Cactorum Isolates Research Articles

Phosphite Is More Effective Against Phytophthora Crown Rot and Leather Rot Caused by Phytophthora cactorum than P. nicotianae.

Phytophthora crown rot (PhCR) and leather rot (LR) caused by Phytophthora spp. are major threats to strawberry production worldwide. In the United States, these diseases are mainly caused by Phytophthora cactorum; however, P. nicotianae has also been recently reported causing PhCR. Growers have relied on three different chemical products (i.e., mefenoxam and phosphites for PhCR and LR, and azoxystrobin for LR). Because resistance to mefenoxam and azoxystrobin has been reported, this study aimed to assess the in vitro sensitivity of Phytophthora spp. isolates from strawberry to phosphites and investigate its efficacy on in vivo assays. In vitro sensitivity of P. cactorum (n = 128) and P. nicotianae (n = 24) isolates collected from 1997 to 2018 was assessed for phosphite at 10, 50, 100, 150, and 300 µg/ml. Regardless of the Phytophthora sp. and isolation organ, most of the isolates (75% for P. cactorum and 54.2% for P. nicotianae) had effective concentration that inhibits pathogen growth by 50% (EC50) values ranging from 50 to 100 µg/ml. In vivo tests with strawberry fruit and plants revealed that commercial formulations of phosphite applied at the highest field rate controlled P. cactorum isolates but failed to control PhCR and LR caused by some isolates of P. nicotianae. In this study, EC50 results from in vitro assay did not truly translate the efficacy of phosphites on controlling LR and PhCR caused by P. cactorum and P. nicotianae. Our findings support the hypothesis that the product acts in a dual way: direct on the pathogen and stimulating the plant immune system. Moreover, this has important implications for disease management, highlighting the importance of a correct diagnosis before phosphite recommendations, because its efficacy varies within Phytophthora spp.

Evaluation of the Ability of Seven Active Ingredients of Fungicides to Suppress Phytophthora cactorum at Diverse Life Stages, and Variability in Resistance Found among Isolates.

Phytophthora cactorum is considered an important plant pathogen which is causing major damage to strawberry plants worldwide. In the current study, the ability of the active ingredients of seven different fungicides, azoxystrobin, cymoxanil, dimethomorph, fenamidone, fluopicolide, metalaxyl and propamocarb, to suppress the mycelial growth, sporangial formation and zoospore release of P. cactorum isolates, was tested. The variation in resistance against various fungicides was found among the isolates. The active ingredients are also unequally efficient against different life stages of P. cactorum, which is probably associated with their different modes of action. A significant level of resistance was recorded against metalaxyl and dimethomorph; however, these were totally inefficient against the zoospore release, while azoxystrobin did not inhibit mycelial growth. The only fungicide efficient against all three P. cactorum life stages tested was fluopicolide, although the calculated resistance factor gives evidence of the rise of resistance in the majority of isolates even against this fungicide. Significant differences were found between responses to fungicides of isolates from strawberry and from other host species. Based on the Mahalanobis distances calculated in the discriminant analysis comprising all of the assays performed, the similarities among isolates were estimated.

Detection and Characterization of Quinone Outside Inhibitor-Resistant Phytophthora cactorum and P. nicotianae Causing Leather Rot in Florida Strawberry.

Phytophthora cactorum and P. nicotianae cause leather rot (LR) of fruit and Phytophthora crown rot (PhCR) in strawberry. LR occurs sporadically but can cause up to 70% fruit loss when weather is conducive. In Florida's annual strawberry winter production system, PhCR can be severe, resulting in plant stunting, mortality, and severe yield loss. Azoxystrobin is labeled for control of LR but not for PhCR. The aims of this research were to determine the sensitivity of P. cactorum and P. nicotianae isolates from strawberry to azoxystrobin and to investigate mechanisms of quinone-outside-inhibitor resistance present in P. cactorum and P. nicotianae based on the known point mutations within the cytochrome b (cytb) gene. Isolates of both Phytophthora spp. causing LR and PhCR were collected from multiple strawberry fields in Florida between 1997 and 2020. Isolates were tested for sensitivity to azoxystrobin at 0, 0.01, 0.1, 1.0, 10, and 50 μg/ml on potato dextrose agar amended with salicylhydroxamic acid (100 μg/ml). Isolates were separated into two groups - sensitive isolates with the 50% effective concentration (EC50) values <1.0 μg/ml, and resistant isolates having EC50 values >50 μg/ml. P. cactorum and P. nicotianae resistance to azoxystrobin was found for isolates collected after 2010. The first 450 nucleotides of the mitochondrial cytb gene were sequenced from a selection of resistant and sensitive isolates of both species. The G143A mutation reported to confer resistance to azoxystrobin was found in all resistant P. cactorum isolates. However, in P. nicotianae, qualitative resistance was observed, but the isolates lacked all the known mutations in the cytb gene. This is the first report of resistance to azoxystrobin in P. cactorum and P. nicotianae.

Resistance to Mefenoxam of Phytophthora cactorum and Phytophthora nicotianae Causing Crown and Leather Rot in Florida Strawberry.

Phytophthora cactorum and Phytophthora nicotianae cause leather rot on fruit and crown rot (PhCR) of strawberry plants. Leather rot is not a common disease in Florida; however, up to 50% yield loss has been reported in harvests after intense rainfall events. PhCR is an important disease worldwide and is characterized by a sudden wilting and collapse of plants. Mefenoxam is the most effective and widely used fungicide to control both diseases. P. cactorum and P. nicotianae isolates from leather rot and PhCR have been collected from multiple strawberry fields in Florida since 1997, and the sensitivity of 185 isolates was tested at 0, 0.05, 0.5, 5, and 100 µg/ml. The 50% effective concentration (EC50) values of sensitive isolates ranged from 0.05 to 1 µg/ml. Resistance to mefenoxam (EC50 values >100 µg/ml) was found among P. cactorum isolates collected after 2015, but no resistance was found in P. nicotianae isolates. During the 2015 to 2016, 2016 to 2017, 2017 to 2018, and 2018 to 2019 seasons, resistance was detected on 9, 10, 21, and 23% of the isolates collected, respectively. Mefenoxam-resistant isolates originated from 3 of the 24 strawberry nurseries monitored. This is the first report of the occurrence of P. cactorum resistance to mefenoxam in Florida, suggesting that alternative control strategies are needed to avoid the increase of mefenoxam-resistant populations of P. cactorum in Florida fields.

Rapid and simple detection of Phytophthora cactorum in strawberry using a coupled recombinase polymerase amplification\u2013lateral flow strip assay

Phytophthora cactorum is a devastating pathogen that infects a wide range of plants and causes Phytophthora rot disease, which has resulted in great economic losses in crop production. Therefore, the rapid and practicable detection of P. cactorum is important for disease monitoring and forecasting. In this study, we developed a lateral flow recombinase polymerase amplification (LF-RPA) assay for the sensitive visual detection of P. cactorum. Specific primers for P. cactorum were designed based on the ras-related protein gene Ypt1; all 10 P. cactorum isolates yielded positive detection results, whereas no cross-reaction occurred in related oomycete or fungal species. The detection limit for the LF-RPA assay was 100 fg of genomic DNA under optimized conditions. Combined with a simplified alkaline lysis method for plant DNA extraction, the LF-RPA assay successfully detected P. cactorum in naturally diseased strawberry samples without specialized equipment within 40 min. Thus, the LF-RPA assay developed in this study is a rapid, simple, and accurate method for the detection of P. cactorum, with the potential for further application in resource-limited laboratories.

Terpenoid and lipid profiles vary in different Phytophthora cactorum – strawberry interactions

Specialized metabolites are essential components in plant defence systems, serving as signalling molecules and chemical weapons against pathogens. The manipulation of plant defence metabolome or metabolites can thus be an important virulence strategy for pathogens. Because of their central role, metabolites can give valuable insights into plant-pathogen interactions. Here, we have conducted nontargeted metabolite profiling with UPLC-ESI-qTOF-MS to investigate the metabolic changes that have taken place in the crown tissue of Fragaria vesca L. (woodland strawberry) and Fragaria × ananassa (Weston) Duchesne ex Rozier (garden strawberry) during 48 h after Phytophthora cactorum challenge. Two P. cactorum isolates were compared: Pc407 is highly virulent to F. × ananassa and causes crown rot, whereas Pc440 is mildly virulent. In total, 45 metabolites differentially accumulated between the treatment groups were tentatively identified. Triterpenoids and various lipid compounds were highly represented. The levels of several triterpenoids increased upon inoculation, some of them showing distinct accumulation patterns in different interactions. Triterpenoids could either inhibit or stimulate P. cactorum growth and, therefore, triterpenoid profiles might have significant impact on disease progression. Of the lipid compounds, lysophospholipids, linoleic acid and linolenic acid were highly accumulated in the most compatible Pc407 - F. × ananassa interaction. As lysophospholipids promote cell death and have been linked to susceptibility, these compounds might be involved in the pathogenesis of crown rot disease. This metabolite analysis revealed potential factors contributing to the outcome of P. cactorum – strawberry interactions. The information is highly valuable, as it can help to find new breeding strategies and new solutions to control P. cactorum in strawberry.

Phytophthora cactorum as a Pathogen Associated with Root Rot on Alfalfa (Medicago sativa) in China.

Alfalfa (Medicago sativa) is the largest grown pasture crop in China due to its economic and ecological importance. During the summer season from June to August in 2018, stunted plants was frequently observed in alfalfa fields that have been established for two years in Jinchang, Gansu Province. The disease incidence of root rot ranged from 40% to 50%. Taproots of stunted plants showed red-brown to dark brown discolorations, and lateral roots were poorly developed. Shoots wilted with rotted taproots and lateral roots in severely affected plants. Twenty symptomatic plants were collected and transported to the laboratory for pathogen isolations. Roots were washed under running tap water, cut into 2 to 3 mm pieces (40 pieces each plant), and then sterilized in 75% ethanol for 2 mins followed by three times washing with autoclaved distilled water. Surface dried pieces on autoclaved filter papers were put onto water agar and also a Phytophthora selective medium P5ARP(H) (Jeffers and Martin 1986). The plates were incubated at 22°C for 3 to 5 days and then the growing hypha were subcultured onto potato dextrose agar (PDA). Thirty-two Phytophthora-like isolates were obtained and showed similar morphologies on PDA. Five isolates picked randomly were purified by single-hyphal-tip and plugs (4 to 5 mm) from PDA cultures were incubated in petri dishes with autoclaved distilled water at 22°C for 5 days. Sporangia, chlamydospores and oospores were examined. Sporangia were usually ovoid and sometimes appeared ellipsoid, with the length of 30.5-39.1 μm and width of 23.4-27.8 μm. The diameter of chlamydospores was 29.6 to 42.5μm. Oospores had a diameter of 23.6 to 30.2 μm. The isolates were tentatively identified as P. cactorum based on these morphology characteristics (Montealegre et al. 2016). DNA of these isolates were extracted and PCR amplifications of the rDNA ITS region and cytochrome oxidase subunit I (Cox I) (Kroon et al. 2004) were conducted. Sequences of these isolates were then compared with reference sequences in GenBank using BLAST search. The 866-bp ITS sequences had a sequence identity of 99% to 100% with P. cactorum (e.g. accession nos. EU662221, KJ128036). In addition, the 663-bp CoxI sequences showed 100% sequence identity with three P. cactorum isolates (accession nos. AB688156, HQ708234, EU660851). The ITS and CoxI sequences of one representative isolate Phy.c2 have been deposited in GenBank with the accession no. MT280033 and MT344138, respectively. Pathogenicity of the five isolates (Phy.c1-Phy.c5) were determined on two-week-old alfalfa seedlings (cv. Longdong) grown from seeds. Inoculums were prepared by subculturing agar plugs from edges of PDA cultures into the flask with autoclaved millet seeds, and incubated at 22°C in darkness for two weeks and shaken by hand every two days to ensure uniform colonization. Seedlings were transplanted into pots (12 cm x 12 cm) filled with autoclaved potting mix infested with millet-seed inoculum of each isolate at a rate of 0.5% (w/w). Control seedlings for comparison were transplanted into pots with uninfested potting mix. There were five seedlings per pot and twelve replicate pots for both inoculated and noninoculated treatments, and pots were kept under controlled environment room (22°C, 12 h photoperiod and 65% relative humidity) that were watered every two days to free draining. 87%~92% of the inoculated plants showed stunted symptoms with poorly developed and brown-discoloured roots three weeks after inoculation while the control plants were healthy with no root disease symptoms. To fulfil Koch's postulates, re-isolated cultures from discoloured root tissues were confirmed as the inoculated isolates by morphological examination and ITS sequencing. The five-purified isolates were submitted to the Grassland Culture Collection Center, Lanzhou University, with the accession nos. LZU-MsR-Phy.c1-Phy.c5. To our knowledge, this is the first report of P. cactorum as a pathogen of root rot on alfalfa in China. Phytophthora spp. has been reported causing root rot on alfalfa in America, Australia and Canada, and other legumes such as chickpea, and many other crops worldwide (Musial et al. 2005; Tan and Tan 1986; Vandemark and Barker 2003), and P. cactorum was reported as a root rot pathogen on lavender in China (Chen et al. 2017). P. cactorum may be a significant pathogen associated with root rot in major commercial alfalfa-producing areas in China where are based on flood-irrigation during the growth season.

PHYTOPHTHORA CACTORUM (LEBERT &amp; COHN) J. SCHRÖT AS CAUSAL AGENT OF DIEBACK OF CHESTNUT AND APPLE TREES IN MACEDONIA

From 2013–2017, 11 chestnut populations and 16 apple orchards/plantations in Macedonia were examined for health; soil, root and bark samples were collected from trees expressing symptoms regarded as Phytophthora specific. Using leaf baits of Prunus laurocerasus and selective V8 Agar (PARPNH), 19 pure Phytophthora sp. cultures were isolated and identified as P. cactorum by ITS sequencing. Sixteen isolates were from apple trees and 3 from chestnut trees. Phylogenetic analyses suggested slight distance between P. cactorum isolates originating from chestnut trees compared to those from apple orchards. Assessment of pathogenicity using chestnuts twigs showed no differences be-tween P. cactorum isolates from the two tree host species.

Quantitative trait loci controlling Phytophthora cactorum resistance in the cultivated octoploid strawberry (Fragaria\u2009\xd7\u2009ananassa)

The cultivated strawberry, Fragaria × ananassa (Fragaria spp.) is the most economically important global soft fruit. Phytophthora cactorum, a water-borne oomycete causes economic losses in strawberry production globally. A bi-parental cross of octoploid cultivated strawberry segregating for resistance to P. cactorum, the causative agent of crown rot disease, was screened using artificial inoculation. Multiple putative resistance quantitative trait loci (QTL) were identified and mapped. Three major effect QTL (FaRPc6C, FaRPc6D and FaRPc7D) explained 37% of the variation observed. There were no epistatic interactions detected between the three major QTLs. Testing a subset of the mapping population progeny against a range of P. cactorum isolates revealed no significant interaction (p = 0.0593). However, some lines showed higher susceptibility than predicted, indicating that additional undetected factors may affect the expression of some quantitative resistance loci. Using historic crown rot disease score data from strawberry accessions, a preliminary genome-wide association study (GWAS) of 114 individuals revealed an additional locus associated with resistance to P. cactorum. Mining of the Fragaria vesca Hawaii 4 v1.1 genome revealed candidate resistance genes in the QTL regions.

Expression of the GAF Sensor, Carbohydrate-Active Enzymes, Elicitins, and RXLRs Differs Markedly Between Two Phytophthora cactorum Isolates.

The phytopathogen Phytophthora cactorum infects economically important herbaceous and woody plant species. P. cactorum isolates differ in host specificity; for example, strawberry crown rot is often caused by a specialized pathotype. Here we compared the transcriptomes of two P. cactorum isolates that differ in their virulence to garden strawberry (Pc407: high virulence; Pc440: low virulence). De novo transcriptome assembly and clustering of contigs resulted in 19,372 gene clusters. Two days after inoculation of Fragaria vesca roots, 3,995 genes were differently expressed between the P. cactorum isolates. One of the genes that were highly expressed only in Pc407 encodes a GAF sensor protein potentially involved in membrane trafficking processes. Two days after inoculation, elicitins were highly expressed in Pc407 and lipid catabolism appeared to be more active than in Pc440. Of the carbohydrate-active enzymes, those that degrade pectin were often more highly expressed in Pc440, whereas members of glycosyl hydrolase family 1, potentially involved in the metabolism of glycosylated secondary metabolites, were more highly expressed in Pc407 at the time point studied. Differences were also observed among the RXLR effectors: Pc407 appears to rely on a smaller set of key RXLR effectors, whereas Pc440 expresses a greater number of RXLRs. This study is the first step toward improving understanding of the molecular basis of differences in the virulence of P. cactorum isolates. Identification of the key effectors is important, as it enables effector-assisted breeding strategies toward crown rot-resistant strawberry cultivars.

Bioinformatic characterisation of the effector repertoire of the strawberry pathogen Phytophthora cactorum.

The oomycete pathogen Phytophthora cactorum causes crown rot, a major disease of cultivated strawberry. We report the draft genome of P. cactorum isolate 10300, isolated from symptomatic Fragaria x ananassa tissue. Our analysis revealed that there are a large number of genes encoding putative secreted effectors in the genome, including nearly 200 RxLR domain containing effectors, 77 Crinklers (CRN) grouped into 38 families, and numerous apoplastic effectors, such as phytotoxins (PcF proteins) and necrosis inducing proteins. As in other Phytophthora species, the genomic environment of many RxLR and CRN genes differed from core eukaryotic genes, a hallmark of the two-speed genome. We found genes homologous to known Phytophthora infestans avirulence genes including Avr1, Avr3b, Avr4, Avrblb1 and AvrSmira2 indicating effector sequence conservation between Phytophthora species of clade 1a and clade 1c. The reported P. cactorum genome sequence and associated annotations represent a comprehensive resource for avirulence gene discovery in other Phytophthora species from clade 1 and, will facilitate effector informed breeding strategies in other crops.

SCR96, a small cysteine-rich secretory protein of Phytophthora cactorum, can trigger cell death in the Solanaceae and is important for pathogenicity and oxidative stress tolerance.

Peptides and small molecules produced by both the plant pathogen Phytophthora and host plants in the apoplastic space mediate the relationship between the interplaying organisms. Various Phytophthora apoplastic effectors, including small cysteine-rich (SCR) secretory proteins, have been identified, but their roles during interaction remain to be determined. Here, we identified an SCR effector encoded by scr96, one of three novel genes encoding SCR proteins in P. cactorum with similarity to the P. cactorum phytotoxic protein PcF. Together with the other two genes, scr96 was transcriptionally induced throughout the developmental and infection stages of the pathogen. These genes triggered plant cell death (PCD) in the Solanaceae, including Nicotiana benthamiana and tomato. The scr96 gene did not show single nucleotide polymorphisms in a collection of P. cactorum isolates from different countries and host plants, suggesting that its role is essential and non-redundant during infection. Homologues of SCR96 were identified only in oomycetes, but not in fungi and other organisms. A stable protoplast transformation protocol was adapted for P. cactorum using green fluorescent protein as a marker. The silencing of scr96 in P. cactorum caused gene-silenced transformants to lose their pathogenicity on host plants and these transformants were significantly more sensitive to oxidative stress. Transient expression of scr96 partially recovered the virulence of gene-silenced transformants on plants. Overall, our results indicate that the P. cactorum scr96 gene encodes an important virulence factor that not only causes PCD in host plants, but is also important for pathogenicity and oxidative stress tolerance.

Transcriptomic analysis of the phytopathogenic oomycete Phytophthora cactorum provides insights into infection-related effectors.

BackgroundPhytophthora cactorum, a hemibiotrophic oomycete pathogen, can cause destructive diseases on numerous crops worldwide, leading to essential economic losses every year. However, little has been known about its molecular pathogenicity mechanisms. To gain insight into its repertoire of effectors, the P. cactorum transcriptome was investigated using Illumina RNA-seq.ResultsWe first demonstrated an in vitro inoculation method that can be used to mimic natural cyst germination on host plants. Over 28 million cDNA reads were obtained for five life cycle stages (mycelium, sporangium, zoospore, cyst and germinating cyst) and de novo assembled into 21,662 unique genes. By comparisons with 11 public databases, 88.99% of the unique genes were annotated, including 15,845 mapped to the gene models of the annotated relative Phytophthora infestans. Using TribeMCL, 5,538 gene families conserved across P. cactorum and other three completely sequenced Phytophthora pathogen species were determined. In silico analyses revealed that 620 P. cactorum effector homologues including 94 RXLR effector candidates matched known or putative virulence genes in other oomycetes. About half of the RXLR effector candidates were predicted to share a conserved structure unit, termed the WY-domain fold. A subset of the effector genes were checked and validated by PCR amplification. Transcriptional experiments indicated that effector genes were differentially expressed during the life cycle and host infection stages of P. cactorum. Ectopic expression in Nicotiana benthamiana revealed that RXLR, elicitin and NLP effectors can trigger plant cell death. These effectors are highly conserved across oomycete species. Single nucleotide polymorphisms for RXLR effectors were detected in a collection of P. cactorum isolates from different countries and hosts.ConclusionsThis study demonstrates the comprehensive sequencing, de novo assembly, and analyses of the transcriptome of P. cactorum life cycle stages. In the absence of genome sequence, transcriptome data is important for infection-related gene discovery in P. cactorum, as demonstrated here for the effector genes. The first look at the transcriptome and effector arsenal of P. cactorum provides valuable data to elucidate the pathogenicity basis of this broad-host-range pathogen.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-980) contains supplementary material, which is available to authorized users.

EVALUATION OF APPLE DWARF CULTIVARS SEEDLINGS FOR SELECTION OF ROOTSTOCKS TOLERANT TO PHYTOPHTHORA CACTORUM, CAUSAL AGENT OF CROWN ROT DISEASE

Crown rot disease in some imported rootstocks is a common problem in establishment of intensive and semi-intensive orchards. Native dwarfing apple cultivars are precious genetic resources for selection of genotypes resistant to crown rot, because of their adaptability to the pedoclimatic conditions of Iran . In previous investigation vegetative and productive characters of native apple cultivars were evaluated and cultivars Azayesh, Morabbaei Isfahan, Morabbaei Kamalabad, Zinati and Northern Spy were selected as parents. The open pollinated fruits and seeds were cultivated. All the obtained seedlings, composed of five different genetic populations were inoculated wih Phytophthora cactorum isolates in a controlled condition in greenhouse. Table for defining damage severity was prepared according to the characteristic symptoms on leaves and stems. According to the damage table seedlings were classified within the six groups of susceptibility. Cultivar Morabbaei Isfahan with higher percentage of germination, lower height and adequate resistance to crown rot was selected as the best cultivar. Cultivar Zinati, inspite of lower germination percentage, showed good dwarfing characteristic and tolerance to crown rot.

Effects of Mefenoxam, Phosphonate, and Paclobutrazol on In Vitro Characteristics of Phytophthora cactorum and P. citricola and on Canker Size of European Beech.

Phytophthora citricola and P. cactorum cause bleeding cankers that lead to the death of mature European beech (Fagus sylvatica) in the northeastern United States. The effects of two fungicides and a plant growth regulator on in vitro pathogen characteristics and on canker expansion were investigated. In the first experiment, 16 single-spore isolates (11 P. citricola and 5 P. cactorum) were grown on clarified V8 juice agar amended with (i) 0 to 20 μg a.i./ml of mefenoxam, (ii) 0 to 301,429 μg a.i./ml phosphonate either with or without a bark-penetrating surfactant at 0.5 mg a.i./ml, or (iii) 0 to 25 mg a.i./ml of the surfactant alone. Radial growth, oospore production, and zoospore germination were observed to be dependent on isolate and treatment. A species effect on growth was also observed, as P. cactorum isolates were 2.5- to sevenfold less sensitive to phosphonate, but 2- to 150-fold more sensitive to mefenoxam than P. citricola isolates (based on 50% inhibition of growth). In the second experiment, bark and soil drenches of mefenoxam (50 mg a.i./ml and 19 μg a.i./ml, respectively), phosphonate (301,429 and 101 μg a.i./ml, respectively), and a soil drench of paclobutrazol (21 mg a.i./ml) were evaluated for their efficacy as curative or preventive treatments against bleeding canker. None of the treatments (curative or preventive) were able to stop canker expansion or prevent infection. However, saplings inoculated with P. citricola and treated with the phosphonate bark drench as either a curative or preventive treatment had cankers that were 36 to 82% shorter than those of inoculated control stems treated with water. For saplings inoculated with P. cactorum, the phosphonate bark drench was only effective when applied as a preventive (38% shorter than inoculated control stems treated with water), and not as a curative treatment. No other treatment was effective at limiting canker expansion.

Virulence and Fungicide Sensitivity of Phytophthora cactorum Isolated from American Ginseng Gardens in Wisconsin and Michigan.

Phytophthora cactorum causes foliar blight and root rot on cultivated ginseng (Panax quinquefolium). Commercial ginseng gardens in Wisconsin and Michigan and research gardens at Michigan State University were sampled and P. cactorum isolated. In all, 114 and 96 P. cactorum isolates were recovered in 2003 and 2004, respectively. The majority (82%) of the isolates screened (210 in total) were resistant to the fungicide mefenoxam. P. cactorum isolates were screened for pathogenicity using 'McIntosh' apple fruit (206 isolates) and ginseng seedlings (38 isolates). Most isolates (98%) produced necrotic lesions on apple fruit and nearly all (97%) isolates resulted in 40 to 100% plant death. Apple fruit and ginseng seedlings were also used to test fungicide efficacy against P. cactorum. Mefenoxam, the industry standard, was ineffective in both the apple fruit and seedling studies when using a mefenoxam-resistant P. cactorum isolate. Only copper hydroxide provided consistent control against P. cactorum when tested on apple fruit and ginseng seedlings.

Effect of season and aggressiveness of isolates on the response of two apple rootstocks toPhytophthora cactoruminfection

Two inoculation techniques were applied to investigate the effects of season and isolate variability on apple rootstocks in response to Phytophthora cactorum infection. Shoots of rootstocks MM106 and MM109 were inoculated in vitro with a single P. cactorum isolate once a month for 26 months. Rootstock susceptibility measured as lesion lengths generally cycled to a low level during dormancy in winter and to a high level during active growth in summer. Direct inoculation on excised stems was used to assess the relative aggressiveness of P. cactorum isolates. Bark was removed and stems were inoculated with mycelial discs of various P. cactorum isolates. Rootstock×isolate interactions complicated this evaluation, causing variable results. However, differences in relative aggressiveness of isolates were apparent during the dormant stage of apple stems. It was shown that the season during which evaluation was performed and variation of isolate aggressiveness affected the host reaction to P. cactorum infection.

Genotypic Variation for Resistance to Phytophthora cactorum in a California Strawberry Breeding Population

One hundred-eighty six strawberry genotypes from the Univ. of California strawberry (Fragaria ×ananassa Duch.) breeding program were evaluated for resistance to Phytophthora cactorum Schroet. in trials conducted over 6 years; 60 of these genotypes were tested in 2 years or more. Mother plants of each genotype were grown in a propagation nursery beginning in June, and runner plants were set into soil infested with inoculum from a mix of four P. cactorum isolates in August or September of the same year. Runner plants of each genotype were harvested from the inoculated nursery, transferred to a fruiting field location, and evaluated for disease symptoms during the winter and spring following inoculation using a disease severity score. Significant variation for the disease severity score was detected due to years, genotypes, and their interaction. Differences among genotypes were responsible for 60.6% of the phenotypic variance, whereas years and year × genotype interactions contributed relatively little to this variance, 8.2% and 9.3%, respectively. A separate analysis conducted using a balanced subset of six cultivars that were present in all trial years detected variance components due to years and year × genotype interaction slightly smaller than those estimated for the complete trial, 5.0% and 3.9%, respectively. These results highlight the utility of the screening system and suggest that stable resistance to P. cactorum is obtainable in California strawberry breeding populations and production systems.

First report of Phytophthora hedraiandra on Viburnum tinus in Spain

During surveys for Phytophthora ramorum carried out at garden centres in Majorca, Spain in 2002, several potted plants of Viburnum tinus showing branch dieback were inspected. The origin of the lesions was traced to basal stem cankers c. 10 cm above-ground. The outer bark was removed aseptically and small pieces of tissue from the lesion front were transferred to P5ARP selective medium (Erwin & Ribeiro, 1996). Two isolates, P3842 (= CBS 117808) and P3942 (= CBS 117809) were obtained and putatively identified from morphological characters as Phytophthora cactorum. However, when sequenced in 2005, the ITS region of the rDNA of P3842 (GenBank accession no. AY881005) was identical, while P3942 (AY961608) had a 1-bp substitution to that of Phytophthora hedraiandra (AY707987) (De Cock & Lévesque, 2004), a recently described species recovered from V. tinus leaves in the Netherlands. Isolates P3842 and P3942 had 3- and 4-bp substitutions, respectively, in the ITS compared with that of P. cactorum (AY943299). Furthermore, the partial sequence of the Cox1 mitochondrial gene of isolates P3842 (DQ220015) and P3942 (DQ220016) showed a 1-bp substitution to that of P. hedraiandra culture ex-type CBS 111725 (AY769115). The morphology of both isolates was therefore re-examined and compared with P. cactorum isolate P1611 (= CBS 117810) (AY943299). Colonies on carrot agar (CA; Brasier, 1967) were slightly stellate with appressed to low tufted aerial mycelium; on cornmeal agar (CMA) radiate and submerged; on malt extract agar uniform or faintly stolonate and velvet; and on potato dextrose agar uniform to slightly petaloid with felt-like to appressed mycelium. Radial growth rates on CA and CMA at 20°C were 8·5 and 5·5 mm day−1, respectively. No chlamydospores or sporangia were formed on any media, but sporangia did appear when mycelial plugs on CA were submerged in soil extract (Moralejo et al., 2004) for 3 days at 20°C. Gametangia formed profusely within 4 days on CA, even on the aerial mycelium. The morphology of sporangia and gametangia of both isolates agreed with the species description of De Cock & Lévesque (2004). The isolates differed from P. cactorum mainly in their slightly higher colony growth rates at 20°C, formation of aerial tufts on CA, predominance of subglobose sporangia, abundance of sessile antheridia with near absence of tangled hyphae below, and larger oogonia (mean diameter 31·2 vs. 28·1 µm for P. cactorum) and oospores (mean diameter 28·4 vs. 24·7 µm for P. cactorum). Pathogenicity was determined by wound-inoculating the undersides of detached leaves and twigs of V. tinus (wound made c. 10 cm below the apex) with mycelial plugs. Controls were wound-inoculated with sterile CA plugs. Both inoculated plant material and controls were incubated under continuous fluorescent white light at 20°C. Excluding the controls, all leaves and twigs developed extensive necrotic lesions 7 and 10 days, respectively, after inoculation, from which the pathogen was reisolated. This is the first finding of P. hedraiandra in Spain.

Susceptibility of Gisela 5 and Maxma 14 cherry rootstocks to four Phytophthora species

Susceptibility of Gisela 5 and Maxma 14 cherry rootstocks to Phytophthora cactorum, P. citrophthora, P. citricola and P. parasitica was evaluated. All Phytophthora species were pathogenic to both of the cherry rootstocks tested. The level of susceptibility, however, varied according to Phytophthora species. P. citrophthora and P. parasitica isolates were the most virulent. P. citricola showed moderately virulence. The least virulence was shown from the P. cactorum isolates. Both rootstocks showed similar susceptibility.