T. Paap

Multiple new Phytophthora species from ITS Clade 6 associated with natural ecosystems in Australia: evolutionary and ecological implications

During surveys of dying vegetation in natural ecosystems and associated waterways in Australia many new taxa have been identified from Phytophthora ITS Clade 6. For representative isolates, the region spanning the internal transcribed spacer region of the ribosomal DNA, the nuclear gene encoding heat shock protein 90 and the mitochondrial cox1 gene were PCR amplified and sequenced. Based on phylogenetic analysis and morphological and physiological comparison, four species and one informally designated taxon have been described; Phytophthora gibbosa, P. gregata, P. litoralis, P. thermophila and P. taxon paludosa. Phytophthora gibbosa, P. gregata and P. taxon paludosa form a new cluster and share a common ancestor; they are homothallic and generally associated with dying vegetation in swampy or water-logged areas. Phytophthora thermophila and P. litoralis are sister species to each other and more distantly to P. gonapodyides. Both new species are common in waterways and cause scattered mortality within native vegetation. They are self-sterile and appear well adapted for survival in an aquatic environment and inundated soils, filling the niche occupied by P. gonapodyides and P. taxon salixsoil in the northern hemisphere. Currently the origin of these new taxa, their pathogenicity and their role in natural ecosystems are unknown. Following the precautionary principle, they should be regarded as a potential threat to native ecosystems and managed to minimise their further spread.

Distribution and diversity of Phytophthora across Australia

The introduction and subsequent impact of Phytophthora cinnamomi within native vegetation is one of the major conservation issues for biodiversity in Australia. Recently, many new Phytophthora species have been described from Australia’s native ecosystems; however, their distribution, origin, and potential impact remain unknown. Historical bias in Phytophthora detection has been towards sites showing symptoms of disease, and traditional isolation methods show variable effectiveness of detecting different Phytophthora species. However, we now have at our disposal new techniques based on the sampling of environmental DNA and metabarcoding through the use of high-throughput sequencing. Here, we report on the diversity and distribution of Phytophthora in Australia using metabarcoding of 640 soil samples and we compare the diversity detected using this technique with that available in curated databases. Phytophthora was detected in 65% of sites, and phylogenetic analysis revealed 68 distinct Phytophthora phylotypes. Of these, 21 were identified as potentially unique taxa and 25 were new detections in natural areas and/or new introductions to Australia. There are 66 Phytophthora taxa listed in Australian databases, 43 of which were also detected in this metabarcoding study. This study revealed high Phytophthora richness within native vegetation and the additional records provide a valuable baseline resource for future studies. Many of the Phytophthora species now uncovered in Australia’s native ecosystems are newly described and until more is known we need to be cautious with regard to the spread and conservation management of these new species in Australia’s unique ecosystems.

Urban trees: bridge-heads for forest pest invasions and sentinels for early detection

Urban trees have been increasingly appreciated for the many benefits they provide. As concentrated hubs of human-mediated movement, the urban landscape is, however, often the first point of contact for exotic pests including insects and plant pathogens. Consequently, urban trees can be important for accidentally introduced forest pests to become established and potentially invasive. Reductions in biodiversity and the potential for stressful conditions arising from anthropogenic disturbances can predispose these trees to pest attack, further increasing the likelihood of exotic forest pests becoming established and increasing in density. Once established in urban environments, dispersal of introduced pests can proceed to natural forest landscapes or planted forests. In addition to permanent long-term damage to natural ecosystems, the consequences of these invasions include costly attempts at eradication and post establishment management strategies. We discuss a range of ecological, economic and social impacts arising from these incursions and the importance of global biosecurity is highlighted as a crucially important barrier to pest invasions. Finally, we suggest that urban trees may be viewed as ‘sentinel plantings’. In particular, botanical gardens and arboreta frequently house large collections of exotic plantings, providing a unique opportunity to help predict and prevent the invasion of new pests, and where introduced pests with the capacity to cause serious impacts in forest environments could potentially be detected during the initial stages of establishment. Such early detection offers the only realistic prospect of eradication, thereby reducing damaging ecological impacts and long term management costs.

The ‘chicken or the egg’: which comes first, forest tree decline or loss of mycorrhizae?

Forest trees are experiencing massive declines globally caused by a multitude of stressors, both abiotic (pollution, fragmentation and climate change) and biotic (fungi, bacteria, viruses and insects). Mycorrhizal fungi aid plants in the requisition of nutrients through their mutualistic relationship with plant roots and are integral to tree health. Stresses affecting tree health will also influence mycorrhizal fungi directly or indirectly, and thus alter the pathways responsible for nutrient absorption. Such an intimate association is a true chicken or egg quandary; do external stressors cause a loss of mycorrhizae which leads to tree decline, does tree decline result in a loss of mycorrhizae, or is it a combination of both? A review of literature has identified six stressors known to contribute to tree decline and to impact directly on mycorrhizae; global climate change, pesticides, heavy metals, excess fertilizer, pathogens and habitat fragmentation. A few review papers have highlighted the link; however, what is missing is irrefutable empirical research. This review documents the known direct impacts of the six stressors on mycorrhizal communities and places this in the context of decline syndromes in long-lived forest trees. We also discuss methodologies available to identify fungi and future research needed to unravel the complex relationships between forest tree declines and their associated mycorrhizal fungi.

Phytophthora Contamination in a Nursery and Its Potential Dispersal into the Natural Environment

A detailed site investigation of a eucalypt nursery suffering disease losses revealed the causal agent to be Phytophthora boodjera. The pathogen was detected in vegetation surrounding the nursery production area, including the lawn, under the production benches during the growing season, and, most importantly, from plant debris in used trays. However, it was not found in the container substrate, water supplies, or production equipment or on the workers themselves. The sterilization methods used by the nursery were shown to be ineffective, indicating that a more rigorous method was required. Boiling trays for 15 min or steaming at 65°C for 60 min eradicated P. boodjera. This pathogen was more pathogenic to the eucalypts tested in their early seedling stage than P. cinnamomi. Tracing of out-planting to revegetation sites showed that P. boodjera was able to spread into the environment. Dispersal via out-planting to native vegetation may affect seedling recruitment and drive long-term shifts in native plant species. Inadequate nursery hygiene increases the risk of an outbreak and can limit the success of biosecurity efforts as well as damage conservation efforts.

The polyphagous shot hole borer (PSHB) and its fungal symbiont Fusarium euwallaceae: a new invasion in South Africa

The polyphagous shot hole borer (PSHB), an ambrosia beetle (Coleoptera: Curculeonidae: Scolytinae) native to Asia, together with its fungal symbiont Fusarium euwallaceae, has emerged as an important invasive pest killing avocado and other trees in Israel and the United States. The PSHB is one of three cryptic species in the Euwallacea fornicatus species complex, the taxonomy of which remains to be resolved. The surge in the global spread of invasive forest pests such as the PSHB has led to the development of programmes utilising sentinel tree plantings to record new host-pest interactions. During routine surveys of tree health in botanical gardens of South Africa undertaken as part of a sentinel project, an ambrosia beetle/fungal associate was detected damaging Platanus x acerifolia (London Plane) in the KwaZulu-Natal National Botanical Gardens, Pietermaritzburg. Identification of the beetle by sequencing part of the mitochondrial cytochrome oxidase c subunit 1 (COI) gene confirmed its identity as PSHB, and specifically one of the invasive haplotypes of the beetle. The associated fungus F. euwallaceae was identified based on phylogenetic analysis of elongation factor (EF 1-α) sequences. Koch’s postulates have confirmed the pathogenicity of fungal isolates to P. x acerifolia. This is the first report of PSHB and its fungal symbiont causing Fusarium dieback in South Africa. This report also represents the first verified case of a damaging invasive forest pest detected in a sentinel planting project, highlighting the importance of such studies. Given the potential impact these species present to urban trees, native biodiversity and agriculture, both the PSHB and its fungal symbiont should be included in invasive species regulations in South Africa.

Phytophthora versiformis sp. nov., a new species from Australia related to P. quercina

During routine surveys of Corymbia calophylla, a widespread tree currently experiencing decline over a large area of its native range of south west Western Australia, a slow growing Phytophthora species was often recovered from roots and rhizosphere soil. This species grew more slowly, produced a wide array of morphological features and was visibly different to all other Phytophthora species present in Western Australia. Phylogenetic analyses of the ITS, cox1, HSP90, β-tubulin and NADH gene regions confirmed this to be a new species closely related to P. quercina and the provisional species P. ‘ohioensis’. The new species is described here as Phytophthora versiformis sp. nov. It produces persistent, papillate sporangia of variable shape, oogonia with thick-walled oospores, and paragynous antheridia. To date this species has only been recovered from C. calophylla. Pathogenicity trials indicate that while P. versiformis can infect the roots of C. calophylla it does not lead to seedling death.