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Tree Decline and Mortality in Selectively Logged Eucalypt Forests in Central Victoria
Abstract
The occurrence, symptoms and impact of a dieback disease in selectively logged, mixed species eucalypt forests in central Victoria are described. E. obliqua E. viminalis and E. globulus subsp. bicostata are the main species affected, although a range of other eucalypt and some Acacia species are also killed. Dieback affects trees at all stages of growth and it is estimated that 1400 ha of high quality production forest are severely affected, with a further 600 ha moderately affected, Stands with severe and moderate dieback are estimated to have respective sawlog volume increments of about one half and two thirds that of an average healthy stand. An unidentified species of A miliaria is consistently associated with dead or dying trees. Disease development is associated with selective cutting in pole stage or mature stands, but no consistent associations between site, other silvicultural factors or disease have yet been established. Past drought stress may have influenced disease development in some areas.
... In Victoria, ARD in natural stands was shown to intensify following partial cutting (Edgar et al. 1976, Kellas et al. 1987. Mature stands with moderate to severe disease were shown to have reduced sawlog volume increments (Edgar et al. 1976) and 30-year-old regrowth stands of E. obliqua L'Hérit. ...
... In Victoria, ARD in natural stands was shown to intensify following partial cutting (Edgar et al. 1976, Kellas et al. 1987. Mature stands with moderate to severe disease were shown to have reduced sawlog volume increments (Edgar et al. 1976) and 30-year-old regrowth stands of E. obliqua L'Hérit. were reported to have Mańka reduced monthly girth increments if 25% or more of their basal circumference was infected with Armillaria (Kile et al. 1982). ...
... Information on the effect of Armillaria infection on the growth of eucalypts in Australia is limited. When compared to healthy stands in central Victoria, Edgar et al. (1976) estimated that mature stands of eucalypts with severe and moderate symptoms of dieback due to Armillaria infection had sawlog volumes of one-half and two-thirds respectively, with growth losses of 0.3-2.0 m 3 ha -1 year -1 . ...
... It is widespread in southwest native forests (Shearer, 1992 ) causing scattered mortality and windthrow. The spread of ARD in eucalypt forests has been associated with logging stumps (Edgar et al., 1976; Podger et al., 1978; Pearce et al., 1986; Kellas et al., 1987). When infected trees are removed, A. luteobubalina rapidly colonizes the cambium of the entire root system and above ground portion of the remaining stump, and may persist on this food base for up to 25 or more years (Kile, 1981). ...
... Reports give mixed results and demonstrate the complex nature of interactions between stand management and root disease devel- opment. Partial cutting has been shown to intensify ARD in natural stands of eucalypts in Victoria (Edgar et al., 1976; Kellas et al., 1987) and conifers in the United States (Koenings, 1969; Byler et al., 1986; Wargo and Harrington, 1991). Precommercial thinning is not recommended in Douglas-®r regenerated on Armillaria-infested sites in western North America (Morrison, 1981; Had®eld et al., 1986; Williams et al., 1989), however, it was recommended in pine and spruce stands in eastern Canada to improve resistance to Armillaria by reducing stress caused by overcrowding (Singh, 1981). ...
... There is strong evidence to suggest that ARD, caused by A. luteobubalina, in southern Australian Eucalypt forests is exacerbated by forest harvesting. (Edgar et al., 1976; Podger et al., 1978; Pearce et al., 1986; Kellas et al., 1987), but there is little information on the effects that thinning per se may have on ARD in Eucalypt forests. Retrospective studies in central Victoria of mixed species Eucalypt forests with different logging regimes indicate that cutting intensity alone did not affect the incidence of disease in residual trees, but frequent low intensity cutting may be the cause of increased infection (Kellas et al., 1987). ...
The incidence of Armillaria root disease was recorded during routine measurement of a silvicultural experiment designed to test the effect of thinning and nitrogen fertilizer application on the growth of karri regenerated after clearfelling. The experiment was established in 1984, when the stand was 12-year-old. Ten years after thinning, the level of disease increased significantly with increased thinning intensity, and disease accounted for 51% of the mortality in the plots thinned to 200 stems/ha. Fifteen years after thinning, the level of infection had increased in all treatments but was still significantly lower in the unthinned treatment. In the thinned treatments, 54–63% of the trees ranked in the largest 200 stems/ha were infected, and 50–100% of the mortality within these trees was attributed to Armillaria luteobubalina. In the unthinned treatment, no mortality within the dominant trees was associated with disease. Ten years after thinning and fertilizer treatments, it could not be determined whether fertilizer application had had any effect on the level of disease. Whole tree thinning, which results in stump and root removal, is discussed as a viable management option in high-quality karri regrowth stands infested with A. luteobubalina.
... Anthropogenic disturbances such as partial cutting can increase the incidence and severity of diseases that promote forest gap formation in tropical as well as in temperate forests (Castello et al. 1995). For example, Armillaria luteobubalina is pathogenic on many canopy Eucalyptus and understory species in Western Australia (Kile 1981); selective cutting has greatly increased the incidence and mortality from this disease by leaving trees in close proximity to inoculum bases of the freshly cut stumps (Edgar et al. 1976;Kellas et al. 1987;Castello et al. 1995). Pathogenic Armillaria species become more aggressive by producing toxins that kill healthy trees if given access to a large food base, such as a cut stump (Leach 1939;Redfern 1975;Cook 1977). ...
... Colonies of cord-forming decomposer basidiomycetes can be extensive, and could therefore be affected by forest fragmentation, roads, and trails. Some cord-forming fungi that kill trees can extend over many hectares (Kile 1983;Smith et al. 1992), are often favored by disturbances such as droughts, selective harvesting, and agroforestry (Edgar et al. 1976;Fagg et al., 1986;Castello et al. 1995), and are themselves a major cause of forest disturbance (Castello et al. 1995). Non-pathogenic cordforming fungi are another potentially keystone guild because they can accelerate the rate of wood decomposition by translocating limiting nutrients over large distances from previous resource bases (Boddy 1993). ...
Fungi and bacteria control many of the vital processes on which the very maintenance and survival of tropical forests depend (Hawksworth and Colwell 1992). An overview of the role of microorganisms in ecosystem functioning as a whole has already been presented (Allsopp et al. 1995). Here, our goal is to identify the functional attributes of microorganisms in tropical forests, and to identify those processes that are most likely to be sensitive to losses of diversity, especially in the face of disturbance or broad environmental changes. In some cases, microbes may influence ecosystem processes indirectly by altering the diversity of other organisms. We also point out where research is needed to determine what effects losses of microbial diversity might have on tropical forest diversity, stability, and regeneration.
... Infection spreads from tree to tree via root contact and the same genotype of A. luteobubalina may infect large numbers of trees within a forest stand. Studies concerning the behaviour of the pathogen and its impact on karri and other regrowth eucalypts show that the spread of the disease in eucalypt forests is associated with infected stumps left following logging operations (Edgar et al. 1976; Pearce et al. 1986; Kellas et al. 1987). Survey results suggest that while ARD has only a scattered presence in most regrowth karri stands, it is widespread in a number of high-quality stands (M. ...
... Roots may be infected for many years before symptoms are expressed in the trees (Edgar et al. 1976). As ground surveys can assess only the aboveground symptoms of the disease, it is very difficult to determine true levels of disease within a stand. ...
Intensive survey based on aboveground symptoms of Armillaria root disease underestimated true levels of disease by at least 20% and sometimes by up to 40% in high-quality karri regrowth stands. The results challenge the reliability of surveys based on aboveground disease symptoms. While most disease was established within the subdominant stratum, a very high proportion (30-60%) of the dominant trees were also infected. Within the study areas 15 distinct genotypes of Armillaria luteobubalina were identified. Individual genotypes existed as clones, with 2-3 clones per hectare. These factors need to be considered in stand management planning and yield predictions. A broader study, including lower-quality sites, is needed to determine whether these findings apply to all types of karri regrowth.
... 'Diebacks' and declines attributed to Armillaria by Kile (1981b) were attributed to drought, logging, fire and poisoning by Edgar et al. (1976). Not all trees showing crown dieback had Armillaria infections and not all seedlings innoculated with Armillaria developed disease (Kile, 1981b). ...
... Conversely, Huettl (1993) suggested that increasing atmospheric carbon dioxide could provide better growing conditions that would increase competition between trees and reduce their vigour. However, E. obliqua declined on their favoured sites, after selfthinning, irrespective of stand density, basal area and the dominance of individual trees (Edgar et al., 1976;Palzer, 1981a;Podger, 1981;Wardlaw, 1989;Jurskis, 2004b). Thinning and fertilizing exacerbated the decline (Wardlaw, 1989) because they exacerbated the changes in soils that had initiated the decline (Jurskis and Turner, 2002). ...
Decline and dieback of eucalypts have been attributed to an exotic pathogen, various native organisms, climatic factors and agricultural or urban pollution. Where particular biotic or abiotic factors could not be singled out, they have been regarded as predisposing, inciting or contributing factors in ‘diseases of complex etiology’. Ongoing monitoring of eucalypt decline during recent droughts in eastern Australia, together with extensive one-time observations across temperate Australia, provided opportunities to further examine some hypotheses of decline and dieback that were largely based on retrospective investigations.Episodes of dieback can be distinguished from the process of chronic decline. Dieback episodes were associated with natural climatic extremes whereas chronic decline was associated with human management. Decline of forests in nature reserves was associated with exclusion of fire and grazing, while decline of rural trees was mostly associated with pasture improvement. Trees growing low in the landscape on soils with poor drainage and aeration were especially predisposed to decline. It appears that chronic abiotic stress causes tree decline when the function of roots is impaired by changes in soils. Climatic extremes can accelerate chronic declines associated with human management. A variety of pests, ‘pathogens’ and parasites can take advantage of trees that are stressed by environmental changes, especially eutrophication. Similarities between diebacks and declines in the Atlantic and Pacific regions suggest a simple unifying concept of tree decline and dieback. The implications for management of forest health are discussed.
... In Australia, drought-related mortality has been recorded in the northern savannah and the western Mediterranean-type forests (Dwyer et al. 2010, Matusick et al. 2013), but less so in temperate forests where a decade of severe drought had no effect on stand live basal area (Zeeman et al. 2014) and forest decline has more clearly been associated with interactive effects of different factors (e.g. pest, drought, management; (Edgar et al. 1976, Jurskis 2005, Carnegie 2007, Horner et al. 2009. While the physiological mechanism of drought-induced tree mortality is not always clear (Sala et al. 2010, Hartmann et al. 2018, our results reveal for the first time a relationship between drought occurrence and tree mortality, as indicated by proportional large DST biomass that is consistent across the large geographical and environmental gradient encompassed in this study. ...
Dead wood, including dead standing trees (DST) and coarse woody debris (CWD), is a critical component of forest ecosystems that provides habitat and refugia for fauna, flora, and microbial communities and plays a key role in carbon and nutrient cycling. However, few studies have modelled the long-term dynamics of dead wood, limiting our ability to predict how the abundance and composition of dead wood may change with climate change or altered fire regimes. Here we analyse DST and CWD data in 884 plots encompassing multiple field campaigns and forest types of varying canopy cover and species composition across the State of Victoria in temperate south-eastern Australia. We use boosted regression tree modelling to examine the relative influence of disturbance history and tree functional traits on dead wood biomass while accounting for the influence of environmental and climatic factors and stand attributes across a broad productivity gradient. We modelled absolute and relative dead wood biomass by size (‘small’ 100 < 200 mm diameter, ‘medium’ 200 < 500 mm diameter, ‘large’ ≥ 500 mm diameter) and decay classes (sound to advanced decay) to evaluate the consistency of predictor effects among different components of dead wood. We found that live tree basal area and mean annual precipitation were influential predictors of both DST and CWD biomass, indicating an over-arching effect of forest productivity on dead wood biomass. Fire history was also an important predictor, with DST biomass decreasing and CWD biomass increasing with time since last wildfire. The proportion of large DST biomass increased with increased tree mortality as a result of fire interval and time in drought. DST biomass also increased and CWD biomass decreased with increasing wood density, and this was relatively more important than the other functional traits we examined (heartwood nitrogen content and bark type). Our study suggests that forest productivity, fire history, drought and wood density are important determinants of dead wood, as they influence dead wood inputs and outputs. Our study reveals the broad-scale drivers of dead wood biomass, and the potential for altered fire regimes and changing climate to influence live- to dead wood dynamics and associated ecosystem functions.
... ARD spreads below ground, from tree to tree, via root contact or rhizomorph infection. In Australia (as in other regions of the world), the spread of ARD has been associated with logging stumps (Edgar et al. 1976, Kellas et al. 1987, Pearce et al. 1986, Podger et al. 1978 and thinning operations (Robinson 2003). When an infected tree is cut down, A. luteobubalina spreads quickly along the cambium of infected roots eventually colonising the entire root system and above ground portion of the stump. ...
Armillaria luteobubalina is an endemic pathogen of dry sclerophyll forests and woodlands of southern Australia. The fungus infects the roots of susceptible hosts causing a root and butt rot, commonly referred to as Armillaria root disease (ARD). In high quality regrowth karri stands, ARD causes significant losses due to mortality and defect, which are exacerbated by intensive management practices such as thinning. This review gives a brief outline of what is known about Armillaria luteobubalina and Armillaria root disease in karri regrowth forests of Western Australia and discusses options available for disease control.
... This idea of a flexible diameter threshold is central to the volume control regulation method that has been applied to southern pines (Guldin and Baker, 1998;Guldin, 2002). We suggest that it may also be applicable to any species or forest type in which vigour loss is not commonly associated with the presence of obvious defects, such as uneven-aged eucalypt forests in Australia for example (Edgar et al., 1976). ...
To enhance the vigour and quality of high-graded hardwood stands, the removal of low-vigour trees is often prioritized during harvesting operations. However, northern red oak (Quercus rubra L.) is rarely affected by defects that are indicative of imminent decline and, therefore, are less likely to be marked for harvesting. Consequently, the red oak volumes harvested during recent years were considerably lower than the estimated annual allowable cut in the public forests of Quebec, Canada. We used data from Quebec's forest inventory to identify variables associated with low-vigour red oak trees. Three groups of explanatory variables were formed to take into account tree size descriptors, inter-tree competition and stand descriptors. Logistic regression revealed that the probability of occurrence of northern red oak of low vigour increased with increasing tree diameter at breast height and dominance. Also, low-vigour oak trees were more likely to be found in stands in which total red oak basal area was low. A cut-point analysis indicated that the maximum diameter threshold for harvesting red oak ranged between 34 and 46 cm. These criteria could help forest managers formulate species-specific tree-marking rules that integrate the need to increase the red oak component in the harvested volume to a level that is closer to the annual allowable cut volume while maintaining stand vigour.
... Stand decline has been observed in Australian native forests and woodlands for many decades (e.g. Edgar et al. 1976; Landsberg & Wylie 1988). Awareness of the extent of eucalypt canopy decline in Australian native forests has increased considerably over recent years (e.g. ...
The extent of eucalypt decline in moist coastal forests of south-eastern Australia is increasing with resultant losses in biodiversity and productivity. This survey aimed to identify factors associated with the decline of Eucalyptus saligna (Sydney Blue Gum) in Cumberland State Forest, a moist sclerophyll forest within urban Sydney. Eucalyptus saligna was the dominant overstorey species in six 20 m radius plots, which differed in floristic composition, structure and crown condition. One plot was colonised by bell miners (Manorina melanophrys). A range of leaf, tree and plot scale parameters were assessed including insect damage and free amino acid content, visual crown condition, floristics and soil chemistry. The plot permanently colonised by bell miners also had Eucalyptus saligna trees in the poorest condition. Both the weed Lantana camara and the soil pathogen Phytophthora cinnamomi were present in some of the plots but neither was strongly consistent with the severity of crown decline. There were, however, significant correlations among the foliar traits of insect damage, free amino acid content and relative chlorophyll content. Free amino acid content differed significantly between leaf age cohorts. Plots differed notably in topsoil organic matter and soil nitrogen, but the plot with the poorest visual crown condition score had intermediate mean values for both soil properties within the ranges presented by the six plots. Overall, crown condition score was weakly negatively correlated with topsoil organic carbon and total nitrogen content. The unhealthiest plot also had the highest density of shrubby understorey. Site factors that could influence both the quantity and quality of foliage (e.g. free amino acid content) in eucalypt crowns, and hence the population dynamics of herbivorous insects and bell miners, are discussed in relation to Eucalyptus saligna crown decline.
... In the study of Baumgartner and Rizzo (2002), most vines only became symptomatic several weeks before death. The same situation is observed for saplings of several eucalypt species (Edgar et al. 1976), young conifer seedlings (Hintikka 1974) and small hardwood trees (Rhoads 1956). The interval between visibly healthy crowns and severe symptoms and the visible presence of G. philippii in 9-year-old A. mangium was 6 weeks (C. ...
... In the study of Baumgartner and Rizzo (2002), most vines only became symptomatic several weeks before death. The same situation is observed for saplings of several eucalypt species (Edgar et al. 1976), young conifer seedlings (Hintikka 1974) and small hardwood trees (Rhoads 1956). The interval between visibly healthy crowns and severe symptoms and the visible presence of G. philippii in 9-year-old A. mangium was 6 weeks (C. ...
The rapid emergence of Acacia mangium as the key industrial plantation species in Indonesia has been followed by the equally rapid emergence of red root rot (Ganoderma philippii) as its potential nemesis. As a consequence, and on severely affected mineral soils in equatorial tropical environments in particular, A. mangium may no longer be capable of producing commercial yields after three rotations. In this experiment,
100-tree plots were established in commercial plantings of A. mangium at five sites which represented the range of growing conditions used for plantation establishment in Indonesia. Repeated monitoring at approximately 6-month intervals of above- and below-ground variables was used to explore relationships between measures of tree health and root rot. Crown colour and density were poor predictors of either the presence or severity of infections. Tree mortality increased more or less linearly with time and led to the progressive coalescence of initially discrete disease gaps. The average rate of disease development was about 0.3% per month, and average time from infection to tree death was conservatively estimated at around 1 year. Trees with more than four dead/missing neighbours had a >50% chance of being dead by the following monitoring. By the end of the monitoring period >40% of trees were classified as dead/missing, although this value varied from 20 to 70% depending on site, tree age and rotation. Canonical discriminant analysis correctly allocated >90% trees into their correct group on the first monitoring and similarly classified whether trees would be dead or missing in the following monitoring.
... Abiotic predisposing factors are often site related and may include certain topographic features , poor drainage (Hennon et al. 1992) heavily eroded or shallow and rocky soils (Manion 1991), or overstocking (Heitzman et al. 2007). Other abiotic stressors that may act as either inciting or contributing factors include moisture stress (Jurskis 2005), highway deicing salts (Horsley et al. 2002;Simini and Leone 1982), atmospheric deposition (Duchesne et al. 2002), and logging damage (Edger et al. 1976), among others (Manion 1991). ...
... These forests had been heavily cut last century, regrowth developed, and logging was resumed on a selection basis from 1947. The expression of the disease seems to be associated with selection cutting in pole or mature regrowth stands; past drought stress may also have influenced disease development in some areas (Edgar et al., 1976). Another, and perhaps more serious example of ecological instability is found in Tasmania's southern regrowth forests. ...
Despite the low yields obtained from them, the eucalypt forests have contributed substantially to Australia's development. During the post-war decades of rapid economic growth, sawlog avail-ability has been run down in order to service the high level of wood demand, with the anticipation that the softwood pianta-tion programme would progressively take over the major wood supply role. As this happens it will be possible to formulate long-term policies and strategies for the eucalypt forests. It is argued in this paper that there will be advantages in maintaining a strong and stable forest products industry based on the eucalpyt forests. This will require, in turn, a more accept-able balance between wood production and conservation of natural environments than has been possible in the past. It will also be essential to take account of the biological nature of the eucalypt, and to develop forest use strategies and practices which will maintain the stability of the eucalypt forest ecosystem.
... Most of these forests have a long history of logging. Although this fungus is pathogenic on eucalypts in unlogged forests, the greatest incidence and severity of disease has been associated with repeated selection-cutting of the older trees (Edgar et al. 1976). Logging frequency rather than intensity appears to be the critical factor. ...
Armillaria root rot of Theobroma cacao was reported for the first time more than a century ago. The pathogen causes a root rot of these trees, resulting in tree death. Mortality commonly occurs in distinct disease centers, often around stumps of dead native trees removed for the establishment of cacao orchards. The cause of Armillaria root rot was, and often continues to be, ascribed to Armillaria mellea. Advances in fungal taxonomy have, however, shown that A. mellea is restricted to eastern Asia, Europe, and North America and that the cause of Armillaria root rot of cacao, in for example Africa, represents a distinct species. Management of Armillaria root rot is often unsuccessful due to the persistence of fungal inoculum in roots, stumps, and soil. Some success has been achieved in managing the disease in other crops, but due to the relatively low acknowledged incidence of Armillaria root rot in cacao plantations, limited experience in controlling the disease exists for this crop. Care should be taken, however, as the impact of root and wood rot pathogens such as Armillaria species results not only in tree death but also in reduced tree vigor and yield. The availability of genomes for both T. cacao and Armillaria species provides valuable opportunities for future endeavors to reduce the impact of this disease in cacao plantations.
Partial cutting has been practised throughout the mixed eucalypt stands of the Wombat Forest for several decades. The introduction of shelterwood management in the 1970s has resulted in the species composition changing from a predominantly messmate stringybark (Eucalyptus obliqua) overstorey (dbhob > 30 cm) to approximately equal proportions of messmate and peppermints (E. dives and E. radiata) in the regrowth and regeneration (dbhob < 30 cm) strata. The change in species composition is associated with high overwood densities, favouring the establishment and development of the more shade tolerant and persistent peppermints. Regeneration of messmate, the preferred timber species, requires lower levels of overwood competition in association with adequate seedbed and seed supply. Further study will be required of these changes in species composition to ascertain if they are temporary or maintained, and hence their effect on future wood quality and yield from the forest.
Colour aerial photography at scales of 1:3000 and 1:6000 was used to identify 86.6% and 68% respectively of the 5.03 ha affected by Armillaria root rot in a 32 ha mixed species eucalypt stand, located in central Victoria. At the 1:3000 scale more of the infected area was detected and correctly mapped but a larger proportion of the healthy area was incorrectly classified as infected.
Summary An interpretation of dieback within eucalypt forests might take account of (i) the consequences of the rapid evolutionary expansion of the eucalypt progenitor associated with continental drift; (ii) the nature of species patterns and pattern-environment relationships within the forests; (iii) the significance of stand structures and ecosystem processes in maintaining stable eucalypt forests; (iv) the destabilisation of forests and ecosystem processes associated with recurrent fires in the post-settlement period and the logging of the forests; and (v) specific site factors which may predispose disturbed ecosystems to decline, insect predation and dieback.
In selectively cut and undisturbed parts of four mature stands, five 0.04-ha plots were established, and trees were measured, mapped, and examined for aboveground symptoms of armillaria root disease. Trees were felled, and stumps and their root systems were removed by an excavator and were measured and examined for Armillaria lesions. Isolates from root lesions, rhizomorphs associated with lesions, and basidiomes collected in or adjacent to plots were of Armillaria ostoyae (Romagn.) Herink. All trees were assigned to one of five tree condition classes based on the location of lesions and host response. The merchantable volume in each class was calculated. In undisturbed plots, incidence of trees with A. ostoyae lesions on roots was about 10% in the dry climatic region compared with about 75% in the moist region and 35% in the wet region. In plots in the selectively cut parts of the stands, 50-100% of stumps were colonized by A. ostoyae. Results of a logistic regression analysis showed that selective cutting was associated with a statistically significant increase in the probability of a tree having A. ostoyae lesions, where the magnitude of the increase depended on tree diameter. The increase in the probability of a tree being diseased was accompanied by an increase in the proportion of primary roots with lesions and the average number of lesions per diseased tree; however, the increases in disease intensity were statistically significant at only two (one dry and one moist) of the four sites. The percentage of merchantable volume threatened or killed by A. ostoyae was usually higher in cutover than undisturbed plots.
An undescribed but native species of Armillaria is a secondary pathogen in declining stands of Eucalyptus obliqua and Eucalyptus regnans in southern Tasmania. The fungus is almost ubiquitous in healthy and diseased forest and occurred epiphytically or parasitically in local lesions on the roots of 74 % of eucalypts partially excavated in 15 forest stands, 25–75 years of age. The fungus infected 85 % of logging stumps and 64 % of fire killed trees. Although the fungus only infected the sapwood of the eucalypts, it survived in infected stumps for at least 70 years. Rhizomorphs initiated most infections but did not grow freely through the soil. Pathogenicity tests confirmed that the Armillaria is a weak pathogen.
In selectively cut and undisturbed parts of four mature stands, five 0.04-ha plots were established, and trees were measured, mapped, and examined for aboveground symptoms of armillaria root disease. Trees were felled, and stumps and their root systems were removed by an excavator and were measured and examined for Armillaria lesions. Isolates from root lesions, rhizomorphs associated with lesions, and basidiomes collected in or adjacent to plots were of Armillaria ostoyae (Romagn.) Herink. All trees were assigned to one of five tree condition classes based on the location of lesions and host response. The merchantable volume in each class was calculated. In undisturbed plots, incidence of trees with A. ostoyae lesions on roots was about 10% in the dry climatic region compared with about 75% in the moist region and 35% in the wet region. In plots in the selectively cut parts of the stands, 50-100% of stumps were colonized by A. ostoyae. Results of a logistic regression analysis showed that selective cutting was associated with a statistically significant increase in the probability of a tree having A. ostoyae lesions, where the magnitude of the increase depended on tree diameter. The increase in the probability of a tree being diseased was accompanied by an increase in the proportion of primary roots with lesions and the average number of lesions per diseased tree; however, the increases in disease intensity were statistically significant at only two (one dry and one moist) of the four sites. The percentage of merchantable volume threatened or killed by A. ostoyae was usually higher in cutover than undisturbed plots.
The cause of tree disease in any location is difficult to diagnose because of the multiplicity of potential causes. Spatial analysis provides useful evidence on the significance, scale and shape of patterns for disease diagnosis. This study explores the causes of dieback through spatial analysis of tree health at Coranderrk Reserve, an example of an important conservation reserve on the urban fringe of Melbourne, Victoria. Ca. 200 trees were sampled at two sites and measured for position, diameter and height in each of two different community types. An index of tree health was developed, and each tree was assigned a value of health. Dead trees were categorized based on state of decay. Correlograms using Moran's I found significant spatial structure at both study sites. The study employed a method of analysis that compares the spatial patterns in dead and living trees. When all dead trees were eliminated from the analysis, significant spatial pattern was found only at one site. The spatial structures obtained were compared with those expected from the different possible causes of tree dieback. Some potential explanations were eliminated. The results illustrate how spatial analysis may make useful contributions to disease diagnosis if employed systematically together with other modes of investigation.
Trees up to 25 m tall died suddenly at the edges of a steadily extending patch of dead and dying trees in a fast-growing plantation of Eucalyptus regnans F. Muell. near Traralgon, Victoria. Measurements of tree height and stem diameter made during the 12 years since the plantation was established indicate that, prior to their death, trees at the margins of the extending patch grew as rapidly as unaffected neighbours. During May 1973 and May 1974, Armillaria luteobubalina Watling & Kile sp.nov. fruited prolifically on dead and infected trees. Few rhizomorphs have been found so that spread of infection seems to have been mainly by mycelial growth within root systems and at root contacts. Infection has spread from inoculum in a single stump of Acacia melanoxylon R. Br. at an average annual rate of 2.5 m. Untreated stumps of healthy 10-year-old trees felled in unaffected parts of the plantation during the fruiting season of 1973 have not become infected. In pathogenicity tests with an isolate from the cap of a fruiting body, E. regnans seedlings have suddenly wilted and died following penetration of the tap root-root collar zone and subsequent girdling of the stem. It is concluded that the deaths at Traralgon are due to primary attack by A. luteobubalina.
The formation and expansion of groups of dead and dying grapevines (disease centers) caused by Armillaria root disease were tracked in a commercial vineyard in Sonoma County, California from 1998 to 2000. Approximately 50% of the vines that died in 1999 and 2000 were adjacent to vines that died in previous years, providing circumstantial evidence of vine-to-vine spread of root disease. To determine if symptomatic and dead vines were infected by vine-to-vine spread or by direct contact with partially decayed tree roots remaining from forest trees that inhabited the site prior to vineyard establishment, pneumatic soil excavation was used to expose the root systems of 30 vines within the oldest disease center. Root system excavation revealed infections on 26 of 30 excavated vines, 27 of which were in direct contact with decayed tree roots. No evidence of vine-to-vine spread was found and rhizomorphs were extremely rare. Therefore, the pattern of disease incidence was due to the patchy distribution of decayed tree roots belowground. Control efforts aimed at reducing vine-to-vine spread of Armillaria root disease in young vineyards, such as the one in this study, may be unnecessary, given the slow rate of spread of the pathogen.
The distribution and symptoms of die-back in the mixed-species eucalypt
hardwood forests of eastern Victoria are described. The disease was recognizedin 1952
in small patches of forest situated in the flat, badly drained sandy coastal soils and has
spread rapidly. This spread was associated with above-average rainfall during spring,
summer, and autumn. Disease symptoms were similar to that caused by drought, and
were observed best during dry periods following summer rain.
The lupin bait of Chee and Newhook was used to isolate Phytophthora cinna-
momi from the diseased areas, and the field symptoms seen on susceptible eucalypts of the
subgenus Renantherae were similar to those observed on young saplings inoculated with
P. cinnamomi in the greenhouse. Species of the subgenus Macrantherae observed in the
forests and tested in the greenhouse are tolerant to the disease. Estimates of population
density indices of the fungus gave highest values in areas where the disease was most
active. The fungus was not isolated from adjacent healthy forests.
Improvements in the baiting and identification of P. cinnamomi with use of
excised lupin radicles floated over a small sample of soil in a petri dish are described.
Several variants of the fungus with differing growth rates were isolated.
A common root rot of conifers in Rhodesia is attributed to Armillaria mellea (Vahl ex Fr.) Kummer. The familiar rhizomorphs typical of this pathogen in Europe are, however, rarely produced under field conditions. Spread of the fungus is thus apparently dependent on the presence of host roots. Rhizomorphs are produced by the fungus in agar culture and in autoclaved forest soils. Their growth in such soils can be inhibited by the addition of sterile water extracts of untreated soil. The absence of rhizomorphs in diseased areas is attributed to the influence of a water soluble inhibitor present in the soil.
Twelfth progress report of the Royal Commission on State Forests and Timber Reserves
- Anon