Cinnamomi Isolates Research Articles

Molecular Detection of Phytophthora cinnamomi by RPA-CRISPR/Cas12a-Mediated Isothermal Amplification

Background: Phytophthora cinnamomi is one of the soil-borne pathogens that causes root rot and stem rot in many plants globally. P. cinnamomi has serious economic, social, and environmental impacts, threatening natural ecosystems and biodiversity. Methods: In this study, a molecular detection method based on Recombinant polymorphic amplification (RPA) combined using the CRISPR/Cas12a system was developed for P. cinnamomi. The method was found to be highly specific for P. cinnamomi. Results: The results showed that 10 P. cinnamomi isolates were positive; however, 21 Phytophthora species, 4 Phytopythium species, 18 fungal species, and 2 Bursaphelenchus species were negative. In total, 10 pg·µL−1 of P. cinnamomi genomic DNA can be detected. The detection process is performed within 20 min at 37 °C, which makes it fast and convenient for use. Discussion: In conclusion, the RPA-CRISPR/Cas12a system in this study is a promising tool for the rapid and sensitive detection of P. cinnamomi in plant samples.

Proteomic analysis revealed that the oomyceticide phosphite exhibits multi-modal action in an oomycete pathosystem

Phytopathogenic oomycetes constitute some of the most devastating plant pathogens and cause significant crop and horticultural yield and economic losses. The phytopathogen Phytophthora cinnamomi causes dieback disease in native vegetation and several crops. The most commonly used chemical to control P. cinnamomi is the oomyceticide phosphite. Despite its widespread use, the mode of action of phosphite is not well understood and it is unclear whether it targets the pathogen, the host, or both. Resistance to phosphite is emerging in P. cinnamomi isolates and other oomycete phytopathogens. The mode of action of phosphite on phosphite-sensitive and resistant isolates of the pathogen and through a model host was investigated using label-free quantitative proteomics. In vitro treatment of sensitive P. cinnamomi isolates with phosphite hinders growth by interfering with metabolism, signalling and gene expression; traits that are not observed in the resistant isolate. When the model host Lupinus angustifolius was treated with phosphite, proteins associated with photosynthesis, carbon fixation and lipid metabolism in the host were enriched. Increased production of defence-related proteins was also observed in the plant. We hypothesise the multi-modal action of phosphite and present two models constructed using comparative proteomics that demonstrate mechanisms of pathogen and host responses to phosphite. SignificancePhytophthora cinnamomi is a significant phytopathogenic oomycete that causes root rot (dieback) in a number of horticultural crops and a vast range of native vegetation. Historically, areas infected with phosphite have been treated with the oomyceticide phosphite despite its unknown mode of action. Additionally, overuse of phosphite has driven the emergence of phosphite-resistant isolates of the pathogen. We conducted a comparative proteomic study of a sensitive and resistant isolate of P. cinnamomi in response to treatment with phosphite, and the response of a model host, Lupinus angustifolius, to phosphite and its implications on infection. The present study has allowed for a deeper understanding of the bimodal action of phosphite, suggested potential biochemical factors contributing to chemical resistance in P. cinnamomi, and unveiled possible drivers of phosphite-induced host plant immunity to the pathogen.

Genomic and transcriptomic analyses of Phytophthora cinnamomi reveal complex genome architecture, expansion of pathogenicity factors, and host-dependent gene expression profiles.

Phytophthora cinnamomi is a hemibiotrophic oomycete causing Phytophthora root rot in over 5,000 plant species, threatening natural ecosystems, forestry, and agriculture. Genomic studies of P. cinnamomi are limited compared to other Phytophthora spp. despite the importance of this destructive and highly invasive pathogen. The genome of two genetically and phenotypically distinct P. cinnamomi isolates collected from avocado orchards in California were sequenced using PacBio and Illumina sequencing. Genome sizes were estimated by flow cytometry and assembled de novo to 140-141 Mb genomes with 21,111-21,402 gene models. Genome analyses revealed that both isolates exhibited complex heterozygous genomes fitting the two-speed genome model. The more virulent isolate encodes a larger secretome and more RXLR effectors when compared to the less virulent isolate. Transcriptome analysis after P. cinnamomi infection in Arabidopsis thaliana, Nicotiana benthamiana, and Persea americana de Mill (avocado) showed that this pathogen deploys common gene repertoires in all hosts and host-specific subsets, especially among effectors. Overall, our results suggested that clonal P. cinnamomi isolates employ similar strategies as other Phytophthora spp. to increase phenotypic diversity (e.g., polyploidization, gene duplications, and a bipartite genome architecture) to cope with environmental changes. Our study also provides insights into common and host-specific P. cinnamomi infection strategies and may serve as a method for narrowing and selecting key candidate effectors for functional studies to determine their contributions to plant resistance or susceptibility.

Short communication: anomalies as a consequence of data transcription error for A1 Phytophthora cinnamomi isolate P15821

Short communication: anomalies as a consequence of data transcription error for A1 Phytophthora cinnamomi isolate P15821

Detection of Phytophthora cinnamomi on declining Araucaria araucana forests

AbstractSince 2016, crown death and mortality of Araucaria araucana trees have been observed at different localities of the Nahuelbuta Coastal Range, Chile. The objective of this study was to determine the presence of Phytophthora spp. in the rhizosphere of A. araucana trees that may explain its decline. Samples of soil rhizosphere were collected from trees with crown death as well as from asymptomatic trees at two visually contrasting severity areas. Phytophthora detection from soil samples was carried out using leaf bait technique, and isolates were identified by morphology and molecular methods. Recovered Phytophthora isolates were used in pathogenicity tests with A. araucana plants. From rhizosphere soil samples of 30 A. araucana trees, 56 isolates of Phytophthora cinnamomi were obtained. Isolations were more frequent in the rhizosphere of trees with severe crown symptoms and originated exclusively from the selected area showing severe crown symptoms. Plant stem inoculation with P. cinnamomi isolates caused 100% mortality; however, through root infestation, mortality was variable depending on the specific isolate. These results are the first report of the pathogenicity of P.cinnamomi A2 in natural forests of A. araucana in Chile.

The Identification and Characterization of Endopolygalacturonases in a South African Isolate of Phytophthora cinnamomi.

Phytophthora cinnamomi is an economically important plant pathogen that has caused devastating losses to the avocado industry worldwide. To facilitate penetration and successful colonization of the host plant, pathogens have been reported to secrete polygalacturonases (PGs). Although a large PG gene family has been reported in P. cinnamomi, in-depth bioinformatics analyses and characterization of these genes is still lacking. In this study we used bioinformatics tools and molecular biology techniques to identify and characterize endopolygalacturonases in the genome of a South African P. cinnamomi isolate, GKB4. We identified 37 PGs, with 19 characteristics of full-length PGs. Although eight PcPGs were induced in planta during infection, only three showed significant up- and down-regulation when compared with in vitro mycelial growth, suggesting their possible roles in infection. The phylogenetic analysis of PcPGs showed both gain and loss of introns in the evolution of PGs in P. cinnamomi. Furthermore, 17 PGs were related to characterized PGs from oomycete species, providing insight on possible function. This study provides new data on endoPGs in P. cinnamomi and the evolution of introns in PcPG genes. We also provide a baseline for future functional characterization of PGs suspected to contribute to P. cinnamomi pathogenicity/virulence in avocado.

An Improved Transformation System for Phytophthora cinnamomi Using Green Fluorescent Protein.

Phytophthora cinnamomi is a destructive pathogen causing root rot and dieback diseases on hundreds of economically and ecologically important plant species. Effective transformation systems enable modifications of candidate genes to understand the pathogenesis of P. cinnamomi. A previous study reported a polyethylene glycol and calcium dichloride (PEG/CaCl2)-mediated protoplast transformation method of P. cinnamomi. However, the virulence of the transformants was compromised. In this study, we selected ATCC 15400 as a suitable wild-type isolate for PEG/CaCl2 transformation using the green fluorescent protein after screening 11 P. cinnamomi isolates. Three transformants, namely, PcGFP-1, PcGFP-3, and PcGFP-5, consistently displayed a green fluorescence in their hyphae, chlamydospores, and sporangia. The randomly selected transformant PcGFP-1 was as virulent as the wild-type isolate in causing hypocotyl lesions on lupines. Fluorescent hyphae and haustoria were observed intracellularly and intercellularly in lupine tissues inoculated with PcGFP-1 zoospores. The potential application of this improved transformation system for functional genomics studies of P. cinnamomi is discussed.

Comparing Intraspecific Aggressiveness in Phytophthora cinnamomi Isolates

Phytophthora cinnamomi is a major plant destructive pathogen with a wide range of hosts and is considered one of the main causes of Quercus spp. decline in the Southwest of the Iberian Peninsula. This study compares two inoculation tests, using simple and fast methodologies, in order to select the most aggressive isolate to use in future Quercus suber inoculation experiments. To evaluate P. cinnamomi intraspecific variability, cork oak excised shoots were inoculated with five isolates, obtained from Q. suber, Quercus rotundifolia and Castanea sativa stands. These isolates were used for in vitro inoculation of a Trifolium subterraneum cultivar available in Portugal in order to assess its usefulness as a "model plant" in rapid screening tests. This study revealed intraspecific variability of P. cinnamomi isolates to cause lesions in Q. suber excised shoots and in the number of T. subterraneum developing lesions on stem and leaves. However, there was no correspondence between the aggressiveness of the isolates in subterranean clover and in the cork oak. Only one isolate showed consistency in aggressiveness regardless of the host species. In conclusion, T. subterraneum is not a good "model plant" for the evaluation of P. cinnamomi isolates aggressiveness.

Morphological and molecular characterization of Phytophthora species associated with root and crown rot of pomegranate in Iran

AbstractThe purpose of this research was to identify the pathogens causing root and crown rot in major pomegranate‐growing areas of Iran. Infected tissue samples were collected from trees with symptoms from 49 pomegranate orchards in four provinces of Iran: Fars, Markazi, Isfahan, and Kohgiluyeh va Boyer‐Ahmad. In total, 23 Phytophthora spp. isolates were obtained, which were identified as P. cryptogea species complex (12 isolates) and P. cinnamomi (11 isolates) based on morphological characters. Molecular confirmation of identification was performed by inference of phylogeny of ITS‐rDNA regions, β‐tubulin gene, and the mitochondrial gene cytochrome c oxidase subunit 1. The results of our phylogenetic analysis confirmed the morphological identification of P. cinnamomi isolates and placed them in Clade 7c of Phytophthora. In addition, the P. cryptogea species complex isolates, despite morphological similarities, were in fact four distinct species including P. cryptogea sensu stricto (two isolates), P. pseudocryptogea (one isolate), P. sp. kelmania (one isolate), and P. erythroseptica (six isolates). This is the first report of pomegranate root and crown rot caused by P. cinnamomi and P. cryptogea species complex.

Prevalence of Phytophthora species in macadamia orchards in Australia and their ability to cause stem canker

AbstractIn Australia, Phytophthora cinnamomi is the only species reported as the causal agent of stem canker and root rot in macadamia. In other countries, five Phytophthora species have been reported to cause diseases in macadamia, which led us to question if more than one Phytophthora species is responsible for poor tree health in macadamia orchards in Australia. To investigate this, samples were collected from the rhizosphere, stem, and root tissues of trees with and without symptoms, nurseries, and water sources from 70 commercial macadamia orchards in Australia. Phytophthora isolates were identified based on morphological characteristics and DNA sequencing. P. cinnamomi was the most predominant and widely distributed species, and was obtained from the different types of samples including symptomless root tissues. In addition to P. cinnamomi, only P. multivora was isolated from diseased tissue (stem canker) samples. Six other Phytophthora species were obtained from the rhizosphere samples: P. pseudocryptogea, P. citrophthora, P. nicotianae, P. gondwanense, P. sojae, and a new Phytophthora taxon. Only P. cinnamomi was obtained from macadamia nursery samples, while five Phytophthora species were obtained from water sources. Of the heterothallic Phytophthora species, mating type A2 isolates were dominant in P. cinnamomi isolates, whereas only mating type A1 isolates were obtained for P. nicotianae, P. pseudocryptogea, and P. citrophthora. Pathogenicity assays revealed that P. cinnamomi and P. multivora caused significantly larger stem and leaf lesions than P. citrophthora, P. nicotianae, and P. pseudocryptogea. Phytophthora sp. and P. sojae were nonpathogenic towards leaves and stems.

Isolation and identification of Phytophthora cinnamomi collected in avocado (Persea americana) from Northeast Colombia

Phytophthora cinnamomi Rands is a root and collar root pathogen of a great number of wooden plants and crops around the world. The species is heterothallic requiring A1 and A2 mating types for sexual reproduction. In Colombia, P. cinnamomi represents one of the most limiting diseases in avocado (Persea americana Mill). In this work, a total of 207 P. cinnamomi isolates were analyzed from 664 symptomatic and asymptomatic avocado roots from Antioquia (Northeast Colombia) from 2014 to 2017 and morphological and molecular characterizations were conducted in 112 isolates. Rosaceous morphology was the most abundant. A haplotype network made with ITS sequences clustered haplotypes according to country of origin and host. H1 and H2 haplotypes represented P. cinnamomi var. cinnamomi and P. c. var. parvispora respectively and H12 haplotype represented P. c. var. robiniae. Non-Mendelian inheritance was found in the species according to microsatellite data corroborating previous reports made in Australia. P. cinnamomi from Antioquia was genetically structured (FST = 0.058, P = 0.001) suggesting reduced gene flow between sampling sites. Clonality estimations (obtained from a PCR-RFLP of the COI locus) in Antioquian P. cinnamomi were higher than in Australia and South Africa. Studies on next generation sequencing and reproductive isolation are required in this species in future investigations in Colombia.

Phytophthora cinnamomi exhibits phenotypic plasticity in response to cold temperatures

Phytophthora cinnamomi has recently been found in highly diverse and fragile alpine and sub-alpine environments previously considered P. cinnamomi and disease free due to low temperatures. In the laboratory, we investigated the ability of P. cinnamomi isolates to adapt to cold and cause disease under conditions comparable to alpine and sub-alpine environments. Initially, the ability of P. cinnamomi isolates to produce sporangia at 10 °C (2 °C lower than previously reported in the literature) was demonstrated in vitro. The lowest temperature limit for host infection was determined (i.e., 8 °C) and the phenotypic plasticity of isolates was then explored in planta in two successive phenotypic plasticity experiments comparing cold 9, 7.5 °C, and ambient temperature 25 (±5 °C). In the phenotypic plasticity experiment-1, three of the five isolates recovered from plants grown at 9 °C produced sporangia and released zoospores (infective propagules) at 7.5 °C, even lower than determined initially, i.e., 10 °C. No changes were observed in the same set of isolates recovered from plants grown at ambient temperature in the glasshouse as a control, which shows that P. cinnamomi can exhibit phenotypic plasticity and responds rapidly to selection pressure and adapts to new environments. Although P. cinnamomi isolates could produce infective propagules at 7.5 °C in vitro, they could not be recovered from inoculated plants grown at 7.5 °C after 3 months in phenotypic plasticity experiment-2. More work is, therefore, needed to establish disease development by P. cinnamomi at 7.5 °C and below.

PHYCI_587572: An RxLR Effector Gene and New Biomarker in A Recombinase Polymerase Amplification Assay for Rapid Detection of Phytophthora cinnamomi

Phytophthora cinnamomi is a devastating pathogen causing root and crown rot and dieback diseases of nearly 5000 plant species. Accurate and rapid detection of P. cinnamomi plays a fundamental role within the current disease prevention and management programs. In this study, a novel effector gene PHYCI_587572 was found as unique to P. cinnamomi based on a comparative genomic analysis of 12 Phytophthora species. Its avirulence homolog protein 87 (Avh87) is characterized by the Arg-Xaa-Leu-Arg (RxLR) motif. Avh87 suppressed the pro-apoptotic protein BAX- and elicitin protein INF1-mediated cell death of Nicotiana benthamiana. Furthermore, a recombinase polymerase amplification-lateral flow dipstick detection assay targeting this P. cinnamomi-specific biomarker was developed. While successfully detected 19 P. cinnamomi isolates of a global distribution, this assay lacked detection of 37 other oomycete and fungal species, including P. parvispora, a sister taxon of P. cinnamomi. In addition, it detected P. cinnamomi from artificially inoculated leaves of Cedrus deodara. Moreover, the RPA-LFD assay was found to be more sensitive than a conventional PCR assay, by detecting as low as 2 pg of genomic DNA in a 50-µL reaction. It detected P. cinnamomi in 13 infested soil samples, while the detection rate was 46.2% using PCR. Results in this study indicated that PHYCI_587572 is a unique biomarker for detecting P. cinnamomi. Although PHYCI_587572 was identified as an effector gene based on the RxLR motif of Avh87 and the avirulence activity on Nicotiana, its exact genetic background and biological function on the natural hosts of P. cinnamomi warrant further investigations.

Aggressiveness of Phytophthora cinnamomi in avocado seedlings and effect of pathogen inoculum concentration and substrate flooding periods on root rot and development of the plants

Abstract The present study evaluated the aggressiveness of Phytophthora cinnamomi isolates and the effect of pathogen inoculum concentration and periods of substrate flooding on root rot and plant development. Twelve pathogen isolates were inoculated on the collar region of avocado seedlings with or without wounding. Only 31.3% of the inoculated plants without wounding developed lesions, compared to 100% of the plants with wounding, while the isolates showed different aggressiveness levels. Avocado seedlings had their substrate inoculated with 0, 0.1% and 1.0% (m/v) wheat seeds colonized by the pathogen per pot, and four periods of 0, 12 and 24 h substrate flooding were produced at fortnightly intervals. The assessed parameters were number of leaves per plant, collar diameter, plant height, leaf area index, visual severity percentage of infected roots, fresh mass (%) of diseased roots and dry mass of shoot and roots. Both pathogen inoculation and substrate flooding caused root rot; however, combination of these two factors produced an additional effect on disease symptoms. Root rot severity was superior to 50% when soilless substrate had 0.1% (m/v) P. cinnamomi inoculum and flooded for 12-24 h after inoculation, conditions that can be recommended for pathogenicity and disease control studies using potted avocado plants.

Resistance to Phytophthora cinnamomi in Castanea spp. is under moderately high genetic control mainly because of additive genetic variance

Susceptibility to Phytophthora cinnamomi is one of the main traits in Castanea sativa breeding programs. An inoculation experiment using 25 control pollinated families, with seedlings of these families cloned by cuttings, was conducted by soil infestation with one P. cinnamomi isolate. Forty-seven days after inoculation, foliar and root collar symptoms and root necrosis were recorded. The data were analyzed using a model based on restricted maximum pseudolikelihood methods of the GLIMMIX procedure to estimate the additive, dominance and epistatic components of the genetic variance, as well as the narrow sense heritability and the breeding values. At the end of the experiment, the percentages of dead plants ranged from 4% to 56% in C. sativa, and 18% to 20% in backcrosses to C. sativa, with much lower percentages in the F1 hybrids (C. crenata × C. sativa). Foliar symptoms were proportional to mortality, affecting 28% of the plants, but root collar lesions and root necrosis were more prevalent, affecting 65% and 84% of the plants, respectively. The proportions of genetic to phenotypic variance, 0.50–0.63, and the estimated values of narrow-sense heritability, 0.30–0.46, indicate that resistance to P. cinnamomi is under moderate to moderately high genetic control caused mainly by additive genetic variance. A high number of backcrosses to C. sativa showed good breeding values for resistance to P. cinnamomi.

Silver Nanoparticles and Polyphenol Inclusion Compounds Composites for Phytophthora cinnamomi Mycelial Growth Inhibition.

Phytophthora cinnamomi, responsible for “root rot” or “dieback” plant disease, causes a significant amount of economic and environmental impact. In this work, the fungicide action of nanocomposites based on silver nanoparticles and polyphenol inclusion compounds, which feature enhanced bioavailability and water solubility, was assayed for the control of this soil-borne water mold. Inclusion compounds were prepared by an aqueous two-phase system separation method through extraction, either in an hydroalcoholic solution with chitosan oligomers (COS) or in a choline chloride:urea:glycerol deep eutectic solvent (DES). The new inclusion compounds were synthesized from stevioside and various polyphenols (gallic acid, silymarin, ferulic acid and curcumin), in a [6:1] ratio in the COS medium and in a [3:1] ratio in the DES medium, respectively. Their in vitro response against Phytophthora cinnamomi isolate MYC43 (at concentrations of 125, 250 and 500 µg·mL−1) was tested, which found a significant mycelial growth inhibition, particularly high for the composites prepared using DES. Therefore, these nanocomposites hold promise as an alternative to fosetyl-Al and metalaxyl conventional systemic fungicides.

Genetic Variation for Resistance to Phytophthora Root Rot in Eastern White Pine Seedlings

Deployment of genetically resistant Eastern white pine (Pinus strobus L.) planting stock could reduce economic losses to root rot caused by Phytophthora cinnamomi Rands in Christmas tree and forest plantations. This study aimed to determine the degree of genetic control of resistance to P. cinnamomi in Eastern white pine and secondarily, to compare the aggressiveness of two P. cinnamomi isolates derived from different host species. Phytophthora isolates from Fraser fir (Abies fraseri (Pursh) Poir.) and Eastern white pine were used in a main and supplemental study, respectively, including 83 and 20 open-pollinated families. In each study, two-year-old seedlings were inoculated twice each of two consecutive years and mortality was assessed biweekly for 16 weeks each year. During the first year, mortality increased over time to 18.6% and 40.4% while family variation in mortality ranged from 1.3% to 60.0% and 12.5% to 73.0% in the main and supplemental studies, respectively. At the end of the first year, individual-tree and family-mean heritability estimates were, respectively, 0.44 ± 0.0935 and 0.85 ± 0.180 for the main study, and 0.57 ± 0.216 and 0.90 ± 0.343 for the supplemental study. The P. cinnamomi isolate from Eastern white pine was more aggressive and there was a large interaction between isolates and pine families. Deploying resistant families will be complicated by this interaction but should, nevertheless, reduce economic losses.

Effect ofPhytophthora cinnamomiisolate, inoculum delivery method, and flood and drought conditions on vigor, disease severity scores, and survival of blueberry plants

Four studies evaluated the effects of Phytophthora cinnamomi isolate, inoculum delivery method, and flood and drought conditions on vigor, disease severity scores, and survival of blueberry plants grown in pots in the greenhouse. Phytophthora cinnamomi isolates were obtained from blueberry plants or the root zone of plants with symptoms of Phytophthora root rot. Rabbiteye 'Tifblue' and southern highbush 'Biloxi' and 'Star' blueberry plants were grown in a potting medium infested with one of five isolates of P. cinnamomi using four inoculum preparation and delivery methods. Inoculum was prepared as a mycelial slurry, zoospore suspension, or grown on rice grains or vermiculite. The effect of flood treatments on root rot symptom development was investigated by immersing the pots containing plants grown in P. cinnamomi infested potting medium for 24 or 48 h at 3 or 6-week intervals. The effect of drought was investigated by watering plants 4, 5 or 7 days each week. Plants were assessed for plant vigor and disease severity at regular intervals. Based on the results of the four studies the following protocol was developed to achieve rapid, consistent root rot symptom development. Blueberry potting medium should be infested with a virulent isolate of P. cinnamomi using the rice grain inoculum delivery method. Rooted cuttings of germplasm to be tested should then be transplanted into the infested medium, and the pots containing the plants should be flooded for 48 h every 6 weeks to allow rapid infection of the root systems. Disease evaluations based on above-ground symptoms should be made beginning six months after transplanting the cuttings into the infested potting medium. Rooted cuttings of a susceptible southern highbush cultivar should be used as the assay plant when evaluating chemical or biological control products or when studying environmental effects. We will use this protocol to study environmental effects on the development of Phytophthora root rot of blueberries and to evaluate blueberry germplasm for tolerance to root rot, and to determine the efficacy of chemical and biological products for root rot control.

Genotypic Diversity of Phytophthora cinnamomi and P. plurivora in Maryland's Nurseries and Mid-Atlantic Forests.

Genetic diversity of two Phytophthora spp.-P. cinnamomi (102 isolates), commonly encountered in Maryland nurseries and forests in the Mid-Atlantic United States, and P. plurivora (186 isolates), a species common in nurseries-was characterized using amplified fragment length polymorphism. Expected heterozygosity and other indices suggested a lower level of diversity among P. cinnamomi than P. plurivora isolates. Hierarchical clustering showed P. cinnamomi isolates separated into four clusters, and two of the largest clusters were closely related, containing 80% of the isolates. In contrast, P. plurivora isolates separated into six clusters, one of which included approximately 40% of the isolates. P. plurivora isolates recovered from the environment (e.g., soil and water) were genotypically more diverse than those found causing lesions. For both species, isolate origin (forest versus nursery or among nurseries) was a significant factor of heterozygosity. Clonal groups existed within P. cinnamomi and P. plurivora and included isolates from both forest and nurseries, suggesting that a pathway from nurseries to forests or vice versa exists.

Response of Turkish and Trojan fir to Phytophthora cinnamomi and P. cryptogea

ABSTRACTPhytophthora root rot, primarily caused by the oomycete Phytophthora cinnamomi, is a large problem for the Christmas tree industry in North Carolina. Fraser fir (Abies fraseri) has no known innate resistance to this pathogen while some exotic fir species, such as Trojan (Abies equi-trojani) and Turkish (Abies bornmuelleriana) fir display varying amounts of resistance. A Phytophthora-resistance screening trial was completed with 1600 seedlings from 12 Turkish and Trojan fir families and Fraser and momi fir (A. firma). Seedlings from each family or species were inoculated with each of eight Phytophthora isolates, six P. cinnamomi and two Phytophthora cryptogea, in an effort to describe variability in isolate aggressiveness. Mortality was assessed as percent shoot necrosis bi-weekly for 16 weeks. Overall, rankings of resistance in fir species confirmed previous single-isolate-based results; momi fir was the most resistant, followed by Turkish, Trojan, and Fraser fir. P. cinnamomi isolates were generally more aggressive than P. cryptogea isolates. The two P. cryptogea isolates resulted in 5.6% and 0.8% mortality on Turkish fir, and 10.9% and 6.7% mortality on Trojan fir, the first reported resistance screen of these host-pathogen combinations. Pearson’s correlation testing identified a high degree of correlation between most isolates and overall mean mortality. Turkish and Trojan fir families appear to possess resistance to Phytophthora species common in North Carolina.