Search

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.

Summary During and after prolonged periods of rainfall in late spring, blighted young twigs of European beech (Fagus sylvatica) were frequently observed in several beech stands in south-western and southern Germany. Long and short shoots of young understorey trees or lower branches up to 1.5 m above the soil level were affected. Symptoms also occurred regularly on twigs in heights up to 2 m and more above the ground. Necroses usually expanded within the current year’s tissue and often also reached into the previous year’s wood. Ponding rain water in the stands or along forest roads or open soil seemed to promote the disease. Of a total of 54 symptomatic twigs collected in four stands, 37 revealed Phytophthora isolates, of which 33 were P. plurivora and four were P. cambivora. Both species caused extensive lesions on beech twigs in laboratory pathogenicity tests. Patterns of the disease indicated that these pathogens, generally considered soilborne species, in most cases are transmitted from the soil to above-ground parts of the trees via rain splash. In larger heights, however, other vectors such as snails might be responsible for transmission. Although Phytophthora spp. are well known as causal agents of seedling blight as well as root and cambium rot and aerial bleeding cankers of mature beech, to our knowledge this is the first report of a twig blight in beech associated with soilborne Phytophthora spp. In particular in periods of high precipitation, this disease might pose an additional threat to Central European beech forests, especially endangering the success of artificial and natural regeneration of beech in affected stands.

Investigations on root and crown status of spruce and beech were carried out on selected trees in the ‘Werdenfelser Land’ area (Bavarian Alps, Southern Germany). In addition, the association of fine root pathogens of the genera Phytophthora and Pythium with the trees’ rhizosphere was studied. In a variety of stands representing various site conditions, soil and root samples were taken from 12 spruce (Picea abies) pairs and eight beech (Fagus sylvatica) pairs. Each pair consisted of a healthy and a declining tree as indicated by crown transparency. The root status was characterized using a set of parameters, and correlations between crown and fine root status were observed. In spruce, most parameters decreased significantly with increasing crown transparency, whereas in beech, correlations were less pronounced. The total number of lateral roots per cm small root (diameter 2–5 mm) was significantly lower in both species for declining trees compared to healthy trees. Pythium spp. were isolated from 15 of 24 soil samples taken from under spruce, and from eight of 16 samples from under beech. Phytophthora citricola was found in two beech stands only. Among the isolated species, Pythium anandrum, Pythium inflatum and Pythium acrogynum were identified according to morphological features. After polymerase chain reaction-restriction fragment length polymorphism analysis, residual Pythium isolates were assigned to four different groups. No crown transparency-dependent differences in isolation frequency were found. In soil infestation tests, all species tested caused root damage on both young spruce and beech plants, with P. citricola being the most aggressive pathogen. Additionally, Pythium-infected beeches showed severe leaf chloroses and necroses. Due to their low isolation frequency, Phytophthora spp. are not considered to play a major role in the decline of spruce and beech in the investigated area. Pythium spp., however, were isolated frequently, showed pathogenicity towards the fine roots of spruce and beech, and are therefore considered to be at least contributing factors in the decline of Bavarian mountain forests. Etude comparee de l’etat des racines fines d’epiceas et de hetres, sains ou deperissants, dans les Alpes bavaroises, et presence d’especes de Phytophthora et de Pythium Les travaux sur l’etat des racines et du houppier de l’epicea et du hetre dans les Alpes bavaroises ont ete realises dans la region ‘Werdenfelser Land’ (sud de l’Allemagne). Les parasites racinaires des genres Phytophthora et Pythium associes a la rhizosphere des arbres ont egalement eteetudies. Dans un certain nombre de peuplements en conditions variees, des echantillons de sol et de racines ont ete preleves sur 12 couples d’epiceas et 8 couples de hetres. Chaque couple comprenait un arbre sain et un arbre deperissant, evalues d’apres la transparence du houppier. L’etat racinaire a ete caracterise en utilisant un ensemble de parametres, et des correlations entre l’etat des racines et celui du houppier ont ete obtenues. Chez l’epicea, la plupart des parametres diminuaient significativement quand la transparence augmentait; chez le hetre, les correlations etaient moins nettes. Le nombre total de racines laterales par cm de petite racine (diametre 2–5 mm) etait significativement plus faible chez les arbres deperissants des deux especes, que chez les arbres sains. Des Pythium spp. ont ete isoles chez 15 echantillons de sol parmi les 24 preleves sous epicea, et chez 8 sur 16 dans le cas du hetre. Phytophthora citricola n’a ete trouve que dans deux hetraies. Parmi les especes isolees, Pythium anandrum, P. inflatum et P. acrogynum ont ete identifies morphologiquement. Apres analyse PCR-RFLP, les autres isolats de Pythium ont ete repartis entre 4 groupes differents. Aucune difference liee a la transparence n’a ete trouvee pour la frequence d’isolement. Dans des tests d’infection par le sol, toutes les especes testees causaient des degâts racinaires a l’epicea et au hetre, P. citricolaetant le plus agressif. Les hetres infectes par Pythium ont presente d’importantes chloroses et necroses foliaires. A cause de leur faible taux d’isolement, on considere que les Phytophthora spp. ne jouent pas un role majeur dans le deperissement de l’epicea et du hetre, dans la zone etudiee. Cependant, les Pythium spp. ayant ete isoles frequemment et s’etant montres parasites vis-a-vis les racines fines de l’epicea et du hetre, ils doivent etre consideres au moins comme des facteurs qui contribuent au deperissement dans les forets de montagne de la Baviere. Vergleichende Untersuchungen zum Feinwurzelzustand von gesunden und kranken Fichten und Buchen in den Bayerischen Alpen sowie zum Vorkommen von Phytophthora- und Pythium-Arten An ausgesuchten Probebaumen im Werdenfelser Land wurde untersucht, ob ein Zusammenhang zwischen Kronen-und Feinwurzelzustand von Fichten und Buchen im Bayerischen Alpenraum besteht. Auch wurde gepruft, ob bodenburtige Wurzelpathogene aus den Gattungen Phytophthora und Pythium an den festgestellten Feinwurzelschaden beteiligt sind. Auf unterschiedlichen Standorten wurden von insgesamt 12 Fichten-und 8 Buchen-Baumpaaren, die unterschiedliche Kronenzustande reprasentierten, Boden-und Wurzelproben entnommen. Nach Charakterisierung des Wurzelzustandes mit Hilfe verschiedener Wurzelparameter liesen sich insbesondere bei der Fichte bei den meisten Parametern signifikant negative Korrelationen zum Verlichtungsgrad der Krone feststellen. Bei der Buche waren diese Zusammenhange weniger straff und nicht statistisch absicherbar. Bei dem Parameter “Seitenwurzeln pro cm Tragerwurzel” waren bei beiden Baumarten signifikante Unterschiede zwischen gesund erscheinenden und verlichteten Βaumen festzustellen. Bei Isolierversuchen konnten aus 15 von 24 unter Fichten, und aus 8 von 16 unter Buchen entnommenen Bodenproben Pythium-Arten isoliert werden, wahrend in nur 2 Bestanden an Buche Phytophthora citricola gefunden wurde. Isoliert wurden u.a. Pythium anandrum, P. inflatum und P. acrogynum. Die ubrigen Pythium-Isolate konnten morphologisch, sowie nach PCR-RFLPs in 4 Gruppen eingeteilt werden. Die Isolierrate zeigte keine Abhangigkeit vom Kronenzustand des jeweiligen Baumes. In Infektionsversuchen losten alle getesteten Arten Schaden am Wurzelsystem junger Fichten und Buchen aus, wobei sich P. citricola als am aggressivsten gegenuber beiden Baumarten erwies. Pythium-Arten losten auserdem deutliche Blattsymptome an den infizierten Buchen aus. In der Diskussion wird der Gattung Phytophthora trotz der Ergebnisse der Infektionsversuche aufgrund der geringen Isolierrate keine Bedeutung fur die Verlichtungserscheinungen in den untersuchten Waldern zugeschrieben. Pythium-Arten dagegen konnten, aufgrund der nachgewiesenen Aggressivitat und der hohen Isolierfrequenz, an den dort beobachteten Schaden beteiligt sein.