Increased Risk of Pitch Canker to Australasia Under Climate Change

Fusarium circinatum is the causal agent of pitch canker, a disease that infects Pinus species. It is considered one of New Zealand and Australia’s most unwanted pathogens because of the high level of threat it poses to the region’s forest industry. In absence of host-associated insects, the severity of pitch canker is influenced by climatic variables. It is currently problematic in humid subtropical and Mediterranean climates. Therefore, climate change may expand the pathogen’s possible range in Australasia, causing large-scale infection in the countries’ pine forests if it is ever introduced in these regions. Ganley et al. (2011) used the process-orientated niche-modeling program CLIMEX to model the climate suitability for pitch canker establishment in Australasia using a current climate dataset and three different global climate models (GCMs) under moderate and high CO2 emissions scenarios. Under Australasia’s current climate, the moist periphery of Australia and the northern coastal areas of New Zealand are climatically suitable for pitch canker. Under both climate change scenarios, the potential range of pitch canker would expand southwards to include most of the forests in the central north island of New Zealand and the northern areas of Tasmania. Currently, these areas are unsuitable because they are too cold to support the pathogen. Therefore, these results emphasize the need to prevent the introduction of pitch canker to Australasia as the climate begins to change and warm within these regions.—Megan Smith
Ganley, R.J., Watt, M.S., Kriticos, D.J., Hopkins, A.J.M., Manning, L.K. 2011. Increased Risk of Pitch Canker to Australasia under Climate Change. Australasian Plant Pathology. 40: 228 – 237. DOI 10.1007/s13313-011-0033-2.

Fusarium circinatum is a wound pathogen. Intact tree tissue is not susceptible to the disease. Therefore, it is primarily associated with wounds created by insects, weather, or mechanical damage. In absence of host-associated insects, the incidence of pitch canker is influenced by climatic variables. The disease is very problematic in humid subtropical and Mediterranean climates, and outbreaks of pitch canker have been previously correlated with severe damage caused by hurricanes. With climate change scenarios projecting an increase in global temperatures, the disease’s range may expand.
Fusarium circinatum is pathogenic to over 60 species of pine and Douglas fir. Pinus radiata is one of the most susceptible species, which places New Zealand’s and Australia’s forest plantations at great risk to the pathogen because this particular tree species accounts for approximately 90% of New Zealand’s pine plantation estates and 50% of Australia’s pine plantation estates. Given the threat this disease poses to New Zealand and Australia’s current and future pine plantations, it is of great use to assess how climate change will influence the distribution of the pathogen.
The authors used a previously developed CLIMEX model for pitch canker to estimate its potential distribution within New Zealand and Australia under current and future climate. CLIMEX integrates modeled weekly responses of a population to climate to create a series of weekly and annual indices. The CLIMEX Compare Locations module uses an annual growth index (GIA) to describe the potential for population growth as a function of soil moisture and temperature during favorable conditions and up to eight stress indices to simulate the ability of the population to survive unfavorable conditions. It also uses a mechanism that defines the minimum amount of thermal accumulation during the growing season that is necessary for population persistence.
The growth and stress indices are calculated weekly and combined into an overall annual index of climatic suitability, the Ecoclimatic Index (EI), that gives an overall measure of the potential of a given location to support a permanent population of the species. It ranges from 0 for locations at which the species is not able to persist to a theoretical maximum of 100 for locations that are climatically perfect for a species. In this study, EI is classified into unsuitable (EI = 0), marginal (EI = 1–5), suitable (EI = 6– 20), and optimal (EI>20) categories for pitch canker establishment. CLIMEX models are fitted to known distribution data. This process involves adjusting growth and stress parameters and then comparing the model results to the known distribution of the species and including any additional information about the pathogen.
Records of pitch canker were compiled from individual point locations and county state and island level observations for countries known to have pitch canker. The records reveal that the disease’s range is primarily humid subtropical and Mediterranean. The disease also spreads into warmer temperate climates and regions with tropical humid rainforest and savannah climates.
A CLIMEX model, constructed using the Compare Locations Module, was utilized to model the current and future distribution of pitch canker. The fit of the model was validated using a set of observed occurrences from locations not used in the original fitted dataset. The results show that almost all the observations of pitch canker occurred in suitable areas for the species.
The current climate dataset used within CLIMEX was a 0.5° of arc dataset. It was generated from the 1961–1990 climate normals provided by the climatic research unit. Six climate change scenarios were used to project the potential distribution of pitch canker under climate change within New Zealand and Australia. These scenarios were developed from three GCMs using two Intergovernmental Panel on Climate Change (IPCC) scenarios, representing medium (A1B) and high (A2) emissions. Selected GCMs included CSIRO Mark 3.0, NCAR-CCSM, and MIROC-H. Data from these three GCMs were pattern scaled to develop individual climate change scenarios for 2080 relative to the base climatology. The climate variables utilized by CLIMEX were taken or calculated form the GCM data.
Areas of New Zealand and Australia with a suitable climate for pitch canker under current and future climate scenarios were estimated with Regional Council administrative areas and state/territory boundaries using ArcGIS. Boundary shapefiles were used to extract EI data by region and state. The areas for each EI value were calculated to obtain the total area per EI value by region and data. The data were classified using suitability values and the total area per suitability classification was determined for current pine and softwood plantations under the current and future climate change scenarios.  Current New Zealand pine plantations were identified within the Land Cover Database layer using ArcGIS. A current Australian softwood plantation shapefile was used to delineate softwood plantation boundaries.
            New Zealand pine plantations are located throughout the country. Most are dispersed in the central North Island, Northland, Gisborne, and the northern South Island. The softwood resource within Australia is located in the southeastern states of South Australia, New South Wales, Victoria, and Tasmania. A map displaying the location of existing Pinus species plantations in New Zealand and Australia was constructed, as was a map displaying the Ecoclimatic Index class for pitch canker under current climate.          The authors determined that under the current climate, the potential distribution of pitch canker in New Zealand included Northland and coastal areas within North Island. The percentage of area within each region projected to be suitable for pitch canker decreased as latitude increased, ranging from 100% in Northland to 0% in South Island regions. The potential distribution only included 43% of current New Zealand plantations. Yet, all of these plantations were projected to have a climate that was optimal for pitch canker. Most of the forests in the central North Island were projected to have an unsuitable climate for pitch canker under current climatic conditions.
            In Australia, the potential pitch canker distributed was limited to the moist periphery of the country in Queensland, New South Wales, Victoria, South Australia and Western Australia. The proportion of each state that was suitable for pitch canker increased with latitude in mainland Australia. Suitable areas closely corresponded to current plantation area. Only 14% of the softwood plantations were projected to be unsuitable for pitch canker. A table displaying the percentage distribution of the softwood plantation area, by EI class for pitch canker, under current and future climate scenarios was constructed.
            Potentially suitable areas in New Zealand increased under all climate change scenarios from 8% for the CSIRO A1B model to 34% for the NCAR A2 model. The future potential distributions and the two emission scenarios demonstrated little variation between the three GCMs. Marked increases in potentially suitable areas occurred in most regions, but were very noticeable in the southern north island regions of Wellington, Manawatu-Wanganui, and Taranaki. There was an increase in EI for the majority of the regions originally designated to be suitable for pitch canker. The area projected to be unsuitable in New Zealand plantations decreased from 57% to 17–21%. Area projected to be optimal for the pathogen increased from 43% to 76% and 78% under climate change scenarios. A graph displaying suitable area for pitch canker, expressed as percent total area and suitable projected area, was displayed for New Zealand and Australia.
            In Australia, the area suitable for pitch canker was reduced for all scenarios. The reductions ranged from 23% for the NCAR A1B model to 51% for the CSIRO A2 model. Reductions were noticeable in Queensland, Western Australia, South Australia, New South Wales, and Victoria. However, climate change resulted in increases in suitable area within Tasmania under all scenarios. The effect of climate change on the area of plantations susceptible to pitch canker was sensitive to the GCM used, but there was little variation between the two emissions scenario used. Areas that were unsuitable showed moderate and substantial increases in suitability under the MIROC and CSIRO models, but showed no change under the NCAR model. Reductions in suitable to optimal area were projected by CSIRO, while reductions in marginal areas were predicted by MIROC. Maps displaying the EI classes for pitch canker under future climate scenarios for each GCM model for New Zealand and Australia were constructed. Graphs displaying the mean EI of suitable areas, by region for Australia and New Zealand, were also constructed.
            In summary, under climate change, pitch canker is expected to pose a greater threat to New Zealand than to mainland Australia as the disease’s preferred climate (wet warm temperate to sub-tropical climates) shifts southward.
            Although the CLIMEX model projects the climatically suitability patterns for pitch canker, it doesn’t indicate the potential severity of the disease. Previous studies suggest that coastal regions or areas with high humidity or subtropical climates would have more severe outbreaks. Moreover, as the frequency of strong winds and extreme rainfall increases with climate change, these climatic environments would provide moist conditions suitable for pitch canker suggestion.
Areas with marginal to suitable climate would be unlikely to have outbreaks of the disease in absence of host-associated insects, while areas that are climatically optimal for pitch canker would be likely to have disease outbreaks regardless of whether specific host insects or wounding agents were present. Therefore, if pitch canker is introduced to New Zealand, the entire North Island and northern coastal regions of the South Island could support outbreaks under future scenarios. Yet, the disease may not develop in the coastal Canterbury region as long as host insects and wounding agents remain absent. If these agents were present in unsuitable areas, the disease would not develop.
New Zealand does not have insects known to be associated with pitch canker disease in the USA. Australia’s bark beetle Ips grandicollis is not currently present in Tasmania, but it could possibly become associated with pitch canker outbreaks under climate change scenarios. Furthermore, other insects present in Australia could become vectors for the pathogen, and it could also be furthered by marsupials, livestock, and birds. Regardless of the role of vectors, the pathogen would still establish in the coastal regions that are climatically optimal for pitch canker.
Pinus radiata is the primary plantation species planted in Australasia and is known to be highly susceptible to pitch canker. However, large plantations of P. pinaster exist in low-medium temperate rainfall zones, and P. elliotii and P. caribaea are also planted in tropical and subtropical regions of Austrialia. Pinus caribaea is moderately susceptible to pitch canker while both P. pinaster and P. elliotiiarea highly susceptible. Overall, outbreaks affecting P. caribaea would possibly be less severe than outbreaks affecting P. pinaster, P. elliottii or P. radiata.
It is expected that the number of countries with pitch canker will increase over the next decade as the disease’s range increases. This is of great concern to New Zealand and Australia, as well as other pitch canker-free countries. Therefore, continued vigilance and monitoring for this disease is of the utmost importance to prevent the establishment of pitch canker under future climate scenarios. 

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