Chapter 6. General discussion
6.7 Concluding statements
Urgent work is required to understand basic aspects of the biology of disease caused by P. multivora (Table 6.1) including the aetiology, epidemiology, distribution and control. Until these basic aspects of the disease are understood, we cannot effectively quarantine and manage its spread, let alone manage infected sites.
The role of the P. multivora in E. gomphocephala decline will vary with time, and location in the landscape, depending upon numerous host, pathogen and environmental characteristics, as described by Manion (1981). An adaptive model is required that shows how different factors influence the E. gomphocephala decline syndrome, that can then be refined as further information becomes available. A conceptual model outlining the development of E. gomphocephala decline following infestation with P. multivora is outlined in Table 6.2. With best available knowledge, the model outlines under what circumstances the current E. gomphocephala decline in Yalgorup National Park may be explained by P. multivora acting as a fine feeder root and minor/major root and/or collar/stem pathogen and the cascading effects of its pathogenicity. Phytophthora multivora is more likely to infect thicker tissue beyond the fine roots during warm wet conditions while damage to root systems is more likely to be expressed during hot extended dry periods of high drought stress (Table 6.2). It is also predicted that P. multivora will be most active immediately following warm rainfall events that follow extended dry periods, which would reduce the density of Phytophthora antagonistic soil micro-organisms. Therefore, P. multivora may be most easy to isolate during the first rains following hot summer dry periods. This model should be updated as more information becomes available, as further work is required to determine what
Table 6.2. Conceptual analysis of core components and those that cascade from these, for the development of Eucalyptus gomphocephala decline attributed to Phytophthora multivora. The relationships are tailored to explain the past and current decline syndrome around Yalgorup; however, it is recommended that these relationships be modified as new evidence about the declines becomes available.
Phytophthora multivora as a fine feeder root pathogen Phytophthora multivora as a minor/major root and/or collar/stem pathogen Continuum
Predisposing pathogen invasion: Host, Pathogen and Environment
Fine feeder root damage associated with regular seasonal variation and extreme weather cycles of warm wet and dry periods: Predisposing conditions include:
increased host stress;
presence of oospores and chlamydospores (resting structures) which can tolerate extreme dry periods; reduce density of soil organisms which are antagonistic to P. multivora; and
sudden warm wet conditions which would favour rapid P. multivora population increase (life strategy r) and infection before antagonistic soil microbes reduce population density.
Minor/major root and/or collar/stem necrosis is more likely, in association with cycles of warm wet periods followed by extreme drought. Most extreme on sites prone to inundation and/or modification to fresh groundwater lenses and hydrology. Predisposing conditions include:
cycles of warmer wet periods followed by drought, including unseasonal cyclonic and El Niño spring, autumn and summer rains;
warm wet winters; and
ponding and anoxic rhizosphere stress.
Primary impacts (direct impact on infected plants)
Fine feeder root damage directly impairs nutrient and water cycling and ectomycorrhizae formation. Cascading primary impacts of fine feeder root damage include:
decreased vascular activity;
decreased mycorrhizal damage and/or decreased mycorrhizal establishment;
reduced nutrient uptake, especially on nutrient limiting sites, including zinc limiting calcareous soils; decreased tolerance to hydrological stress;
increased likelihood of secondary infections; and increased opportunity for pathogen population increase.
Minor/major root and/or collar/stem necrosis will cause more damage than fine root necrosis: however, infection of thicker plant material will be limited by less frequent extreme weather events.
Cascading primary impacts of infection of thicker plant material include: decreased vascular activity;
increased likelihood of secondary infection; increased fine root impacts;
increased pathogen protection from unfordable conditions; and
increased opportunity for pathogen population increase, especially after dry periods.
Secondary impacts (outcomes of direct impacts on plants)
Cascading secondary impacts of fine feeder root damage include: decreased critical plant functions including foliar and seed production; greater dry and dead material which is more flammable; and death resulting from chronic deterioration.
Cascading secondary impacts of infection of thicker plant material include: secondary impacts as indicated for fine feeder root damage; and death resulting from chronic deterioration and girdling.
Tertiary impacts (ecosystem impacts)
Cascading tertiary impacts of fine feeder root damage include:
impaired site hydrology, nutrient cycling, recruitment, community plant population dynamics, fauna population dynamics; and
increased fire frequency until sites become degraded, then decreased fire frequency and more shrub fires.
Cascading tertiary impacts of infection of thicker plant material include:
tertiary impacts as indicated for fine feeder root damage, occurring more frequently during periods of sudden tree death.
Appendices
Appendix 1. Study sites for research chapters 2, 3, and 5, and sites from which Phytophthora multivora isolates WAC 13200, 13201, 13202, 13203 and 13204, where isolated as described in Chapter 4 and Scott et al. (2009). Phytophthora multivora isolates stored at Department of Agriculture and Food Western Australia Plant Pathogen Collection, Perth, Australia.
a
b
Grid Percentage cover
Average fine root
density (D) Fine root (DSG)
Area inside each grid within the total reach (m2) (Ag) 1 100 20 20 0.05 2 65 35 22.75 0.5 3 100 25 25 0.35 4 100 35 35 0.08 5 90 40 36 0.95 6 95 40 38 1.0 7 100 35 35 0.6 8 100 25 25 0.5 9 100 15 15 0.05 10 40 10 4 0.8 11 40 15 6 1.0 12 70 20 14 1.0 13 90 30 27 0.6 14 100 20 20 0.1 15 95 20 19 0.7 16 95 18 17.1 0.8 17 100 18 18 0.75 18 100 15 15 0.5 19 100 10 10 0.05 20 100 10 10 0.08
Area within total reach (Atr) (m²)= 10.46 Total density score (fine roots) = ((0.05/10.46)*20)+((0.5/10.46)*22.75)... = 21.10
Appendix 2. (a) Schematic representation of a theoretical fine root system (< 2 mm in diameter) attached to the largest main lateral root. The dashed line delineates the area inside the perimeter of the outer root tips defined as the „total reach‟. Each square represents 1m2. (b) Worked example showing how the fine root total
176
a
b
Grid Percentage cover
Average
ectomycorrhizal density
(D) Ectomycorrhizal (DSG)
Area inside each grid within the total reach (m2) (Ag) 1 100 15 15 0.35 2 100 30 30 0.08 3 70 25 17.5 0.8 4 70 20 17.5 0.8 5 100 25 25 0.45 6 100 20 20 0.3 7 15 5 1.5 0.8 8 20 10 2 1 9 60 15 9 0.95 10 80 25 20 0.6 11 100 35 35 0.3 12 85 35 29.75 1 13 95 35 33.25 1 14 85 30 25.5 1 15 85 25 21.25 0.6 16 100 20 20 0.05 17 100 25 25 0.15 18 100 30 30 0.15 19 100 25 25 0.1 20 100 20 20 0.1
Area within total reach (Atr) (m²)= 10.58 Total density score (fine roots) = ((0.35/10.58)*15)+((0.08/10.58)*30)... = 18.78
Appendix 3. (a) Schematic representation of a theoretical ectomycorrhizal system attached to the largest main lateral root. The dashed line delineates the area inside the perimeter of the outer root tips defined as the „total reach‟. Each square represents 1m2. (b) Worked example showing how the ectomycorrhizal total density score
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