A molecular phylogeny of Phytophthora and related oomycetes.

Essential fatty acids (EFA) remain one of the least well-understood and enigmatic nutrients in aquaculture nutrition. Of all dietary nutrients none has a greater direct impact on the composition of its consumer. Their importance stems not only to their impact on animal growth, but also to factors such as reproduction, immunity and product quality. Docosahexaenoic acid (DHA; 22:6n-3) has consistently been shown to provide the greatest EFA value to most species. However, the nutritional value of eicosapentaenoic (EPA; 20:5n-3) and arachidonic (ARA; 20:4n-6) acids has also been significantly greater than that exhibited by linolenic (LNA; 18:3n-3) and linoleic (LOA; 18:2n-6) acids. All five fatty acids have been shown to provide EFA value to most aquaculture species, although the optimal dietary inclusion levels and balance among the fatty-acid classes (n-3 and n-6) and fatty-acid chain lengths (18-C, 20-C or 22-C) vary among species. Environmental origin (freshwater, estuarine or marine) appears to be a primary factor influencing the difference in EFA requirements. The role that EFA play in osmoregulation clearly shows how these nutrients affect animals from different aquatic environments. The influence of EFA on growth also appears to be greatest in larval fish and crustaceans, possibly because of their reduced ability to digest and absorb lipids, but also because of a proportionally higher demand for EFA in the development of, in particular, neural tissues. Despite an abundance of research since the 1970s on the EFA requirements of aquaculture species there remains a need to better define the EFA requirements of most aquaculture species. Of all major aquaculture species only the penaeid shrimp has a comprehensively documented assessment of its nutritional requirements for EFA. The nutritional requirements for EFA in most fish species have not been comprehensively studied and those species that were fully examined in the 1970s and 1980s now need to be reassessed in light of recent changes to the use of high-nutrient-density diets that were not routinely used in either practice or research during that earlier period. In addition to changes in dietary specification strategies, declining dependence on marine-origin lipid sources in recent years has placed an increased imperative on understanding the dietary need for long-chain polyunsaturated fatty acids (lcPUFA). As aquaculture continues to grow there will be an increased use of alternative lipid resources, such as grain, algal and rendered oils, to provide dietary lipids. In addition to dietary dilution of natural EFA sources through the use of these raw materials, they will also bring new challenges, such as increased levels of n-6 and 18-C polyunsaturated fatty acids (PUFA). Introduction of these n-6 and PUFA fatty acids to the diet of aquaculture species will not only influence the nutritional demands of these animals, but will also affect their flesh quality characteristics by reducing their level of n-3 lcPUFA. This dilemma will demand an increased prioritisation on the value of lipid sources rich in n-3 lcPUFA, but is also likely to stimulate the development of alternative sources of lcPUFA.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1753-5131.2009.01006.x

Exploring the nutritional demand for essential fatty acids by aquaculture species

Soilborne root diseases caused by plant pathogenic Pythium species cause serious losses in a number of agricultural production systems, which has led to a considerable effort devoted to the development of biological agents for disease control. In this article we review information on the ecology and biological control of these pathogens with the premise that a clear understanding of the ecology of the pathogen will assist in the development of efficacious biocontrol agents. The lifecycles of the pathogens and etiology of host infection also are reviewed, as are epidemiological concepts of inoculum-disease relationships and the influence of environmental factors on pathogen aggressiveness and host susceptibility. A number of fungal and bacterial biocontrol agents are discussed and parallels between their ecology and that of the target pathogens highlighted. The mechanisms by which these microbial agents suppress diseases caused by Pythium spp., such as interference with pathogen survival, disruption of the...

Soilborne plant diseases caused by pythium spp: ecology, epidemiology, and prospects for biological control

In order to investigate phylogenetic relationships of the Peronosporomycetes (Oomycetes), nuclear large subunit ribosomal DNA sequences containing the D1 and D2 region were analyzed of 92 species belonging to the orders Peronosporales, Pythiales, Leptomitales, Rhipidiales, Saprolegniales and Sclerosporales. The data were analyzed applying methods of neighbor-joining as well as maximum parsimony, both statistically supported using the bootstrap method. The results confirm the major division between the Pythiales and Peronosporales on the one hand and the Saprolegniales, Leptomitales, and Rhipidiales on the other. The Sclerosporales were shown to be polyphyletic; while Sclerosporaceae are nested within the Peronosporaceae, the Verrucalvaceae are merged within the Saprolegniales. Within the Peronosporomycetidae, Pythiales as well as Peronosporales as currently defined are polyphyletic. The well supported Albugo clade appears to be the most basal lineage, followed by a Pythium-Lagenidium clade. The third, highly supported clade comprises the Peronosporaceae together with Sclerospora, Phytophthora, and Peronophythora. Peronophythora is placed within Phytophthora, indicating that both genera should be merged. Bremiella seems to be polyphyletic within the genus Plasmopara, suggesting a transfer to Plasmopara. The species of Peronospora do not appear as a monophyletic group. Peronospora species growing on Brassicaceae form a highly supported clade.

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Phylogenetic relationships of the downy mildews (Peronosporales) and related groups based on nuclear large subunit ribosomal DNA sequences

Pythium insidiosum sp. nov., the etiologic agent of pythiosis, a cosmopolitan disease of horses, cattle, and dogs, is described and illustrated. Images

Pythium insidiosum sp. nov., the etiologic agent of pythiosis.

A new medium, VP 3 medium (containing vancomycin, PCNB, penicillin and pimaricin), modified from Mircetich's MPVM medium, has been devised and tested. The VP 3 medium is much cheaper because of the small quantity of vancomycin used. It has been shown suitable for the collection and isolation of a wide range of diverse Pythium species and to give high counts for propagule densities from a range of cultivated and uncultivated soils. Surface-spread dilutions are recommended in preference to the incorporation of the dilution in cool molten agar. The degree of replication needed for retrieval of particular kinds of data has been assessed.

An improved method and medium for quantitative estimates of populations of Pythium species from soil

The identity of the oomycete causing ‘Kikuyu Yellows’, with a reclassification of the downy mildews

Isolation of Saprolegnia parasitica from the crayfish Astacus leptodactylus

An improved method for the isolation of Pythium from soil has been devised. Using this technique 45 species of Pythium have been isolated from the Reading area. Up to 8600 propagules g d.w. −1 of Pythium have been recorded and population levels exceeding 2000 propagules g d.w. −1 were frequent. Community structure of the component species populations has been assessed by Braun-Blanquet analysis and Sorensen coefficients. The former revealed species associations which could be related to taxonomic diversity within each association. The qualitative assessments from both methods and the quantitative assessment using the Sorensen coefficient indicate that locality and land management influence community structure. Soil pH is regarded as being the most important of the physico-chemical soil characteristics tested.

M.W. Dick

Papers

An improved method and medium for quantitative estimates of populations of Pythium species from soil

The identity of the oomycete causing ‘Kikuyu Yellows’, with a reclassification of the downy mildews

Isolation of Saprolegnia parasitica from the crayfish Astacus leptodactylus

Distribution and frequency of Pythium species in parkland and farmland soils

A new genus of the Verrucalvaceae (Oomycetes)