Pythium, Phytophthora, and Phytopythium spp. Associated with Soybean in Minnesota, Their Relative Aggressiveness on Soybean and Corn, and Their Sensitivity to Seed Treatment Fungicides

Characterization of the Species of Pythium Associated with Corn Seedlings in Delaware

Corn (Zea mays L.) is the top grain crop by hectares grown in Delaware (DE). Increased pre- and postemergence damping-off in corn caused by Pythium species have been observed in recent seasons. To date, no characterization studies have been performed to understand Pythium species diversity in DE. Symptomatic corn seedlings were collected from 33 fields across DE in 2019 and 2020. Isolates were obtained from infected root tissues and identified by molecular sequencing. In total, 14 species were recovered: P. aristosporum, P. arrhenomanes, P. attrantheridium, P. catenulatum, P. dissotocum, P. graminicola, P. inflatum, P. irregulare, P. oligandrum, P. periplocum, P. spinosum, P. sylvaticum, P. tardicrescens, and P. torulosum. The dominant species recovered was P. graminicola, accounting for 66% of the isolates. Five species were screened for fungicide sensitivity to calculate the EC50 of mefenoxam, ethaboxam, and picarbutrazox which are the most commonly used active ingredients to manage species of Pythium in corn seed treatment packages. All species tested were sensitive to mefenoxam and picarbutrazox. For ethaboxam, P. torulosum was the only species with an EC50 value >10 μg ml-1. Ten species were screened for fungicide sensitivity to mefenoxam, ethaboxam, or picarbutrazox at 18, 21, and 25°C. As temperatures increased, percent inhibition was reduced in 40, 70, and 80% of the species screened for sensitivity to ethaboxam, picarbutrazox, and mefenoxam, respectively. Nine of these species were screened for isolate aggressiveness in greenhouse panels to examine impact on emergence, shoot height, shoot weight, and root weight. While all species were recovered from inoculated roots, P. arrhenomanes, P. attrantheridium, P. spinosum, and P. sylvaticum had the greatest root weight reduction.

Picarbutrazox Effectiveness Added to a Seed Treatment Mixture for Management of Oomycetes that Impact Soybean in Ohio

None of the current oomycota fungicides are effective towards all species of Phytophthora, Phytopythium, Globisporangium, and Pythium that affect soybean seed and seedlings in Ohio. Picarbutrazox is a new oomyceticide with a novel mode of action towards oomycete pathogens. Our objectives were to evaluate picarbutrazox to determine (i) baseline sensitivity (EC50) to 189 isolates of 29 species, (ii) the efficacy with a base seed treatment with three cultivars with different levels of resistance in 14 field environments; and (iii) if the rhizosphere microbiome was affected by the addition of the seed treatment on a moderately susceptible cultivar. The mycelial growth of all isolates was inhibited beginning at 0.001 μg, and the EC50 ranged from 0.0013 to 0.0483 μg of active ingredient (a.i.)/ml. The effect of seed treatment was significantly different for plant population and yield in eight of 14 and six of 12 environments, respectively. The addition of picarbutrazox at 1 and 2.5 g of a.i./100 kg seed to the base seed treatment compared to the base alone was associated with higher plant populations and yield in three and one environments, respectively. There was limited impact of the seed treatment mefenoxam 7.5 g of a.i. plus picarbutrazox 1 g of a.i./100 kg seed on the oomycetes detected in the rhizosphere of soybean seedlings collected at the V1 growth stage. Picarbutrazox has efficacy towards a wider range of oomycetes that cause disease on soybean, and this will be another oomyceticide tool to combat early season damping-off in areas where environmental conditions highly favor disease development.

Effect of Infection Timing by Four Pythium spp. on Soybean Damping-Off Symptoms with and Without Cold Stress

Pythium spp. cause damping-off of soybean, especially when soil conditions at or shortly after planting are cool and wet. Soybean planting dates continue to shift to earlier dates, so germinating seed and seedlings are exposed to periods of cold stress at a time which favors infection by Pythium, and seedling disease occurs. The objective of this study was to assess infection timing and cold stress on soybean seedling disease severity caused by four Pythium spp. prevalent in Iowa, namely P. lutarium, P. oopapillum, P. sylvaticum, and P. torulosum. Each species was used individually to inoculate soybean cultivar 'Sloan' using a rolled towel assay. Two temperature treatments (continuous 18°C [C18]; a 48-h cold stress period at 10°C [CS]) were applied. Soybean seedling age was divided into five growth stages (GS1 to GS5). Root rot severity and root length were assessed at 2, 4, 7, and 10 days after inoculation (DAI). At C18, root rot was greatest when soybean was inoculated with P. lutarium or P. sylvaticum at GS1 (seed imbibes water) and with P. oopapillum or P. torulosum at GS1, GS2 (radicle elongation), and GS3 (hypocotyl emergence). After CS, soybean susceptibility to P. lutarium and P. sylvaticum was reduced compared to C18 for inoculation at all GSs except GS5 (unifoliate leaf emergence). Conversely, root rot by P. oopapillum and P. torulosum was greater after CS compared to C18. Data from this study demonstrate that greater root rot, and consequently more damping-off, is likely if infection occurs at early germination stages before seedling emergence.

Long-Term Tillage and Crop Rotation Regimes Reshape Soil-Borne Oomycete Communities in Soybean, Corn, and Wheat Production Systems

Soil-borne oomycetes include devastating plant pathogens that cause substantial losses in the agricultural sector. To better manage this important group of pathogens, it is critical to understand how they respond to common agricultural practices, such as tillage and crop rotation. Here, a long-term field experiment was established using a split-plot design with tillage as the main plot factor (conventional tillage (CT) vs. no till (NT), two levels) and rotation as the subplot factor (monocultures of soybean, corn, or wheat, and corn-soybean-wheat rotation, four levels). Post-harvest soil oomycete communities were characterized over three consecutive years (2016-2018) by metabarcoding the Internal Transcribed Spacer 1 (ITS1) region. The community contained 292 amplicon sequence variants (ASVs) and was dominated by Globisporangium spp. (85.1% in abundance, 203 ASV) and Pythium spp. (10.4%, 51 ASV). NT decreased diversity and community compositional structure heterogeneity, while crop rotation only affected the community structure under CT. The interaction effects of tillage and rotation on most oomycetes species accentuated the complexity of managing these pathogens. Soil and crop health represented by soybean seedling vitality was lowest in soils under CT cultivating soybean or corn, while the grain yield of the three crops responded differently to tillage and crop rotation regimes.

Synthesis of 3,4-dihydroisoquinolin-1(2H)-one derivatives and their antioomycete activity against the phytopathogen Pythium recalcitrans†

In an effort to exploit the bioactive natural scaffold 3,4-dihydroisoquinolin-1(2H)-one for plant disease management, 59 derivatives of this scaffold were synthesized using the Castagnoli–Cushman reaction. The results of bioassay indicated that their antioomycete activity against Pythium recalcitrans was superior to the antifungal activity against the other 6 phytopathogens. Compound I23 showed the highest in vitro potency against P. recalcitrans with an EC₅₀ value of 14 μM, which was higher than that of the commercial hymexazol (37.7 μM). Moreover, I23 exhibited in vivo preventive efficacy of 75.4% at the dose of 2.0 mg/pot, which did not show significant differences compared with those of hymexazol treatments (63.9%). When the dose was 5.0 mg per pot, I23 achieved a preventive efficacy of 96.5%. The results of the physiological and biochemical analysis, the ultrastructural observation and lipidomics analysis suggested that the mode of action of I23 might be the disruption of the biological membrane systems of P. recalcitrans. In addition, the established CoMFA and CoMSIA models with reasonable statistics in the three-dimensional quantitative structure–activity relationship (3D-QSAR) study revealed the necessity of the C4-carboxyl group and other structural requirements for activity. Overall, the above results would help us to better understand the mode of action and the SAR of these derivatives, and provide crucial information for further design and development of more potent 3,4-dihydroisoquinolin-1(2H)-one derivatives as antioomycete agents against P. recalcitrans.

A collection of 3,4-dihydroisoquinolin-1(2H)-one derivatives were synthesized by Castagnoli–Cushman reaction to screen antioomycete agents against Pythium recalcitrans.

Efficacy and Timing of Application of Fungicides, Biofungicides, Host-Plant Defense Inducers, and Fertilizer to Control Phytophthora Root Rot of Flowering Dogwood in Simulated Flooding Conditions in Container Production

Phytophthora root rot, caused by Phytophthora cinnamomi Rands, is one of the major diseases of flowering dogwood (Cornus florida L.). The severity of root rot disease increases when the plants are exposed to flooding conditions. A study was conducted to determine the efficacy and timing of application of different fungicides, biofungicides, host-plant defense inducers, and fertilizer to manage Phytophthora root rot in month-old seedlings in simulated flooding events for 1, 3, and 7 days. Preventative treatments were drench applied 3 weeks and 1 week before flooding whereas curative treatments were applied 24 h after flooding. Dogwood seedlings were inoculated with P. cinnamomi 3 days before the flooding. Plant height and width were recorded at the beginning and end of the study. At the end of the study, plant total weight and root weight were recorded and disease severity in the root was assessed using a scale of 0 to 100%. Root samples were plated using PARPH-V8 medium to determine the percent recovery of the pathogen. Empress Intrinsic, Pageant Intrinsic, Segovis, and Subdue MAXX, as preventative and curative applications, were able to suppress the disease severity compared with the inoculated control in all flooding durations. All treatments, with the exception of Stargus as a preventative application 3 weeks before flooding and Orkestra Intrinsic as a curative application, were able to suppress the disease severity compared with the inoculated control for a 1-day flooding event. Aliette and ON-Gard were effective in the first trial when applied preventatively at both 1 week and 3 weeks before flooding but not in the second trial. Signature Xtra was effective as a preventative application but not as a curative application. Interface was effective as a curative application but not as a preventative application. The findings of this study will help nursery growers to understand the performance of fungicides, biofungicides, host-plant defense inducers, and fertilizer at different time intervals and repeated applications to manage Phytophthora root rot in flooding conditions.

Identification and characterization of pleiotropic and epistatic QDRL conferring partial resistance to Pythium irregulare and P. sylvaticum in soybean

Pleiotropic and epistatic quantitative disease resistance loci (QDRL) were identified for soybean partial resistance to different isolates of Pythium irregulare and Pythium sylvaticum. Pythium root rot is an important seedling disease of soybean [Glycine max (L.) Merr.], a crop grown worldwide for protein and oil content. Pythium irregulare and P. sylvaticum are two of the most prevalent and aggressive Pythium species in soybean producing regions in the North Central U.S. Few studies have been conducted to identify soybean resistance for management against these two pathogens. In this study, a mapping population (derived from E13390 x E13901) with 228 F4:5 recombinant inbred lines were screened against P. irregulare isolate MISO 11-6 and P. sylvaticum isolate C-MISO2-2-30 for QDRL mapping. Correlation analysis indicated significant positive correlations between soybean responses to the two pathogens, and a pleiotropic QDRL (qPirr16.1) was identified. Further investigation found that the qPirr16.1 imparts dominant resistance against P. irregulare, but recessive resistance against P. sylvaticum. In addition, two QDRL, qPsyl15.1, and qPsyl18.1 were identified for partial resistance to P. sylvaticum. Further analysis revealed epistatic interactions between qPirr16.1 and qPsyl15.1 for RRW and DRX, whereas qPsyl18.1 contributed resistance to RSE. Marker-assisted resistance spectrum analysis using F6:7 progeny lines verified the resistance of qPirr16.1 against four additional P. irregulare isolates. Intriguingly, although the epistatic interaction of qPirr16.1 and qPsyl15.1 can be confirmed using two additional isolates of P. sylvaticum, the interaction appears to be suppressed for the other two P. sylvaticum isolates. An 'epistatic gene-for-gene' model was proposed to explain the isolate-specific epistatic interactions. The integration of the QDRL into elite soybean lines containing all the desirable alleles has been initiated.

Reduction of Pythium Damping-Off in Soybean by Biocontrol Seed Treatment

Pythium spp. is one of the major groups of pathogens that cause seedling diseases on soybean, leading to both preemergence and postemergence damping-off and root rot. More than 100 species have been identified within this genus, with Pythium irregulare, P. sylvaticum, P. ultimum var ultimum, and P. torulosum being particularly important for soybean production given their aggressiveness, prevalence, and abundance in production fields. This study investigated the antagonistic activity of potential biological control agents (BCAs) native to the U.S. Midwest against Pythium spp. First, in vitro screening identified BCAs that inhibit P. ultimum var. ultimum growth. Scanning electron microscopy demonstrated evidence of mycoparasitism of all potential biocontrol isolates against P. ultimum var. ultimum and P. torulosum, with the formation of appressorium-like structures, short hyphal branches around host hyphae, hook-shaped structures, coiling, and parallel growth of the mycoparasite along the host hyphae. Based on these promising results, selected BCAs were tested under field conditions against six different Pythium spp. Trichoderma afroharzianum 26 used alone and a mix of T. hamatum 16 + T. afroharzianum 19 used as seed treatments protected soybean seedlings from Pythium spp. infection, as BCA-treated plots had on average 15 to 20% greater plant stand and vigor than control plots. Our results also indicate that some of these potential BCAs could be added with a fungicide seed treatment with minimum inhibition occurring, depending on the fungicide active ingredient. This research highlights the need to develop tools incorporating biological control as a facet of soybean seedling disease management programs. The harnessing of native BCAs could be integrated with other management strategies to provide efficient control of seedling diseases.

Oxathiapiprolin Alone or Mixed with Metalaxyl Seed Treatment for Management of Soybean Seedling Diseases Caused by Species of Phytophthora, Phytopythium, and Pythium

Species of Phytophthora, Phytopythium, and Pythium affect soybean seed and seedlings each year, primarily through reduced plant populations and yield. Oxathiapiprolin is effective at managing several foliar diseases caused by some oomycetes. The objectives of these studies were to evaluate oxathiapiprolin in a discriminatory dose assay in vitro; evaluate oxathiapiprolin as a soybean seed treatment on a moderately susceptible cultivar in 10 environments; compare the impact of seed treatment on plant populations and yields in environments with low and high precipitation; and compare a seed treatment mixture on cultivars with different levels of resistance in four environments. There was no reduction in growth in vitro among 13 species of Pythium at 0.1 µg ml-1. Soybean seed treated with the base fungicide plus oxathiapiprolin (12 and 24 µg a.i. seed-1) alone, oxathiapiprolin (12 µg a.i. seed-1) plus mefenoxam (6 µg a.i. seed-1), or oxathiapiprolin (24 µg a.i. seed-1) plus ethaboxam (12.1 µg a.i. seed-1) had greater yields in environments that received ≥50 mm of precipitation within 14 days after planting compared with those that received less. Early plant population and yield were significantly higher for seed treated with oxathiapiprolin (24 µg a.i. seed-1) + metalaxyl (13.2 µg a.i. seed-1) compared with nontreated for six of seven cultivars in at least one of four environments. Oxathiapiprolin combined with another Oomycota fungicide applied to seed has the potential to be used to protect soybean plant establishment and yield in regions prone to poor drainage after high levels of precipitation.

Diversity and Pathogenicity of Pythium Species Associated with Reduced Yields of Processing Tomatoes (Solanum lycopersicum) in Victoria, Australia

Yield decline associated with poor crop establishment, stunting, wilting, and diminished root systems was reported in processing tomato crops in Victoria, Australia. During surveys between 2016 and 2018 Pythium species were isolated by soil baiting and by culturing from the diseased roots and collars of plants exhibiting these symptoms. Eleven species of Pythium were identified based on cultural characteristics and phylogenetic analysis with ITS, Cox-1, and Cox-2 gene sequences. None of the 11 Pythium species had been reported previously from processing or fresh tomatoes in Australia. Pythium dissotocum was the most abundant and widespread species isolated during surveys in each of two growing seasons. In pathogenicity tests, these Pythium species ranged from nonpathogenic to highly aggressive. P. aphanidermatum, P. ultimum, and P. irregulare were consistently the most aggressive species, causing serious damage or death at the pregermination, postgermination, and later stages of plant growth. Five processing tomato cultivars varied significantly in their susceptibility to Pythium disease. These results suggest that Pythium species could be contributing to yield loss in processing tomatoes in Victoria both in the crop establishment phase and through the season.

Peronosporales Species Associated with Strawberry Crown Rot in the Czech Republic

The symptoms of crown rot on strawberry plants are considered typical for the pathogen Phytophthora cactorum, which causes high losses of this crop. However, an unknown number of related species of pathogens of Peronosporales cause symptoms quite similar to those caused by P. cactorum. To determine their spectrum and importance, strawberry plants were sampled from 41 farms in the Czech Republic. The cultures were isolated from the symptomatic plants using the baiting method, with subsequent cultivation on a semiselective medium. Isolates were identified to the species level using nuclear ribosomal internal transcribed spacer (ITS) barcoding after preliminary morphological determination. In total, 175 isolates of 24 species of Phytophthora, Phytopythium, Pythium, and Globisporangium were detected. The most represented was Phytophthora cactorum, with 113 (65%) isolates, which was recorded in 61% of farms, and the Pythium dissotocum complex with 20 (11%) isolates, which was recorded in 27% of farms. Other species were represented in units of percent. Large differences between farms in the species spectra were ascertained. The differences between species in cardinal growth temperatures and different management of the farms are discussed as a main reason for such a diversification. Regarding the dissimilar sensitivity of various species of Peronosporales against fungicides, the proper determination of the cause of disease is of crucial significance in plant protection.

Pathotype Complexity and Genetic Characterization of Phytophthora sojae Populations in Illinois, Indiana, Kentucky, and Ohio

Phytophthora sojae, the causal agent of Phytophthora root and stem rot of soybean, has been managed with single Rps genes since the 1960s but has subsequently adapted to many of these resistance genes, rendering them ineffective. The objective of this study was to examine the pathotype and genetic diversity of P. sojae from soil samples across Illinois, Indiana, Kentucky, and Ohio by assessing which Rps genes were still effective and identifying possible population clusters. There were 218 pathotypes identified from 473 P. sojae isolates with an average of 6.7 out of 15 differential soybean lines exhibiting a susceptible response for each isolate. Genetic characterization of 103 P. sojae isolates from across Illinois, Indiana, Kentucky, and Ohio with 19 simple sequence repeat markers identified 92 multilocus genotypes. There was a moderate level of population differentiation between these four states, with pairwise FST values ranging from 0.026 to 0.246. There were also moderate to high levels of differentiation between fields, with pairwise FST values ranging from 0.071 to 0.537. Additionally, cluster analysis detected the presence of P. sojae population structure across neighboring states. The level of pathotype and genetic diversity, in addition to the identification of population clusters, supports the hypothesis of occasional outcrossing events that allow an increase in diversity and the potential to select for a loss in avirulence to specific resistance genes within regions. The trend of suspected gene flow among neighboring fields is expected to be an ongoing issue with current agricultural practices.

Phytophthora sojae Pathotype Distribution and Fungicide Sensitivity in Michigan

Identifying the pathotype structure of a Phytophthora sojae population is crucial for the effective management of Phytophthora stem and root rot of soybean (PRR). P. sojae has been successfully managed with major resistance genes, partial resistance, and fungicide seed treatments. However, prolonged use of resistance genes or fungicides can cause pathogen populations to adapt over time, rendering resistance genes or fungicides ineffective. A statewide survey was conducted to characterize this pathotype structure and fungicide sensitivity of P. sojae within Michigan. Soil samples were collected from 69 fields with a history of PRR and fields having consistent plant stand establishment issues. Eighty-three isolates of P. sojae were obtained, and hypocotyl inoculations were performed on 14 differential soybean cultivars, all of which carry a single Rps gene or no resistance gene. The survey identified a loss of effectiveness of Rps genes 1b, 1k, 3b, and 6, compared with a previous survey conducted in Michigan from 1993 to 1997. Three effective resistance genes were identified for P. sojae management in Michigan; Rps 3a, 3c, and 4. Additionally, the effective concentration of common seed treatment fungicides to inhibit mycelial growth by 50% (EC50) was determined. No P. sojae isolates were insensitive to the tested chemistries with mean EC50 values of 2.60 × 10-2 μg/ml for ethaboxam, 3.03 × 10-2 μg/ml for mefenoxam, 2.88 × 10-4 μg/ml for oxathiapiprolin, and 5.08 × 10-2 μg/ml for pyraclostrobin. Results suggest that while there has been a significant shift in Rps gene effectiveness, seed treatments are still effective for early season management of this disease.

Phytopythium vexans Associated with Apple and Pear Decline in the Saïss Plain of Morocco

An extensive survey conducted in the Saïss plain of Morocco during the 2017-2018 growing season revealed that 35 out of 50 apple and pear orchards were infested with a pathogen that causes the decline disease. Morphological and phylogenetic tree analyses using the cox II gene allowed us to identify the pathogen as Phytopythium vexans. Interestingly, no Phytophthora and Pythium species were isolated. The occurrence and prevalence of the disease varied between locations; the most infested locations were Meknes (100%), Imouzzer (83%), and Sefrou (80%). To fulfill Koch's postulate, a greenhouse pathogenicity test was performed on the stem and collar of one-year-old healthy seedlings of apple rootstock M115. Symptoms similar to those observed in the field were reproduced in less than 4 months post-inoculation with root rot disease severity ranging from 70 to 100%. The survey results evidenced that apple rootstocks, soil type, and irrigation procedure may contribute significantly to the occurrence of the disease. The disease was most prevalent in drip water irrigation and sandy-clay soil on wild apple rootstock. Accordingly, a rational drip advanced watering system and good sanitation practices could eliminate water stagnation and help prevent the onset of this disease. It was concluded that Pp. vexans occurrence may be strongly influenced by irrigation mode and type of soil. Therefore, the obtained findings of this study could help to better understand the recurrence of this disease and to develop a reliable integrated strategy for its management.

Extracellular metabolites of endophytic fungi from Azadirachta indica inhibit multidrug-resistant bacteria and phytopathogens

Aim: To evaluate antimicrobial activity of extracellular metabolites (EMs) of endophytic fungal isolates (EFIs) from Azadirachta indica. Materials & methods: EFIs were identified by internal transcribed spacer (ITS) sequencing. Antimicrobial activity, and minimum inhibitor concentration (MIC) and minimum bactericidal concentration (MBC) were determined using agar diffusion and microdilution method, respectively. Results: Seventeen EFIs were isolated from different organs of A. indica. Eight of them were identified based on ITS sequencing. The EMs of EFIs inhibited the growth of six multidrug-resistant (MDR) bacterial superbugs and three phytopathogenic fungi. The MDR bacterial superbugs are resistant to six commercial antibiotics of different generations but susceptible to EMs of EFIs. The MIC (0.125-1.0 μg/μl), MBC (0.5-4.0 μg/μl) and minimum fungicidal concentration (1.0-4.0 μg/μl) of the EMs from EFIs are lower enough. Conclusion: The EMs of the EFIs have promising antimicrobial activity against MDR bacteria and phytopathogenic fungi.

Influence of Soybean Tissue and Oomicide Seed Treatments on Oomycete Isolation

Soybean seedlings are vulnerable to different oomycete pathogens. Seed treatments containing the two antioomycete (oomicide) chemicals, metalaxyl-M (mefenoxam) and ethaboxam, are used for protection against oomycete pathogens. This study aimed to evaluate the influence of these two oomicides on isolation probability of oomycetes from soybean taproot or lateral root sections. Soybean plants were collected between the first and third trifoliate growth stages from five Midwest field locations in 2016 and four of the same fields in 2017. Oomycetes were isolated from taproot and lateral root. In 2016, 369 isolation attempts were completed, resulting in 121 isolates from the taproot and 154 isolates from the lateral root. In 2017, 468 isolation attempts were completed, with 44 isolates from the taproot and 120 isolates from the lateral roots. In three of nine site-years, the probability of isolating an oomycete from a taproot or lateral root section was significantly different. Seed treatments containing a mixture of ethaboxam and metalaxyl significantly reduced the probability of oomycete isolation from lateral roots in Illinois in 2016 and 2017, but not in other locations, which may have been related to the heavy soil type (clay loam). Among the 439 isolates collected from the two years sampled, 24 oomycete species were identified, and community compositions differed depending on location and year. The five most abundant species were Pythium sylvaticum (28.9%), P. heterothallicum (14.3%), P. ultimum var. ultimum (11.8%), P. attrantheridium (7.9%), and P. irregulare (6.6%), which accounted for 61.7% of the isolates collected. Oomicide sensitivity to ethaboxam and mefenoxam was assessed for >300 isolates. There were large differences in ethaboxam sensitivity among oomycete species, with effective concentrations to reduce optical density at 600 nm by 50% compared with the nonamended control (EC50 values) ranging from <0.01 to >100 μg/ml and a median of 0.65 μg/ml. Isolates with insensitivity to ethaboxam (>12 μg/ml) belonged to the species P. torulosum and P. rostratifingens but were sensitive to mefenoxam. Oomicide sensitivity to mefenoxam ranged from <0.01 to 0.62 μg/ml with a median of 0.03 μg/ml. The mean EC50 value of the five most abundant species to ethaboxam ranged from 0.35 to 0.97 μg/ml of ethaboxam and from 0.02 to 0.04 μg/ml of mefenoxam. No shift in sensitivity to mefenoxam or ethaboxam was observed as a result of soybean seed treatment or year relative to the nontreated seed controls. In summary, this study contributed to the understanding of the composition of oomycete populations from different soybean root tissues, locations, years, and seed treatments. Finally, seed treatments containing mefenoxam or metalaxyl plus ethaboxam can be effective in reducing the probability of oomycete isolation from soybean roots.

Pythium huanghuaiense sp. nov. isolated from soybean: morphology, molecular phylogeny and pathogenicity

Background

Soybean (Glycine max) is a major source of edible oil and protein. A novel species of the genus Pythium, Pythium huanghuaiense, isolated from soybean seedlings in China, is described and illustrated on the basis of morphological characters and molecular evidence.

New information

Pythium huanghuaiense sp. nov. is closely related to species of the genus Pythium in clade F, as evidenced by the presence of hyphal swellings and its relatively rapid morphological growth. However, it differs by having relatively small sporangia and plerotic or nearly plerotic and thin-walled oospores. A pathogenicity test confirmed the newly-identified species as a pathogen of soybean.

Identification and Chemical and Biological Management of Phytopythium vexans, the Causal Agent of Phytopythium Root and Crown Rot of Woody Ornamentals

Soilborne diseases caused by pathogens such as Phytophthora, Rhizoctonia, Fusarium, Verticillium, and Pythium species are the most important diseases of woody ornamentals. Ginkgo (Ginkgo biloba) and red maple (Acer rubrum 'October Glory') plants grown in containers and fields in Tennessee showed root and crown rot symptoms with dark brown to black lesions in 2017 and 2018. The objective of this research was to isolate and identify pathogens affecting ginkgo and red maple plants in Tennessee nurseries and to develop fungicide/biofungicide management recommendations for nursery producers. Isolations were made from the infected roots. Several Phytophthora-like colonies with spherical zoospores, filamentous to globose oogoni, and whitish mycelium were isolated on V8-PARPH medium. To confirm identity, total genomic DNA was extracted, followed by sequence analysis of the internal transcribed spacer regions, large subunit of nuclear rRNA, and cytochrome c oxidase subunits I and II of mitochondrial DNA. Based on morphological and molecular analysis, Phytopythium vexans was described as a causal agent of crown and root rot from the infected ginkgo and red maple plants. To complete Koch's postulates, a pathogenicity test was performed by drenching 100 ml of V8 agar medium slurry of Phytopythium vexans inoculum on 1-year-old potted ginkgo plant root systems as well as red maple October Glory. Necrotic lesion development was observed in the root system 45 days after inoculation and Phytopythium vexans was reisolated from the roots of both ginkgo and red maple. All control ginkgo and red maple plants remained disease free and no pathogen was reisolated. In addition, the efficacy of fungicides, biofungicides, fertilizer, and host plant defense inducers (traditionally recommended for management of oomycete diseases) for control of Phytopythium crown and root rot was evaluated on ginkgo and red maple October Glory seedlings in greenhouse and field trials. Fungicides such as Empress Intrinsic, Pageant Intrinsic, Segovis, and Subdue MAXX were effective in both greenhouse and field trials, and the biofungicide Stargus reduced disease severity caused by pathogen Phytopythium vexans on ginkgo and red maple plants in greenhouse trials. These results will help nursery producers make proper management decisions for newly reported Phytopythium crown and root rot disease of ginkgo and red maple plants.

Effect of 6-Methoxy-2-Benzoxazolinone (MBOA) on Pythium Species and Corn Seedling Growth and Disease

Corn yield reduction following a cereal rye cover crop has been attributed to, among other factors, allelochemicals released from decomposing cereal rye residue. The allelopathic effect of 6-methoxy-2-benzoxazolinone (MBOA) was evaluated on corn seedling growth, mycelial growth of seven pathogenic species of Pythium, and root rot of corn seedlings caused by Pythium spp. at 13, 16, and 22 to 23°C (room temperature) using a plate assay. Mycelial growth of all Pythium spp. tested was slower with MBOA at 0.25 mg/ml compared with MBOA at 0.125 and 0.0625 mg/ml and the check (4% V8 juice medium containing neomycin sulfate and chloramphenicol with 0.5% dimethyl sulfoxide). Therefore, no further tests were done with MBOA at 0.25 mg/ml. In general, MBOA reduced corn radicle length and did not cause root rot across all temperatures. However, greater root rot severity in corn was observed on corn seedlings grown in the presence of Pythium lutarium and P. oopapillum on media amended with MBOA compared with the check at all temperatures. Similarly, more root rot caused by P. torulosum and P. spinosum was observed when MBOA was present at 16°C compared with the check with no MBOA. These data suggest that corn seedling disease caused by Pythium spp. could be more severe when corn is planted following a cover crop of winter cereal rye due to the presence of allelochemicals that are released from the cover crop.

Identification of Quantitative Disease Resistance Loci Toward Four Pythium Species in Soybean

In this study, four recombinant inbred line (RIL) soybean populations were screened for their response to infection by Pythium sylvaticum, Pythium irregulare, Pythium oopapillum, and Pythium torulosum. The parents, PI 424237A, PI 424237B, PI 408097, and PI 408029, had higher levels of resistance to these species in a preliminary screening and were crossed with "Williams," a susceptible cultivar. A modified seed rot assay was used to evaluate RIL populations for their response to specific Pythium species selected for a particular population based on preliminary screenings. Over 2500 single-nucleotide polymorphism (SNP) markers were used to construct chromosomal maps to identify regions associated with resistance to Pythium species. Several minor and large effect quantitative disease resistance loci (QDRL) were identified including one large effect QDRL on chromosome 8 in the population of PI 408097 × Williams. It was identified by two different disease reaction traits in P. sylvaticum, P. irregulare, and P. torulosum. Another large effect QDRL was identified on chromosome 6 in the population of PI 408029 × Williams, and conferred resistance to P. sylvaticum and P. irregulare. These large effect QDRL will contribute toward the development of improved soybean cultivars with higher levels of resistance to these common soil-borne pathogens.

Taxonomic and pathogenic characterization of Phytopythium species from West Azarbaijan, Iran, and description of two new species

Phytopythium is a globally distributed genus, species of which occur in various ecological niches. Despite their importance as plant pathogens, information on species diversity and their distribution in West Azarbaijan Province of Iran is limited. This study aimed to investigate the taxonomy and plant pathogenicity of Phytopythium isolates recovered from soils in the province. A polyphasic taxonomy based on morphological, cultural, and multilocus sequence data revealed two new species, described here as P. babaiaharii and P. longitubum. In addition, a putatively new species and five known species, P. boreale, P. carbonicum, P. mercuriale, P. ostracodes, and P. vexans, were found in the studied region. Phytopythium ostracodes, P. mercuriale, and P. boreale were the three most frequent species isolated from soil, although P. mercuriale was only found in one field. Among the studied species, P. mercuriale, P. ostracodes, and P. vexans exhibited various levels of pathogenicity on sugar beet, sunflower, and tomato, and P. boreale was documented for first time as a plant pathogen. Phytopythium ostracodes was highly aggressive on sugar beet and sunflower but lowly aggressive on tomato, whereas P. vexans exhibited high aggressiveness toward the three crop plants. Both P. boreale and P. mercuriale were found to be lowly aggressive on the host plants. The results indicated that members of Phytopythium, particularly plant-pathogenic species, are common in arable soils of West Azarbaijan Province and can pose a threat to agricultural crops in the region.

Sensitivity of Pythium spp. and Phytopythium spp. and tolerance mechanism of Pythium spp. to oxathiapiprolin

BACKGROUND

Oxathiapiprolin, developed by DuPont, is the only commercial oxysterol-binding protein inhibitor (OSBPI) of oomycete pathogens. Although the activity of oxathiapiprolin on some Pythium spp. and Phytopythium spp. has been reported, it has not been tested on many other species, and little is known about the mechanisms of Pythium spp. that are tolerant to it.

RESULTS

Oxathiapiprolin exhibited a strong inhibitory effect on mycelial growth of Phy. litorale, Phy. helicoides and Phy. chamaehyphon, with EC₅₀ values ranging from 0.002 to 0.013 μg mL^-1 . It also showed good effectiveness against Py. splendens and two Py. ultimum isolates, with EC₅₀ values ranging from 0.167 to 0.706 μg mL^-1 , but showed no activity against 14 other Pythium spp. Oxathiapiprolin provoked a slight upregulation of PuORP1 in Py. ultimum, but it did not lead to PaORP1-1 or PaORP1-2 overexpression in Py. aphanidermatum. Transformation and expression of PuORP1, PaORP1-1 or PaORP1-2 in the sensitive wild-type Phytophthora sojae isolate P6497 confirmed that either the PuORP1, PaORP1-1 or PaORP1-2 was responsible for the observed oxathiapiprolin tolerance.

CONCLUSION

This study showed that oxathiapiprolin had excellent activity against Phytopythium spp. but displayed a differentiated activity against different Pythium spp. ORP1s in Pythium spp. are positively related to the tolerance of Pythium species to oxathiapiprolin. © 2020 Society of Chemical Industry.

Phytopythium litorale: A Novel Killer Pathogen of Plane (Platanus orientalis) Causing Canker Stain and Root and Collar Rot

Decline symptoms associated with lethal stem and branch canker stain along with root and collar rots were observed on 5- to 7-year-old roadside oriental plane trees (Platanus orientalis) in Diyarbakır, Turkey. Above-ground symptoms included leaf necrosis, leaf curling, extensive bluish or blackish staining of shoots, branches, stem bark, and wood surfaces, as well as stem cankers and exfoliation of branch bark scales. A general decline of the trees was distinctly visible from a distance. A Phytophthora/Pythium-like oomycete species with globose to ovoid, often papillate and internally proliferating sporangia was consistently isolated from the fine and coarse roots and stained branch parts and shoots. The pathogen was identified as Phytopythium litorale based on several morphological features. Partial DNA sequences of three loci, including nuclear rDNA internal transcribed spacer (ITS) and the large ribosomal subunit (LSU), and mitochondrial cytochrome c oxidase subunit II (coxII) confirmed the morphological identification. All P. litorale isolates were homothallic, developing gametangia, ornamented oogonia with elongate to lobate antheridia. Pathogenicity of P. litorale was tested by inoculation on excised shoots and by root inoculation on seedlings. P. litorale produced large lesions and blights on shoots in just 5 days and killed 100% of the seedlings in a month. This paper presents the first confirmed report of P. litorale as an important pathogen on a plant species causing branch and stem cankers, and root and collar rot, in and on P. orientalis, resulting in a rapid decline of trees and suggesting a threat to plane.

The Efficacy of Ethaboxam as a Soybean Seed Treatment Toward Phytophthora, Phytopythium, and Pythium in Ohio

Phytophthora, Phytopythium, and Pythium species that cause early-season seed decay and pre-emergence and post-emergence damping off of soybean are most commonly managed with seed treatments. The phenylamide fungicides metalaxyl and mefenoxam, and ethaboxam are effective toward some but not all species. The primary objective of this study was to evaluate the efficacy of ethaboxam in fungicide mixtures and compare those with other fungicides as seed treatments to protect soybean against Pythium, Phytopythium, and Phytophthora species in both high-disease field environments and laboratory seed plate assays. The second objective was to evaluate these seed treatment mixtures on cultivars that have varying levels and combinations of resistance to these soilborne pathogens. Five of eight environments received adequate precipitation in the 14 days after planting for high levels of seedling disease development and treatment evaluations. Three environments had significantly greater stands, and three had significantly greater yield when ethaboxam was used in the seed treatment mixture compared with treatments containing metalaxyl or mefenoxam alone. Three fungicide formulations significantly reduced disease severity compared with nontreated in the seed plate assay for 17 species. However, the combination of ethaboxam plus metalaxyl in a mixture was more effective than either fungicide alone against some Pythium and Phytopythium species. Overall, our results indicate that the addition of ethaboxam to a fungicide seed treatment is effective in reducing seed rot caused by these pathogens commonly isolated from soybean in Ohio but that these effects can be masked when cultivars with resistance are planted.

Is the Phenomenon of Mefenoxam-Acquired Resistance in Phytophthora infestans Universal?

Phytophthora infestans is the causal agent of late blight disease of potatoes and tomatoes. This disease causes devastating economic losses each year, and control is mainly achieved by the use of fungicides. Unfortunately, populations of P. infestans resistant to fungicides have been documented. Furthermore, studies have reported that sensitive isolates to the phenylamide fungicide, mefenoxam, become less sensitive in vitro after a single passage through sublethal concentrations of fungicide-amended medium. The first objective of this study was to investigate if isolates of P. infestans are capable of acquiring resistance to two additional systemic fungicides, fluopicolide (benzamide) and cymoxanil (cyanoacetamide-oxime). In contrast to the situation with mefenoxam, exposure of isolates to sublethal concentrations of fluopicolide and cymoxanil did not induce reduced sensitivity to these two fungicides. The second objective was to assess if reduced sensitivity to mefenoxam could occur in naturally sensitive isolates of other Phytophthora species and of Phytopythium sp., another oomycete plant pathogen. All Phytophthora spp. assessed (P. infestans, P. betacei, and P. pseudocryptogea) as well as Phytopythium sp. acquired resistance to mefenoxam after previous exposure through medium containing 1 µg ml^-1 of mefenoxam. Interestingly, isolate 66 of Phytopythium sp. and the isolate of P. pseudocryptogea tested do not seem to be acquiring resistance to mefenoxam after exposure to medium containing 5 µg ml^-1 of this fungicide. The tested isolates of P. palmivora and P. cinnamomi were extremely sensitive to mefenoxam, and thus it was not possible to perform a second transfer to access acquisition of resistance to this fungicide.

Convergent Evolution of C239S Mutation in Pythium spp. β-Tubulin Coincides with Inherent Insensitivity to Ethaboxam and Implications for Other Peronosporalean Oomycetes

Ethaboxam is a benzamide antioomycete chemical (oomicide) used in corn and soybean seed treatments. Benzamides are hypothesized to bind to β-tubulin, thus disrupting microtubule assembly. Recently, there have been reports of corn- and soybean-associated oomycetes that are insensitive to ethaboxam despite never having been exposed. Here, we investigate the evolutionary history and molecular mechanism of ethaboxam insensitivity. We tested the sensitivity of 194 isolates representing 83 species across four oomycete genera in the Peronosporalean lineage that were never exposed to ethaboxam. In all, 84% of isolates were sensitive to ethaboxam (effective concentration to reduce optical density at 600 nm by 50% when compared with the nonamended control [EC₅₀] < 5 μg ml^-1), whereas 16% were insensitive (EC₅₀ > 11 μg ml^-1). Of the insensitive isolates, two different transversion mutations were present in the 239th codon in β-tubulin within three monophyletic groups of Pythium spp. The transversion mutations lead to the same amino acid change from an ancestral cysteine to serine (C239S), which coincides with ethaboxam insensitivity. In a treated soybean seed virulence assay, disease severity was not reduced on ethaboxam-treated seed for an isolate of Pythium aphanidermatum containing a S239 but was reduced for an isolate of P. irregulare containing a C239. We queried publicly available β-tubulin sequences from other oomycetes in the Peronosporalean lineage to search for C239S mutations from other species not represented in our collection. This search resulted in other taxa that were either homozygous or heterozygous for C239S, including all available species within the genus Peronospora. Evidence presented herein supports the hypothesis that the convergent evolution of C239S within Peronosporalean oomycetes occurred without selection from ethaboxam yet confers insensitivity. We propose several evolutionary hypotheses for the repeated evolution of the C239S mutation.

Phytopythium and Pythium Species (Oomycota) Isolated from Freshwater Environments of Korea

Oomycetes are widely distributed in various environments, including desert and polar regions. Depending upon different habits and hosts, they have evolved with both saprophytic and pathogenic nutritional modes. Freshwater ecosystem is one of the most important habitats for members of oomycetes. Most studies on oomycete diversity, however, have been biased mostly towards terrestrial phytopathogenic species, rather than aquatic species, although their roles as saprophytes and parasites are essential for freshwater ecosystems. In this study, we isolated oomycete strains from soil sediment, algae, and decaying plant debris in freshwater streams of Korea. The strains were identified based on cultural and morphological characteristics, as well as molecular phylogenetic analyses of ITS rDNA, cox1, and cox2 mtDNA sequences. As a result, we discovered eight oomycete species previously unknown in Korea, namely Phytopythium chamaehyphon, Phytopythium litorale, Phytopythium vexans, Pythium diclinum, Pythium heterothallicum, Pythium inflatum, Pythium intermedium, and Pythium oopapillum. Diversity and ecology of freshwater oomycetes in Korea are poorly understood. This study could contribute to understand their distribution and ecological function in freshwater ecosystem.

Differentiation of Pythium spp. from vegetable crops with molecular markers and sensitivity to azoxystrobin and mefenoxam

BACKGROUND

Pythium species attack various vegetable crops causing seed, stem and root rot, and 'damping-off' after germination. Pythium diseases are prevalently controlled by two classes of fungicides, QoIs with azoxystrobin and phenlyamides with mefenoxam as representatives. The present study aimed to test the sensitivity of six Pythium species from different vegetable crops to azoxystrobin and mefenoxam and differentiating species based on ITS, cytochrome b and RNA polymerase I gene sequences.

RESULTS

The inter- and intra-species sensitivity to azoxystrobin was found to be stable, with the exception of one Pythium paroecandrum isolate, which showed reduced sensitivity and two cytochrome b amino acid changes. For mefenoxam, the inter-species sensitivity was quite variable and many resistant isolates were found in all six Pythium species, but no RNA polymerase I amino acid changes were observed in them. ITS and cytochrome b phylogenetic analyses permitted a clear separation of Pythium species corresponding to globose- and filamentous-sporangia clusters.

CONCLUSION

The results document the necessity of well-defined chemical control strategies adapted to different Pythium species. Since the intrinsic activity of azoxystrobin among species was stable and no resistant isolates were found, it may be applied without species differentiation, provided it is used preventatively to also control highly aggressive isolates. For a reliable use of mefenoxam, precise identification and sensitivity tests of Pythium species are crucial because its intrinsic activity is variable and resistant isolates may exist. Appropriate mixtures and/or alternation of products may help to further delay resistance development. © 2018 Society of Chemical Industry.

Molecular characterization of genomic regions for resistance to Pythium ultimum var. ultimum in the soybean cultivar Magellan

KEY MESSAGE

Two novel QTL for resistance to Pythium ultimum var. ultimum were identified in soybean using an Illumina SNP Chip and whole genome re-sequencing. Pythium ultimum var. ultimum is one of numerous Pythium spp. that causes severe pre- and post-emergence damping-off of seedlings and root rot of soybean [Glycine max (L.) Merr.]. The objective of this research was to identify quantitative trait loci (QTL) for resistance to P. ultimum var. ultimum in a recombinant inbred line population derived from a cross of 'Magellan' (moderately resistant) and PI 438489B (susceptible). Two different mapping approaches were utilized: the universal soybean linkage panel (USLP 1.0) and the bin map constructed from whole genome re-sequencing (WGRS) technology. Two genomic regions associated with variation in three disease-related parameters were detected using both approaches, with the bin map providing higher resolution. Using WGRS, the first QTL were mapped within a 350-kbp region on Chr. 6 and explained 7.5-13.5% of the phenotypic variance. The second QTL were positioned in a 260-kbp confidence interval on Chr. 8 and explained 6.3-16.8% of the phenotypic variation. Candidate genes potentially associated with disease resistance were proposed. High-resolution genetic linkage maps with a number of significant SNP markers could benefit marker-assisted breeding and dissection of the molecular mechanisms underlying soybean resistance to Pythium damping-off in 'Magellan.' Additionally, the outputs of this study may encourage more screening of diverse soybean germplasm and utilization of genome-wide association studies to understand the genetic basis of quantitative disease resistance.

Evaluation of Major Ancestors of North American Soybean Cultivars for Resistance to Three Pythium Species that Cause Seedling Blight

Pythium seedling blight, which can be caused by a number of Pythium spp., is a disease that affects soybean (Glycine max) in the United States and Canada. Pythium ultimum var. ultimum, one of the most common pathogenic species, is favored by cool, wet conditions in early spring and causes seed decay, root rot, and seedling damping-off. In all, 102 major ancestors of modern North American cultivars and "first progeny" cultivars developed directly from ancestral lines were evaluated for resistance to P. ultimum var. ultimum and two other species of Pythium in greenhouse assays. Several ancestors and first progeny cultivars, as well as the resistant check Archer, had varying levels of partial resistance to an Illinois isolate of P. ultimum var. ultimum. In a subsequent experiment, four of the most resistant lines (PI 84637, Maple Isle, Fiskeby III, and Fiskeby 840-7-3) and the susceptible cultivar Kanro were screened for resistance against isolates of P. irregulare and P. sylvaticum, and resistance to P. ultimum var. ultimum was confirmed. The lines that were partially resistant to P. ultimum var. ultimum in the first experiment were also partially resistant to P. irregulare and P. sylvaticum. The P. ultimum var. ultimum isolate was the most aggressive of the three isolates, followed by the P. irregulare and P. sylvaticum isolates. Modern cultivars descended from the soybean lines with partial resistance to these pathogens could be useful sources of resistance to Pythium seedling blight if they are found to have similar levels of resistance.

Mapping Quantitative Trait Loci for Tolerance to Pythium irregulare in Soybean (Glycine max L.)

Pythium root rot is one of the significant diseases of soybean (Glycine max (L.) Merr.) in the United States. The causal agent of the disease is a soil-borne oomycete pathogen Pythium irregulare, the most prevalent and aggressive species of Pythium in North Central United States. However, few studies have been conducted in soybean for the identification of quantitative trait loci (QTL) for tolerance to P. irregulare In this study, two recombinant inbred line (RIL) populations (designated as POP1 and POP2) were challenged with P. irregulare (isolate CMISO2-5-14) in a greenhouse assay. POP1 and POP2 were derived from 'E09014' × 'E05226-T' and 'E05226-T' × 'E09088', and contained 113 and 79 lines, respectively. Parental tests indicated that 'E05226-T' and 'E09014' were more tolerant than 'E09088', while 'E09088' was highly susceptible to the pathogen. The disease indices, root weight of inoculation (RWI) and ratio of root weight (RRW) of both populations showed near normal distributions, with transgressive segregation, suggesting the involvement of multiple QTL from both parents contributed to the tolerance. All the lines were genotyped using Illumina Infinium BARCSoySNP6K iSelect BeadChip and yielded 1373 and 1384 polymorphic markers for POP1 and POP2, respectively. Notably, despite high density, polymorphic markers coverage was incomplete in some genomic regions. As such, 28 and 37 linkage groups were obtained in POP1 and POP2, respectively corresponding to the 20 soybean chromosomes. Using RRW, one QTL was identified in POP1 on Chromosome 20 that explained 12.7-13.3% of phenotypic variation. The desirable allele of this QTL was from 'E05226-T'. Another QTL was found in POP2 on Chromosome 11. It explained 15.4% of the phenotypic variation and the desirable allele was from 'E09088'. However, no QTL were identified using RWI in either population. These results supported that RRW was more suitable to be used to evaluate P. irregulare tolerance in soybean.

Genetic Diversity and Population Structure of Pythium irregulare from Soybean and Corn Production Fields in Ohio

High levels of genetic diversity have been described within the Pythium irregulare complex from several host plants; however, little is known about the population structure in fields used for grain production. Therefore, the objective of this study was to evaluate the genetic diversity and population structure of 53 isolates baited from 28 soybean and corn production fields from 25 counties in Ohio. Genetic diversity was characterized based on sequence analysis of the internal transcribed spacer (ITS1-5.8S-ITS2) region and with 21 simple sequence repeat (SSR) markers. In addition, aggressiveness on soybean, optimum growth temperature, and sensitivity to metalaxyl fungicide were determined. ITS sequence analysis indicated that four isolates clustered with P. cryptoirregulare, whereas the remaining isolates clustered with P. irregulare that was subdivided into two groups (1 and 2). Cluster analysis of SSR data revealed a similar subdivision, which was also supported by structure analysis. The isolates from group 2 grew at a slower rate, but both groups of P. irregulare and P. cryptoirregulare recovered in this study had the same optimum growth at 27°C. Variability of aggressiveness and sensitivity toward metalaxyl fungicide was also observed among isolates within each group. The results from this study will help in the selection of isolates to be used in screening for resistance, assessment of fungicide efficacy, and disease management recommendations.

Effect of potassium and manganese phosphites in the control of Pythium damping-off in soybean: a feasible alternative to fungicide seed treatments

BACKGROUND

Use of fungicide seed treatments for control of soybean soilborne diseases such as Pythium damping-off has increased worldwide. However, emergence of Pythium strains resistant to metalaxyl-M has prompted the need for alternative technologies to fungicides for damping-off control. The use of phosphites (Phis) has been proposed as a method to control oomycetes, but their use as seed treatments in soybean is limited by the lack of information on their efficacy. The effect of potassium (K) and manganese (Mn) Phis (as seed treatments) in the control of Pythium damping-off in soybean was evaluated in vitro and in vivo. In vitro, treated seeds and a control were placed on potato dextrose agar and the damping-off severity caused by Pythium aphanidermatum (Edson) Fitzpatrick, Pythium irregulare Buisman, and Pythium ultimum Trow was assessed 5 days after incubation using an ordinal scale. In vivo, treated seeds and a control were planted in polystyrene pots and emergence was evaluated 21 days after planting.

RESULTS

Analysis of the in vitro data using a multinomial generalized linear model showed that the probabilities of non-germinated, dead seeds ranged from 0.64 to 1.00 in the control and from 0 to 0.13 in the Phi treatments in each of the Pythium species. Probabilities of seed germination without or with damping-off symptoms were significantly higher for seeds treated with the Phi products than for the control. In the in vivo experiment, the Phi-based products increased seedling emergence by up to 29% on average compared with the untreated control.

CONCLUSION

Mn and K Phis are feasible alternatives as seed treatments to control Pythium damping-off in soybean. This study is the first, worldwide, to document the efficacy of K and Mn Phis in the control of soybean Pythium damping-off. © 2017 Society of Chemical Industry.

Seed Rot and Damping-off of Alfalfa in Minnesota Caused by Pythium and Fusarium Species

Globally, 15 Pythium species have been found to cause damping-off and seed rot of alfalfa, although surveys of species causing disease on alfalfa in the midwestern United States are lacking. Pathogens were isolated by a seedling baiting technique from soil samples of five alfalfa fields in Minnesota with high levels of damping-off. Of the 149 organisms isolated, 93 (62%) were identified as Pythium spp. and 43 (29%) were identified as Fusarium species. Pythium sylvaticum, P. irregulare, and P. ultimum var. ultimum were aggressive pathogens on germinating alfalfa seedlings. Strains of seven Pythium spp. pathogenic on soybean and corn were also pathogenic on alfalfa. The majority of the Fusarium isolates were identified as F. solani and F. oxysporum with a low number of F. redolens and F. incarnatum-equiseti. The F. oxysporum and F. incarnatum-equiseti strains were the most aggressive in causing seed and root rot. Pythium strains were sensitive to Apron XL (mefenoxam) and pyraclostrobin in vitro but efficacy varied when the fungicides were applied as a seed treatment. Seed treatments with Apron XL were more effective than treatments with Stamina against Pythium. The presence of aggressive, broad-host-range pathogens causing seed rot and damping-off suggests that new strategies are needed for managing this disease in alfalfa production systems.