662 research outputs found
Quarantine is important in restricting the spread of exotic seed-borne tree pathogens in the southern hemisphere
No full textThe lack or destruction of indigenous forests in the southern hemisphere has in some cases led to extensive afforestation with exotic softwoods that subsequently developed into large successful forestry industries. Pinus radiana is the predominant softwood species in the southern hemisphere, with over 3 million ha planted. In its natural environment on the West coast of the United States, P. radiata has been decimated by an epidemic outbreak of pitch canker caused by Fusarium circinatum. The potential devastation of softwood industries in the southern hemisphere, as a consequence of introducing this pathogen, has resulted in strict quarantine regulations. However, this may be too little, too late. F. circinatum is already present in South Africa and it appears that only the lack of an insect vector that has prevented its movement from seedlings to mature trees in plantations. This review considers the risk of introducing new pathogens into the southern hemisphere softwood plantations. The pine pathogen Sphaeropsis sapinea is used as a model to assess future risks, particularly of introducing seed-borne pathogens. The genotypic diversity observed in the pine endophyte S. sapinea, is consistent with historical records of the frequency and quantity of seed and germplasm importation to and within the southern hemisphere. The diversity in South Africa is high, moderate in New Zealand and low in Australia. Many of the genotypes observed were probably introduced before the advent of quarantine. However, new introduced genotypes may be more pathogenic than existing genotypes, potentially leading to more severe disease outbreaks. In pathogens capable of sexual reproduction such as F. circinatum, newly introduced genotypes crossing with existing genotypes would allow for more gene diversity and a greater risk of the pathogen overcoming the trees' resistance. Thus, quarantine is now more important than ever, as limiting the genetic diversity of an exisiting introduced pathogen can be as important as excluding new pathogens
A simple and rapid method to determine vegetative compatibility groups in fungi
No full textVegetative compatibility in fungi reflects phenotypic differences (or similarity) among individuals representing the population of a species (Leslie 1993). Thus, individuals (genotypes) of a fungal species having the same heterokaryon (het) or vegetative incompatibility (vic) loci can fuse to form a heterokaryon (Glass et al 2000). Fungal isolates that form stable heterokaryons are then considered to belong to same vegetative compatible group (VCG). In contrast, isolates that are different at one or some or more of these loci will not anastomose. Rather, programmed cell death or apoptosis occurs in the mycelial cells that are in contact with an isolate representing a different VCG (Anagnostakis 1987, Leslie 1993). In the case of fungi which have coloured or dark mycelium in culture (such as most Botryosphaeriaceae and Cryphonectriaceae) failure to anastomose is observed as a thick barrage line between the two different isolates. For such species, tests in Petri dishes make it relatively easy to determine the VCG’s for a population of isolates and this provides a robust view of population diversity (Burgess et al 2001, van Heerden and Wingfield 2001). For fungi with light coloured mycelium, for example species of Fusarium, barrage zones between isolates having different VCG’s are difficult to discern. In such cases, it is necessary to produce nit mutants to define the individuals in culture (Klittich and Leslie 1988, Swift 2002)
Multigene phylogeny and mating tests reveal three cryptic species related to Calonectria pauciramosa
No full textCalonectria pauciramosa is a pathogen of numerous plant hosts worldwide. Recent studies have indicated that it included cryptic species, some of which are identified in this study. Isolates from various geographical origins were collected and compared based on morphology, DNA sequence data of the ß-tubulin, histone H3 and translation elongation factor-1 regions and mating compatibility. Comparisons of the DNA sequence data and mating compatibility revealed three new species. These included Ca. colombiana sp. nov. from Colombia, Ca. polizzii sp. nov. from Italy and Ca. zuluensis sp. nov. from South Africa, all of which had distinguishing morphological features. Based on DNA sequence data, Ca. brasiliensis is also elevated to species leve
Mycosphaerella and Teratosphaeria diseases of Eucalyptus; easily confused and with serious consequences
No full textThe Mycosphaerella complex accommodates thousands of taxa. Many of these species are economically important plant pathogens, notably on native and commercially propagated Eucalyptus species where they cause a wide range of disease symptoms including leaf spot, leaf blotch, shoot blight and stem cankers. Some of these diseases represent major impediments to sustainable Eucalyptus forestry in several countries where infection by Mycosphaerella and Teratosphaeria species can result in reduction of wood volume and in severe cases tree death. Extensive research has been conducted on these disease complexes over the past 40 years. The incorporation of DNA-based molecular techniques has made it possible to define and to better understand the differences between the Mycosphaerella and Teratosphaeria species occurring on Eucalyptus. These studies have also enabled refinement of anamorph and teleomorph generic concepts for the genera and thus facilitated the more accurate identification of species. They have also promoted a more lucid understanding of the biology, life cycles, population biology and epidemiology of the most important pathogens in the group
Distribution of Diplodia pinea and its genotypic diversity within asymptomatic Pinus patula trees
No full textDiplodia pinea (= Sphaeropsis sapinea) is an endophytic fungus and opportunistic canker pathogen of Pinus spp. The diversity of this fungus has been studied at broad geographic scales, but little is known regarding its population structure at smaller spatial scales such as within a single tree. This is despite the importance that diversity in a single tree might hold for understanding the biology of the fungus, especially the role of the endophytic or asymptomatic phase in disease development. Moreover there was not information regarding the distribution of the fungus within healthy trees and its persistence. The genotypic diversity of these isolates was investigated using microsatellite markers. Five polymorphic markers were developed and these were used together with eight previously developed markers and vegetative compatibility tests to study the genotypic diversity of D. pinea isolates. In this study, D. pinea was isolated for the first time in the well structured stems of healthy P. patula trees along with branches and cones. From a total of 44 isolates collected from five trees, 39 microsatellite haplotypes and 32 vegetative compatibility groups (VCG's) were identified. The results indicate high genotypic diversity of D. pinea within individual asymptomatic trees which will lead to disease outbreak when trees are physiologically stressed
Calonectria species associated with cutting rot of Eucalyptus
No full textDecline in the productivity of Eucalyptus hybrid cutting production in the Guangdong Province of China is linked to cutting rot associated with several Calonectria spp. The aim of this study was to identify these fungi using morphological and DNA sequence comparisons. Two previously undescribed Calonectria spp., Ca. pseudoreteaudii sp. nov. and Ca. cerciana sp. nov. were identified together with Ca. pauciramosa. Calonectria pseudoreteaudii resides in the Ca. reteaudii complex and Ca. cerciana is closely related to Ca. morganii. Connected to the discovery of Ca. pseudoreteaudii, species in the Ca. reteaudii complex were re-considered and the group is shown to accommodate two cryptic species. These originate from Australia and are described as Ca. queenslandica sp. nov. and Ca. terrae-reginae sp. nov
Climate change and forest insect pests
No full textClimate change and the underlying causal factors have been thoroughly described in the scientific literature. Although well understood and documented in general, we are only beginning to understand the effects of climate change on biotic systems, such as insect communities. It seems obvious that insect pest problems should be be more important in a warmer climate; temperature has a strong impact on insect physiology and demography. It must be remembered, however, that insect distribution and abundance are controlled by many factors other than temperature, e.g. host plant, natural enemies. This chapter summarizes empirical evidence for climate change-induced insect pest problems, i.e. changed distribution range and frequency of insect outbreaks. The chapter reviews cases of forest pests for which there is sufficient scientific evidence of climate change-effects on their population dynamics leading to outbreaks, strong enough to be a concern for forest ecology and management
DNA sequence incongruence and inconsistent morphology obscure species boundaries in the Teratosphaeria suttonii species complex
No full textTeratosphaeria suttonii (=Kirramyces epicoccoides) is a leaf pathogen that can cause premature defoliation, reduced growth and vigor, and subsequent tree death of many Eucalyptus species. Although the fungus primarily infects mature leaves in the lower canopy, infections can spread to younger leaves during continued epidemics or when trees are stressed. Teratosphaeria suttonii has a wide distribution in Australia and has been introduced to many other parts of the world, most probably with germplasm used to establish plantations. The aim of this study was to establish the phylogenetic relationships between T. suttonii isolates from different countries and to consider whether cryptic species exist in a species complex. DNA from parts of the nuclear ribosomal internal transcribed spacer, beta-tubulin, and elongation factor-1 alpha genes was sequenced and analyzed for isolates from throughout the range of T. suttonii in Australia, and from six countries (China, Indonesia, South Africa, Uruguay, United States, and Vietnam) where the pathogen is introduced. Morphometrics of conidia produced both in vivo and in vitro were also considered. Analysis of the sequence data resulted in incongruent genealogies. Furthermore, groups of isolates in the genealogies could not be linked to area of origin. Similarly, differences in conidial morphology could not be linked to any of the phylogenetic groups. There was no evidence of distinct species boundaries, and isolates from Australia were closely related to those from other parts of the world. The results of this study support the treatment of T. suttonii as a morphologically and genetically diverse species in its natural range in Australia. The diversity is reflected in introduced populations
Comparison of three varieties of Leptographium wageneri using Random Amplified Polymorphic DNA
No full textLeptographium wageneri (Kendrick) Wingfield is the cause of black stain root disease in western North America. The fungus has been separated into three host-specialised morphological variants, namely L. wageneri var. wageneri (Kendrick) Wingfield, var. pseudotsugae Harrington & Cobb and var. ponderosum (Harrington & Cobb) Harrington & Cobb. These varieties have been well-characterised on the basis of both morphological and molecular data. The aim of this study was to determine whether HAPD analysis might be used in distinguishing between the three varieties of L. wageneri. Screening was performed for thirty primers, and four primers that produced unique DNA fingerprints for each variety were selected. RAPD analysis separated the three varieties of L. wageneri into distinct groups. The RAPD analysis was reliable, reproducible and might be useful in separating the three varieties of L. wageneri, which are morphologically very similar
Two new Phytophthora species from South African Eucalyptus plantations
No full textA recent study to determine the cause of collar and root rot disease outbreaks of cold tolerant Eucalyptus species in South Africa resulted in the isolation of two putative new Phytophthora species. Based on phylogenetic comparisons using the ITS and β-tubulin gene regions, these species were shown to be distinct from known species. These differences were also supported by robust morphological characteristics. The names, Phytophthora frigida sp. nov. and Phytophthora alticola sp. nov. are thus provided for these taxa, which are phylogenetically closely related to species within the ITS clade 2 (P. citricola, P. tropicali and P.multivesiculata) and 4 (P. arecae and P. megakarya), respectively. Phytophthora frigida is heterothallic, and produces stellate to rosaceous growth patterns on growth medium, corraloid hyphae, sporangia with a variety of distorted shapes and has the ability to grow at low temperatures. Phytophthora alticola is homothallic and has a slower growth rate in culture. Both P. frigida and P. alticola are pathogenic to Eucalyptus dunnii. In pathogenicity tests, they were, however, less pathogenic than P. cinnamomi, which is a well-known pathogen of Eucalyptus in South Africa
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