Toxicity of metalaxyl, azoxystrobin, dimethomorph, cymoxanil, zoxamide and mancozeb to Phytophthora infestans isolates from Serbia

Cymoxanil disrupts RNA synthesis through inhibiting the activity of dihydrofolate reductase

The agricultural fungicide cymoxanil (CMX) is commonly used in the treatment of plant pathogens, such as Phytophthora infestans. Although the use of CMX is widespread throughout the agricultural industry and internationally, the exact mechanism of action behind this fungicide remains unclear. Therefore, we sought to elucidate the biocidal mechanism underlying CMX. This was accomplished by first performing a large-scale chemical-genomic screen comprising the 4000 haploid non-essential gene deletion array of the yeast Saccharomyces cerevisiae. We found that gene families related to de novo purine biosynthesis and ribonucleoside synthesis were enriched in the presence of CMX. These results were confirmed through additional spot-test and colony counting assays. We next examined whether CMX affects RNA biosynthesis. Using qRT-PCR and expression assays, we found that CMX appears to target RNA biosynthesis possibly through the yeast dihydrofolate reductase (DHFR) enzyme Dfr1. To determine whether DHFR is a target of CMX, we performed an in-silico molecular docking assay between CMX and yeast, human, and P. infestans DHFR. The results suggest that CMX directly interacts with the active site of all tested forms of DHFR using conserved residues. Using an in vitro DHFR activity assay we observed that CMX inhibits DHFR activity in a dose-dependent relationship.

A Survey of Phytophthora spp. in Eastern Indian Nurseries and Their Sensitivity to Six Oomycete-Targeted Commercial Fungicides

A survey of the flori-horticultural nurseries in eastern India found Phytophthora nicotianae to be the most widespread Phytophthora species associated with different foliar symptoms of nursery plants and identified the presence of P. palmivora in eastern Indian nurseries for the first time. The survey also led to the first worldwide finding of P. nicotianae on Dipteracanthus prostratus (Poir.) Nees; Ocimum tenuiflorum L. (syn. Ocimum sanctum L.); Philodendron xanadu Croat, Mayo & J. Boos; and Pyrostegia venusta (Ker-Gawl.) Miers and P. palmivora on Episcia cupreata (Hook.) Hanst., as well as the first report from India of P. nicotianae on Spathiphyllum wallisii Regel; Anthurium andraeanum Linden ex André; and Adenium obesum (Forsk.) Roem. & Schult. Sensitivity to commercial fungicides Glazer 35WS, Rallis India (metalaxyl, FRAC code 4); Ridomil Gold, Syngenta (mefenoxam + mancozeb); Revus, Syngenta (mandipropamid, FRAC code 40); Aliette Bayer (fosetyl-Al, FRAC code 33); Acrobat, BASF (dimethomorph, FRAC code 40); and Amistar, Syngenta (azoxystrobin, FRAC code 11) was analyzed, showing EC50 values ranging from 0.75 to 16.39 ppm, 0.74 to 1.45 ppm, 2.43 to 17.21 ppm, 63.81 to 327.31 ppm, 8.88 to 174.69 ppm, and 0.1 to 1.13 ppm, respectively, with no cross-resistance of the isolates to the fungicides. The baseline information produced about these Phytophthora spp. from ornamental and horticultural host associations could help prevent the pathogens from becoming primary drivers of new disease outbreaks and their large-scale distribution beyond their natural endemic ranges.

Phenotypic and Genotypic Characterization of Phytophthora infestans Isolates Associated with Tomato and Potato Crops in Colombia

Late blight disease, caused by the plant pathogen Phytophthora infestans, is one of the major threats for tomato and potato crops. Monitoring the populations of P. infestans is important to determine if there are changes in the sensitivity to fungicides and host preference. In this study, microsatellite markers and mitochondrial haplotypes were used to assess the genotype of isolates of P. infestans collected from tomato and potato plants in Colombia. Furthermore, sensitivity to the three fungicides cymoxanil (penetrant fungicide), mefenoxam, and fluopicolide (systemic fungicides), and tomato-potato host preference, were evaluated. Mitochondrial haplotyping showed that isolates collected on tomato were from the genetic groups Ia and Ib, while isolates collected on potatoes belonged to group IIa. Microsatellite analyses showed that isolates from tomato form two groups, including the Ib mitochondrial haplotype (which is genetically close to the US-1 clonal lineage) and the Ia haplotype (related to the EC-3 lineage), whereas Colombian isolates from potato formed a separate group. Furthermore, differences in sensitivity to fungicides were observed. Eighty-one percent of the isolates tested were resistant to mefenoxam with an EC₅₀ >10 μg ml^-1. Forty-two percent of the isolates showed an intermediate resistance to cymoxanil. The EC₅₀ values ranged between 1 and 10 μg ml^-1. For fluopicolide, 90% of the isolates were sensitive, with EC₅₀ <1 μg ml^-1. Host preference assays showed that potato isolates infected both host species. Thus, isolates that infect potatoes may pose a risk for tomato crops nearby.

Phytophthora cinnamomi causing root rot on Rhododendron lapponicum and control it using potential biocontrol agents

Rhododendron lapponicum (R. lapponicum) is a dwarf Rhododendron species, which is severely infected with root rot and wilt in Yunnan province, China. However, the causal agent causing these symptoms was unknown. An isolate, Pci-1 was identified as Phytophthora cinnamomi, based on its morphology and the sequences of β-tubulin, internal transcribed spacer, and Ypt1 genes and verified according to the Koch's postulate. We found that this pathogen could infect 14 species of plants, including Althaea rosea, Viburnum cylindricum, and Brassica napus. Strain Pci-1 could cause R. lapponicum to wither and die; and it grows best in an oat medium with pH 7.0 - 8.0 and an optimum temperature of 27°C. We suggest that the rhizosphere of R. lapponicum treated with biocontrol strains Paenibacillus polymyxoides P2-5 and Trichoderma asperellum Tv-1 showed a significant inhibitory effect on pathogen Pci-1. The inhibitory effect of 70% dimethomorph + cymoxanil was significantly higher with EC₅₀ and EC₉₀ values of 0.1894 and 0.3618 a.i. µg/ml, respectively. Greenhouse experiments revealed that the pathogen load is decreased in the presence of potential antagonists. This study provides fundamentals on risk assessment and theoretical support for the management of P. cinnamomi pathogen and contributes significantly to the planting of forest and horticultural crops in a disease-free environment.

Phytophthora infestans: An Overview of Methods and Attempts to Combat Late Blight

Phytophthora infestans (Mont.) de Bary is one of the main pathogens in the agricultural sector. The most affected are the Solanaceae species, with the potato (Solanum tuberosum) and the tomato (Solanum lycopersicum) being of great agricultural importance. Ornamental Solanaceae can also host the pests Petunia spp., Calibrachoa spp., as well as the wild species Solanum dulcamara, Solanum sarrachoides, etc. Annual crop losses caused by this pathogen are highly significant. Although the interaction between P. infestans and the potato has been investigated for a long time, further studies are still needed. This review summarises the basic approaches in the fight against the late blight over the past 20 years and includes four sections devoted to methods of control: (1) fungicides; (2) R-gene-based resistance of potato species; (3) RNA interference approaches; (4) other approaches to control P. infestans. Based on the latest advances, we have provided a description of the significant advantages and disadvantages of each approach.

Increasing temperature elevates the variation and spatial differentiation of pesticide tolerance in a plant pathogen

Climate change and pesticide resistance are two of the most imminent challenges human society is facing today. Knowledge of how the evolution of pesticide resistance may be affected by climate change such as increasing air temperature on the planet is important for agricultural production and ecological sustainability in the future but is lack in scientific literatures reported from empirical research. Here, we used the azoxystrobin-Phytophthora infestans interaction in agricultural systems to investigate the contributions of environmental temperature to the evolution of pesticide resistance and infer the impacts of global warming on pesticide efficacy and future agricultural production and ecological sustainability. We achieved this by comparing azoxystrobin sensitivity of 180 P. infestans isolates sampled from nine geographic locations in China under five temperature schemes ranging from 13 to 25°C. We found that local air temperature contributed greatly to the difference of azoxystrobin tolerance among geographic populations of the pathogen. Both among-population and within-population variations in azoxystrobin tolerance increased as experimental temperatures increased. We also found that isolates with higher azoxystrobin tolerance adapted to a broader thermal niche. These results suggest that global warming may enhance the risk of developing pesticide resistance in plant pathogens and highlight the increased challenges of administering pesticides for effective management of plant diseases to support agricultural production and ecological sustainability under future thermal conditions.

4-Ethylphenol, A Volatile Organic Compound Produced by Disease-Resistant Soybean, Is a Potential Botanical Agrochemical Against Oomycetes

Oomycetes, represented by Phytophthora, are seriously harmful to agricultural production, resulting in a decline in grain quality and agricultural products and causing great economic losses. Integrated management of oomycete diseases is becoming more challenging, and plant derivatives represent effective alternatives to synthetic chemicals as novel crop protection solutions. Biologically active secondary metabolites are rapidly synthesized and released by plants in response to biotic stress caused by herbivores or insects, as well as pathogens. In this study, we identified groups of volatile organic compounds (VOCs) from soybean plants inoculated with Phytophthora sojae, the causal agent of soybean root rot. 4-Ethylphenol was present among the identified VOCs and was induced in the incompatible interaction between the plants and the pathogen. 4-Ethylphenol inhibited the growth of P. sojae and Phytophthora nicotianae and had toxicity to sporangia formation and zoospore germination by destroying the pathogen cell membrane; it had a good control effect on soybean root rot and tobacco black shank in the safe concentration range. Furthermore, 4-Ethylphenol had a potent antifungal activity against three soil-borne phytopathogenic fungi, Rhizoctonia solani, Fusarium graminearum, and Gaeumannomyces graminis var tritici, and four forma specialis of Fusarium oxysporum, which suggest a potential to be an eco-friendly biological control agent.

Effective Remediation Strategy for Xenobiotic Zoxamide by Pure Bacterial Strains, Escherichia coli, Streptococcus pyogenes, and Streptococcus pneumoniae

Zoxamide, a class IV hazardous fungicide, is perilous for the environment due to its highly persistent nature. Up till the current date, there are no reports on the biodegradation of zoxamide. The scarcity of knowledge in this domain led to the present research to evaluate the biodegradation of this benzamide fungicide by three bacterial strains, Escherichia coli (EC), Streptococcus pyogenes (SPy), and Streptococcus pneumoniae (SP). Biotransformation of zoxamide was scrutinized in nutrient broth assemblies for a period of 28 days followed by UV-visible spectrophotometer and GC-MS analysis of the metabolites. The results exhibited a low to medium biodegradation potential of the bacterial cells to metabolize zoxamide. The highest biotransformation percentage was observed by E. coli to be 29.8%. The order of half-life calculated for the degradation results was EC (42.5) < SPy (58.7) < SP (67.9) days. GC-MS analysis indicated the formation of several metabolites including, 2-(3,5-dichloro-4-methylphenyl)-4-ethyl-4-methyl-4H-1,3-oxazin-5(6H)-one, 3,5-dichloro-N-(3-hydroxy-1-ethyl-1-methyl--2-oxopropyl)-4-methylbenzamide and 3,5-dichloro-4-methylbenzamide. The research could influence the biotreatment strategies for the environmentally friendly eradication of xenobiotics.

Slow and temperature-mediated pathogen adaptation to a nonspecific fungicide in agricultural ecosystem

The spread of antimicrobial resistance and global change in air temperature represent two major phenomena that are exerting a disastrous impact on natural and social issues but investigation of the interaction between these phenomena in an evolutionary context is limited. In this study, a statistical genetic approach was used to investigate the evolution of antimicrobial resistance in agricultural ecosystem and its association with local air temperature, precipitation, and UV radiation. We found no resistance to mancozeb, a nonspecific fungicide widely used in agriculture for more than half a century, in 215 Alternaria alternata isolates sampled from geographic locations along a climatic gradient and cropping system representing diverse ecotypes in China, consistent with low resistance risk in many nonspecific fungicides. Genetic variance accounts for ~35% of phenotypic variation, while genotype-environment interaction is negligible, suggesting that heritability plays a more important role in the evolution of resistance to mancozeb in plant pathogens than phenotypic plasticity. We also found that tolerance to mancozeb in agricultural ecosystem is under constraining selection and significantly associated with local air temperature, possibly resulting from a pleiotropic effect of resistance with thermal and other ecological adaptations. The implication of these results for fungicide and other antimicrobial management in the context of global warming is discussed.

In vitro growth response of Phytophthora cactorum, P. nicotianae and P. × pelgrandis to antibiotics and fungicides

The reactions of isolates of Phytophthora cactorum, P. nicotianae and P. × pelgrandis to metalaxyl, mancozeb, dimethomorph, streptomycin and chloramphenicol were tested to obtain information about the variability of resistance in these pathogens. Distinct genetic groups showed significant differences in resistance to all tested substances except streptomycin. In response to streptomycin, the growth inhibition rates of distinct groups did not differ significantly. The most remarkable differences were detected in the reactions to chloramphenicol and metalaxyl. Discriminant analysis evaluating the effect of all substances confirmed the differences among the groups, which are in agreement with the differences revealed by earlier DNA analyses.

Diverse mechanisms shape the evolution of virulence factors in the potato late blight pathogen Phytophthora infestans sampled from China

Evolution of virulence in plant pathogens is still poorly understood but the knowledge is important for the effective use of plant resistance and sustainable disease management. Spatial population dynamics of virulence, race and SSR markers in 140 genotypes sampled from seven geographic locations in China were compared to infer the mechanisms driving the evolution of virulence in Phytophthora infestans (P. infestans). All virulence types and a full spectrum of race complexity, ranging from the race able to infect the universally susceptible cultivar only to all differentials, were detected. Eight and two virulence factors were under diversifying and constraining selection respectively while no natural selection was detected in one of the virulence types. Further analyses revealed excesses in simple and complex races but deficiency in intermediate race and negative associations of annual mean temperature at the site from which pathogen isolates were collected with frequency of virulence to differentials and race complexity in the pathogen populations. These results suggest that host selection may interact with other factors such as climatic conditions in determining the evolutionary trajectory of virulence and race structure in P. infestans and global warming may slow down the emergence of new virulence in the pathogen.

Comparative analyses of fungicide sensitivity and SSR marker variations indicate a low risk of developing azoxystrobin resistance in Phytophthora infestans

Knowledge of the evolution of fungicide resistance is important in securing sustainable disease management in agricultural systems. In this study, we analyzed and compared the spatial distribution of genetic variation in azoxystrobin sensitivity and SSR markers in 140 Phytophthora infestans isolates sampled from seven geographic locations in China. Sensitivity to azoxystrobin and its genetic variation in the pathogen populations was measured by the relative growth rate (RGR) at four fungicide concentrations and determination of the effective concentration for 50% inhibition (EC50). We found that all isolates in the current study were sensitive to azoxystrobin and their EC50 was similar to that detected from a European population about 20 years ago, suggesting the risk of developing azoxystrobin resistance in P. infestans populations is low. Further analyses indicate that reduced genetic variation and high fitness cost in resistant mutations are the likely causes for the low evolutionary likelihood of developing azoxystrobin resistance in the pathogen. We also found a negative correlation between azoxystrobin tolerance in P. infestans populations and the mean annual temperature of collection sites, suggesting that global warming may increase the efficiency of using the fungicide to control the late blight.

Fungicide Sensitivity of U.S. Genotypes of Phytophthora infestans to Six Oomycete-Targeted Compounds

Phytophthora infestans causes potato late blight, an important and costly disease of potato and tomato crops. Seven clonal lineages of P. infestans identified recently in the United States were tested for baseline sensitivity to six oomycete-targeted fungicides. A subset of the dominant lineages (n = 45) collected between 2004 and 2012 was tested in vitro on media amended with a range of concentrations of either azoxystrobin, cyazofamid, cymoxanil, fluopicolide, mandipropamid, or mefenoxam. Dose-response curves and values for the effective concentration at which 50% of growth was suppressed were calculated for each isolate. The US-8 and US-11 clonal lineages were insensitive to mefenoxam while the US-20, US-21, US-22, US-23, and US-24 clonal lineages were sensitive to mefenoxam. Insensitivity to azoxystrobin, cyazofamid, cymoxanil, fluopicolide, or mandipropamid was not detected within any lineage. Thus, current U.S. populations of P. infestans remained sensitive to mefenoxam during the displacement of the US-22 lineage by US-23 over the past 5 years.

Isolation and characterization of an antifungal protein from Bacillus licheniformis HS10

Bacillus licheniformis HS10 is a good biocontrol agent against Pseudoperonospora cubensis which caused cucumber downy disease. To identify and characterize the antifungal proteins produced by B.licheniformis HS10, the proteins from HS10 were isolated by using 30-60% ammonium sulfate precipitation, and purified with column chromatography on DEAE Sepharose Fast Flow, RESOURCE Q and Sephadex G-75. And the SDS-PAGE and MALDI-TOF/TOF-MS analysis results demonstrated that the antifungal protein was a monomer with molecular weight of about 55 kDa, identified as carboxypeptidase. Our experiments also showed that the antifungal protein from B. licheniformis HS10 had significantly inhibition on eight different kinds of plant pathogenic fungi, and it was stable with good biological activity at as high as 100°C for 30 min and in pH value ranged from 6 to 10. The biological activity was negatively affected by protease K and 10mM metal cations except Ca(2+).