Biology • Year 12 • Module 7 • Lesson 6
Disease in Agriculture — Plants
Apply knowledge of plant pathogen types, spread mechanisms and economic effects to real data, case studies and cause-and-effect reasoning.
1. Interpret disease severity data — myrtle rust impact on nursery species
The table below shows infection severity ratings (0 = no infection; 5 = severe infection causing plant death) for five commercially important Myrtaceae species exposed to Austropuccinia psidii (myrtle rust) under controlled conditions in a 2017 Australian nursery trial. Ratings are averaged across 50 plants per species. 7 marks
| Species | Common name | Mean severity rating | % plants showing pustules |
|---|---|---|---|
| Rhodamnia rubescens | Scrub turpentine | 4.8 | 100% |
| Melaleuca quinquenervia | Broad-leaved paperbark | 4.1 | 96% |
| Callistemon viminalis | Weeping bottlebrush | 3.2 | 84% |
| Syzygium luehmannii | Riberry | 2.0 | 60% |
| Leptospermum laevigatum | Coastal tea tree | 0.9 | 22% |
Source: Hypothetical trial data, after Pegg et al. (2017), Australasian Plant Pathology.
1.1 Identify the species most severely affected by myrtle rust and state two pieces of data that support this conclusion. 2 marks
1.2 Using the concept of host resistance, explain the difference in infection severity between Rhodamnia rubescens and Leptospermum laevigatum. 3 marks
1.3 The scrub turpentine is listed as critically endangered as a result of myrtle rust. Predict one ecological consequence that could follow widespread death of this species in coastal NSW rainforests, and justify your prediction. 2 marks
2. Interpret graph — estimated global wheat yield loss due to stem rust
The figure below shows estimated percentage of global wheat yield lost to Puccinia graminis (wheat stem rust) under three management scenarios over a 20-year period, modelled from 2005 outbreak data in East Africa. 6 marks
Figure 2.1. Modelled wheat yield lost to Puccinia graminis under three management scenarios 2005–2025. Adapted from Singh et al. (2011), Current Opinion in Plant Biology.
2.1 Describe the trend in wheat yield loss for the “No management” scenario from 2005 to 2025. Include at least two data values in your description. 2 marks
2.2 Calculate the approximate difference in yield loss between “Fungicide only” and “Resistant varieties” at the year 2025. Explain which approach provides better long-term protection and why. 2 marks
2.3 The graph models only one pathogen. Explain one limitation of using this data to make conclusions about the economic impact of plant disease on Australian wheat production generally. 2 marks
3. Trace the cause-and-effect chain — Banana bunchy top virus outbreak
Starting from the arrival of an aphid carrying Banana bunchy top virus (BBTV) to a Queensland plantation, complete the cause-and-effect chain below. Write the missing effect in each box. 5 marks
4. Case study — fire blight detection in Australian apple orchards
5 marks
Stimulus. In March 2021, Erwinia amylovora (fire blight) was confirmed in an apple orchard in the Harcourt region of Victoria — one of Australia’s most productive apple-growing districts, contributing approximately $120 million annually to the regional economy. The orchard was placed under a destruction order: all infected and at-risk trees within a containment zone were to be destroyed within 14 days. An emergency biosecurity response was activated, including restrictions on movement of plant material, equipment and people from the affected property. Three neighbouring orchards were also placed under surveillance. Japan, which imports around 40% of Australia’s exported apples, immediately suspended imports from the affected district pending a risk assessment.
4.1 Using the stimulus and your lesson knowledge, assess the direct and indirect economic effects of this fire blight detection on the Harcourt apple industry. Refer to specific examples from the stimulus in your answer. 4 marks
4.2 Explain why the biosecurity response emphasised prevention of movement rather than treatment of infected trees. 1 mark
5. Predict and justify — nematode management decision
A cotton grower in the Darling Downs (Queensland) has confirmed that a paddock is severely infested with root-knot nematodes (Meloidogyne spp.). The grower must decide between two management options: (A) fumigate the soil with a nematicide before replanting cotton, or (B) rotate the paddock to a non-host grain crop (e.g. sorghum) for two seasons before returning to cotton. 4 marks
5.1 Predict the short-term and long-term advantages of Option B (crop rotation) over Option A (fumigation). Justify your prediction using your knowledge of nematode biology from the lesson. 4 marks
Q1.1 — Most severely affected species (2 marks)
Rhodamnia rubescens (scrub turpentine) is the most severely affected species. Supporting data: (1) mean severity rating of 4.8 out of 5 — the highest of all five species tested; (2) 100% of plants showed pustules — every plant in the sample was infected. [1 mark for naming species; 1 mark for two specific data values].
Q1.2 — Host resistance difference (3 marks)
Rhodamnia rubescens has very low or no host resistance to Austropuccinia psidii (severity 4.8, 100% infected), meaning the fungal spores germinate successfully and hyphae penetrate its tissue causing severe damage [1]. Leptospermum laevigatum shows significantly higher host resistance (severity 0.9, only 22% infected), which means most spores that land on its tissue either fail to germinate or cannot penetrate effectively [1]. Both species are Australian Myrtaceae with no co-evolutionary history with myrtle rust, but the lower severity in L. laevigatum suggests this species has some pre-existing physical or biochemical barrier that provides partial resistance — this is used in management programs to identify naturally resistant individuals [1].
Q1.3 — Ecological consequence (2 marks)
Accept any well-justified ecological consequence, e.g.: Loss of scrub turpentine from coastal NSW rainforest would reduce the canopy, letting more light reach the understorey and favouring invasive weed species that outcompete native seedlings [1]. Justified because scrub turpentine is a structurally dominant rainforest species and its removal creates gaps that alter the light and moisture regime, changing competitive outcomes for other plants [1]. Also accept: loss of habitat for dependent fauna (birds, insects using canopy), altered soil chemistry from reduced leaf litter, changed water cycling.
Q2.1 — Trend description (2 marks)
Under no management, wheat yield losses due to stem rust increase steadily and consistently from approximately 3% in 2005 to approximately 10% in 2025 — more than tripling over the 20-year period [1]. The rate of increase appears roughly linear with no plateau, suggesting ongoing spread of the pathogen without any check [1].
Q2.2 — Difference and better approach (2 marks)
In 2025, “Fungicide only” shows approximately 6.2% yield loss while “Resistant varieties” shows approximately 1.5% — a difference of approximately 4.7 percentage points [1]. Resistant varieties provide better long-term protection because they incorporate genetic host resistance into the wheat plants themselves, permanently reducing susceptibility without requiring repeated chemical applications; fungicide use, by contrast, must be sustained each season and may become less effective if new rust strains (races) emerge that overcome it [1].
Q2.3 — Limitation of the data (2 marks)
The graph models only one pathogen (Puccinia graminis) and therefore does not capture the cumulative economic impact of all pathogens affecting Australian wheat (e.g. barley yellow dwarf virus, Fusarium crown rot, stripe rust) [1]. Australian conditions also differ from the East African outbreak used to derive the model, so yield loss values may not accurately reflect losses in Australian wheat-growing regions with different climate, varieties and management practices [1]. Accept any single well-explained limitation that identifies a constraint on generalisation.
Q3 — Cause-and-effect chain — sample answer
Effect 1: The aphid feeds on the phloem of healthy banana plants and injects the virus into the plant’s vascular system. Effect 2: The virus replicates in plant cells, causing characteristic symptoms (stunted, bunched leaves with striped foliage) and preventing the plant from producing fruit. Effect 3: As there is no cure, infected plants must be identified and immediately destroyed (removed and burned) to prevent the aphid from acquiring the virus and spreading it further. Effect 4: The grower loses all infected plants and their potential harvest; quarantine zones restrict movement of planting material into or out of the affected property. Outcome: Both direct economic loss (yield) and indirect losses (replanting costs, potential export restrictions) reduce the profitability of the plantation; the Queensland banana industry faces ongoing control costs for aphid vector management and use of certified virus-free planting material.
Q4.1 — Direct and indirect economic effects (4 marks)
Direct effects: The destruction order means all infected and at-risk trees in the containment zone must be physically removed within 14 days — this represents an immediate direct loss of productive trees, the capital investment they represent, and any fruit they would have produced in the upcoming and future seasons. Replanting costs will be substantial [1]. Three neighbouring orchards are under surveillance and may face similar orders, further extending direct losses [1]. Indirect effects: Japan, which imports 40% of Australia’s exported apples, immediately suspended imports from the district — this indirect economic effect arises not from additional tree damage but from the reputation of the region; growers in the entire district lose access to the most valuable export market even if their trees are unaffected [1]. The restriction on movement of equipment and people from the affected property disrupts normal farm operations and may extend to neighbouring properties, compounding indirect costs through reduced operational efficiency and potential compliance costs [1].
Q4.2 — Why movement restriction (1 mark)
Fire blight is a bacterial disease that spreads via insects, rain splash and contaminated pruning equipment — and once an orchard is infected, treatment is limited (some copper-based bactericides can slow spread but cannot eliminate the bacterium from established infections). Since eradication after establishment is extremely difficult and costly, the biosecurity response correctly prioritises containment — preventing the bacterium from being physically carried to new orchards on boots, tools, vehicles or plant material. Prevention is far more cost-effective than management after spread.
Q5.1 — Crop rotation advantages (4 marks)
Short-term advantage: Crop rotation to sorghum (a non-host) removes the food source that root-knot nematode juveniles need to complete their life cycle. Without a susceptible root to penetrate, newly hatched juveniles in the soil die before becoming reproducing adults, and the nematode population declines significantly in the soil over 1–2 seasons [1]. This is a short-term advantage because it progressively reduces the pathogen load during the rotation period [1]. Long-term advantage: Unlike fumigation, which must be repeated with each planting cycle, a successfully implemented rotation reduces nematode egg density in the soil to levels where cotton can be replanted with substantially lower yield loss — this is sustainable and does not carry the environmental and health risks associated with nematicide chemicals [1]. Additionally, nematode eggs can remain viable in soil for years, so fumigation alone may not eliminate the egg bank; rotation addresses this gradually without reliance on chemicals that nematodes can develop tolerance to over time [1].