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Pathogenic oomycetes represented mainly by the species of Phytophthora genus are among the most dangerous plant pathogens. They pose a serious threat for agricultural as well as wild plants, and are involved in forest decline worldwide. Over 140 pathogenic Phytophthora species have been identified so far. The common infection symptoms include rotting of below− and aboveground parts of plants, causing weakness and slow decline of infected trees. The economic losses caused by certain Phytophthora species may rich even 100%. Globalization and border opening have facilitated the transport of plant material between countries and continents, thus increased the risk of transfer of various Phytophthora genus representatives to new geographical locations. Global warming (e.g. mild winters) have facilitated the expansion of species from southern to northern Europe. Among Polish Phytophthora isolates are species that have previously been known only in nurseries (e.g. P. cactorum), but nowadays they are also isolated from forests (e.g. oak stand in the Krotoszyn Plateau). It suggests the pathway from nurseries to stands with plant for plantings and attached soil. There are also new species, that have not been isolated so far in the world (P. polonica) or found far away from Poland (P. fragaliaefolia in Japan on strawberry). The possible natural pathways are birds and water courses. In Mazowsze and Wielkopolska regions (C and W Poland) the polyphagous P. cinnamomi was found on pedunculate oaks (Quercus robur). This species causes significant damage to red oak forest in France, but also threats Jarrah forests in the Australian ecosystem (it attacks ca. 1000 species of plants). Fortunately, in addition to time consuming and laborious classic methods of Phytophthora identification, based on morphology and physiological properties, molecular methods that are based on immunological tests and chromosomal or mitochondrial DNA markers identification have come into common use. Despite morphological similarity to true fungi, oomycetes are more closely related to diatoms and brown algae, and have several structural features that differentiate them from fungi, including the cell wall composed of cellulose instead of chitin. That is one of the reasons that fungicides have a limited use in the fight with Phytophthora infections. Additionally, type of ecological niches that are settled by pathogenic oomycetes (root remnants in soil, watercourses) hinders the chemical combating. Biocontrol, i.e. the use of interspecies interactions between microorganisms (bacteria, fungi) to limit the growth and development of pathogens, seems to be a reasonable alternative.
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