Giles Hardy

The most important oomycete forest pathogens comprise two genera: Pythium and the formidable genus Phytophthora, whose name appropriately means 'plant destroyer'. Pythium spp. cause seed and root rots and damping off diseases that thwart seedling establishment, and have been implicated in helping to drive forest diversity patterns through increased disease pressures on seedlings closest to their mother tree (Janzen, 1970; Connell, 1971). In contrast, Phytophthora spp. can cause disease at every life stage of forest trees, from root to crown, and from trunk cankers to foliar blights (Erwin and Ribeiro, 1996). They are remarkably flexible and effective pathogens with an unusual genetic architecture that may favour the rapid evolution of pathogenicity (Jiang et al., 2008; Raffaele et al., 2010; Seidl et al., 2011). Outbreaks of disease caused by Phytophthora spp. (especially when they have been introduced to new systems) have been documented with dramatic, and sometimes disastrous, effects since the mid 1800s, when European and North American chestnuts (Castanea spp.) began dramatic declines from chestnut ink disease, a root rot caused in part by the extreme generalist Phytophthora cinnamomi Rands (Crandall et al., 1945; Anagnostakis, 1995). P. cinnamomi is notorious for the massive mortality it has caused in jarrah (Eucalyptus marginata Donn ex Sm.) forests in Western Australia, where it was first observed in the 1920s (Podger, 1972). P. cinnamomi causes root disease in agricultural and forest systems worldwide with varying degrees of virulence, but as Phytophthora dieback it has been seen to kill 50-75% of the species in sites in Western Australia, in some cases leaving every tree and much of the understorey dead (Weste, 2003). Shearer et al. (2004) estimate that of the 5710 described plant species in the South West Botanical Province of Western Australia, approximately 2300 species are susceptible, 800 of which are highly susceptible.

As far back as the 1920s patches of dead trees were visible in the hills surrounding Perth, Western Australia (Dell et al., 2005.). By 1964, when the causal agent was identified as Phytophthora cinnamomi, the disease had spread and was causing widespread decline of the dominant forest species Eucalyptus marginata (jarrah) (see Hee et al., Chapter 14, this volume). The disease is known as 'jarrah dieback'; a particularly misleading title for a disease that has decimated extensive regions in this fragile biodiversity hotspot. In Western Australia P. cinnamomi is known as a biological bulldozer and 2284 of the 5710 described plant species are susceptible or highly susceptible (Shearer et al., 2004) (see Hee et al., Chapter 14, this volume). This is just one example of the impact caused by invasive Phytophthora species and there are many additional examples from natural ecosystems, agriculture and agroforestry worldwide. The common thread is human-mediated movement, and the origin of many species remains a mystery.

Phytophthora cinnamomi has recently been recognized as a key threatening process to biodiversity in Australia. The impact of this introduced microscopic water mould on destruction of forests and heath land communities has been observed since 1921 in southwestern Australia. It took over 40 years for the causal agent to be identified in 1964. Over the next 40 years, State Government Departments formulated policy and implemented management measures to deal with the problem. These measures have changed greatly over time as new knowledge about the host range and extent of the epidemic have become available. Unfortunately, the pathogen had spread over large areas of estate prior to the identification of the causal agent and the development of a management response. The spread of P.cinnamomi into significant areas of the conservation estate, including biodiversity hotspots, highlights the urgency of ensuring that Phytophthora dieback and its management is adequately resourced and is underpinned by appropriate research and communication programs. This review describes the main historical events leading up to the formulation of the 2004 State Phytophthora Dieback Response Framework. These include: quarantining half a million hectares of State Forest in 1976/7 in order to map the extent of the disease and implement hygiene measures; developing policy and management practices for the conservation estate; the acceptance by the Hon. Minister for the Environment in 1996 of the 33 recommendations in the WA Dieback Review Panel Report; the establishment of community based Dieback Working Groups; the preparation of the National Threat Abatement Plan in 2001, and in 2004 the development of National Best Practices Management Guidelines and a risk assessment methodology suitable for national adoption. In spite of these actions, much remains to be done. Flora and fauna remain threatened by the continued expansion and impact of Phytophthora dieback. We have few tools available to reduce the extension, spread and impact of the pathogen and the diseases it causes. The community needs to be better informed of the direct and indirect impacts this disease has had on individual species and ecosystem function and health, and encouraged to take greater ownership of an environmental problem that encompasses all types of land tenure. Recent developments in policy development are encouraging but need to be underpinned by much further research and collaboration.

The condition of the tuart tree (Eucalyptus gomphocephala), a coastal southwestern Australian woodland species, has declined dramatically within parts of its distribution over the last decade, particularly within Yalgorup National Park. Prior to the park being gazetted in 1968, some of the woodlands were used for cattle grazing. Frequent, light, understorey burns were carried out to encourage grass fodder growth. Earlier, Aboriginal use is believed to have involved a similar regime to facilitate hunting and access. Since gazettal, the majority of the park has either been excluded from fire, or burnt infrequently by wildfire and prescribed fire. Consequently, from 1968 to the present, most fires are thought to have been more intense due to increased fuel loads. Alterations in disturbance patterns (particularly fire) elsewhere, have been linked with vegetation changes (composition and structure) and in some instances, declining tree health. For tuart woodland, it has been proposed that increased abundance and vigour of the lower storey peppermint tree (Agonis flexuosa) and a decline in the health of tuart trees are consequences of reduced fire frequency. Sample plot data from the mid – late 1970s and photographs from 1957 are contrasted with the 2003/2004 situation to describe changes in tuart woodland. Declining tuart health, changes in the health and abundance of some understorey species (for example, fewer Banksia attenuata) and a shift towards peppermint dominance are revealed. The contribution of changing fire regimes to these trends is explored. While a link between fire and changes to the woodland may be established, factors underlying the loss of tuart dominance remain to be determined. An integrated research project is in progress to examine the range of decline factors.

As a genus Phytophthora can be considered as the most devastating group of plant pathogens on earth. Members of the genus cause huge economic losses in agricultural crops annually and are extremely destructive in a range of forest ecosystems worldwide. Their direct affect on plant losses is really quite well documented. The genus name is aptly derived from the Greek that means phyto (plant) and phthora (destroyer). There is still much debate over the taxonomy of Phytophthora and there are over 64 species, but this constantly changes with new species being described and others amalgamated as molecular diagnostics tools are used to characterize the genus. Some species such as P. cinnamomi have a wide host range whilst others have one to only a few hosts, consequently as a genus it is extremely plastic in terms of the range of plant species it impacts on. This article will provide a general but not a comprehensive overview of the major Phytophthora diseases of forest trees and cover impacts, threats, and methods of control.

Although the impacts of the introduced plant pathogen Phytophthora cinnamomi on Australian forest flora are well documented, the indirect impact on forest fauna through changes in floristics and plant structure are less clear. A review of the literature on the responses of forest faunal communities to P. cinnamomi suggests there is evidence of declines in abundance and distribution of mammals and invertebrates. Mammals were mostly affected by reductions in shelter and food resources, while invertebrates probably declined because of changes in litterfall patterns. Mammal declines could also disrupt important ecosystem processes by reducing burrowing and digging which are vital to soil turnover and plant propagation. The trends for birds and herpetofauna were less clear. Overall, the review indicated that the impacts of P. cinnamomi infestations were serious for a range of forest fauna, or were plausible but not yet demonstrated. This highlights the need for an integrated approach to managing P. cinnamomi, which adopts both precautionary and preventive measures.

The eucalypt plantation forestry industry in China has expanded rapidly in the last decade. With this expansion, it is expected that pest and disease problems will increase. Devastating eucalypt pathogens already present in Asia include Phytophthora cinnamomi, Cryphonectria cubensis, Coniothyrium spp., Botryosphaeria spp., Mycosphaerella spp. and Cylindrocladium spp. There is an urgent need to train Chinese foresters and scientists to recognise eucalypt diseases and to establish a database for easy and rapid identification of disease problems. Good identification procedures in association with quarantine and breeding programs should help to reduce the impact of eucalypt diseases in Chinese plantations. This article gives an overview of the potential major threats to the Chinese eucalypt plantation industry and some strategies to manage disease incursions and their spread.

Plant species recovery programs are enacted when naturally occurring diversity of a species or population falls close to or below what is considered to be a sufficient size for the species to continue to exist without human intervention. The purpose of these plans is to ensure the long-term survival of the (axon concerned, and where possible, to re-establish self-sustaining populations in their natural habitat. In such an endeavour to increase plant numbers, there exists a risk that seed, soil, machinery and plant material used in the introduction of plants to native habitats will include the introduction of phytopathogens capable of destroying the very population that we are attempting to increase. Alternatively, changes to environmental conditions created by the activity of the program may favour disease development by native or naturalised pathogens. For these reasons, phytosanitation (literally, plant-health-process) procedures are employed to minimise the threat of pathogens already at a site and, more importantly, to prevent infected or infested material entering a site.