Sensitivity of Pythium irregulare, P. sylvaticum, and P. ultimum from Forest Nurseries to Mefenoxam and Fosetyl-Al, and Control of Pythium Damping-off

The Prospect of Hydrolytic Enzymes from Bacillus Species in the Biological Control of Pests and Diseases in Forest and Fruit Tree Production

Plant diseases and insect pest damage cause tremendous losses in forestry and fruit tree production. Even though chemical pesticides have been effective in the control of plant diseases and insect pests for several decades, they are increasingly becoming undesirable due to their toxic residues that affect human life, animals, and the environment, as well as the growing challenge of pesticide resistance. In this study, we review the potential of hydrolytic enzymes from Bacillus species such as chitinases, β-1,3-glucanases, proteases, lipases, amylases, and cellulases in the biological control of phytopathogens and insect pests, which could be a more sustainable alternative to chemical pesticides. This study highlights the application potential of the hydrolytic enzymes from different Bacillus sp. as effective biocontrol alternatives against phytopathogens/insect pests through the degradation of cell wall/insect cuticles, which are mainly composed of structural polysaccharides like chitins, β-glucans, glycoproteins, and lipids. This study demonstrates the prospects for applying hydrolytic enzymes from Bacillus sp. as effective biopesticides in forest and fruit tree production, their mode of biocidal activity and dual antimicrobial/insecticidal potential, which indicates a great prospect for the simultaneous biocontrol of pests/diseases. Further research should focus on optimizing the production of hydrolytic enzymes, and the antimicrobial/insecticidal synergism of different Bacillus sp. which could facilitate the simultaneous biocontrol of pests and diseases in forest and fruit tree production.

Influence of Culture Media and Environmental Factors (Water Potential and Temperature) on Mycelial Growth of Phytopythium vexans (de Bary), the Causal Agent of Dieback Disease in Apple Trees

This study aimed at evaluating the effects of culture media and environmental factors (temperature and water potential (Ψw)) on the growth of the pathogenic fungus Phytopythium vexans (de Bary) associated with root rot and dieback disease in apple trees. Tomato agar, potato dextrose agar (PDA), and soybean agar were the most favourable for rapid mycelial growth, with optimum growth recorded for PDA medium. To determine the environmental conditions that promoted the development of this phytopathogen, the effects of temperature (5–30 °C), water potential (Ψw) (−15.54; −0.67 MPa) (0.89–0.995 aw), and their interaction were evaluated on the in vitro radial growth rates of the five isolates of P. vexans and on their latency phase (time period prior to growth). The results of this study showed that temperature, water potential, and their interaction had significant effects (p < 0.001) on the radial growth rates and latency phases of all tested P. vexans isolates. All isolates were able to grow throughout the temperature range (5 to 30 °C), with the maximum radial growth rate being observed at the highest temperatures, 25–30 °C. Growth was seen to be faster at −0.67 MPa (0.995 aw) at 25 °C and 30 °C. No growth was observed at Ψw < −5.44 MPa (0.96 aw), regardless of the temperature. It was found that the length of the latency phase depended significantly on both environmental factors. The longest latency phases (5 days on average) were recorded at a temperature of 5 °C and Ψw of −0.67 MPa (0.995 aw) and −2.69 MPa (0.98 aw), while the shortest latency phases were observed at a temperature of 30 °C and a Ψw of −0.67 MPa (0.995 aw), with an average of 0.2 days. The findings from this study could help to understand the impact of these environmental factors on the occurrence of diseases caused by P. vexans and more likely to design a reliable preventive control strategy based on the avoidance of conditions that play in favour of the phytopathogen.

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.

Growth, Sporulation, and Pathogenicity of the Raspberry Pathogen Phytophthora rubi Under Different Temperature and Moisture Regimes

Phytophthora root rot of raspberry, which is mostly caused by Phytophthora rubi, is a significant issue for the Washington State red raspberry industry. Considered a cool weather pathogen, it is often assumed that it is most active and infective during the cool, wet winters of the region when soil temperatures range from 5 to 10°C; however, there are little data to support this view. More recent research has found that symptoms of root disease during late summer were strongly associated with P. rubi. Therefore, experiments were conducted at four temperatures from 5 to 20°C to evaluate the effects of temperature on P. rubi mycelial growth and sporulation and the effects of both temperature and soil moisture on the pathogenicity of P. rubi on red raspberry. At 20°C, P. rubi grew fastest and sporulated the most heavily. However, disease was most severe at both 15 and 20°C. The soil moisture parameters tested did not affect the pathogenicity results. These results show that P. rubi is more likely to infect during the spring and summer months (from May through September), when soil temperatures are consistently in the range of 15 to 20°C.

Evaluating Fungicide Selections to Manage Pythium Root Rot on Poinsettia Cultivars with Varying Levels of Partial Resistance

Pythium aphanidermatum is the predominant species causing Pythium root rot of commercially grown poinsettia (Euphorbia pulcherrima Willd. ex Kotzch) in North Carolina. Pythium root rot is managed primarily through a combination of sanitation practices and preventative fungicide applications of mefenoxam or etridiazole. Insensitivity to mefenoxam is common but growers continue to rely on it from lack of inexpensive and efficacious alternatives. This research was conducted to identify alternative fungicides for Pythium root rot control and to evaluate their efficacy on poinsettia cultivars with varying levels of partial resistance. Greenhouse studies were conducted to assess efficacy of fungicide treatments in seven poinsettia cultivars inoculated with a mefenoxam-sensitive isolate of P. aphanidermatum. One study examined control with a single fungicide drench made at transplant and a second study examined repeat fungicide applications made throughout the experiment. Treatments containing etridiazole, mefenoxam, fenamidone, and cyazofamid provided control of Pythium root rot across all cultivars in both experiments whereas Fosetyl-al, potassium phosphite, and Trichoderma spp. failed to offer satisfactory control. Azoxystrobin, pyraclostrobin, and propamocarb reduced disease on some cultivars but failed to control Pythium root rot on highly susceptible cultivars. Four isolates of P. aphanidermatum cultured from plants growing in commercial greenhouses were evaluated for in vitro sensitivity to fungicides labeled for Pythium root rot control at four rates. Etridiazole, fosetyl-al, and potassium phosphite completely inhibited mycelial growth, whereas isolates varied in response to mefenoxam, cyazofamid, propamocarb, fenamidone, azoxystrobin, and pyraclostrobin in vitro. Twenty-one additional isolates then were evaluated at label rates of these fungicides. Seven isolates were insensitive to label rates of all three quinone outside inhibitors and one isolate was insensitive to the quinone outside inhibitors and mefenoxam. These results provide guidelines for selecting fungicides to maximize control of Pythium root rot on poinsettia cultivars.

Characterization of Pythium Species Collected from a Multiple Time-Point Sampling of Cucurbits in South Carolina

Species of Pythium cause root and stem rot in cucurbits, but no formal surveys have been conducted in the United States to identify which species are responsible. The cucurbit hosts bottle gourd, cucumber, Hubbard squash, and watermelon were transplanted in May, July, September, and November into sentinel plots in four and five different fields in 2017 and 2018, respectively, in South Carolina. Eight of the nine fields were replanted in March 2019. Isolates (600) were collected and identified by sequencing DNA of the mitochondrial cytochrome oxidase I region. The four most common species were P. spinosum (45.6% of all isolates), P. myriotylum (20.0%), P. irregulare (15.3%), and P. aphanidermatum (12.8%). P. myriotylum and P. aphanidermatum were predominantly isolated in May, July, and September, whereas P. spinosum and P. irregulare were predominantly isolated in November and March. Isolates of P. ultimum, P. irregulare, and P. spinosum were more virulent than isolates of P. myriotylum and P. aphanidermatum at 25°C. Representative isolates were screened in vitro for sensitivity to three fungicides: mefenoxam, propamocarb, and oxathiapiprolin. All isolates were sensitive to mefenoxam and propamocarb, but these same isolates were insensitive to oxathiapiprolin, except those classified taxonomically in Pythium clade I.

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.

Talaromyces variabilis interferes with Pythium aphanidermatum growth and suppresses Pythium-induced damping-off of cucumbers and tomatoes

Pythium-induced damping-off disease is a major disease limiting cucumber and tomato production in different parts of the world. The current study investigated the efficiency of Talaromyces variabilis and its bioactive metabolites in suppressing Pythium-induced damping-off of cucumbers and tomatoes. T. variabilis inhibited the in vitro growth of P. aphanidermatum in solid and liquid media. In addition, abnormalities in P. aphanidermatum hyphae were observed as a result of T. variabilis. Extracts from T. variabilis induced cellular leakage and suppressed oospore production of P. aphanidermatum. Biochemical analyses of T. variabilis metabolites showed that T. variabilis produces glucanase, cellulase and siderophores, suggesting the contribution of these metabolites in the inhibition of P. aphandermatum growth and in hyphal abnormalities. Treating cucumber seeds with spore and mycelial suspension of T. variabilis isolates led to a significant improvement in the seedling survival of P. aphanidermatum-inoculated seedlings from 18 to 52% (improvement by 34%) for isolate 48 P and from 30–66% (improvement by 36%) for isolate 28 R. Similarly, treating tomato seeds with spore and mycelial suspension of T. variabilis isolates led to a significant improvement in the seedling survival of P. aphanidermatum-inoculated seedlings from 7 to 36% (improvement by 29%) for isolate 28 R and from 20 to 64% (improvement by 44%) for isolate 48 P. Differences in the percent improvement in seedling survival between experiments may be related to difference in the efficacy of the two different isolates or their interaction with the hosts and pathogen. The use of T. variabilis in the biocontrol of Pythium-induced diseases may offer alternatives to the currently used chemical control.

Variation in Disease Severity Caused by Phytophthora cinnamomi, P. plurivora, and Pythium cryptoirregulare on Two Rhododendron Cultivars

Rhododendrons are an important crop in the ornamental nursery industry, but are prone to Phytophthora root rot. Phytophthora root rot is a continuing issue on rhododendrons despite decades of research. Several Phytophthora species are known to cause root rot, but most research has focused on P. cinnamomi, and comparative information on pathogenicity is limited for other commonly encountered oomycetes, including Phytophthora plurivora and Pythium cryptoirregulare. In this study, three isolates each of P. cinnamomi, P. plurivora, and Py. cryptoirregulare were used to inoculate rhododendron cultivars Cunningham's White and Yaku Princess at two different inoculum levels. All three species caused disease, especially at the higher inoculum level. P. cinnamomi and P. plurivora were the most aggressive pathogens, causing severe root rot, whereas Py. cryptoirregulare was a weak pathogen that only caused mild disease. Within each pathogen species, isolate had no influence on disease. Both P. cinnamomi and P. plurivora caused more severe disease on Cunningham's White than on Yaku Princess, suggesting that the relative resistance and susceptibility among rhododendron cultivars might be similar for both pathogens. Reisolation of P. cinnamomi and P. plurivora was also greater from plants exhibiting aboveground symptoms of wilting and plant death and belowground symptoms of root rot than from those without symptoms. Results show that both P. cinnamomi and P. plurivora, but not Py. cryptoirregulare, are important pathogens causing severe root rot in rhododendron. This study establishes the risks for disease resulting from low and high levels of inoculum for each pathogen. Further research is needed to evaluate longer term risks associated with low inoculum levels on rhododendron health and to explore whether differences among pathogen species affect disease control.

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.

Aquatic Pseudomonads Inhibit Oomycete Plant Pathogens of Glycine max

Seedling root rot of soybeans caused by the host-specific pathogen Phytophthora sojae, and a large number of Pythium species, is an economically important disease across the Midwest United States that negatively impacts soybean yields. Research on biocontrol strategies for crop pathogens has focused on compounds produced by microbes from soil, however, recent studies suggest that aquatic bacteria express distinct compounds that efficiently inhibit a wide range of pathogens. Based on these observations, we hypothesized that freshwater strains of pseudomonads might be producing novel antagonistic compounds that inhibit the growth of oomycetes. To test this prediction, we utilized a collection of 330 Pseudomonas strains isolated from soil and freshwater habitats, and determined their activity against a panel of five oomycetes: Phytophthora sojae, Pythium heterothalicum, Pythium irregulare, Pythium sylvaticum, and Pythium ultimum, all of which are pathogenic on soybeans. Among the bacterial strains, 118 exhibited antagonistic activity against at least one oomycete species, and 16 strains were inhibitory to all pathogens. Antagonistic activity toward oomycetes was significantly more common for aquatic isolates than for soil isolates. One water-derived strain, 06C 126, was predicted to express a siderophore and exhibited diverse antagonistic profiles when tested on nutrient rich and iron depleted media suggesting that more than one compound was produced that effectively inhibited oomycetes. These results support the concept that aquatic strains are an efficient source of compounds that inhibit pathogens. We outline a strategy to identify other strains that express unique compounds that may be useful biocontrol agents.

The use of newly isolated Streptomyces strain TN258 as potential biocontrol agent of potato tubers leak caused by Pythium ultimum

An actinomycete strain designated TN258, was isolated from Tunisian Sahara soil and selected for its antagonistic activity, especially against Pythium ultimum (P. ultimum) causing potato tubers leak. Based on the results of cultural characteristic of TN258 strain, the 16S rRNA gene nucleotide sequence (1433 bp, accession n° HE600071) and the phylogenetic analysis, we propose the assignment of our new isolate bacterium as Streptomyces TN258 strain. After culture optimization, the inhibitory effect of TN258 free cell supernatant against P. ultimum was evaluated. As result, by application of 50% (v/v) from 25 mg ml^-1 of concentration, mycelial growth was totally inhibited with hyphal destruction. At the same concentration, the oospores were distorted and the germination was completely stopped. In potato tubers, Streptomyces TN258 filtrated supernatant, applied 24 h before inoculation by P. ultimum (preventive treatment group) was able to significantly decrease pathogen penetration by 62% and to reduce the percentage of weight loss by 59.43%, in comparison with non-treated group.

Sensitivity of Phakopsora pachyrhizi Isolates to Fungicides and Reduction of Fungal Infection Based on Fungicide and Timing of Application

Soybean rust (SBR), caused by Phakopsora pachyrhizi, is a damaging foliar fungal disease in many soybean-growing areas of the world. Strategies to manage SBR include the use of foliar fungicides. Fungicide types, the rate of product application, and the number and timing of applications are critical components for successful rust management. The objectives of this study were to determine i) the sensitivity of P. pachyrhizi isolates collected in the U.S. to a range of fungicides and ii) the reduction of fungal infection based on fungicide type and timing of applications on soybean. There were differences (P < 0.05) in effective concentration (EC₅₀) values among the fungicides tested. Azoxystrobin had low EC₅₀ values for both urediniospore germination and fungal sporulation on inoculated leaflets. There were differences (P < 0.05) in fungal sporulation for application times, fungicide treatments, and their interaction when the fungus was inoculated on plants. All application times and nearly all fungicide treatments reduced (α = 0.05) fungal infection compared with the nonfungicide control. Information on fungicide sensitivity of P. pachyrhizi isolates and the preventive and curative effects of different fungicides are important in the management of SBR.

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

Pythium spp. cause seed decay, damping-off, and root rot in soybean and corn; however, their diversity and importance as pathogens in Minnesota are unknown. Our objectives were to identify the Pythium spp. present in Minnesota soybean fields, determine their aggressiveness on corn and soybean, and investigate their sensitivity to seed treatment fungicides. For identification, sequences obtained using internal transcribed space ITS4 and ITS1 primers were compared with reference sequences in the National Center for Biotechnology Information database. Seedling and soil samples yielded over 30 oomycete species. Aggressiveness was determined using two methods; a seed assay, which also examined temperature effects on aggressiveness, and a seedling assay. Of 21 species evaluated, seven Pythium spp. were pathogenic on both soybean and corn, reducing root growth by 20% or more while two Pythium and one Phytopythium spp. were pathogenic only on soybean. Aggressiveness of many isolates increased as temperature increased from 15°C to 25°C. The sensitivity of 10 pathogenic species to azoxystrobin, ethaboxam, mefenoxam, pyraclostrobin, or trifloxystrobin was tested. EC₅₀ values for mefenoxam and ethaboxam were 10^-2 of those to strobilurin fungicides. Pythium spp. in Minnesota are diverse and a significant cause of seedling disease on soybean and corn. Most Pythium spp. isolated in this study were more sensitive to mefenoxam and ethaboxam than to strobilurin fungicides.

Identification and Characterization of Isolates of Pythium and Phytophthora spp. from Snap Beans with Cottony Leak

Cottony leak is an important disease of snap bean in Oklahoma and nearby states. Oomycete pathogens isolated from diseased pods collected from commercial fields and research plots consisted of both Pythium spp. (n = 131) and Phytophthora spp. (n = 46). Isolates were identified to species by morphological characteristics and by sequencing a portion of the internal transcribed spacer region of representative isolates. The most common Pythium spp. were Pythium ultimum var. ultimum; Pythium 'group HS', a self-sterile form of P. ultimum that produces hyphal swellings in lieu of sporangia (n = 74); and P. aphanidermatum (n = 50). Phytophthora spp. included Phytophthora drechsleri (n = 41) and P. nicotianae (n = 5). Nearly all of the isolates (95%) and all species were pathogenic on detached pods but Pythium ultimum var. ultimum and Pythium 'group HS' were most aggressive. Phytophthora drechsleri was most aggressive on seedlings, causing preemergence damping off and seed rot. Pythium ultimum var. ultimum, Pythium 'group HS', and P. aphanidermatum were intermediate in virulence to seedlings, causing root rot, stunting, and limited postemergence damping off. Phytophthora nicotianae and Pythium diclinum (n = 4) were not pathogenic on seedlings. Most (87%) isolates were sensitive to metalaxyl-M (concentration that caused a 50% reduction in mycelial growth [EC₅₀] < 1 μg/ml) and the rest were intermediate in sensitivity (EC₅₀ > 1 to < 100 μg/ml). Phytophthora drechsleri was the most sensitive species (EC₅₀ = 0.06 μg/ml) compared with Pythium aphanidermatum, which was least sensitive (EC₅₀ = 1.3 μg/ml). Cottony leak is a disease complex caused by several oomycete species that should include Phytophthora drechsleri, a newly reported pathogen of snap bean in the United States.

Characterization of Pythium Species Associated With Greenhouse Floriculture Crops in Michigan

Michigan ranks third in the United States for the wholesale value of floriculture products, with an estimated value of $375.7 million. Seedling damping-off and root and crown rot are commonly caused by Pythium spp. and are important problems for greenhouse growers. Pythium spp. associated with Michigan's floriculture crops were characterized as a means to improve current management strategies. During 2011 and 2012, potted poinsettias with root rot symptoms were sampled from nine greenhouses located in Kent, Kalamazoo, and Wayne counties. In 2013, from the same three counties, symptomatic geranium and snapdragon bedding plants were sampled from 12 greenhouses. Additionally, symptomatic hibiscus and lantana plants were sampled at one greenhouse facility. Isolates were confirmed to be Pythium spp. via morphology and sequencing of the ITS region. A total of 287 Pythium spp. isolates were obtained from poinsettias and 726 isolates from geranium, snapdragon, hibiscus, and lantana. Seven Pythium spp., and a group of isolates determined as Pythium sp. 1 were identified. The most prevalent species were P. irregulare, P. ultimum, and P. aphanidermatum. A subset of isolates was chosen for pathogenicity and mefenoxam sensitivity testing. Six of the species were virulent to germinating geranium seeds. Most P. ultimum and P. cylindrosporum isolates tested were intermediate to highly resistant to mefenoxam, whereas most P. aphanidermatum isolates were sensitive. This study suggests that Pythium spp. recovered from Michigan greenhouses may vary depending on the host, and that mefenoxam may not be effective to control P. ultimum or P. cylindrosporum.

Temperature Affects Aggressiveness and Fungicide Sensitivity of Four Pythium spp. that Cause Soybean and Corn Damping Off in Iowa

Damping off of soybean and corn, caused by Pythium spp., is favored by cool temperatures and wet soil conditions and is primarily managed using fungicide seed treatments. The goal of this research was to determine the effect of temperature on aggressiveness and fungicide sensitivity of Pythium spp. recovered from soybean and corn in Iowa. A total of 21 isolates of four of the most prevalent Pythium spp. in Iowa were screened. Seed and seedling assays were used to quantify the aggressiveness of P. lutarium, P. oopapillum, P. sylvaticum, and P. torulosum on soybean and corn at 13, 18, and 23°C. Isolates recovered from soybean or corn were equally pathogenic on both hosts. P. torulosum was more aggressive at 13°C compared with 18 and 23°C. Conversely, P. sylvaticum was more aggressive at 18 and 23°C than at 13°C. A plate assay was used to assess fungicide sensitivity to seven fungicides that are commonly used as seed treatments, and EC₅₀ values at each of the three temperatures were determined and compared. EC₅₀ values for P. torulosum were higher for all fungicides tested at 13°C, compared with 18 or 23°C, whereas EC₅₀ values for P. sylvaticum were higher for all fungicides at 18 and 23°C compared with 13°C. These data contribute to our understanding of the effect of soil temperature on the risk of soybean and corn damping off, which may aid in the development of more effective management practices.

Population Structure of Pythium irregulare, P. ultimum, and P. sylvaticum in Forest Nursery Soils of Oregon and Washington

Pythium species are important soilborne pathogens occurring in the forest nursery industry of the Pacific Northwest. However, little is known about their genetic diversity or population structure and it is suspected that isolates are moved among forest nurseries on seedling stock and shared field equipment. In order to address these concerns, a total of 115 isolates of three Pythium species (P. irregulare, P. sylvaticum, and P. ultimum) were examined at three forest nurseries using simple sequence repeat (SSR) and amplified fragment length polymorphism (AFLP) markers. Analyses revealed distinct patterns of intraspecific variation for the three species. P. sylvaticum exhibited the most diversity, followed by P. irregulare, while substantial clonality was found in P. ultimum. For both P. irregulare and P. sylvaticum, but not P. ultimum, there was evidence for significant variation among nurseries. However, all three species also exhibited at least two distinct lineages not associated with the nursery of origin. Finally, evidence was found that certain lineages and clonal genotypes, including fungicide-resistant isolates, are shared among nurseries, indicating that pathogen movement has occurred.