Phytophthora Agathidicida Research Articles

Is provenance or phylogeny a better predictor of growth and survival of a soil pathogen in leaf litter?

Exotic plants have the potential to increase pathogen inoculum that can affect native plants. New Zealand’s iconic kauri tree (Agathis australis) is threatened by disease caused by Phytophthora agathidicida, which is most prevalent in fragmented forests that have been invaded by or are adjacent to populations of exotic species. Exotic plants have been introduced intentionally (i.e., plantations and pastures) and unintentionally along the margins of kauri forests, yet it is unclear if invasive species play a role in pathogen spread. To determine the extent to which native and exotic plant litter supports P. agathidicida inoculum, we performed a phylogenetically controlled detached leaf assay. We inoculated 60 native and 44 invasive species’ leaves with three isolates of P. agathidicida collected from two different geographical regions of New Zealand, measured disease symptoms and re-isolated the pathogen from infected leaves. Lesions grew larger and faster on exotic leaves across all three isolates tested. However, pathogen recovery was not necessarily more likely from exotic leaves. In contrast, one of the three isolates grew faster when recovered from native compared with exotic leaves. Phylogeny did not predict disease expression. This data suggests that native and exotic plant litter may be reservoirs for P. agathidicida, but reservoir potential varies among isolates. These results also support key management tools used in New Zealand aimed at reducing pathogen spread by foot traffic in fragmented kauri forests, such as hygiene stations for shoe cleaning at trailheads and boardwalks in sensitive forest areas. Further, these tools may benefit forest management worldwide, as pathogens and exotic, invasive species increase at a global scale.

Sensitivity of the soil-borne pathogen Phytophthora agathidicida, the causal agent of kauri dieback, to the anti-oomycete fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin

The oomycete Phytophthora agathidicida is the causal agent of kauri dieback, which threatens the survival of endemic kauri (Agathis australis) forests in Aotearoa|New Zealand. Current chemical control of P. agathidicida involves the application of either a mixture of halogenated tertiary amines or phosphite salts with some success, but neither treatment cures the disease. Recently, four anti-oomycete fungicides, all with different modes of action, have become commercially available. Here, we determined the inhibition potential of these fungicides on three P. agathidicida isolates, using agar dilution assays. The average concentration required to inhibit mycelial growth by 50% (EC50) for ethaboxam, fluopicolide, and mandipropamid was 0.0916, 0.372, and 0.0196 µg/mL, respectively. Inhibition of P. agathidicida mycelia by oxathiapiprolin and its commercial formulation, Zorvec® Enicade®, was 0.000152 and 0.000309 µg/mL, respectively. Based on the EC50 values reported in this study, these fungicides are the most effective inhibitors of P. agathidicida mycelia when compared to previously screened fungicides, natural products, and plant extracts. Thus, their performance in this initial screening supports further research into their potential use as a kauri dieback management tool.

Public Rāhui and Road Blocks in Aotearoa: Navigating Iwi/Hapū Perspectives and Mana Motuhake

During the COVID-19 pandemic there have been wide-spread public debates over public access for walkers, cyclists and vehicles in relation to the land rights and the safety of iwi/hapu (Māori tribes and sub-tribes) in Aotearoa/New Zealand. This includes checkpoints or road blocks set up by iwi/hapu in rural areas to stop the spread of the pandemic (Harris and Williams, 2020; Ngata, 2020), and the ongoing rāhui or temporary ban on all forms of public access in regional conservation forests by Te Kawerau ā Maki iwi of Tāmaki Makaurau/Auckland to stop the growth of the ecological pest kauri dieback by visitors. This article reflects on the complexities of these bans by considering public responses to them, including settler politics, discourses of rights and freedoms, and white supermacy. Consideration is given to the perspectives of iwi/hapu themselves and contingent histories (for instance Te Kawerau a Maki, 2020). This is reflected on through mana motuhake (authority, self-determination and agency), alongside Te Tiriti o Waitangi (Māori perspectives of the Treaty of Waitangi; Orange, 1987; Walker, 2004) and mātauranga Māori (Māori knowledge).

Variability in phosphite sensitivity observed within and between seven Phytophthora species

Phosphite is used to control and manage many phytophthora diseases in horticultural systems worldwide and natural ecosystems in Australia, Africa, New Zealand and parts of Northern America and Europe. Phosphite does not kill Phytophthora species, but inhibits growth while also stimulating host defence responses. Phytophthora species differ in their underlying tolerance to phosphite and isolates have been shown to acquire tolerance after prolonged exposure. Intra- and inter-specific variability in phosphite sensitivity is of interest to determine the efficacy and sustainability of phosphite for the treatment of phytophthora diseases, which continue to spread globally. Seven Phytophthora species were tested for their sensitivity to phosphite in vitro in a mycelial growth experiment. Phytophthora agathidicida was the species most sensitive to phosphite, being inhibited by 98.7% on average at the lowest phosphite treatment (15 µg/mL phosphite), followed by P. aleatoria, P. cinnamomi, P. pluvialis, P. multivora, P. kernoviae and P. citricola. Huge intraspecific variability was observed with P. kernoviae, which raises the question of whether diseases caused by P. kernoviae such as phytophthora needle blight of Pinus radiata could be managed effectively with phosphite. Further work is required to determine the phosphite sensitivity of different introduced and native Phytophthora species growing in key hosts and whether tolerance observed in vitro is also expressed in vivo.

Cross-Cultural Leadership Enables Collaborative Approaches to Management of Kauri Dieback in Aotearoa New Zealand

In Aotearoa/New Zealand, the soilborne pathogen Phytophthora agathidicida threatens the survival of the iconic kauri, and the ecosystem it supports. In 2011, a surveillance project to identify areas of kauri dieback caused by Phytophthora agathidicida within the Waitākere Ranges Regional Park (WRRP) highlighted the potential impact of the pathogen. A repeat of the surveillance in 2015/16 identified that approximately a quarter of the kauri area within the Regional Park was infected or possibly infected, an increase from previous surveys. The surveillance program mapped 344 distinct kauri areas and showed that 33.4% of the total kauri areas were affected or potentially affected by kauri dieback and over half (58.3%) of the substantial kauri areas (above 5 ha in size) were showing symptoms of kauri dieback. Proximity analysis showed 71% of kauri dieback zones to be within 50 m of the track network. Spatial analysis showed significantly higher proportions of disease presence along the track network compared to randomly generated theoretical track networks. Results suggest that human interaction is assisting the transfer of Phytophthora agathidicida within the area. The surveillance helped trigger the declaration of a cultural ban (rāhui) on recreational access. Te Kawerau ā Maki, the iwi of the area, placed a rāhui over the kauri forest eco-system of the Waitākere Forest (Te Wao Nui o Tiriwa) in December 2017. The purpose of the rāhui was to help prevent the anthropogenic spread of kauri dieback, to provide time for investment to be made into a degraded forest infrastructure and for research to be undertaken, and to help protect and support forest health (a concept encapsulated by the term mauri). Managing the spread and impact of the pathogen remains an urgent priority for this foundation species in the face of increasing pressures for recreational access. Complimentary quantitative and qualitative research programs into track utilization and ecologically sensitive design, collection of whakapapa seed from healthy and dying trees, and remedial phosphite treatments are part of the cross-cultural and community-enabled biosecurity initiatives to Kia Toitu He Kauri “Keep Kauri Standing”.

Assessing the effectiveness of oxathiapiprolin toward Phytophthora agathidicida, the causal agent of kauri dieback disease.

Phytophthora species cause disease and devastation of plants in ecological and horticultural settings worldwide. A recently identified species, Phytophthoraagathidicida, infects and ultimately kills the treasured kauri trees (Agathis australis) that are endemic to New Zealand. Currently, there are few options for managing kauri dieback disease. In this study, we sought to assess the efficacy of the oomycide oxathiapiprolin against several life cycle stages of two geographically distinct P. agathidicida isolates. The effective concentration to inhibit 50% of mycelial growth (EC50) was determined to be ∼0.1ng/ml, indicating that P. agathidicida mycelia are more sensitive to oxathiapiprolin than those from most other Phytophthora species that have been studied. Oxathiapiprolin was also highly effective at inhibiting the germination of zoospores (EC50 = 2-9ng/ml for the two isolates) and oospores (complete inhibition at 100ng/ml). In addition, oxathiapiprolin delayed the onset of detached kauri leaf infection in a dose-dependent manner. Collectively, the results presented here highlight the significant potential of oxathiapiprolin as a tool to aid in the control of kauri dieback disease.

Identification of Burkholderia and Penicillium isolates from kauri (Agathis australis) soils that inhibit the mycelial growth of Phytophthora agathidicida

Phytophthora agathidicida is a highly virulent pathogen of kauri (Agathis australis) and the causal agent of dieback disease in New Zealand’s kauri forests. This study aimed to identify microbial isolates isolated from kauri forest soils that inhibited the growth of P. agathidicida. Three different forms of in vitro bioassays were used to assess the inhibition of each isolate on the mycelial growth of P. agathidicida. Furthermore, head space (HS) solid-phase micro-extraction coupled with gas chromatography-mass spectrometry (SPME-GCMS) was performed to identify if the microbial isolates emitted volatile organic compounds (VOCs), which may be contributing to inhibition. This research identified several bacterial isolates belonging to the genus Burkholderia that inhibited the mycelial growth of P. agathidicida. Furthermore, several VOCs produced by these isolates were putatively identified, which may be responsible for the inhibition observed in the bioassays. Several isolates of Penicillium were identified that inhibit Phytophthora agathidicida, with the culture filtrate of one isolate being found to strongly inhibit P. agathidicida mycelial growth. These isolates of Burkholderia and Penicillium appear to exhibit multiple modes of antagonism against P. agathidicida, including microbial competition and the production of diffusible and volatile anti-microbial compounds. Although further research is needed to better define their mechanisms of inhibition, these findings have identified candidate microbial antagonists of P. agathidicida.

The Fatty Acid Methyl Ester (FAME) profile of Phytophthora agathidicida and its potential use as diagnostic tool.

ABSTRACTPhytophthora diseases cause devastation to crops and native ecosystems worldwide. In New Zealand, Phytophthora agathidicida is threatening the survival of kauri, an endemic, culturally and ecologically important tree species. The current method for detecting P. agathidicida is a soil bating assay that is time-consuming and requires high levels of expertise to assess, thus limiting the analytical sample throughput. Here, we characterized the fatty acid methyl ester (FAME) profile of P. agathidicida. We also compared it with the FAME profile of P. cinnamomi and assessed the efficacy of FAME analysis as a diagnostic tool for detecting the pathogen in soil samples. In FAME analysis, the total fatty acid content is isolated from a sample and converted to FAMEs for analysis, a process that takes less than a day. Unique fatty acid acyl chains can serve as biomarkers for specific organisms. We detected 12 fatty acids in P. agathidicida, two of which (20:4ω6 and 20:5ω3) show promise as potential Phytophthora specific biomarkers. Collectively, these findings advance our fundamental understanding of P. agathidicida biology and provide a promising technique to increase the rate of sample processing and the speed of pathogen detection for P. agathidicida in soil.

The response of soil microbial communities to the infection of kauri (Agathis australis) seedlings with Phytophthora agathidicida

AbstractNew Zealand's ancient kauri (Agathis australis) forests are under threat from the spread of dieback disease, caused by the soil‐borne pathogen Phytophthora agathidicida. Characterizing the response of the soil microbiota to the infection of kauri with P. agathidicida is essential to identify how they may form a protective response to pathogen invasion and disease expression. This study infected 18‐month‐old kauri seedlings with a standardized inoculum load of P. agathidicida for 6 weeks under controlled environmental conditions. Following this, changes in the diversity, composition and biomass of soil microbial communities associated with kauri seedlings were characterized using high‐throughput 16S rRNA and ITS gene region sequencing and phospholipid fatty acid analysis. Significant differences were found in the composition of soil bacterial communities associated with inoculated and non‐inoculated kauri seedlings. Furthermore, soils of inoculated seedlings had a significantly higher relative abundance of bacteria previously reported to be associated with plant disease suppression, which included several members of the Firmicutes. Significant reductions were found in the fungal: bacterial biomass of soils from inoculated seedlings. This finding contrasts to previous field‐based research that observed an increased diversity of soil fungal communities associated with diseased kauri in old growth kauri forests. Further research that aims to isolate members of the kauri soil microbiota and study their interactions with P. agathidicida is required for us to begin selecting potential biocontrol agents against kauri dieback.

The mitogenome of Phytophthora agathidicida: Evidence for a not so recent arrival of the "kauri killing" Phytophthora in New Zealand.

Phytophthora agathidicida is associated with a root rot that threatens the long-term survival of the iconic New Zealand kauri. Although it is widely assumed that this pathogen arrived in New Zealand post-1945, this hypothesis has yet to be formally tested. Here we describe evolutionary analyses aimed at evaluating this and two alternative hypotheses. As a basis for our analyses, we assembled complete mitochondrial genome sequences from 16 accessions representing the geographic range of P. agathidicida as well as those of five other members of Phytophthora clade 5. All 21 mitogenome sequences were very similar, differing little in size with all sharing the same gene content and arrangement. We first examined the temporal origins of genetic diversity using a pair of calibration schemes. Both resulted in similar age estimates; specifically, a mean age of 303.0-304.4 years and 95% HPDs of 206.9-414.6 years for the most recent common ancestor of the included isolates. We then used phylogenetic tree building and network analyses to investigate the geographic distribution of the genetic diversity. Four geographically distinct genetic groups were recognised within P. agathidicida. Taken together the inferred age and geographic distribution of the sampled mitogenome diversity suggests that this pathogen diversified following arrival in New Zealand several hundred to several thousand years ago. This conclusion is consistent with the emergence of kauri dieback disease being a consequence of recent changes in the relationship between the pathogen, host, and environment rather than a post-1945 introduction of the causal pathogen into New Zealand.

Screening a Natural Product-Inspired Library for Anti-Phytophthora Activities.

Phytophthora is a genus of microorganisms that cause devastating dieback and root-rot diseases in thousands of plant hosts worldwide. The economic impact of Phytophthora diseases on crops and native ecosystems is estimated to be billions of dollars per annum. These invasive pathogens are extremely difficult to control using existing chemical means, and the effectiveness of the few treatments available is being jeopardized by increasing rates of resistance. There is an urgent need to identify new chemical treatments that are effective against Phytophthora diseases. Natural products have long been regarded as “Nature’s medicine chest”, providing invaluable leads for developing front-line drugs and agrochemical agents. Here, we have screened a natural product-inspired library of 328 chemicals against two key Phytophthora species: Phytophthora cinnamomi and Phytophthora agathidicida. The library was initially screened for inhibition of zoospore germination. From these screens, we identified twenty-one hits that inhibited germination of one or both species. These hits were further tested in mycelial growth inhibition studies to determine their half-maximal inhibitory concentrations (IC50s). Four compounds had IC50 values of approximately 10 µM or less, and our best hit had IC50s of approximately 3 µM against both Phytophthora species tested. Overall, these hits may serve as promising leads for the development of new anti-Phytophthora agrochemicals

Impact of Phytophthora agathidicida infection on canopy and forest floor plant nutrient concentrations and fluxes in a kauri-dominated forest.

Kauri dieback, caused by Phytophthora agathidicida, is a biotic disturbance that poses a recent threat to the survival of kauri (Agathis australis) forests in New Zealand. Previous studies have shown that throughfall and stemflow play an important role in the kauri forests’ internal nutrient cycle. However, the effects of P. agathidicida infection on canopy and forest floor nutrient fluxes in kauri forests remain unknown. Here, we measured throughfall, stemflow and forest floor water yield, nutrient (potassium, calcium, magnesium, manganese, silicon, sulfur, sodium, iron) concentrations and fluxes of ten kauri trees differing in soil P. agathidicida DNA concentration, and health status. We did not observe an effect of soil P. agathidicida DNA concentration on throughfall, stemflow, and forest floor water yield. Throughfall and forest floor nutrient concentrations and fluxes decreased (up to 50%) with increasing soil P. agathidicida DNA concentration. We found significant effects on potassium and manganese fluxes in throughfall; calcium and silicon fluxes in forest floor leachate. A decline in canopy and forest floor nutrient fluxes may result in soil nutrient imbalances, which in turn may negatively impact forest productivity and may increase the susceptibility of trees to future pathogen infection in these ecologically unique kauri forests. Given our findings and the increasing spread of Phytophthora species worldwide, research on the underlying physiological mechanisms linking dieback and plant–soil nutrient fluxes is critical.

Early infection by Phytophthora agathidicida up‐regulates photosynthetic activity in Agathis australis seedlings

AbstractNew Zealand kauri (Agathis australis) trees are culturally, socially and ecologically significant within northern New Zealand's nutrient‐poor podocarp forest ecosystems. Phytophthora agathidicida is an aggressive oomycete pathogen, capable of killing A. australis across its ecological range, causing the disease known as kauri dieback. The pathogen, like many other forest Phytophthora species, commences as a fine root rot which progresses to collar rot and resinous cankers on the lower trunk. In this study, we investigated the eco‐physiological and fine root responses of kauri to infection by P. agathidicida and tested whether the foliar application of micronutrients (manganese and zinc combined, iron alone, or a trace element mix) affects dieback disease expression. Fortnightly assessments of chlorophyll‐a‐fluorescence were conducted over 12 weeks, and fine root length and tip numbers were recorded at the end of the experiment. None of the micronutrient treatments had a significant physiological effect regardless of pathogen infection. However, contrary to expectations, pathogen infection caused a significant upregulation in photosynthetic activity over time, as the electron transport rate of infected plants was approximately 26% higher than that of control plants at the end of the trial. These results indicate that an increase in the strength of the below‐ground carbon sink through pathogen consumption of labile carbohydrates (sugars), together with pathogen‐induced root damage, triggered upregulation of photosynthesis in the seedlings. Understanding how P. agathidicida infection affects the physiology and resource allocation in kauri is critical to determining the disease aetiology and management options.

Reducing risk to wild ecosystems in nursery production

The implication of the nursery trade in the historic movement of the pathogen that causes kauri dieback (Phytophthora agathidicida) instigated a review of nursery biosecurity procedures at Auckland Botanic Gardens. The nursery grows 65 000 native plants a year which are planted at restoration sites throughout Auckland. Threatened plants are also grown in the nursery and returned to the wild. The nursery biosecurity project resulted in a manual of protocols, supported by infrastructure upgrades, to ensure the nursery takes all practicable steps to ensure there is as little risk as possible to natural ecosystems through nursery practices. This work also demonstrates the important role of Botanic Gardens in providing education and advocacy for such schemes in all nurseries.

Soil microbial community restructuring and functional changes in ancient kauri (Agathis australis) forests impacted by the invasive pathogen Phytophthora agathidicida

New Zealand's culturally iconic, ancient kauri (Agathis australis) forests are threatened with extinction as a result of dieback caused by an invasive and highly virulent soil-borne pathogen (Phytophthora agathidicida). Kauri trees function as a foundation species in their forests, supporting an ecologically distinct plant and soil environment. The impacts of disease outbreak and subsequent tree dieback on kauri forest ecosystems, in particular soil microbial communities, remains unknown. Using high throughput amplicon sequencing and GeoChip 5 S microarray analysis we analysed the differences in diversity, taxonomic composition and functional genes related to C and N cycling of soil microbial communities associated with asymptomatic and symptomatic mature kauri. Our results found significant differences in the fungal diversity and both fungal and bacterial community composition between asymptomatic and symptomatic kauri. Several microbial taxa known in the literature for their roles in disease suppression, such as Penicillium, Trichoderma, Enterobacteriacae and Pseudomonas, were identified as being significantly higher in asymptomatic kauri soils. These findings have provided a promising direction for the discovery of disease suppressive microbial taxa against kauri dieback. In addition, the composition and abundances of microbial genes related to C and N cycling also differed significantly. These findings highlight the potential long term impacts that dieback disease may have on the health and functioning of kauri forests.

Functional analysis of RXLR effectors from the New Zealand kauri dieback pathogen Phytophthora agathidicida.

New Zealand kauri is an ancient, iconic, gymnosperm tree species that is under threat from a lethal dieback disease caused by the oomycete Phytophthora agathidicida. To gain insight into this pathogen, we determined whether proteinaceous effectors of P. agathidicida interact with the immune system of a model angiosperm, Nicotiana, as previously shown for Phytophthora pathogens of angiosperms. From the P. agathidicida genome, we defined and analysed a set of RXLR effectors, a class of proteins that typically have important roles in suppressing or activating the plant immune system. RXLRs were screened for their ability to activate or suppress the Nicotiana plant immune system using Agrobacterium tumefaciens transient transformation assays. Nine P. agathidicida RXLRs triggered cell death or suppressed plant immunity in Nicotiana, of which three were expressed in kauri. For the most highly expressed, P. agathidicida (Pa) RXLR24, candidate cognate immune receptors associated with cell death were identified in Nicotiana benthamiana using RNA silencing‐based approaches. Our results show that RXLRs of a pathogen of gymnosperms can interact with the immune system of an angiosperm species. This study provides an important foundation for studying the molecular basis of plant–pathogen interactions in gymnosperm forest trees, including kauri.

Stress Detection in New Zealand Kauri Canopies with WorldView-2 Satellite and LiDAR Data

New Zealand kauri trees are threatened by the kauri dieback disease (Phytophthora agathidicida (PA)). In this study, we investigate the use of pan-sharpened WorldView-2 (WV2) satellite and Light Detection and Ranging (LiDAR) data for detecting stress symptoms in the canopy of kauri trees. A total of 1089 reference crowns were located in the Waitakere Ranges west of Auckland and assessed by fieldwork and the interpretation of aerial images. Canopy stress symptoms were graded based on five basic stress levels and further refined for the first symptom stages. The crown polygons were manually edited on a LiDAR crown height model. Crowns with a mean diameter smaller than 4 m caused most outliers with the 1.8 m pixel size of the WV2 multispectral bands, especially at the more advanced stress levels of dying and dead trees. The exclusion of crowns with a diameter smaller than 4 m increased the correlation in an object-based random forest regression from 0.85 to 0.89 with only WV2 attributes (root mean squared error (RMSE) of 0.48, mean absolute error (MAE) of 0.34). Additional LiDAR attributes increased the correlation to 0.92 (RMSE of 0.43, MAE of 0.31). A red/near-infrared (NIR) normalised difference vegetation index (NDVI) and a ratio of the red and green bands were the most important indices for an assessment of the full range of stress symptoms. For detection of the first stress symptoms, an NDVI on the red-edge and green bands increased the performance. This study is the first to analyse the use of spaceborne images for monitoring canopy stress symptoms in native New Zealand kauri forest. The method presented shows promising results for a cost-efficient stress monitoring of kauri crowns over large areas. It will be tested in a full processing chain with automatic kauri identification and crown segmentation.

Soil microbial diversity in adjacent forest systems - contrasting native, old growth kauri (Agathis australis) forest with exotic pine (Pinus radiata) plantation forest.

ABSTRACTGlobally, the conversion of primary forests to plantations and agricultural landscapes is a common land use change. Kauri (Agathis australis) is one of the most heavily impacted indigenous tree species of New Zealand with <1% of primary forest remaining as fragments adjacent to pastoral farming and exotic forest plantations. By contrasting two forest systems, we investigated if the fragmentation of kauri forests and introduction of pine plantations (Pinus radiata) are significantly impacting the diversity and composition of soil microbial communities across Waipoua kauri forest, New Zealand. Using next generation based 16S rRNA and ITS gene region sequencing, we identified that fungal and bacterial community composition significantly differed between kauri and pine forest soils. However, fungal communities displayed the largest differences in diversity and composition. This research revealed significant shifts in the soil microbial communities surrounding remnant kauri fragments, including the loss of microbial taxa with functions in disease suppression and plant health. Kauri dieback disease, caused by Phytophthora agathidicida, currently threatens the kauri forest ecosystem. Results from this research highlight the need for further investigations into how changes to soil microbial diversity surrounding remnant kauri fragments impact tree health and disease expression.

Monitoring of Canopy Stress Symptoms in New Zealand Kauri Trees Analysed with AISA Hyperspectral Data

The endemic New Zealand kauri trees (Agathis australis) are under threat by the deadly kauri dieback disease (Phytophthora agathidicida (PA)). This study aimed to identify spectral index combinations for characterising visible stress symptoms in the kauri canopy. The analysis is based on an aerial AISA hyperspectral image mosaic and 1258 reference crowns in three study sites in the Waitakere Ranges west of Auckland. A field-based assessment scheme for canopy stress symptoms (classes 1–5) was further optimised for use with RGB aerial images. A combination of four indices with six bands in the spectral range 450–1205 nm resulted in a correlation of 0.93 (mean absolute error 0.27, RMSE 0.48) for all crown sizes. Comparable results were achieved with five indices in the 450–970 nm region. A Random Forest (RF) regression gave the most accurate predictions while a M5P regression tree performed nearly as well and a linear regression resulted in slightly lower correlations. Normalised Difference Vegetation Indices (NDVI) in the near-infrared / red spectral range were the most important index combinations, followed by indices with bands in the near-infrared spectral range from 800 to 1205 nm. A test on different crown sizes revealed that stress symptoms in smaller crowns with denser foliage are best described in combination with pigment-sensitive indices that include bands in the green and blue spectral range. A stratified approach with individual models for pre-segmented low and high forest stands improved the overall performance. The regression models were also tested in a pixel-based analysis. A manual interpretation of the resulting raster map with stress symptom patterns observed in aerial imagery indicated a good match. With bandwidths of 10 nm and a maximum number of six bands, the selected index combinations can be used for large-area monitoring on an airborne multispectral sensor. This study establishes the base for a cost-efficient, objective monitoring method for stress symptoms in kauri canopies, suitable to cover large forest areas with an airborne multispectral sensor.

A LAMP at the end of the tunnel: A rapid, field deployable assay for the kauri dieback pathogen, Phytophthora agathidicida.

The root rot causing oomycete, Phytophthora agathidicida, threatens the long-term survival of the iconic New Zealand kauri. Currently, testing for this pathogen involves an extended soil bioassay that takes 14-20 days and requires specialised staff, consumables, and infrastructure. Here we describe a loop-mediated isothermal amplification (LAMP) assay for the detection of P. agathidicida that targets a portion of the mitochondrial apocytochrome b coding sequence. This assay has high specificity and sensitivity; it did not cross react with a range of other Phytophthora isolates and detected as little as 1 fg of total P. agathidicida DNA or 116 copies of the target locus. Assay performance was further investigated by testing plant tissue baits from flooded soil samples using both the extended soil bioassay and LAMP testing of DNA extracted from baits. In these comparisons, P. agathidicida was detected more frequently using the LAMP test. In addition to greater sensitivity, by removing the need for culturing, the hybrid baiting plus LAMP approach is more cost effective than the extended soil bioassay and, importantly, does not require a centralised laboratory facility with specialised staff, consumables, and equipment. Such testing will allow us to address outstanding questions about P. agathidicida. For example, the hybrid approach could enable monitoring of the pathogen beyond areas with visible disease symptoms, allow direct evaluation of rates and patterns of spread, and allow the effectiveness of disease control to be evaluated. The hybrid LAMP bioassay also has the potential to empower local communities to evaluate the pathogen status of local kauri stands, providing information for disease management and conservation initiatives.