Evaluating aggressiveness and host range of Alternaria dauci in a controlled environment

The aggressiveness of Alternaria dauci isolates was investigated in greenhouse conditions. Twenty‐seven isolates were pre‐selected from a large collection to represent high diversity according to geographic or host origins and intergenic spacer (IGS) polymorphism. IGS sequence analysis revealed that isolates were grouped within three different clusters. Eleven isolates were selected and inoculated on a susceptible carrot cultivar. Three criteria (mean lesion number, mean necrotic leaf area and mean disease index) were used to assess the aggressiveness of isolates. Continuous variation in aggressiveness was shown and no clear division into isolate classes was evident. For the host range study, two isolates were inoculated under greenhouse conditions onto nine cultivated Apiaceae species, two wild Daucus species and six cultivated non‐Apiaceae species representing six botanical families. Lesions varying in severity were observed on all dicot species (Apiaceae and non‐Apiaceae), but no symptoms developed on the two monocots studied (leek and sweetcorn). Plant species were also differentiated on the basis of expanding lesions (cultivated and wild carrot, dill and fennel) or non‐expanding lesions (other dicot species). Typical A. dauci conidia were observed after in vitro incubation of leaves with symptoms. Fungal structures were isolated from lesions and A. dauci was confirmed on the basis of conidial morphology and specific conventional PCR results. Genotyping of individual isolates performed with microsatellite markers confirmed the presence of the inoculated isolate. The results clearly showed that, in controlled conditions, the host range of A. dauci is not restricted to carrot.     

Characteristics and distribution of Colletotrichum species in coffee plantations in Hainan,China

Anthracnose caused by Colletotrichum species is a serious disease on a range of economically important hosts. To determine the Colletotrichum species in coffee plantations in Hainan, China, 55 isolates were obtained from Coffea arabica (arabica) and C. canephora var. robusta (robusta) in five counties. Initially, partial sequences of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were used to measure fungal genetic diversity. Then a subset of 23 isolates was selected to represent the range of genetic diversity, varieties and geographic origin for further multilocus phylogenetic analyses. These isolates belong to eight known Colletotrichum species from three Colletotrichum species complexes, including gloeosporioides (C. endophytica, C. fructicola, C. ledongense, C. siamense and C. tropicale), boninense (C. karstii), gigasporum (C. gigasporum), and one singleton species (C. brevisporum). Of these, C. siamense was isolated in all sampled counties and C. fructicola was identified in three counties. The other six species were isolated only in one or two counties. Only C. siamense and C. fructicola were isolated from arabica, whereas all eight species were isolated from robusta. Occurrence of C. brevisporum, C. endophytica, C. ledongense and C. tropicale in coffee has not been reported previously. Pathogenicity tests showed that all eight species were pathogenic to coffee leaves and fruit. In vitro tests showed that Colletotrichum isolates from coffee in Hainan were most sensitive to prochloraz, less sensitive to carbendazim, propiconazole and difenoconazole, and least sensitive to myclobutanil.     

Early detection of Neophysopella tropicalis in grapevine leaves and on spore traps by qPCR

Neophysopella tropicalis, one of the causal agents of Asian grapevine leaf rust (AGLR), can cause severe epidemics in Brazil that lead to yield losses in commercial vineyards. An early detection of the pathogen by air sampling of urediniospores on spore traps or in symptomless leaves would be valuable to multiple studies, such as epidemics modelling, risk forecasting, monitoring of pathogen introductions in rust-free areas, and predicting the beginning of epidemics. This study developed a quantitative PCR (qPCR) protocol to quantify N. tropicalis urediniospores attached to adhesive tapes and in grapevine leaves before symptom appearance. A specific primer pair was designed based on the internal transcribed spacer (ITS) sequence region of the AGLR pathogen. Standard amplification curves using genomic DNA from urediniospores of N. tropicalis and from urediniospores attached to adhesive tapes were established. Grapevine leaves inoculated with N. tropicalis were collected at 2, 5, and 7 days postinoculation (dpi). One primer pair (580F/720R) amplified a 140 bp product in all AGLR isolates but did not amplify products of other rust genera, such as Phakopsora, Puccinia, Hemileia, Tranzschelia, Cerotelium, and Coleosporium. As little as 0.1 pg DNA and 10 urediniospores of N. tropicalis attached to adhesive tapes could be detected. qPCR enabled the detection of the pathogen as early as 2 dpi, before symptom appearance. This method can be used to monitor N. tropicalis inoculum in grapevine-growing areas and to quantify symptomless infections of the AGLR pathogen.     

Response of selected South Australian native plant species to Phytophthora cinnamomi

Thirty‐seven South Australian native plant species from 11 families, including 15 threatened species in the state (of which six are listed as threatened under the federal Environment Protection and Biodiversity Conservation Act 1999) were assessed for response to infection by Phytophthora cinnamomi. Seedlings, 3–6 months old and grown in a greenhouse, were inoculated by placing infested pine wood plugs in the potting mix, maintained in moist conditions and assessed for mortality and disease symptoms for between 3 and 10 months. Thirty species were found to be susceptible, of which nine were highly susceptible, 15 moderately susceptible and six slightly susceptible. Three species were found to be resistant and results for four species were inconclusive. Six of the 15 threatened, rare or locally endangered species tested (Eucalyptus viminalis var. viminalis, Correa aemula, C. calycina, Olearia pannosa ssp. pannosa, Pomaderris halmaturina ssp. halmaturina and Prostanthera eurybioides) were moderately susceptible, while two (Allocasuarina robusta and Pultenaea graveolens) were highly susceptible. Significant populations of at least five of the threatened species susceptible to the disease are located close to confirmed or suspected Phytophthora‐infested areas or growing in areas conducive for P. cinnamomi. An effective management strategy is therefore required to avoid extinction of such species due to infection by the phytophthora dieback pathogen.     

Citrus black spot intensity and yield losses on sweet orange are affected by Phyllosticta citricarpa inoculum concentration and fruit developmental stage

Citrus black spot (CBS) symptom expression on sweet oranges has been reported to be affected by the fruit developmental stage and inoculum concentration in greenhouse conditions. However, there is little information concerning the period in which the fruit is susceptible in commercial orchards. This study assessed the influence of fruit age and inoculum concentration of Phyllosticta citricarpa on CBS intensity and fruit drop in the field. Two field trials were conducted in commercial ‘Valencia’ orchards in São Paulo state, Brazil. Fruit were inoculated from October (petal fall stage) to July (c.6.5 cm diameter), with 10¹, 10³ and 10⁵ pycnidiospores/ml. CBS symptoms and fruit drop were observed in higher levels for fruit inoculated from October to February than from March to July. In both trials, when fruit were inoculated 10 times from October to July or single-inoculated in November or December, high CBS severities of 11%–22% were observed and the proportion of fruit remaining on the tree was 48%–77%; in contrast, noninoculated fruit had severity below 1.1% and 90% remained on the tree. Inoculations in November or December of green fruit with 1.5–3.0 cm diameter using 10⁵ pycnidiospores/ml were associated with higher CBS intensity and fruit drop and shorter incubation period than inoculations with lower concentrations. This work provides a better understanding of fruit stages and P. citricarpa inoculum concentrations most related to symptom expression and losses under natural conditions and may be helpful for improvement of CBS management in the São Paulo citrus belt.     

Characterization of weeds and rotational crops as alternative hosts of Spongospora subterranea,the causal agent of powdery scab in Israel

Alternative hosts of Spongospora subterranea may allow multiplication and survival of the pathogen over time; thus, host range is important from an epidemiological aspect. Weeds and rotational crops, such as wheat and barley, were sampled from potato fields with a history of powdery scab (PS) and examined for the presence of S. subterranea by root staining followed by microscopic observations and by qPCR analysis after DNA extraction. The pathogen was detected in plants of 16 weed species from eight families and in volunteer plants of potato and wheat. The ability of the pathogen to infect weeds and rotational crops was further examined by artificial inoculations with sporosori in pot experiments. Successful inoculations occurred with 13 weed species from eight families and with 12 rotational crops from five families. The findings of this study indicate a wide host range in Israel; the families Malvaceae and Zygophyllaceae and the following species are reported for the first time as S. subterranea hosts: Solanum elaeagnifolium, Triticum aestivum, Cynodon dactylon, Phalaris paradoxa, Phalaris minor, Setaria verticillata, Rostaria cristata, Sinapis nigra, Arachis hypogaea, Medicago sativa, Astragalus hauraensis, Amaranthus albus, Chenopodium murale, Chenopodium opulifolium, Salsola soda, Malva nicaeensis, Chrysanthemum segetum, Verbesina encelioides, Ammi majus and Tribulus terrestris. Controlling weeds and avoiding the relevant rotational crops observed to be S. subterranea-positive and thus potential hosts, should be taken into consideration in the management of PS, to reduce pathogen inoculum build-up.     

Survival of Fusarium circinatum in soil and Pinus radiata needle and branch segments

Survival of Fusarium circinatum in colonized pine needles and wood pieces was measured. Naturally colonized branches and their needles were cut into small pieces and placed in mesh bags on the soil surface at two locations in northern Spain. Pieces were recovered periodically, cultured on a selective medium, and microscopically examined to identify the species. After 507 days, F. circinatum was recovered from 0 to 27% of the wood pieces and from none of the needles. After 858 days, F. circinatum was not recovered from any wood pieces but was found to be present on 1 out of 220 needle pieces analysed. Artificially infested pieces of wood and needles were placed on 5‐mm sieved soil either in plastic boxes at controlled temperature or in mesh bags under field conditions. No survival was recorded after 794 days under field conditions and the decline over time occurred more rapidly in inoculated pieces under field conditions. Soil was also infested with conidia of F. circinatum and survival was estimated. No conidia were recovered after 224 days at 30 °C, although at 20 and 5 °C the respective populations were 20 and 3700 cfu/g soil. Fusarium circinatum was not recovered from 2‐mm‐sieved soil collected under pitch canker‐infected pines. Results indicate that branch segments and needles naturally colonized by F. circinatum will not be a potential source of inoculum, and the fungus in soil is not likely to contribute to reinfection of new plantations after 2 years.     

Susceptibility of native New Zealand Myrtaceae to the South African strain of Austropuccinia psidii: A biosecurity threat

Austropuccinia psidii, cause of myrtle rust, has spread globally where Myrtaceae occur. Multiple strains of A. psidii have been identified, including a unique strain found only in South Africa. The South African strain is a biosecurity concern for species of Myrtaceae worldwide. This is because preliminary testing of South African Myrtaceae suggests it could have a wide host range, and thus has the potential to be invasive. In this study, we assessed the ability of the South African strain to infect other species of Myrtaceae by testing the susceptibility of New Zealand provenance Myrtaceae. Seedlings of four native New Zealand Myrtaceae species (Metrosideros excelsa, Leptospermum scoparium, Kunzea robusta, and Kunzea linearis) were artificially inoculated in South Africa with a single-uredinium isolate of the South African strain. Fourteen days after inoculation, uredinia, and in many cases telia, had developed on the young leaves and stems of all four host species, which led to shoot tip dieback in the more severe cases. When comparisons were made between the species, K. robusta was the least susceptible to the South African strain of A. psidii, while L. scoparium and M. excelsa were the most susceptible. While only a limited number of seed families were tested, only a small proportion of the seedlings showed resistance to infection by the South African strain. This preliminary testing highlights the potential invasive risk the South African strain poses to global Myrtaceae communities, including New Zealand Myrtaceae.     

Screening and identification of Cucurbita germplasm resistant to Xanthomonas cucurbitae,incitant of cucurbit bacterial spot

Bacterial spot of cucurbits, caused by Xanthomonas cucurbitae, is an emerging disease of cucurbits. This study was conducted to identify Cucurbita species that are resistant to X. cucurbitae. We developed a reliable method for inoculating cucurbit plants by spraying plants with X. cucurbitae suspensions containing carborundum. Symptoms of the disease developed within 7 days after inoculation. Subsequently, we evaluated the virulence of six isolates of X. cucurbitae in the greenhouse and observed that X. cucurbitae isolated from cucurbit fields in Illinois, Michigan, Kansas, Ohio, and Wisconsin were more virulent than the reference ATCC 23378 strain. Then, we screened 81 commercial cultivars of gourds, pumpkins, and squashes, as well as 300 Cucurbita spp. accessions, for their resistance to X. cucurbitae under greenhouse and field conditions. In the greenhouse study, all commercial cultivars and some of the accessions developed typical symptoms of bacterial spot disease, while some of the accessions developed fewer lesions. In the field studies, infection of leaves and fruits was caused by both natural inoculum and spray-inoculation of plants with a suspension of the X. cucurbitae isolate from Illinois. Among 300 accessions tested, 9 and 21 accessions were classified as resistant and less resistant, respectively. Resistant and less resistant accessions belong to the species Cucurbita maxima, C. maxima subsp. maxima, C. maxima subsp. andreana, and C. okeechobeensis subsp. martinezii. This is the first report of potential resistance to bacterial spot of cucurbits.     

Intraspecific variation in susceptibility to dothistroma needle blight within native Scottish Pinus sylvestris

Dothistroma needle blight (DNB), caused by Dothistroma septosporum, is the most important disease currently affecting pine plantations in Britain. Intraspecific variation in susceptibility to DNB has been observed in several pine species, but it is not clear if similar variation occurs in Pinus sylvestris (Scots pine), Britain's only native pine. In three separate experiments 2‐ and 3‐year‐old Scots pine saplings from six native Scottish populations were artificially inoculated with D. septosporum conidial suspensions and incubated under conditions optimal for disease development. Conidial suspensions were produced using a single isolate from northeast Scotland. In one experiment, plants were also treated with various spore suspension concentrations to assess the impact of inoculum load on disease severity. There were no significant interactions between host population, plant height, and experiment/inoculum load (anova , P > 0·05), but population, height and inoculum load all significantly affected disease severity (anova , P < 0·05). Among the 2‐year‐old trees, those from Amat were less susceptible than those from Glen Loyne and Glen Cannich (anova , P < 0·05). Among the 3‐year‐old trees, those from Beinn Eighe were less susceptible than those from Abernethy. Plant height and DNB susceptibility had a slightly negative relationship. The use of a spore suspension with a concentration of 1·6 × 10⁶ spores mL^?1 was optimum for disease development. In an in vitro experiment, production of conidia was greater when cultures were incubated in darkness. This paper is the first to report intraspecific variation in DNB susceptibility within Scots pine.     

Components of partial resistance in the slash pine-fusiform rust pathosystem

ABSTRACT Progeny of rust-resistant, open-pollinated slash pine families exhibited components of partial resistance in greenhouse tests. Nine-month-old seedlings of some resistant families had (i) a greater frequency of short galls (相似文献   

Mendelian inheritance of rust resistance to Melampsora larici‐epitea in crosses between Salix sachalinensis and S. viminalis

Two F₁, two F₂ and two backcross (BC) full‐sib families of Salix sachalinensis × S. viminalis were tested for resistance to two pathotypes of Melampsora larici‐epitea in leaf‐disc inoculation experiments. Two single‐pustule isolates, VM and ST, belonging to pathotypes LET1 and LET5, respectively, were used in the tests. Disease was scored based on the number of uredinia, uredinial diameter and inoculum densities. Both F₁ families were completely resistant to both VM and ST. Resistance to VM segregated at a 9:6:1 ratio in the F₂ families and at a 1:2:1 ratio in the BC families, suggesting that two independently segregating genes controlled rust resistance, with resistance dominant over susceptibility. This also indicates incomplete dominance of the resistance alleles over the susceptibility to VM. For ST, the equivalent ratios were 3:1 and 1:1, showing that a single dominant gene was responsible for rust resistance. The broad sense heritabilities were >0·91 for uredinial diameter and 0·1–0·33 for the number of uredinia. There were significant overall correlations between data from inoculations with VM and those from inoculations with ST in the number of uredinia, uredinial diameter and disease scores (Spearman’s rank correlation coefficients = 0·31–0·75).     

Pathogenicity of Diaporthe spp. on two blueberry cultivars (Vaccinium corymbosum)

Diaporthe vaccinii causes twig blight and fruit rot and is currently listed as a quarantine organism for the European Union. In the Netherlands, two species from the same genus, Diaporthe eres and Diaporthe rudis, are regularly isolated from blighted twigs of Vaccinium corymbosum. This study compared the pathogenicity of these two species to D. vaccinii. To develop a pathogenicity test a field experiment was performed at an experimental station, testing isolates of D. rudis and D. eres. Most of the isolates tested did not cause statistically significantly larger lesions compared to the control treatment (P > 0.05). In a greenhouse experiment a similar test was conducted, comparing D. vaccinii with the closely related species D. eres. Two cultivars of blueberry, Duke and Liberty, were inoculated using two D. vaccinii and two D. eres isolates. Stem canker lesions caused by D. vaccinii were larger than these caused by D. eres isolates. The majority of the lesions caused by D. vaccinii did not pass the first node of the inoculated shoot, which might be an explanation for the ‘mild’ symptoms of D. vaccinii in V. corymbosum, which appear not to cause crop losses. The results of this study suggest that D. vaccinii may not be a major threat to blueberry production in Europe. Further data is required to be able to conclude whether the quarantine status of D. vaccinii is still appropriate.     

Plasmodiophora brassicae resting spore dynamics in clubroot resistant canola (Brassica napus) cropping systems

The soilborne pathogen Plasmodiophora brassicae, causal agent of clubroot of canola (Brassica napus), is difficult to manage due to the longevity of its resting spores, ability to produce large amounts of inoculum, and the lack of effective fungicides. The cropping of clubroot resistant (CR) canola cultivars is one of the few effective strategies for clubroot management. This study evaluated the impact of the cultivation of CR canola on P. brassicae resting spore concentrations in commercial cropping systems in Alberta, Canada. Soil was sampled pre-seeding and post-harvest at multiple georeferenced locations within 17 P. brassicae-infested fields over periods of up to 4 years in length. Resting spore concentrations were measured by quantitative PCR analysis, with a subset of samples also evaluated in greenhouse bioassays with a susceptible host. The cultivation of CR canola in soil with quantifiable levels of P. brassicae DNA resulted in increased inoculum loads. There was a notable lag in the release of inoculum after harvest, and quantifiable P. brassicae inoculum peaked in the year following cultivation of CR canola. Rotations that included a ≥2-year break from P. brassicae hosts resulted in significant declines in soil resting spore concentrations. A strong positive relationship was found between the bioassays and qPCR-based estimates of soil infestation. Results suggest that CR canola should not be used to reduce soil inoculum loads, and crop rotations in P. brassicae infested fields should include breaks of at least 2 years away from B. napus, otherwise the risk of selecting for virulent pathotypes may increase.     

Development of three fusarium crown rot causal agents and systemic translocation of deoxynivalenol following stem base infection of soft wheat

Fusarium pseudograminearum, F. culmorum and F. graminearum are the most important fusarium crown rot (FCR) causal agents. They have the common ability to biosynthesize deoxynivalenol (DON). To elucidate the behaviour of each of the three species, a comparative study was carried out to investigate symptom progression, fungal systemic growth and translocation of DON following stem base inoculation of soft wheat. FCR symptoms were mainly localized in the inoculated area, which extended up to the second node for all inoculated species. Only the most aggressive strains caused symptoms up to the third node. Real‐time quantitative PCR showed that fungal colonization reached the third node for all the tested species, but a low percentage of plants showed colonization above the third node following inoculation with the most aggressive strains. Fungal growth was detected in symptomless tissues but none of the three species was able to colonize as far as the head tissues. However, even if the pathogens were not detected in the heads, DON was detected in head tissues of the plants inoculated with the most aggressive strains. These results demonstrate that F. pseudograminearum, F. culmorum and F. graminearum, under the same experimental conditions, follow a similar pattern of symptom progression, fungal colonization and DON translocation after stem base infection. Differences in the extent of symptoms, fungal colonization and mycotoxin distribution were mainly attributable to strain aggressiveness. These findings provide comparative information on the events following infection of the stem base of wheat by three of the most important FCR casual agents.     

An Ulva armoricana extract protects plants against three powdery mildew pathogens

The protective activity of a crude extract prepared from the green macroalga, Ulva armoricana, previously shown to induce plant defence responses, was evaluated on three plant species, common bean, grapevine and cucumber, cultivated in the greenhouse and inoculated with three powdery mildew pathogens Erysiphe polygoni, E. necator and Sphareotheca fuliginea respectively. Chemical analyses showed that the extract was enriched in ulvans, which are green algae polysaccharides essentially composed of uronic acid and sulphated rhamnose. Weekly applications were performed by spraying of the green algal extract at various dilutions on bean, grapevine and cucumber leaves. A significant effect (50% protection) was observed using a dilution corresponding to about 3 g l⁻¹ dry matter and up to 90% reduction of symptom severity was obtained for the highest concentration (1/9 dilution, 6 g l⁻¹ dry matter) for the three plant species. To study the natural variability of the protective activity, five extracts prepared from algae batches harvested at different year periods were evaluated. Although polysaccharide composition varied among batches, all extracts elicit a reporter gene regulated by a defence-gene promoter in a transgenic tobacco line, and protect cucumber plants against powdery mildew infection. Together, these data demonstrate that U. armoricana is a reproducible source of active compounds which can be used to efficiently protect crop plants against powdery mildew diseases.     

Significant in vitro antagonism of the laurel wilt pathogen by endophytic fungi from the xylem of avocado does not predict their ability to control the disease

The ascomycete Raffaelea lauricola causes laurel wilt, a lethal vascular disease of avocado, Persea americana, and other members of the Lauraceae plant family. Few effective control measures for laurel wilt exist and new measures are needed. In this study, biological control of the disease with endophytic fungi from avocado was examined. Thirty‐two endophytes (24 operational taxonomic units or OTUs) isolated from the xylem of healthy trees (the infection court of R. lauricola) were evaluated against R. lauricola with in vitro dual‐culture assays. Nine OTUs that showed strong in vitro antagonism of the pathogen were tested in planta against laurel wilt. In three greenhouse experiments, grafted avocado plants of Simmonds or Russell cultivars, which are both susceptible to laurel wilt, were inoculated with endophytes and, after 10–16 days, inoculated with the same isolate of R. lauricola that was used in the in vitro assays. Within 14 days of inoculation with R. lauricola, laurel wilt developed in plants that were not treated with endophytes (positive controls) but also developed in endophyte‐treated plants to the extent observed in the positive controls (P = 0.05). The pathogen colonized plants rapidly and systemically, but endophytes generally did not colonize xylem more than 2 cm above the point at which plants were inoculated. Although the tested endophytes strongly antagonized the pathogen in vitro, this did not translate to an ability to reduce development of laurel wilt. The management of laurel wilt and other plant diseases with fungal endophytes is discussed.     

Evaluation of mulberry germplasm for resistance to powdery mildew in the field and greenhouse

Powdery mildew caused by the ascomycete Phyllactinia guttata (syn. P. corylea) is a major foliar disease worldwide of the unique mulberry (Morus spp.) for silkworm feed. Genetic resistance to powdery mildew, the most sustainable and economical strategy for disease control, is still elusive for tropical mulberry. About 147 germplasm sources, representing 18 countries of origin, were screened for resistance to P. guttata in six seasonal fields and greenhouse trials after exposure to natural and artificial inoculum, respectively. In the field, the level of plant responsiveness to disease was assessed from 30 to 62 days after pruning in each season as variations in the disease severity index (DSI), disease incidence (DI%) and area under the disease progress curve (AUDPC). These measures differed significantly among the germplasm. Of 147 germplasm sources, ~6.8% had useful resistance (two high and nine moderately resistant) to the powdery mildew pathogen on the basis of DSI. The AUDPC values were 13.5-fold higher in the most susceptible accession—(Philippines) than the least responsive (Vietnam-2). The results of DSI were strongly correlated with the obtained DI values (r = 0.92; P < 0.01) and AUDPC (r = 0.89; P < 0.01). Moreover, field screening results were highly correlated (R ² = 0.839) with values from the greenhouse evaluation using artificial inoculum. However, the DSI values in field and greenhouse screenings for three sources (Non-nayapati, Nao-khurkul and Tista Valley) varied significantly. A relatively low disease reaction of 09 resources (Vietnam-2, Ankara and 07 others) using different assessment scales after natural and artificial inoculation prove, for the first time, that they have potential in breeding for resistance in tropical mulberry to powdery mildew.     

Occurrence and ITS diversity of wood‐associated Bursaphelenchus nematodes in Scots pine forests in Switzerland

The quarantine pathogen Bursaphelenchus xylophilus (pine wood nematode, PWN) represents a serious threat for Pinus species in Europe. To exclude its presence in Switzerland, in 2010 and 2011 a countrywide survey was conducted in 102 Pinus sylvestris stands, chosen according to whether they contained dying or dead trees or were located in areas at risk of PWN introduction. In total, 285 trees (1–5 per site) were sampled. Nematodes were extracted from wood chips using a standard procedure, and identified to species by internal transcribed spacer (ITS) sequencing. Bursaphelenchus species were present in 34% of the trees, but no B. xylophilus was identified, i.e. PWN is still not present in Switzerland. The nematodes found belonged to seven different species, with B. vallesianus the most frequent species, followed by B. sexdentati, B. mucronatus kolymensis and B. eggersi. Three other species (B. borealis, B. pinophilus, B. poligraphi) were each only present in one or two trees. Three groups of sequences could not be assigned to a species because of the lack of matching reference sequences. The species composition found in Switzerland suggests co‐existence of southern and central European Bursaphelenchus species. Intraspecific ITS variability differed considerably among the four most common species. Bursaphelenchus eggersi, B. mucronatus kolymensis and B. sexdentati had several variable sites in the ITS region, resulting in multiple ITS genotypes in each species. In contrast, all 99 B. vallesianus isolates had an identical ITS region. This could indicate a founder effect, and possibly that B. vallesianus is not native to the Alpine region.     

Epiphytic growth of Xanthomonas arboricola and Xanthomonas citri on non‐host plants

The population dynamics of Xanthomonas arboricola pv. pruni (Xap) and X. citri subsp. citri (Xcc) was assessed on over three dozen plant species/genotypes under field and greenhouse conditions. Both Xap and Xcc multiplied on red nightshade, black nightshade, bindweed, Chenopodium, common bean and wheat up to 20 days post‐inoculation (dpi) under greenhouse conditions. A high bacterial growth rate was observed on all (alfalfa, bindweed, Chenopodium, field mustard, millet and prickly lettuce) but one (liquorice) plant species tested under field conditions. Xap successfully proliferated on both lemon and sweet lemon up to 140 dpi, attaining a population density even higher than that of Xcc. The latter showed an increased growth rate on GxN, GF677, Ghisella 6 and Mariana 2624 rootstocks up to 140 dpi. While Xap and Xcc did not grow on pomegranate and common fig, they had a steady population growth on apple and pear plants up to 140 dpi, although the final population sizes were smaller than those observed on lemon and sweet lemon plants. The results suggest that a large number of non‐host plant species could support epiphytic populations of Xap or Xcc, which may have implications for plant disease epidemiology.