Phytophthora species associated with dieback of sweet chestnut in Western Turkey

Sweet chestnut (Castanea sativa) is an important tree species in the Marmara and Aegean regions of Turkey as these two regions produce the great majority of edible nuts, especially those used for marron glacé production. Chestnut forests and orchards in these regions showing severe dieback symptoms not associated with chestnut blight were investigated to determine the role of Phytophthora spp. in the decline syndrome. Soil samples were collected from around 108 symptomatic chestnut trees at 29 sites and Phytophthora spp. isolated using soil baiting technique and selective medium. Species isolated were identified by cultural characteristics and ITS sequencing. Phytophthora x cambivora was the dominant species detected in 13 sites, followed by P. cinnamomi (5 sites), P. plurivora (3 sites) and P. cryptogea (1 site). Phytophthora x cambivora was present in both regions, while P. cinnamomi was found only in the Marmara region in coastal areas around Istanbul. When inoculated at the stem bases of 3‐year‐old chestnut saplings, P. cinnamomi produced significantly longer necrotic lesions (7.8–12.0 cm) than P. x cambivora (2.6–6.3 cm) by 12 days after inoculation. Phytophthora plurivora was the least aggressive species causing only small lesions. Phytophthora cryptogea, which represents the first record on chestnut in Turkey, produced intermediate sized lesions in between P. x cambivora and P. plurivora. These results indicate that P. x cambivora and in some areas P. cinnamomi play major roles in the observed dieback of sweet chestnut in western Turkey.     

Phytophthora citrophthora,a new pathogen causing decline on horse chestnut in Turkey

Dieback symptoms were observed on horse chestnut trees planted approximately 40 years ago in Ankara, Turkey. Lesions at the stem bases of the affected trees were similar to those of ink disease on sweet chestnut. A Phytophthora sp. was isolated from the fine roots and soil samples collected around the stem bases by baiting using chestnut leaves. The pathogen was identified as Phytophthora citrophthora based on several morphological features and DNA sequences of the ITS region. Pathogenicity of P. citrophthora was tested by stem inoculation on 3‐year‐old horse chestnut saplings. P. citrophthora produced large cankers in 20 days and killed 40% of the saplings. This is the first report of P. citrophthora causing dieback on horse chestnut.     

Involvement of Phytophthora species in white oak (Quercus alba) decline in southern Ohio

This study was initiated to investigate the possible role of Phytophthora species in white oak decline (Quercus alba) in southern Ohio at Scioto Trail State Forest. Surveys demonstrated the presence of four species of Phytophthora including one novel species. By far, the most common species was P. cinnamomi; P. citricola and P. cambivora were isolated infrequently. In few instances, P. cinnamomi was isolated from fine roots and necroses on larger roots. No special pattern of incidence was found, but P. cinnamomi was more commonly isolated from greater Integrated Moisture Index values suggesting moist lower bottomlands favour this Phytophthora species. When tree crown condition was examined relative to the presence of Phytophthora, no significant association was found. However, roots of declining P. cinnamomi‐infested trees had 2.5 times less fine roots than non‐infested and healthy trees, which was significantly different. The population densities of P. cinnamomi from declining trees were significantly greater than from healthy trees, suggesting increased pathogen activity that has the potential to cause dieback and decline and possibly the cause of a reduced fine root amount found on declining trees.     

Occurrence and pathogenicity of Phytophthora × cambivora on Prunus laurocerasus in Serbia

Cherry laurel (Prunus laurocerasus) is a native plant species in Serbian forests, but is also widely used for ornamental plantings. Following two extremely wet summers in 2014 and 2015, in spring and summer of 2016 and 2017, numerous cherry laurel plants with symptoms indicative for Phytophthora diseases, like wilting and chlorosis of leaves, dieback and bleeding bark necroses, were recorded in a park in Belgrade and in two ornamental nurseries in central Serbia. From necrotic bark samples and rhizosphere soil, self‐sterile Phytophthora isolates with woolly colonies were obtained. Due to the production of ellipsoid and elongated, non‐papillate sporangia in water and of ornamented oogonia with two‐celled antheridia in mating tests with tester strains of both Phytophthora × cambivora and P. cryptogea, these isolates were identified as P. ×cambivora which was confirmed by ITS sequence analysis. Pathogenicity of P. ×cambivora from cherry laurel (PCCL) was tested by inoculating one‐year‐old seedlings of cherry laurel under the bark. P. ×cambivora from European beech (PCB), and isolates of P. cactorum (CAC), P. cryptogea (CRY), P. plurivora (PLU) and P. ×serendipita (SER) were included as comparison. Three and a half months after inoculation, nine of the twelve plants in PCB, three in PCCL and CAC and two in PLU declined with longitudinal necroses and chlorosis, wilting and premature shedding of leaves. These results demonstrate the ability of P. ×cambivora to infect and cause decline of cherry laurel plants. The particularly high aggressiveness of the P. ×cambivora isolate from beech shows that this pathogen poses a serious risk to cherry laurel in the rare natural communities of cherry laurel and beech in Serbia.     

Occurrence of Phytophthora species in oak stands in Italy and their association with declining oak trees

Soil‐borne species of Phytophthora were isolated from 19 of 30 examined oak forest areas in Italy. The frequency of isolated Phytophthora spp. (35.2%) was significantly correlated with soil pH and longitude of the sites. Eleven Phytophthora species were detected. Phytophthora cambivora, P. cinnamomi and P. cactorum were recovered from sites in central and southern Italy whereas P. quercina was isolated in the northern and central part of the country. Phytophthora citricola occurred all over Italy. Phytophthora quercina was the only species significantly associated with declining oak trees.     

Pathogenicity of Phytophthora species and Pythium undulatum isolated from Abies procera Christmas trees in Ireland

Phytophthora cryptogea, Phytophthora cinnamomi, Phytophthora cambivora, Phytophthora megasperma and Pythium undulatum were isolated from diseased Noble fir (Abies procera) seedlings and soil associated with dead Noble fir in Ireland. Seedlings of four Christmas tree species (A. procera, Picea sitchensis, Picea abies and Pinus contorta) were inoculated with these oomycetes to test their pathogenicity and the susceptibility of the various tree species. Phytophthora spp. and Pythium undulatum caused root rot on all tree species. Disease symptoms included reddish brown cambial discoloration, crown symptoms, brown foliage, dark brown roots, root rot and seedling mortality. These symptoms were similar to those observed on Noble fir in naturally infested plantations. Pythium undulatum appeared as the most virulent pathogen followed by P. cinnamomi, P. cambivora, P. megasperma and P. cryptogea. Noble fir showed to be most susceptible and lodgepole pine most tolerant while Sitka spruce and Norway spruce were intermediate.     

Pathogenicity to alder of Phytophthora species from riparian ecosystems in western Oregon

Described as one of the most destructive pathogens of agricultural crops and forest trees, Phytophthora is a genus of microorganisms containing over 100 known species. Phytophthora alni has caused collar and root disease in alders throughout Europe, and a subspecies has recently been isolated in North America. Reports of canopy dieback in red alder, Alnus rubra, prompted a survey of their overall health and to determine whether P. alni was present in western Oregon riparian ecosystems. Over 1100 Phytophthora isolates were recovered, representing 20 species and 2 taxa. Phytophthora‐type cankers were observed in many trees, and their incidence was positively correlated with canopy dieback. High levels of mortality for red alder were not observed, which suggests these Phytophthora species are not aggressive pathogens. To test this hypothesis, three stem wound inoculations and one root dip were conducted on red alder seedlings using 13 Phytophthora species recovered from the riparian survey. Ten of the 13 Phytophthora species produced significant lesions in at least one pathogenicity test. Phytophthora siskiyouensis produced the largest lesions on red alder from the two stem wound inoculation tests conducted under summer conditions, while P. taxon Pgchlamydo caused the largest lesions during the winter stem wound inoculation test. Phytophthora gonapodyides, P. taxon Pgchlamydo and P. siskiyouensis have previously been found associated with necrotic alder roots and bole cankers in the field, and with the pathogenicity results reported here, we have established these species as causes of Phytophthora root disease and Phytophthora bole canker of alder in Oregon. While none of the Phytophthora species were especially aggressive towards red alder in the pathogenicity tests, they did cause localized disease symptoms. By weakening the root systems or boles of alders, the Phytophthoras could be leaving alders more susceptible to other insects and pathogens.     

Pathogenicity of three Phytophthora spp. causing late seedling rot of Quercus ilex ssp. ballota

Within a research project on quality of plants for forestation of agricultural lands, we studied the aetiology of a late seedling rot affecting holm oak (Quercus ilex ssp. ballota) in two forest nurseries in southern Spain. Major disease symptoms were foliage wilting and necrosis of feeder roots. Phytophthora cinnamomi, Phytophthora cryptogea and Phytophthora drechsleri were isolated from necrotic roots of holm oaks. Selected isolates of the three Phytophthora species were pathogenic to Quercus ilex ssp. ballota and Quercus suber seedlings in artificial inoculations. Soil flooding conditions were essential for infection and root rot development. There was no host specificity among the species, the isolates of P. cinnamomi being the most virulent in all inoculated plants. In these inoculations, Q. ilex ssp. ballota plants were more susceptible than those of Q. suber. This work is the first report of P. cinnamomi, P. drechsleri and P. cryptogea affecting Q. ilex ssp. ballota in forest nurseries.     

New records of Phytophthora on trees in Britain

Phytophthora megasperma var. megasperma and unidentified isolates of Pbytophthora rescmbling P. cambivora were obtained from dead and dying roots of Horsc chestnut trccs with sparse foliage, small chlorotic leaves and branch dieback. P. citricola was isolatcd from soil around dead roots of similarly affected trccs and from around dcad feeder roots of a tree suffering from the common but unexplained Leaf scorch (marginal leaf necrosis) of Horse chestnut. Both P. cactorum and P. citricola were isolated from oozing acrial stem lesions on Horse chestnut trees. Inoculations with these species reproduced the damage.     

Biomass,nutrient and pigment content of beech (Fagus sylvatica) saplings infected with Phytophthora citricola,P. cambivora,P. pseudosyringae and P. undulata

Fagus sylvatica saplings were infected with Phytophthora citricola, Phytophthora cambivora, Phytophthora pseudosyringae and Phytophthora undulata to study the influence of these root pathogens on total belowground and aboveground biomass, on the nutrient distribution within plants, on the concentration of plastid pigments, including tocopherol and on components of the xanthophyll cycle. Phytophthora citricola and P. cambivora infection significantly reduced total biomass of beech when compared with control plants and finally most of these plants died at the end of the experiment. However, beech invaded by the other two Phytophthora spp. did not differ from control plants and none of them was killed. Fine root length as well as the number of root tips of all infected beeches were reduced between 30 and 50%. The excellent growth of beech infected with P. pseudosyringae and P. undulata when compared with control plants was correlated with a strong increase of important root efficiency parameters. Phytophthora citricola and P. cambivora caused a significant reduction in nitrogen concentration of leaves in comparison with control and other infected plants, whereas this nutrient was slightly increased in fine and coarse roots. Furthermore, the phosphorus and potassium concentrations in leaves were impaired after infection with P. citricola. However, foliar concentrations of Ca and Mg were not affected by the different Phytophthora spp., whereas fine and coarse roots were significantly enriched with Ca in beech infected with P. citricola or P. cambivora. The concentrations of α‐tocopherol and xanthophyll cycle pigments were increased in plants infected by P. citricola and P. cambivora, indicating that several reactive oxygen species might be formed in leaves during infection.     

Large‐scale fuzzy rule‐based prediction for suitable chestnut ink disease sites: a case study in north‐east Italy

In the past few decades, economic interest in the cultivation of chestnuts for both timber and nut production has resurfaced in the Mediterranean area. However, chestnut cultivation has suffered in recent years from the spread of exotic pests, such as the gall wasp Dryocosmus kuriphilus, and from the resurgence of previously present diseases, most likely due to anomalous climate dynamics. This is the case with chestnut ink disease, caused by the soilborne pathogens Phytophthora cinnamomi and P. cambivora. Scientific and technical support in monitoring and management, that utilizes new forecasting approaches incorporating related environmental variables, is therefore essential. The main aim of this study was to develop a mathematical model assessing the potential for the establishment of chestnut ink disease at a large scale. Towards this goal, fuzzy rule‐based theory was applied to the environmental variables associated with host presence, pathogens' ecological niches and ink disease symptoms expression. The effectiveness of the rule‐based modelling outcomes, provided with uncertainty maps to facilitate their correct interpretation, was confirmed by detailed field data collected from a large chestnut‐growing area where ink disease has been increasing in recent years. The final model gave consistent predictions for disease presence. For this reason, it represents a flexible and valuable decision‐support tool to forecast which sites are at risk from CID.     

Foliar susceptibility of eastern oak species to Phytophthora infection

Seven different Phytophthora species were used to test the foliar susceptibility of the common eastern US oak species and understory plants to Phytophthora infection. The Phytophthora species employed were Phytophthora cambivora, Phytophthora cinnamomi, Phytophthora citricola, Phytophthora europaea, Phytophthora quercetorum, Phytophthora quercina‐like and Phytophthora sp1. Inoculation of detached‐leaves with agar plugs containing mycelia of Phytophthora provided an estimate of their relative susceptibility. Lesions were always greater when foliage was wounded and young. On deciduous plants, lesion sizes were considerably reduced with the increasing foliar age, although with evergreen plants lesion sizes remained similar regardless of foliar age when more aggressive isolates were tested. Infections seldom resulted when foliage was not wounded. With young and mature foliage, P. citricola usually produced the largest lesions. Young foliage of Quercus rubra was the most susceptible to infection followed by Castanea dentata for both wounded and non‐wounded inoculations. Mature foliage of Hamamelis virginiana, Kalmia latifolia and Quercus alba were the most susceptible to wound and non‐wound inoculations.     

Fine root dynamics of oak saplings in response to Phytophthora cinnamomi infection under different temperatures and durations

The belowground effects of Phytophthora cinnamomi on 1‐year‐old saplings of two common oak species in mid‐Atlantic US forests, white (Quercus alba) and red oak (Q. rubra), were examined after incubation in pathogen‐infested soilless potting mix. Fine root lengths (0–1.5 mm in diameter) of both oak species were quantified after incubation at successive 30‐day intervals up to 300 days, for a total of 10 incubation periods. In addition, colony‐forming units (CFU) of P. cinnamomi were quantified after white oak saplings were incubated in infested soilless potting mix at different temperature/duration combinations that reflect soil conditions present in the mid‐Atlantic United States. Impact of P. cinnamomi on fine root lengths of red and white oak saplings varied considerably over time. Significant periods of fine root loss occurred primarily during spring, when bud break and leaf flush began for both oak species. Red oaks had 17% fine root loss on average, while white oaks appeared more resistant to P. cinnamomi infection with a 2% decrease in fine roots over the course of the experiment. Phytophthora cinnamomi CFU declined significantly with exposure to all incubation temperatures except 8°C. This was in contrast to in vitro experiments, where the optimum temperature for mycelial growth was determined to be 21°C and above. Significant fine root loss caused by P. cinnamomi depended on plant phenology and the oak species tested. Extreme soil temperatures have a significant adverse impact on temporal changes of P. cinnamomi population.     

New insights into the survival strategy of the invasive soilborne pathogen Phytophthora cinnamomi in different natural ecosystems in Western Australia

Despite its importance as one of the most notorious, globally distributed, multihost plant pathogens, knowledge on the survival strategy of Phytophthora cinnamomi in seasonally dry climates is limited. Soil and fine roots were collected from the rhizosphere of severely declining or recently dead specimens of 13 woody species at 11 dieback sites and two dieback spots and from healthy specimens of five woody species at four dieback‐free sites in native forests, woodlands and heathlands of the south‐west of Western Australia (WA). Phytophthora cinnamomi was recovered from 80.4, 78.1 and 100% of tested soil, fine root and soil–debris slurry samples at the 11 dieback sites, in some cases even after 18‐month storage under air‐dry conditions, but not from the small dieback spots and the healthy sites. Direct isolations from soil–debris slurry showed that P. cinnamomi colonies exclusively originated from fine roots and root fragments not from free propagules in the soil. Microscopic investigation of P. cinnamomi‐infected fine and small woody roots and root fragments demonstrated in 68.8, 81.3 and 93.8% of samples from nine woody species the presence of thick‐walled oospores, stromata‐like hyphal aggregations and intracellular hyphae encased by lignitubers, respectively, while thin‐walled putative chlamydospores were found in only 21.2% of samples from five woody species. These findings were confirmed by microscopic examination of fine roots from artificially inoculated young trees of 10 woody species. It is suggested that (i) the main function of chlamydospores is the survival in moderately dry conditions between consecutive rain events and (ii) selfed oospores, hyphal aggregations, and encased hyphae and vesicles in infected root tissue of both host and non‐host species are the major long‐term survival propagules of P. cinnamomi during the extremely dry summer conditions in WA.     

Susceptibility of Eucalyptus grandis and Acacia mearnsii seedlings to five Phytophthora species common in South African plantations

Eucalyptus grandis and its hybrids, as well as Acacia mearnsii, are important non‐native trees commonly propagated for forestry purposes in South Africa. In this study, we conducted pathogenicity trials to assess the relative importance of five commonly isolated Phytophthora spp. (Phytophthora alticola, P. cinnamomi, P. frigida, P. multivora and P. nicotianae) from the plantation environment on E. grandis and A. mearnsii seedlings. Overall E. grandis was more susceptible to the tested Phytophthora spp. than A. mearnsii. Phytophthora cinnamomi was the only pathogen that had a significant negative effect on both the host tree species, leading to a reduction in root and shoot weight as well as to death in the case of E. grandis. Phytophthora alticola and P. nicotianae exclusively affected E. grandis and A. mearnsii, respectively. This study updated the current knowledge on the pathogenicity of Phytophthora spp. on two important non‐native commercially propagated tree species from South Africa.     

Occurrence of Phytophthora species in riparian stands of black alder (Alnus glutinosa) in Slovakia

During the monitoring period of alder decline phenomenon in Slovakia, indicative symptoms of Phytophthora diseases were observed in riverside stands in Slovakia. The study aimed to test the presence and diversity of Phytophthora species in declining alder stands. The samples were collected from six stands situated by rivers/streams in the central and eastern parts of Slovakia. Phytophthora plurivora and P. cactorum were detected in soil, root and water samples. The most isolated species was P. plurivora. Both Phytophthora species have been recognized in the literature as the perpetrators of black alder dieback together with other species, such as P. alni or P. polonica. To our knowledge, this is the first report of P. plurivora and P. cactorum in riverside stands with the main share of black alder in Slovakia.     

Multispecies Phytophthora disease patterns in declining beech stands

Decline diseases are typically caused by complex abiotic and biotic interactions and characterized by a suite of symptoms indicative of low plant vigour. Diseased trees are frequently infected by Phytophthora, but the complex interactions between pathogen, host and the heterogeneous forest environment mask a comprehensive understanding of the aetiology. In the present study, we surveyed European beech (Fagus sylvatica) stands in Swiss forests with recent increases in bleeding lesions for the presence of Phytophthora. We used a combined approach of analysing soil and bark samples from trees displaying bleeding lesions and trees free from bleeding lesions. Soil baiting revealed a higher prevalence of Phytophthora spp. around trees with bleeding lesions than around trees without bleeding lesions. For the bark samples from bleeding lesions, we used several detection methods. Phytophthora spp. were detected in 74% of the trees by an immunological on‐site diagnostic kit, in 64% by a specific PCR assay, and 38% by isolation on selective media. All samples tested were negative for P. ramorum using qPCR. Overall, nine Phytophthora species were identified by ITS sequencing, the most common of which were P. plurivora, P. gonapodyides, P. × cambivora and P. syringae. We identified distinct species in bleeding lesions and the rhizosphere of the same host tree which suggests a multispecies Phytophthora disease patterns in these declining beech. Among the recovered species, P. × cambivora and P. × serendipita were identified as hybrid genotypes with the former abundant in bleeding lesions.     

First report of Phytophthora cinnamomi in forest stands in Germany

Phytophthora cinnamomi was repeatedly isolated from the rhizosphere of recently dead sweet chestnut trees in the upper Rhine valley. Identification was genetically confirmed by RFLP and sequencing of the ITS‐region. This is the first record of P. cinnamomi on forest land in Germany. Mode of introduction of this invasive species and potential factors favouring establishment are discussed.