Potato miR394 targets StA/N-INVE and StLCR to negatively regulate late blight resistance.

MicroRNAs (miRNAs) are small noncoding RNAs in eukaryotes. Plant endogenous miRNAs play pivotal roles in regulating plant development and defense responses. MicroRNA394 (miR394) has been reported to regulate plant development, abiotic stresses and defense responses. Previous reports showed that miR394 responded to P. infestans inoculation in potato, indicating that miR394 may be involved in defense responses. In this study, we further investigated its role in potato defense against P. infestans. Stable expression of miR394 in tobacco and potato enhances the susceptibility to P. infestans, which is accompanied with the reduced accumulation of ROS and down-regulation of the PTI (pattern-triggered immunity) marker genes. Besides well-known target StLCR, miR394 also targets StA/N-INVE, which encodes a chloroplast Alkaline/Neutral Invertases (A/N-INVE). Both StLCR and StA/N-INVE positively regulate late blight resistance, while miR394 degrades them. Interestingly, StA/N-INVE is located in the chloroplast, indicating that miR394 may manipulate chloroplast immunity. Degradation of StA/N-INVE may affect the chloroplast function and hence lead to the compromised ROS (reactive oxygen species) burst and reduced retrograde signaling from the chloroplast to the nucleus and cytoplasm. In summary, this study provides new information that miR394 targets and degrades StA/N-INVE and StLCR, which are positive regulators, to enhance potato susceptibility to P. infestans.

A RabGAP negatively regulates plant autophagy and immune trafficking.

Plants rely on autophagy and membrane trafficking to tolerate stress, combat infections, and maintain cellular homeostasis. However, the molecular interplay between autophagy and membrane trafficking is poorly understood. Using an AI-assisted approach, we identified Rab3GAP-like (Rab3GAPL) as a key membrane trafficking node that controls plant autophagy negatively. Rab3GAPL suppresses autophagy by binding to ATG8, the core autophagy adaptor, and deactivating Rab8a, a small GTPase essential for autophagosome formation and defense-related secretion. Rab3GAPL reduces autophagic flux in three model plant species, suggesting that its negative regulatory role in autophagy is conserved in land plants. Beyond autophagy regulation, Rab3GAPL modulates focal immunity against the oomycete pathogen Phytophthora infestans by preventing defense-related secretion. Altogether, our results suggest that Rab3GAPL acts as a molecular rheostat to coordinate autophagic flux and defense-related secretion by restraining Rab8a-mediated trafficking. This unprecedented interplay between a RabGAP-Rab pair and ATG8 sheds new light on the intricate membrane transport mechanisms underlying plant autophagy and immunity.

Fitness difference between two synonymous mutations of Phytophthora infestans ATP6 gene.

BACKGROUND: Sequence variation produced by mutation provides the ultimate source of natural selection for species adaptation. Unlike nonsynonymous mutation, synonymous mutations are generally considered to be selectively neutral but accumulating evidence suggests they also contribute to species adaptation by regulating the flow of genetic information and the development of functional traits. In this study, we analysed sequence characteristics of ATP6, a housekeeping gene from 139 Phytophthora infestans isolates, and compared the fitness components including metabolic rate, temperature sensitivity, aggressiveness, and fungicide tolerance among synonymous mutations. RESULTS: We found that the housekeeping gene exhibited low genetic variation and was represented by two major synonymous mutants at similar frequency (0.496 and 0.468, respectively). The two synonymous mutants were generated by a single nucleotide substitution but differed significantly in fitness as well as temperature-mediated spatial distribution and expression. The synonymous mutant ending in AT was more common in cold regions and was more expressed at lower experimental temperature than the synonymous mutant ending in GC and vice versa. CONCLUSION: Our results are consistent with the argument that synonymous mutations can modulate the adaptive evolution of species including pathogens and have important implications for sustainable disease management, especially under climate change.

Genetic structure and population diversity of Phytophthora infestans strains in Pacific western Canada.

Late blight caused by Phytophthora infestans is an economically important disease of potato and tomato worldwide. In Canada, an increase in late blight incidence and severity coincided with changes in genetic composition of P. infestans. We monitored late blight incidence on tomato and potato in Pacific western and eastern Canada between 2019 and 2022, identified genotypes of P. infestans, and examined their population genetic diversity. We identified four major existing genotypes US11, US17, US8, and US23 as well as 25 new genotypes. The US11 genotype was dominant in Pacific western Canada, accounting for 59% of the total population. We discovered the US17 genotype for the first time in Canada. We revealed a higher incidence of late blight and quite diverse genotypes of P. infestans in Pacific western Canada than in eastern Canada. We found high genetic diversity of P. infestans population from Pacific western Canada, as evidenced by the high number of multilocus genotypes, high values of genetic diversity indices, and emergence of 25 new genotypes. Considering the number of disease incidence, the detection of diverse known genotypes, the emergence of novel genotypes, and the high number of isolates resistant to metalaxyl-m (95%) from Pacific western Canada, the region could play a role in establishing sexual recombination and diverse populations, which could ultimately pose challenges for late blight management. Therefore, continuous monitoring of P. infestans populations in Pacific western region and across Canada is warranted. KEY POINTS: • Genotypes of P. infestans in Pacific western were quite diverse than in eastern Canada. • We discovered US17 genotype for the first time in Canada and identified 26 novel genotypes. • Approximately 95% of P. infestans isolates were resistant to metalaxyl-m.

Reconstructing historic and modern potato late blight outbreaks using text analytics.

In 1843, a hitherto unknown plant pathogen entered the US and spread to potato fields in the northeast. By 1845, the pathogen had reached Ireland leading to devastating famine. Questions arose immediately about the source of the outbreaks and how the disease should be managed. The pathogen, now known as Phytophthora infestans, still continues to threaten food security globally. A wealth of untapped knowledge exists in both archival and modern documents, but is not readily available because the details are hidden in descriptive text. In this work, we (1) used text analytics of unstructured historical reports (1843-1845) to map US late blight outbreaks; (2) characterized theories on the source of the pathogen and remedies for control; and (3) created modern late blight intensity maps using Twitter feeds. The disease spread from 5 to 17 states and provinces in the US and Canada between 1843 and 1845. Crop losses, Andean sources of the pathogen, possible causes and potential treatments were discussed. Modern disease discussion on Twitter included near-global coverage and local disease observations. Topic modeling revealed general disease information, published research, and outbreak locations. The tools described will help researchers explore and map unstructured text to track and visualize pandemics.

The lncRNA20718-miR6022-RLPs module regulates tomato resistance to Phytophthora infestans.

KEY MESSAGE: Sl-lncRNA20718 acts as an eTM of Sl-miR6022 regulating its expression thereby affecting SlRLP6/10 expression. SlRLP6/10 regulate PRs expression, ROS accumulation, and JA/ET content thereby affecting tomato resistance to P. infestans. Tomato (Solanum lycopersicum) is an important horticultural and cash crop whose yield and quality can be severely affected by Phytophthora infestans (P. infestans). Long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) are widely involved in plant defense responses against pathogens. The involvement of Sl-lncRNA20718 and Sl-miR6022 in tomato resistance to P. infestans as well as the targeting of Sl-miR6022 to receptor-like protein genes (RLPs) were predicted in our previous study. However, uncertainty exists regarding their potential interaction as well as the molecular processes regulating tomato resistance. Here, we found that Sl-lncRNA20718 and Sl-miR6022 are positive and negative regulators of tomato resistance to P. infestans by gain- and loss-of-function experiments, respectively. Overexpression of Sl-lncRNA20718 decreased the expression of Sl-miR6022, induced the expression of PRs, reduced the diameter of lesions (DOLs), thereby enhanced disease resistance. A six-point mutation in the binding region of Sl-lncRNA20718 to Sl-miR6022 disabled the interaction, indicating that Sl-lncRNA20718 acts as an endogenous target mimic (eTM) of Sl-miR6022. We demonstrated that Sl-miR6022 cleaves SlRLP6/10. Overexpression of Sl-miR6022 decreases the expression levels of SlRLP6/10, induces the accumulation of reactive oxygen species (ROS) and reduces the content of JA and ET, thus inhibiting tomato resistance to P. infestans. In conclusion, our study provides detailed information on the lncRNA20718-miR6022-RLPs module regulating tomato resistance to P. infestans by affecting the expression of disease resistance-related genes, the accumulation of ROS and the phytohormone levels, providing a new reference for tomato disease resistance breeding.

Phytophthora infestans RXLR effector Pi23014 targets host RNA-binding protein NbRBP3a to suppress plant immunity.

Phytophthora infestans is a destructive oomycete that causes the late blight of potato and tomato worldwide. It secretes numerous small proteins called effectors in order to manipulate host cell components and suppress plant immunity. Identifying the targets of these effectors is crucial for understanding P. infestans pathogenesis and host plant immunity. In this study, we show that the virulence RXLR effector Pi23014 of P. infestans targets the host nucleus and chloroplasts. By using a liquid chromatogrpahy-tandem mass spectrometry assay and co-immunoprecipitation assasys, we show that it interacts with NbRBP3a, a putative glycine-rich RNA-binding protein. We confirmed the co-localization of Pi23014 and NbRBP3a within the nucleus, by using bimolecular fluorescence complementation. Reverse transcription-quantitative PCR assays showed that the expression of NbRBP3a was induced in Nicotiana benthamiana during P. infestans infection and the expression of marker genes for multiple defence pathways were significantly down-regulated in NbRBP3-silenced plants compared with GFP-silenced plants. Agrobacterium tumefaciens-mediated transient overexpression of NbRBP3a significantly enhanced plant resistance to P. infestans. Mutations in the N-terminus RNA recognition motif (RRM) of NbRBP3a abolished its interaction with Pi23014 and eliminated its capability to enhance plant resistance to leaf colonization by P. infestans. We further showed that silencing NbRBP3 reduced photosystem II activity, reduced host photosynthetic efficiency, attenuated Pi23014-mediated suppression of cell death triggered by P. infestans pathogen-associated molecular pattern elicitor INF1, and suppressed plant immunity.

Potato calcium sensor modules StCBL3-StCIPK7 and StCBL3-StCIPK24 negatively regulate plant immunity.

BACKGROUND: Potato late blight, caused by Phytophthora infestans, is the most devastating disease on potato. Dissecting critical immune components in potato will be supportive for engineering P. infestans resistance. Upon pathogens attack, plant Ca2+ signature is generated and decoded by an array of Ca2+ sensors, among which calcineurin B-like proteins (CBLs) coupled with plant specific CBL-interacting protein kinases (CIPKs) are much less explored in plant immunity. RESULTS: In this study, we identified that two differential potato CBL-CIPK modules regulate plant defense responses against Phytophthora and ROS production, respectively. By deploying virus-induced gene silencing (VIGS) system-based pathogen inoculation assays, StCBL3 was shown to negatively regulate Phytophthora resistance. Consistently, StCBL3 was further found to negatively regulate PTI and ETI responses in Nicotiana benthamiana. Furthermore, StCIPK7 was identified to act together with StCBL3 to negatively regulate Phytophthora resistance. StCIPK7 physically interacts with StCBL3 and phosphorylates StCBL3 in a Ca2+-dependent manner. StCBL3 promotes StCIPK7 kinase activity. On the other hand, another StCBL3-interacting kinase StCIPK24 negatively modulating flg22-triggered accumulation of reactive oxygen species (ROS) by interacting with StRBOHB. CONCLUSIONS: Together, these findings demonstrate that the StCBL3-StCIPK7 complex negatively modulates Phytophthora resistance and StCBL3-StCIPK24 complex negatively regulate ROS production. Our results offer new insights into the roles of potato CBL-CIPK in plant immunity and provide valuable gene resources to engineer the disease resistance potato in the future.

Impact of Argemone mexicana L. on tomato plants infected with Phytophthora infestans.

Background: Fungal diseases can cause significant losses in the tomato crop. Phytophthora infestans causes the late blight disease, which considerably affects tomato production worldwide. Weed-based plant extracts are a promising ecological alternative for disease control. Methods: In this study, we analyzed the plant extract of Argemone mexicana L. using chromatography-mass spectrometry analysis (GC-MS). We evaluated its impact on the severity of P. infestans, as well as its effect on the components of the antioxidant defense system in tomato plants. Results: The extract from A. mexicana contains twelve compounds most have antifungal and biostimulant properties. The findings of the study indicate that applying the A. mexicana extract can reduce the severity of P. infestans, increase tomato fruit yield, enhance the levels of photosynthetic pigments, ascorbic acid, phenols, and flavonoids, as well as decrease the biosynthesis of H2O2, malondialdehyde (MDA), and superoxide anion in the leaves of plants infected with this pathogen. These results suggest that using the extract from A. mexicana could be a viable solution to control the disease caused by P. infestans in tomato crop.

Synthesis of Acrylopimaric Acid Triazole Derivatives and Their Antioomycete Activity against Phytophthora capsici.

To develop new antioomycete agents against plant pathogens, two series of acrylopimaric acid triazole derivatives from rosin were synthesized. The in vitro antioomycete activity of these derivatives was evaluated and screened against Pseudoperonospora cubensisi, Plasmopara viticola, Phytophthora sojae, Phytophthora infestans, and Phytophthora capsici. Compound 5m showed the highest antioomycete activity against P. capsici, with a half-maximal effective concentration (EC50) value that was lower than that of the positive control metalaxyl (1.391 and 1.815 mg/L, respectively). Compound 5m demonstrated satisfactory protective and curative efficacy against P. capsici in pepper in in vivo antioomycete activity studies. Physiological and biochemical testing showed that the action mechanism of compound 5m on P. capsici involved altering the morphology and ultrastructure of the mycelium, increasing cell membrane permeability, inducing dysfunction of the nucleus and mitochondria, and ultimately causing cell necrosis. In addition, the analysis of three-dimensional quantitative structure-activity relationship (3D-QSAR) revealed the significance of the molecular structure and charge distribution in the interaction between compound 5m and its target. Collectively, these findings indicate that compound 5m has the potential as an antioomycete candidate.

Antifungal activity of the botanical compound rhein against Phytophthora capsici and the underlying mechanisms.

BACKGROUND: Phytophthora capsici is an extremely destructive phytopathogenic oomycete that causes huge economic losses. However, due to the drug resistance risk and environmental threat of chemical fungicides, it is necessary to develop environmentally friendly biocontrol alternatives. Rhein is a major medicinal ingredient of traditional Chinese herbs, and it is widely used in the medical field. However, its inhibitory effect against phytopathogens is unknown. Herein, the antifungal spectrum of rhein and its possible action mechanism against P. capsici were investigated. RESULTS: Rhein possessed broad-spectrum antifungal activity against phytopathogens, particularly P. capsici, Phytophthora infestans, Helminthosporium maydis, and Rhizoctonia solani. Rhein inhibited the mycelial growth as well as the spore germination of P. capsici with mean 50% effective concentration (EC50 ) values of 4.68 µg mL-1 and 6.57 µg mL-1 against 117 P. capsici isolates, respectively. Rhein effectively suppressed the occurrence and spread of Phytophthora blight and significantly destroyed the cell membrane permeability and integrity of P. capsici, corroded its cell wall integrity, and damaged its morphology and ultrastructure. Moreover, rhein caused a considerable reduction in the phospholipid and cellulose contents. Genome-wide transcriptional profiling of P. capsici in response to rhein indicated significant reduction in the expression levels of genes participating in glycerolipid metabolism and starch and sucrose metabolism. Additionally, rhein strengthened the disease defense system of pepper by enhancing related enzyme activities. CONCLUSION: This study demonstrated that rhein could effectively inhibit P. capsici using multiple mechanisms of action. Rhein has the potential to be an efficient alternative to control diseases caused by P. capsici. © 2023 Society of Chemical Industry.

Oomycete effector AVRblb2 targets cyclic nucleotide-gated channels through calcium sensors to suppress pattern-triggered immunity.

Transient and rapid increase in cytosolic Ca2+ plays a crucial role in plant-pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). Cyclic nucleotide-gated channels (CNGCs) have been implicated in mediating this Ca2+ influx; however, their regulatory mechanisms remain poorly understood. Here, we have found that AVRblb2 requires the calmodulin (CaM) and calmodulin-like (CML) proteins as co-factors to interact with the NbCNGCs, resulting in the formation of AVRblb2-CaM/CML-NbCNGCs complex. Furthermore, CaM and CML are dissociated from NbCNGC18 during PTI response to increase Ca2+ influx; however, Avrblb2 inhibits calcium channel activation by disrupting the release of CaM and CML from NbCNGC18. Following recognition of PAMP, NbCNGC18 forms active heteromeric channels with other NbCNGCs, which may give selectivity of CNGC complex against diverse signals for fine-tuning of cytosolic Ca2+ level to mediate appropriate responses. Silencing of multiple NbCNGCs compromised the function of AVRblb2 on the pathogenicity of Phytophthora infestans, confirming that AVRblb2 contributes to pathogen virulence by targeting CNGCs. Our findings provide new insights into the regulation of CNGCs in PTI and the role of pathogen effectors in manipulating host cell physiology to promote infection.

The WY Domain of an RxLr Effector Drives Interactions with a Host Target Phosphatase to Mimic Host Regulatory Proteins and Promote Phytophthora infestans Infection.

Plant pathogens manipulate the cellular environment of the host to facilitate infection and colonization that often lead to plant diseases. To accomplish this, many specialized pathogens secrete virulence proteins called effectors into the host cell, which subvert processes such as immune signaling, gene transcription, and host metabolism. Phytophthora infestans, the causative agent of potato late blight, employs an expanded repertoire of RxLR effectors with WY domains to manipulate the host through direct interaction with protein targets. However, our understanding of the molecular mechanisms underlying the interactions between WY effectors and their host targets remains limited. In this study, we performed a structural and biophysical characterization of the P. infestans WY effector Pi04314 in complex with the potato Protein Phosphatase 1-c (PP1c). We elucidate how Pi04314 uses a WY domain and a specialized C-terminal loop carrying a KVxF motif that interact with conserved surfaces on PP1c, known to be used by host regulatory proteins for guiding function. Through biophysical and in planta analyses, we demonstrate that Pi04314 WY or KVxF mutants lose their ability to bind PP1c. The loss of PP1c binding correlates with changes in PP1c nucleolar localization and a decrease in lesion size in plant infection assays. This study provides insights into the manipulation of plant hosts by pathogens, revealing how effectors exploit key regulatory interfaces in host proteins to modify their function and facilitate disease. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Genome-wide identification and expression profiles of the Phytophthora infestans responsive CYPome (cytochrome P450 complement) in Solanum tuberosum.

Cytochrome P450s represent one of the largest protein families across all domains of life. In plants, biotic stress can regulate the expression of some P450 genes. However, the CYPome (cytochrome P450 complement) in Solanum tuberosum and its response to Phytophthora infestans infection remains unrevealed. In this study, 488 P450 genes were identified from potato genome, which can be divided into 41 families and 57 subfamilies. Responding to the infection of P. infestans, 375 potato P450 genes were expressed in late blight resistant or susceptible cultivars. A total of 14 P450 genes were identified as resistant related candidates, and 81 P450 genes were identified as late blight responsive candidates. Several phytohormone biosynthesis, brassinosteroid biosynthesis, and phenylpropanoid biosynthesis involved P450 genes were differentially expressed during the potato-pathogen interactions. This study firstly reported the CYPome in S. tuberosum, and characterized the expression patterns of these P450 genes during the infection of P. infestans.

Effectiveness of potato late blight (Phytophthora infestans) forecast by meteorological estimation in mountainous terrain based on CARAH rules.

Potato late blight (PLB) caused by Phytophthora infestans (Mont.) de Bary, its incidence and development are highly dependent on meteorological conditions. To solve the problem of PLB in mountainous terrain under the condition of limited meteorological monitoring capability, the air temperature and humidity was estimated based on the basic meteorological datasets, the forecast effect of the onset period and infection cycle of PLB based on CARAH rules was evaluated. The average MAE, RMSE and CI of the estimated air temperature and observations were 1.17 °C, 1.52 °C and 0.95, respectively. The average MAE, RMSE and CI of the estimated relative humidity and observations were 8.0 %, 10.7 % and 0.53, respectively. The curve of the infection cycle of PLB at different locations were estimated from the basic meteorological datasets based on the CARAH rules, and the false alarm and missing ratios were 8.8 % and 4.6 % respectively. It may be delayed by 1 or 2 fungal generations compared to the observations, and then the protective fungicide should be adjusted to a systemic fungicide. The false alarm of the infection cycle of PLB may increase in dry air conditions, and the missing report may occur in humid condition.

Anti-Oomycete Effect and Mechanism of Salicylic Acid on Phytophthora infestans.

Pathogenic oomycetes infect a wide variety of organisms, including plants, animals, and humans, and cause massive economic losses in global agriculture, aquaculture, and human health. Salicylic acid (SA), an endogenous phytohormone, is regarded as an inducer of plant immunity. Here, the potato late blight pathogen Phytophthora infestans was used as a model system to uncover the inhibitory mechanisms of SA on pathogenic oomycetes. In this research, SA significantly inhibited the mycelial growth, sporulation, sporangium germination, and virulence of P. infestans. Inhibition was closely related to enhanced autophagy, suppression of translation initiation, and ribosomal biogenesis in P. infestans, as shown by multiomics analysis (transcriptomics, proteomics, and phosphorylated proteomics). Monodansylcadaverine (MDC) staining and Western blotting analysis showed that SA promoted autophagy in P. infestans by probably targeting the TOR signaling pathway. These observations suggest that SA has the potential to control late blight caused by P. infestans.

Harnessing Plant's Arsenal: Essential Oils as Promising Tools for Sustainable Management of Potato Late Blight Disease Caused by Phytophthora infestans-A Comprehensive Review.

Potato late blight disease is caused by the oomycete Phytophthora infestans and is listed as one of the most severe phytopathologies on Earth. The current environmental issues require new methods of pest management. For that reason, plant secondary metabolites and, in particular, essential oils (EOs) have demonstrated promising potential as pesticide alternatives. This review presents the up-to-date work accomplished using EOs against P. infestans at various experimental scales, from in vitro to in vivo. Additionally, some cellular mechanisms of action on Phytophthora spp., especially towards cell membranes, are also presented for a better understanding of anti-oomycete activities. Finally, some challenges and constraints encountered for the development of EOs-based biopesticides are highlighted.

The secreted PAMP-induced peptide StPIP1_1 activates immune responses in potato.

Treatment of potato plants with the pathogen-associated molecular pattern Pep-13 leads to the activation of more than 1200 genes. One of these, StPIP1_1, encodes a protein of 76 amino acids with sequence homology to PAMP-induced secreted peptides (PIPs) from Arabidopsis thaliana. Expression of StPIP1_1 is also induced in response to infection with Phytophthora infestans, the causal agent of late blight disease. Apoplastic localization of StPIP1_1-mCherry fusion proteins is dependent on the presence of the predicted signal peptide. A synthetic peptide corresponding to the last 13 amino acids of StPIP1_1 elicits the expression of the StPIP1_1 gene itself, as well as that of pathogenesis related genes. The oxidative burst induced by exogenously applied StPIP1_1 peptide in potato leaf disks is dependent on functional StSERK3A/B, suggesting that StPIP1_1 perception occurs via a receptor complex involving the co-receptor StSERK3A/B. Moreover, StPIP1_1 induces expression of FRK1 in Arabidopsis in an RLK7-dependent manner. Expression of an RLK from potato with high sequence homology to AtRLK7 is induced by StPIP1_1, by Pep-13 and in response to infection with P. infestans. These observations are consistent with the hypothesis that, upon secretion, StPIP1_1 acts as an endogenous peptide required for amplification of the defense response.

Anti-oomycete activities from essential oils and their major compounds on Phytophthora infestans.

Botanicals are various plant-based products like plant extracts or essential oils. Anti-fungal activities of selected essential oils were tested on the pathogen causing potato and tomato late blight (Phytophthora infestans). Tests to evaluate anti-oomycete activities of commercial essential oils and their major compounds were carried out in vitro in microplate in liquid media. Anti-oomycete activities on Phytophthora infestans strain were obtained from essential oils/major compounds: Eucalyptus citriodora/citronellal; Syzygium aromaticum (clove)/eugenol; Mentha spicata/D-Carvone, L-Carvone; Origanum compactum/carvacrol; Satureja montana (savory)/carvacrol; Melaleuca alternifolia (tea tree)/terpinen-4-ol, and Thymus vulgaris/thymol. As an active substance of mineral origin, copper sulfate was chosen as a control. All selected essential oils showed an anti-oomycete activity calculated with IC50 indicator. The essential oils of clove, savory, and thyme showed the best anti-oomycete activities similar to copper sulfate, while oregano, eucalyptus, mint, and tea tree essential oils exhibited significantly weaker activities than copper sulfate. Clove essential oil showed the best activity (IC50 = 28 mg/L), while tea tree essential oil showed the worst activity (IC50 = 476 mg/L). For major compounds, three results were obtained: they were statistically more active than their essential oils (carvacrol for oregano, D- and L-Carvone for mint) or as active as their essential oils sources (thymol for thyme, carvacrol for savory, terpinen-4-ol for tea tree) or less active than their original essential oils (eugenol for clove, citronellal for eucalyptus). Microscopical observations carried out with the seven essential oils showed that they were all responsible for a modification of the morphology of the mycelium. The results demonstrated that various essential oils show different anti-oomycete activities, sometimes related to a major compound and sometimes unrelated, indicating that other compounds must play a role in total anti-oomycete activity.