Summary

摘要

The total area planted to tropical acacias is now approaching 2 million ha. The largest of these estates (about 1.2 million ha) is in Indonesia where the major species planted is Acacia mangium Willd. Acacia mangium is generally grown as an exotic. It was first introduced into Malaysia in 1966 and into Indonesia in 1979 where it was used in South Sumatra as a fire break, for land rehabilitation and for reforestation of alang-alang grassland. Its excellent adaptability to degraded sites, rapid growth and wood properties quickly led to its commercial exploitation. In Indonesia, pulp wood production from A. mangium plantations is currently more than 9 million m~3/year, while the potential for solid-wood production is around 165,000 m~3/year. Thus, domestication of the species has progressed rapidly with extensive research on its genetics, silviculture and wood utilisation. These proceedings add a new focus by providing an insight into two diseases that threaten the sustainability of pulp and solid-wood industries based on tropical acacias. The sustainability of planted forests in the tropics is threatened by their improper management, fire, and the illegal logging of native forests. Combating these threats is a challenge that should be met through effective forest management and a multi-stakeholder approach: the latter, in particular, is essential in the drive against illegal logging. Sustainable management therefore embraces the interests of the planet -conserving biodiversity and preventing environmental degradation; the people - providing opportunities for social development and poverty alleviation; and profit - ensuring a steady supply of renewable, high-quality, internationally cost-effective fibre. These interests are all threatened if monocultures are not managed to minimise the effects of diseases on sustained production across rotations. Heart rot is a major disease problem in A. mangium, particularly where trees are grown for solid-wood products. Heart-rot fungi are wound parasites that enter through broken branches and branch stubs after self pruning, singling and manual pruning. The fungi attack cellulose and lignin, causing a typical white rot that is associated with changes in colour, texture and quality of the wood. These changes have been used to rapidly assess the incidence and severity of heart rot on harvested log-ends in the field. An assessment of two trials within different commercial growing areas in Indonesia (Riau and South Sumatra) showed that provenance could influence the incidence of heart rot. The basis for this observation remains unclear, as heart-rot incidence was not correlated with the content of polyphenolic wood extractives which are a known antifungal defence. Root-rot diseases of A. mangium are associated with crown dieback, reduced growth and tree death. When first detected, infected trees or disease foci tend to be randomly distributed but then enlarge and may aggregate. The rate of disease progress appears to be positively correlated with current levels of root rot. Accurate surveys to investigate spread are required and should record above-ground symptoms, inspect the extent of root infection, and observe patterns of disease infection. It is recognised that such surveys are operationally laborious and costly. Hence, survey options based on remote sensing should be considered. Root-rot diseases are by no means isolated to acacias, and surveys to identify the disease organisms are being conducted in forest plantations of Azadirachta excelsa, Tectona grandis and Khaya ivorensis throughout Peninsular Malaysia. Several significant root diseases that affect plantations in tropical Asia are caused by certain species of basidiomycetes. This group of fungi produces sexual spores on the outside of microscopic structures called basidia which are held on macroscopic fruit bodies such as mushrooms, toadstools, puff balls, earthstars, and bracket, shelf, crust and coral fungi. Basidiomycetes occupy many niches, e.g. decomposing litter, decaying wood and soil organic matter, in a variety of habitats that includes forests. Some species form beneficial mycorrhizal relationships with the roots of host trees, but others are pathogens of the foliage, stems or root systems of many tree species, including acacias. Basidiomycete species are traditionally identified by the form and microscopic structure of their fruitbodies, and by their appearance when isolated in laboratory culture. Important root diseases of trees in Indonesia are caused by the basidiomycete fungi Rigidoporus microporus, Junghuhnia vincta, Phellinus noxius, and certain species of Ganoderma. In Peninsular Malaysia, Rigidoporus lignosus and P. noxius are the two major root diseases of A. excelsa, T. grandis and K. ivorensis. Root rots are characterised according to the colour of the infected fungal tissues/roots. DNA-based molecular methods have identified Ganoderma philippii as the causative agent of red root rot in A. mangium. This disease is considered a major threat to the efficiency of wood production by the pulpwood industry in Indonesia. Traditional taxonomic methods for describing fungi causing heart rot and root rot continue to play an important role in identifying the causal agents of these diseases. However, molecular techniques for fungal identification are gaining in popularity, value and effectiveness. DNA provides an abundance of taxonomic characters for the identification of organisms that have inadequate morphological characters, or possess distinguishing features only during particular stages of their life cycle. At present, because of their speed, sensitivity and high throughput, variations of PCR-based techniques are popular for assessing these DNA characteristics. Nevertheless, the exact method selected for a particular application will depend on several factors, including the number of samples and number of candidate species. For heart rot and root rot, large numbers of fungal species are implicated, so a technique that can simultaneously identify many species is more efficient than one based on species-specific probes or primers. In addition, as no comprehensive database of root- and heart-rot fungi from A. mangium exists, techniques that require an exact match to a known species will provide only limited information. All DNA-based techniques depend on an adequate herbarium source of carefully prepared specimens with detailed morphological descriptions to verify the DNA-based protocol. Although a new challenge for growers of tropical acacia plantations, root-rot diseases have a long history of association with forest monocultures. This has inevitably attracted research initiatives to understand the dynamics of the diseases involved and options for disease control. In Peninsular Malaysia, poor land preparation and areas with a previous history of root disease were strongly associated with the incidence of R. lignosus and P. noxius in forest plantations. Based on experience gained from the management of root-rot disease in rubber plantations, good land management, the construction of isolation trenches and the application of fungicides are considered valuable tools in the control of root-rot disease in forest tree plantations. To date, cost-effective management tools based on biological and chemical treatments to soil and stumps have yet to be developed for controlling root rot in A. mangium planta- tions, though Trichoderma spp. have been shown to act as biological control agents of Ganoderma. Biological control of plant pathogens aims to reduce dependence on chemical treatments that may cause environmental pollution and the development of resistant strains. Filamentous fungi such as Trichoderma are mycoparasites of plant pathogens and thus have potential for the biocontrol of plant disease: species of Trichoderma are among the most widely tested agents. Although the mechanism of mycopara-sitism is not fully understood, expression of extracellular cell-wall degrading enzymes is assumed to be involved, including the action of chitinolytic and glu-canolytic enzymes. As reported for other chitinolytic systems, endochitinase (EC 3.2.1.14) is among the most effective for both antifungal and lytic activities in comparison with other types of chitinolytic enzymes. Recently, a 32-kDa endochitinolytic enzyme has been purified from Trichoderma reesei. These Trichoderma isolates have an antagonistic ability against some plant pathogenic fungi, such as Ganoderma spp., R. microporus, Rhizoctonia spp., Fusarium spp., and Sclerotium rolfsii and can effectively suppress the development of these fungal pathogens in vitro and in glasshouse experiments. Disease management is not just an issue out in the plantations. The foliar diseases Pestalotiopsis leaf spot, Phaeotrichoconis leaf spot, bacterial leaf blight caused by Xanthomonas, phyllode rust disease caused by Atelocauda digitata, and anthracnose disease and tip necrosis caused by Colletotrichum sp. are all associated with tropical Acacia seedlings being raised in nurseries, as are Pythium, Rhizoctonia, and Fusarium fungi that commonly cause damping-off. Disease control requires integrated mnagement strategies based on a detailed knowledge of these pathogens and their interactions with the seedlings and their environment, including the wider context of the nursery operation. Acacia mangium wood has proved to be not only suitable for producing high-quality pulp and paper, but also an excellent material for solid-wood products. In Indonesia, there has been growing interest in utilising wood of A. mangium for solid wood, corresponding with the declining availability of logs from native forests. When managed for pulpwood, A. mangium plantations are established at around 1000 stems/ha and are clear-felled at age 6-7 years. Silvicultural techniques that incorporate thinning and pruning from below are of crucial importance in growing plantations for solid wood, as there is poten- tial for large and persistent branches to develop in plantations: both are associated with the development of heart rot. Thinning systems must ensure that final-crop trees retain green branches until pruning is completed as well as maintaining acceptable rates of growth of the retained trees. Form-pruning - the selective removal of large branches or those competing with the leader - ahead of lift-pruning, has been shown to increase the straightness of trees in a silvicultural system based on a final crop of about 300 stems/ha pruned to 4.5 m. Acacia mangium can be grown for solid-wood products with a rotation of around 10 years, which is expected to produce a total stem volume of more than 200 m~3 per ha: about 30% of it will be for solid wood. The minimum tree diameter at breast height will be 30 cm. Heart rot is exacerbated by pruning when the plant material is susceptible and a sufficient source of fungal inoculum is present to invade pruning wounds. The workshop from which these proceedings are derived was billed as 'a synthesis of research progress': what have we learnt about heart rot and root rot to date and what are the challenges that must be confronted? The incidence of heart rot will probably exclude the sustainable production of A. mangium for its solid-wood values on some sites. While we now have a quick way of assessing the incidence and severity of heart rot that will help determine disease risk, what defines a high-risk site remains unknown. As with other tree crops, good silviculture that includes pruning prescriptions based on live-branch pruning should reduce disease incidence. Use of selected lines and the best seed source - one producing straight-growing and small-branched trees, may also contribute to managing disease incidence. However, in the absence of better information, A. mangium is a species that appears very susceptible to heart rot and, if acacia wood is to be grown on high-risk sites, it may be necessary to consider alternative species or hybrids that demonstrate an inherent resistance to heart rot. This need to focus on the host rather than the pathogen to manage the disease makes even more sense now that it has been established that a suite of fungi causes heart rot. We did not need a workshop to come to the conclusion that root rot is a more intractable problem than heart rot. Not only does root rot kill A. mangium, other organisms that fall into the same basket of diseases kill trees across a range of species in the temperate as well as the tropical zone. Solutions have been difficult to find: to date, biological control has been shown to work effectively for only one root-rot disease, that caused by Heterobasidion annosum. If there is a challenge here, it is to integrate the breadth of skills and resources that are available in both the private and public sectors in the region to at least understand disease behaviour and then do the right things on the ground to contain the disease and, if possible, begin moving towards disease management based on biological control. The heart-rot project demonstrated it was possible through research to make progress by embracing a multi-stakeholder approach on how to manage a more tractable disease. Are we now ready to meet the more difficult challenge?