Bernard Dell

Stem rot disease caused by Sclerotinia sclerotiorum has emerged as a serious problem for canola (Brassica napus L.) production in Western Australia (WA) over the past few years where crop losses can be up to 40% in the worst affected crops. The biological characteristics and pathogenicity of the pathogen in WA is poorly understood. Also the potential for local biological control agents (BCAs) to be used in the management of the disease has not been explored. This paper provides preliminary data in these fields. One hundred and forty isolates of S. sclerotiorum were collected from WA canola growing regions for identification of biological characteristics which include colour of mycelia, growth rate, production of sclerotia, and pathogenicity. Other fungal isolates with potential biological control activity were collected from southern regions of WA. Colour of mycelia of Sclerotinia isolates varied from white, yellowish white, greyish white, brownish white, grey, dark grey to brown. Each isolate had its 24 and 48 hour growth rate recorded after sub-culture on PDA + ampicillin medium. ANOVA showed highly significant differences between growth rates of isolates 24 and 48 hours after being sub-cultured (P≤0.001). There were significant differences in number of sclerotia produced by each isolate. Two potential fungal biological control agents were found in WA, namely isolate KEN1 and isolate MTB1. These local fungal BCAs were found to be effective in inhibiting in vitro both the growth and ability to produce sclerotia of S. sclerotiorum.

The plantation area in Vietnam of Acacia auriculiformis, A. mangium and their hybrid has expanded greatly in the last decade. Recently, a new stem canker disease causing symptoms of crown wilt, followed by wood discoloration then death of infected trees has occurred in many ecological zones. Ascomata were obtained by incubating discolored wood pieces in moist chambers or by carrot baiting. Isolates of fungi were obtained on PDA medium by taking spores emerging from the tips of ascomata necks. Ceratocystis was identified based on ascospore morphology and conidial types. Twenty six isolates of Ceratocystis were used for pathogenicity assessment on 8-month old seedlings of A. mangium in a nursery, with 5 seedlings per isolate. Stems were inoculated by inserting an 8 mm diameter PDA plug covered with 15-day old mycelia onto the cambium about 50 cm above the ground. Five seedlings were inoculated with sterile PDA plugs to serve as the control. The wounds and plugs were sealed with parafilm to protect them against desiccation and rain. After 60 days of inoculation, based on lesion development and tree death, the pathogenicity of the isolates were identified: 2 isolates (AA8, AMH12) nil, 4 isolates (AAHX1, AMH40, AMD26, AHDL1) low, 4 isolates (AA22, AMH9, AMMB7, AHXL3) moderate, 3 isolates (AMBL3, AMPL2, AMH5) high, and 13 isolates (AA54, AA62, AMH24, AMH26, AMH41, AMHX1, AMQN1, AMBL4, AHBB1, AHBD1, AHBP1, AHXL1 and AHXL2) very high level of pathogenicity causing plant death. This is the first record of Ceratocystis causing damage to Acacia plantations in Vietnam. The origin of the pathogen is unknown. Work is progressing to determine whether the species is the same as that known to cause damage to A. mangium plantations in Indonesia.

Surveys of nurseries and plantations of Eucalyptus species were conducted within Lao PDR in 2009. A range of pathogens were isolated including species within Phytophthora, Pythium, Fusarium, Colletotrichum, Neofusicoccum, Lasiodiplodia, Pilidiella, Calonectria, Cryptosporiopsis, Corticium and Teratosphaeria. Some diseases caused significant defoliation and loss of stock within nurseries and plantations. The presence of these diseases in combination with a changing climate poses many challenges for the future sustainable and profitable management of plantations in Lao PDR.

Australasia to identify emerging plant diseases, carriers of these diseases, and the role of hosts in the transmission of disease. In Australia, a cause for concern and currently listed as a category 1 emergency pest pathogen is Phytophthora ramorum. It has a wide host range and causes widespread damage in nurseries and private estates across Europe and is devastating coastal forest ecosystems of western USA, mainly in California (Rizzo and others 2002, Werres and others 2001). Several Australasian plant species, including Griselinia littoralis (New Zealand broadleaf), Eucalyptus haemastoma (Australian scribbly gum), and Pittosporum undulatum (Victorian box), have already been listed as natural hosts of P. ramorum based on field observations and pathogenicity tests in the USA and Europe (Hüberli and others 2006, RAPRA 2007). While P. ramorum has not been detected in New Zealand or Australia, a preliminary study has identified ecosystems that could be conducive to disease development in Australia (W. Smith unpublished data). It is a pathogen that the region cannot afford as the threat and management implications of this pathogen on natural ecosystems, agriculture and horticulture may potentially be far worse than that currently posed by P. cinnamomi (O’Gara and others 2005). The study aims to provide knowledge of potential hosts and therefore carriers of the pathogen, provide data for the establishment of robust quarantine practices and reduce the risk of an introduction of P. ramorum into Australasia.

The aim of our experiments was to develop protocols that can be used to contain and eradicate spot infestations of P. cinnamomi that, if untreated, are likely to threaten extensive areas of native vegetation or areas of high conservation value. Treatment regimes were guided by two assumptions: 1) within the selected sites, transmission of the pathogen is by root-to-root contact, and 2) the pathogen is a weakly competitive saprotroph. In Western Australia (WA), treatment and control plots were set-up along an active disease front within scrub-heath vegetation dominated by Banksia spp. Treatments, applied sequentially and in combination, included: 1) destruction of the largest plants within disease free vegetation forward of the disease front; 2) destruction of all plants to create a ‘dead zone’; 3) installation of physical root barriers and subsurface irrigation for the application of fungicide/s; 4) surface applications of fungicides selective against oomycetes (triadiazole and metalaxyl-M), and 5) surface injection and deep (± 1 m) treatments with Metham-sodium. In a separate experiment in Tasmania (TAS), combined treatments including vegetation removal, Ridomil and Metham-sodium and root barriers, or Ridomil and root barriers alone, were applied to experimental plots within active disease centres in Eucalyptus-Banksia woodland. In the WA experiment, P. cinnamomi was not recovered (by soil baiting) from plots after treatment with Ridomil and metham-sodium. In the TAS experiment, similar results were achieved with combined treatments (vegetation removal + Ridomil + metham sodium) but in plots treated with Ridomil alone, recoveries of P. cinnamomi increased after initially showing a significant reduction in recoveries.

The most serious foliar disease of eucalypt plantations in WA is Mycosphaerella leaf disease (MLD) (1). Since the commencement of the plantation industry, several fungal species contributing to MLD, previously known only in eastern Australia or overseas, have been reported on E. globulus in WA. Initially only three species were identified (2). More recently, five new records from WA (M. aurantia, M. ellipsoidea, M. mexicana and M. fori) have been identified that have not been recorded elsewhere in Australia (1, 3). Currently, 13 species of Mycosphaerella have been recorded in WA from Eucalyptus (3). Re‐examination of cultures adds six new species that have yet to be described from E. globulus in WA. The impact of MLD on growth of E. globulus plantations in WA was examined in a chemical exclusion trial at two plantations in the Albany region.

Large areas of indigenous forests, Banksia woodlands and heathlands in Australia are devastated by Phytophthora dieback disease caused by Phytophthora cinnamomi (Weste 1994). In southwestern Australia, some 50 percent of the 5710 plants endemic to the region are susceptible (Shearer and others 2004a). Phosphite has been shown to be effective in controlling this pathogen’s impact on a wide range of plant species across different families (Hardy and others 2001). Recently, disease extension was reduced after phosphite treatment even after fire (Shearer and others 2004b). However, very little is known about the influence of a plant’s physiological status at the time of phosphite application on the subsequent efficacy of phosphite treatment to control Phytophthora dieback disease. The key seasonal stresses in an Australian ecosystem of fire and flooding are explored.

Plant reproduction has higher boron (B) requirements than vegetative growth and generally reproductive plant parts contain more B, but the form and function of the extra B remains unexplained. In this paper we review the literature on B in plant reproductive development and add to it recent results on B distribution and forms in reproductive parts (silks, pollen) and vegetative parts (youngest open leaves) of maize grown in B-buffered solution culture. Cell wall binding of B in the silk and pollen of maize does not explain the higher B concentrations in reproductive than vegetative tissue. Given the rather high proportion of B present in the non-cell wall fraction in pollen and silk, the high B requirement for plant reproduction suggests an additional role for B other than in cell wall formation. The identity of non-cell wall B binding substrates in pollen and carpel tissue awaits further study. The higher sensitivity of plant reproduction to B deficiency is also related to weaker B transport into floral organs, especially where transpiration is suppressed in reproductive plant parts by enclosure of sheaths (e.g. wheat ear) or husks (e.g. maize ear) during the critical stage of development.

Many diseases of Eucalyptus species have emerged as pathogens in exotic plantations. Guava rust (Puccinia psidii), cryphonectria canker (Crysoporthe cubensis) coniotherium canker (Colletogloeopsis zuluensis) and Kirramyces leaf blight (Kirramyces destructans) are all serious pathogens that have not been found in native forests or in plantations in Australia (Burgess & Wingfield 2002; Cortinas et al. 2006; Glen et al. 2007; Wingfield et al. 2001). The susceptibility to these pathogens of Eucalyptus spp. commonly used in exotic plantations is known; however the susceptibility of many Eucalyptus spp. found only in natural ecosystems in Australia is unknown. There are two main uses of sentinel plantations. Firstly, tree species known to be susceptible to different pathogens can be planted within the natural environment to try and trap pathogens from their surroundings. In Australia, taxa trials planted in different environments act as sentinel plantings. By surveying these taxa trials we have collected and described a number of new eucalypt pathogens and reported the presence in Australia of Kirramyces destructans. The second use for sentinel planting is where many tree species are planted in a region known to harbour certain pathogens. In this manner the susceptibility of the different tree species can be determined.

The long-term productivity of short-rotation plantation eucalypts and acacias in Asia requires ongoing investment in the biology of current and new genetics and constraints that climate change may impose. Understanding site fertility constraints has the potential to further lift productivity through precision application of inorganic fertilizers, management of beneficial bacteria and fungi and appropriate slash management. It remains a concern that nutrient disorders, especially micronutrients, which could be easily corrected in acid soils, are still being widely reported in the region. The low genetic diversity in many plantations heightens the risk of productivity loss due to incursions of pathogens and pests. In the past decade, a number of new biotic eucalypt threats have appeared within the region and are rapidly spreading. No plantation estate can be considered to be risk-free from such incursions. Furthermore, abiotic stresses (e.g. water, nutrients) are likely to exacerbate damage from biotic agents. With climate change, plantations established on marginal lands and sites with poor soils are likely to experience stresses that are more frequent and a more diverse group of stress agents than those in areas that are more resilient to climate change. Future breeding programs should include selection for tolerance to key abiotic and biotic stress agents that have the highest risk to plantation productivity.