Summary
In recent years, Common ash (Fraxinus excelsior) throughout Europe has been severely impacted by a leaf and twig dieback caused by the hyphomycete Chalara fraxinea. The reasons for its current devastating outbreak, however, still remain unclear. Here, we report the presence of four Phytophthora taxa in declining ash stands in Poland and Denmark. Phytophthora cactorum, Phytophthora plurivora, Phytophthora taxon salixsoil and Phytophthora gonapodyides were isolated from rhizosphere soil samples and necrotic bark lesions on stems and roots of mature declining ash trees in four stands. The first three species proved to be aggressive to abscised roots, twigs and leaves of F. excelsior in inoculation experiments. Soil infestation tests also confirmed their pathogenicity towards fine and feeder roots of ash seedlings. Our results provide first evidence for an involvement of Phytophthora species as a contributing factor in current decline phenomena of F. excelsior across Europe. Specifically, they may act as a predisposing factor for trees subsequently infected by C. fraxinea. Phytophthora species from ash stands also proved to be aggressive towards a wide range of tree and shrub species commonly associated with F. excelsior in mixed stands. Although damage varied considerably depending on the Phytophthora species/isolate–host plant combination, these results show that many woody species may be a potential source for survival and inoculum build‐up of soilborne Phytophthora spp. in ash stands and forest ecosystems in general.
<em>Phytophthora plurivora</em> was the most often detected species from water using rhododendron baits. The species was isolated from water of two rivers, Jasieniec and Korabiewka, a water pond and a drainage canal from March to November, 2008 (in Korabiewka river also in December). The highest population density of <em>P. plurivora</em> was observed in March and April in water pond and canal, and in May in both analysed rivers. In laboratory trials all tested isolates colonized rhododendron and poplar leaves. Isolates from drainage canal were the most pathogenic for rhododendron. Isolates detected in March from water pond and two rivers caused the quickest spread of necrosis on leaf blades. On poplar leaves the fastest development of necrotic spots was observed when isolates obtained in June and November were used for inoculation, while the isolate from September sample was less pathogenic.
The plant pathogen, Phytophthora cryptogea, was recovered from 4/5 of the 75 poinsettias collected from two greenhouse farms. The poinsettias showed stunting and wilting as well as stem base and root rot symptoms. Representative isolates of the species from two poinsettia cultivars, colonized stem parts, leaf petioles, leaf blades and root parts. Stem parts of 6 cultivars were colonized at a slower rate than other poinsettia organs. Isolate from Euphorbia pulcherrima also colonized stem parts and leaf blades of E. amygdaloides, E. cyparissias, and E. polychroma. Isolates of P. cryptogea from the other 6 plant hosts, colonized the overground parts of poinsettia cv. Allegro. This is the first report describing symptoms and colonization of poinsettia cultivars by P. cryptogea in Poland.
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