens habitat for more than 50 bird species and alters wetland water cycling and soil chemistry. Most starkly, Rhodamnia rubescens (scrub turpentine) and two Xanthostemon species have been listed as critically endangered specifically because of myrtle rust — the first Australian plant species to be listed as critically endangered due to a disease. Unlike agricultural losses, which can be compensated through replanting or industry support, extinction is permanent and represents an irreversible loss of biodiversity. Assessed overall, myrtle rust is exceptional in scope and severity among Australian plant diseases: it is simultaneously the largest threat to the native Myrtaceae flora in Australian ecological history, a significant ongoing economic cost to multiple agricultural industries, and a demonstration of why biosecurity failure at the point of entry carries consequences that are permanent and continental in scale.

Marking criteria.

• 1 mark — Names a specific plant disease and correctly identifies the pathogen type (fungal/bacterial/viral/nematode) with the pathogen’s scientific or common name.
• 1 mark — Describes the mechanism of infection accurately (how the pathogen enters the plant and causes damage at the cellular or tissue level).
• 1 mark — Describes the spread mechanism accurately (how the pathogen moves from plant to plant or property to property).
• 1 mark — Assesses at least one direct economic effect on agricultural production with specific detail (e.g. yield reduction, named industry, quantified impact).
• 1 mark — Assesses at least one additional economic effect (either a second direct effect or an indirect effect such as export ban, increased control costs, or land value reduction).
• 1 mark — Assesses ecological consequences with a specific named example, distinguishing them from agricultural consequences.
• 1 mark — Identifies a qualitative distinction between agricultural and ecological consequences (e.g. ecological losses are permanent / cannot be compensated financially).
• 1 mark — Reaches a coherent, evidence-based overall evaluative judgement about the severity or significance of the disease using specific evidence from the lesson (not a summary).

Sample response.

Flaw 1: “myrtle rust killing native trees is an environmental problem, not an agricultural one.” This is incorrect. Myrtle rust directly affects multiple agricultural industries: the nursery industry (valued at over $900 million annually, with Myrtaceae ornamentals as a major component), the bush food industry (which relies on native Myrtaceae fruits commercially), and research/management costs borne by growers and government. The distinction between “environmental” and “agricultural” is false in this case; the ecological and agricultural impacts of myrtle rust are deeply intertwined. [1 mark for identifying the flaw; 1 mark for correct biology explaining why it is wrong].

Flaw 2: “introduced pathogens are no more dangerous than diseases we already have; any plant that dies just wasn’t well adapted.” This misrepresents the biology. Introduced pathogens are often more dangerous precisely because the host plants have had no co-evolutionary history with them and therefore no natural selection has occurred for resistance. In the pathogen’s native range (e.g. South America for Austropuccinia psidii), host Myrtaceae species have developed at least partial resistance through millions of generations of natural selection. Australian Myrtaceae have none — not because they are “poorly adapted” in general, but because they have never encountered this pathogen. The danger is the absence of co-evolutionary history, not any inherent weakness in Australian plants. [1 mark for identifying the flaw; 1 mark for correct biology explaining why it is wrong].

Marking notes. 2 marks per flaw: 1 for correctly identifying and stating the flawed claim, 1 for explaining the correct biology. Other valid flaw to accept: “chemical sprays have plant disease well under control” — this is incorrect because many diseases (e.g. viral diseases) cannot be controlled with sprays, and fungicide resistance is an ongoing challenge; myrtle rust has no cure.