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Pythium

Pythium is a genus of oomycetes, fungus-like eukaryotic microorganisms belonging to the kingdom Stramenopila, characterized by their coenocytic (non-septate) hyphae, cellulose-based cell walls, and production of biflagellate zoospores for dispersal.^[1] Comprising over 200 described species, Pythium occupies diverse aquatic and terrestrial habitats worldwide, where it exhibits a range of nutritional modes including saprotrophy, parasitism on plants and animals, and even mycoparasitism on other fungi.^[1]^[2]

Classification and Phylogeny

The genus Pythium was established by Ferdinand Pringsheim in 1858 and is classified within the phylum Oomycota, class Peronosporomycetes, order Pythiales, and family Pythiaceae, alongside related genera such as Phytophthora.^[1] Oomycetes like Pythium are not true fungi but stramenopiles, more closely related to brown algae and diatoms than to the Fungi kingdom, as evidenced by their diploid-dominant life cycles and distinct cell wall composition featuring β-1,3-glucans and cellulose rather than chitin.^[1]^[3] Recent phylogenetic studies have led to the reclassification of some species into the segregate genus Globisporangium, but Pythium sensu stricto remains a major group of soil- and water-inhabiting organisms.^[4]

Morphology and Life Cycle

Pythium species exhibit filamentous, branching hyphae that lack septa, enabling rapid growth in moist environments.^[1] Asexual reproduction occurs through sporangia, which can be filamentous or globose and may release motile zoospores or germinate directly into hyphae; not all species produce zoospores, with some relying on chlamydospores or hyphal swellings for survival.^[1]^[5] Sexual reproduction involves the formation of oogonia (female structures) fertilized by antheridia (male structures), resulting in thick-walled oospores that serve as resting spores capable of enduring adverse conditions for years.^[1] Species vary in mating type: homothallic (self-fertile) or heterothallic (requiring compatible strains), with oosporogenesis often triggered by specific environmental cues like low carbon-to-nitrogen ratios or salinity.^[1]

Ecology and Pathogenicity

Ecologically diverse, Pythium species thrive in wet, poorly drained soils and aquatic systems, where they decompose organic matter or parasitize hosts; some, like Pythium oligandrum, act as beneficial biocontrol agents by preying on harmful fungi and oomycetes.^[2]^[6] However, the majority are necrotrophic plant pathogens, causing significant diseases such as damping-off (pre- and post-emergence seedling blight), root rot, and stem rot in a wide array of crops including corn, soybeans, turfgrasses, ornamentals, and vegetables.^[5]^[7] Common pathogenic species include Pythium ultimum (cool-temperature root rot), Pythium aphanidermatum (warm-temperature root and crown rot), Pythium irregulare (damping-off), and Pythium myriotylum (severe root rots in warm climates).^[5]^[7] These pathogens infect via zoospores that encyst and penetrate roots, thriving under excessive moisture and high soluble salts, leading to symptoms like wilting, stunting, discolored roots with sloughing cortex, and plant death.^[5]^[8] Economically, Pythium diseases cause substantial losses in agriculture, particularly in greenhouses and nurseries, with management relying on cultural practices (e.g., sanitation, proper drainage), resistant varieties, and fungicides like mefenoxam, though resistance is emerging in some isolates.^[5]^[2] Beyond plants, certain species such as Pythium insidiosum are zoonotic, causing pythiosis—a severe infectious disease in mammals including horses, dogs, and humans—primarily in tropical and subtropical regions through cutaneous or gastrointestinal infections from contaminated water.^[9] This highlights the genus's broad host range, spanning algae, insects, fish, and amphibians, underscoring its role as an opportunistic pathogen in flooded or irrigated ecosystems.^[10]

Taxonomy

Classification

Pythium belongs to the domain Eukarya, kingdom Stramenopila, phylum Oomycota, class Peronosporomycetes, order Pythiales, and family Pythiaceae.^[11] This placement reflects its position among the stramenopiles, a diverse group of heterokont protists that includes diatoms and brown algae, rather than the true fungi in the kingdom Fungi.^[12] Although superficially resembling fungi in their filamentous growth and ecological roles, Pythium and other oomycetes are distinguished by key cellular features: their cell walls consist primarily of cellulose and β-1,3-glucans instead of chitin, their mycelium is predominantly diploid rather than haploid, and they produce biflagellate, motile zoospores for dispersal.^[13] These traits underscore the oomycetes' evolutionary divergence from fungi, aligning them more closely with photosynthetic chromists.^[14] Molecular phylogenetics has revealed that Pythium sensu lato is polyphyletic, divided into five monophyletic clades based on analyses of the large subunit ribosomal DNA D1/D2 region and the mitochondrial cytochrome c oxidase subunit II (coxII) gene.^[15] These clades highlight significant genetic divergence, prompting taxonomic revisions; for instance, species in clades B, E, and G (from prior 11-clade analyses) were reclassified into the new genus Globisporangium in 2010, with further refinements in subsequent studies including segregation into Elongisporangium, Ovatisporangium (synonym Phytopythium), and Pilasporangium as of 2024.^[16]^[12] Identification of Pythium species often relies on sequencing the ITS rDNA region, coxII, and β-tubulin genes, which provide robust markers for resolving phylogenetic relationships and distinguishing closely related taxa.^[17]

History

The genus Pythium was established by Nathanael Pringsheim in 1858, initially as a subgroup subordinate to the Saprolegniaceae within the fungi, based on observations of its reproductive structures resembling those in the alga Vaucheria.^[18] Pringsheim's description highlighted the formation of swarm spores, distinguishing the group from other saprolegnian forms, though it was firmly placed among true fungi at the time.^[19] In the 1860s, Heinrich Anton de Bary advanced the understanding of oomycete reproduction through detailed studies on life cycles of related pathogens like Phytophthora infestans, laying foundational work for Pythium systematics, while the formal recognition of oomycetes as a distinct class emerged in the 1880s with de Bary's comparative morphology and Winter's nomenclature in 1880.^[20] Early classifications maintained Pythium within fungal lineages due to morphological similarities, such as filamentous growth and absorptive nutrition, perpetuating confusion until cytological differences became evident.^[21] Twentieth-century advancements began with electron microscopy in the 1960s, which revealed ultrastructural features of Pythium hyphae and zoospores, including cellulose-based cell walls and tubular cristae in mitochondria, further separating oomycetes from chitin-walled true fungi.^[22] By the 1990s, molecular analyses using 18S rRNA sequences shifted nomenclature, integrating the family Pythiaceae—including Pythium—into the order Peronosporales within the oomycetes, based on phylogenetic affinities with downy mildews and Phytophthora.^[20] From the 2000s onward, comprehensive molecular phylogenies refined Pythium's evolutionary position; for instance, Lévesque and de Cock's 2004 analysis of ITS regions delineated 11 clades, while Uzuhashi et al.'s 2010 study using LSU rDNA and coxII genes segregated divergent lineages into new genera like Globisporangium, prompting further reclassifications such as Phytopythium in 2015.^[19]^[15] These developments underscored Pythium's polyphyletic nature and its basal placement in peronosporalean oomycetes.^[23]

Species Diversity

The genus Pythium displays substantial species diversity, encompassing over 300 described species within the sensu lato group, with molecular surveys indicating the potential for hundreds more undescribed taxa based on environmental sampling.^[12] This diversity is particularly pronounced across phylogenetic clades, with Clade A featuring numerous pathogenic species adapted to terrestrial substrates and Clade B dominated by saprophytic forms prevalent in aquatic environments.^[24] Such cladistic divisions highlight the genus's ecological breadth, though ongoing taxonomic revisions—splitting Pythium into genera like Globisporangium and Phytopythium (now Ovatisporangium)—continue to refine these groupings.^[12] Among the most notable species are Pythium ultimum (reclassified as Globisporangium ultimum), a widespread agent of damping-off in seedlings across various crops; P. aphanidermatum, which causes root rot predominantly in warm, moist soils; P. irregulare (now Globisporangium irregulare), linked to seedling blight in temperate agriculture; P. violae, responsible for bulb rot in ornamentals and vegetables; and P. kashmirense, an emerging soil-borne pathogen recently documented in diverse host plants including herbs and cereals.^[25]^[10] These examples illustrate the genus's impact on agriculture, with species distribution reflecting adaptations to specific niches—roughly half soil-associated and about a third aquatic—concentrated in hotspots of temperate and tropical zones globally.^[26] Identification of Pythium species traditionally involves morphological assessment of traits like sporangia size, oospore characteristics, and hyphal structure, but these features often overlap, complicating differentiation.^[24] Modern approaches integrate molecular tools, such as sequencing of the internal transcribed spacer (ITS) region of ribosomal DNA, to resolve cryptic species complexes and confirm phylogenetic placement, though challenges persist due to intraspecific variation and the need for comprehensive reference databases.^[12] This combined methodology has been essential in uncovering hidden diversity, particularly in underexplored ecosystems.^[24]

Biology

Morphology

Pythium species are characterized by coenocytic hyphae that lack cross-walls, appearing as continuous, multinucleate filaments. These hyphae are hyaline, transparent, and typically measure 5-10 μm in diameter, though widths can vary slightly among species. Septa are generally absent except in aging cultures or during the formation of reproductive structures such as sporangia.^[27]^[28] Asexual reproductive structures in Pythium consist of sporangia, which develop terminally or intercalarily on the hyphae. These sporangia exhibit diverse morphologies, ranging from spherical (often 10-30 μm in diameter) to filamentous forms that can extend up to 50 μm or more, with some species showing inflated, toruloid, or proliferating types. In zoosporogenic species, encysted zoospores within sporangia or their discharge vesicles are typically 8-14 μm in size; non-zoosporogenic species rely on direct germination of sporangia or other structures like chlamydospores or hyphal swellings for survival and dispersal.^[27]^[28] Sexual reproduction involves the formation of oogonia containing oospores, which serve as thick-walled, spherical resting spores measuring 15-40 μm in diameter. Oospores are classified as either aplerotic (with the protoplast not filling the entire spore wall) or plerotic (protoplast filling the wall), featuring a smooth surface in most species and walls 1-7 μm thick for durability.^[27]^[28] Antheridia, the male gametangia, arise from hyphal branches and fertilize the oogonia. They are typically hyphal in structure, either diclinous (from a different hypha) or monoclinous (from the same oogonial hypha), and often encircle or appress the oogonium with 1-12 antheridia per oogonium, measuring 10-40 μm in length depending on the species.^[27]^[28] The motile zoospores of zoosporogenic Pythium species display a characteristic ultrastructure typical of oomycetes, with a bean- or pear-shaped body approximately 8-14 μm long. They are biflagellate, possessing an anterior tinsel flagellum adorned with mastigonemes (hairs) for propulsion and an posterior whiplash flagellum that is smooth. An eyespot is present near the anterior end, aiding in phototactic responses, while a ventral groove runs along the body, housing the flagellar insertion points and contributing to the zoospore's asymmetrical shape.^[27]^[29]

Life Cycle

Pythium species exhibit a complex life cycle characterized by both asexual and sexual reproduction, enabling rapid dissemination and long-term survival in diverse environments. The cycle typically initiates from dormant oospores in the soil, which germinate under favorable conditions to produce germ tubes that develop into coenocytic hyphae. These hyphae colonize substrates and can lead to the formation of reproductive structures. The organism is primarily diploid throughout its vegetative phase, with meiosis occurring briefly during gamete formation to produce haploid gametes, followed by fertilization that restores the diploid state.^[30]^[2] Asexual reproduction predominates under moist conditions and facilitates quick infection cycles. In zoosporogenic species, hyphae differentiate into sporangia, which form in response to free water availability; these structures release biflagellate zoospores that are motile in aqueous environments. Not all Pythium species produce zoospores, with some relying on direct germination of sporangia, chlamydospores, or hyphal swellings as additional asexual survival propagules. The zoospores (where produced) swim toward potential hosts, encyst upon attachment, and germinate to produce infection hyphae via germ tubes or appressoria. This phase allows for proliferation without a sexual partner and can complete an infection cycle in 48-72 hours under optimal wet conditions, particularly affecting vulnerable seedlings. Flooding or soil saturation serves as a key environmental trigger for zoospore release and dispersal in applicable species.^[1]^[31] Sexual reproduction occurs under nutrient-limiting conditions, such as low carbon-to-nitrogen ratios, and contributes to genetic diversity and persistence. Female oogonia develop alongside male antheridia, which are often paragynous (attached laterally to the oogonium). Fertilization involves the transfer of haploid nuclei from the antheridium to the oogonium, resulting in thick-walled oospores that serve as resting structures capable of surviving adverse conditions, including overwintering in soil for years. Most Pythium species are homothallic (self-fertile), though some require opposite mating types (heterothallic). Oospore germination resumes the cycle by producing new hyphae. Temperature optima for growth and reproduction vary by species; for instance, Pythium aphanidermatum thrives at around 34°C, with activity from 10°C to over 43°C. Overall, the life cycle's efficiency in wet, cool-to-warm soils (typically 20-30°C for many species) underscores Pythium's role as an opportunistic pathogen.^[1]^[31]^[32]

Ecology

Habitats and Distribution

Pythium species primarily inhabit saturated soils, irrigation water, and aquatic environments such as rivers and ponds, where they thrive as saprophytes or pathogens.^[33] They are cosmopolitan in agricultural fields, greenhouses, and natural wetlands, often associated with moist conditions that facilitate zoospore motility and dispersal.^[34] These oomycetes are commonly recovered from poorly drained media and standing water, contributing to their prevalence in both natural and managed ecosystems.^[26] The genus Pythium exhibits a global distribution, occurring worldwide from tropical to temperate regions and even in polar areas like the Arctic and Antarctic. Highest species diversity is observed in humid tropical and subtropical zones, such as in agroecosystems of Southeast Asia and Latin America, where environmental conditions support a wide array of taxa.^[35] For instance, Pythium ultimum is widespread in temperate crop fields, while species like P. aphanidermatum predominate in warmer, subtropical settings.^[36] Recent studies indicate that climate change, through shifts in temperature, humidity, and precipitation, is altering Pythium distribution and ecology, potentially expanding ranges or enhancing disease risks in affected regions as of 2025.^[37]^[38] In microhabitats, Pythium colonizes the rhizosphere and decaying plant matter, where organic substrates provide nutrients for saprophytic growth.^[33] Oospores enable long-term survival in dry or unfavorable soils, persisting for several years as dormant structures until conditions improve.^[39] Distribution and abundance are influenced by soil pH in the range of 5 to 7, organic-rich soils that enhance persistence, and spread through contaminated irrigation water or seeds.^[40]

Ecological Roles

Pythium species fulfill important saprophytic functions in ecosystems by decomposing organic matter and facilitating nutrient cycling in soils. As saprophytes, they colonize decaying plant material and fresh organic substrates, surviving as mycelium in soil environments where they break down complex carbohydrates. This decomposition is enabled by a repertoire of carbohydrate-active enzymes (CAZymes), including cellulases and glycoside hydrolases, which target cellulose and other cell wall polysaccharides, thereby releasing nutrients such as carbon and nitrogen for recycling by other organisms.^[41]^[42]^[43] Certain Pythium species engage in symbiotic interactions, occasionally functioning as endophytes or associates with mycorrhizal fungi, while competing with other microbes for resources. For instance, Pythium oligandrum can colonize plant roots without causing disease, promoting growth through induction of host defenses and mycoparasitism against pathogenic fungi, thus shaping beneficial rhizosphere communities. This species competes effectively with other soil microbes by limiting nutrient availability and producing antimicrobial compounds, enhancing overall microbial balance in the rhizosphere.^[44]^[45]^[46] Pythium influences biodiversity through predator-prey dynamics with bacteria and protozoa, acting both as a predator and prey in soil food webs. Bacterial predators such as Lysobacter species target Pythium via enzymatic degradation of its cell walls, while protozoan grazing on associated bacteria indirectly modulates Pythium populations, promoting microbial diversity and preventing dominance by any single taxon. In wetland ecosystems, Pythium contributes to nutrient flux by accelerating organic matter decomposition in saturated conditions, aiding the release of phosphorus and nitrogen into water columns and supporting biogeochemical cycles.^[47]^[48]^[49] As an indicator species, Pythium signals soil moisture levels and health status in agroecosystems, thriving in waterlogged conditions that reflect excessive irrigation or poor drainage. High Pythium activity often correlates with reduced soil suppressiveness, indicating imbalances in microbial communities and potential vulnerabilities to root diseases, whereas suppressive soils with low Pythium prevalence suggest robust health and resilience.^[50]^[51]^[52]

Pathogenicity

Diseases in Plants

Pythium species are responsible for several devastating diseases in plants, most notably damping-off, root rot, and stem canker. Damping-off manifests in two forms: pre-emergence damping-off, where seeds rot before germination, and post-emergence damping-off, characterized by the collapse of seedlings shortly after emergence due to infection at the soil line. For instance, in tomatoes, Pythium-induced damping-off leads to widespread seedling collapse, particularly in high-density nursery settings. Root rot primarily affects the root system, causing decay that impairs water and nutrient uptake, while stem canker involves necrotic lesions on lower stems, often progressing to girdling and plant death. These diseases are especially prevalent in herbaceous crops grown in moist environments, such as greenhouses and field nurseries.^[53]^[43] The infection mechanism of Pythium relies on motile zoospores, which are produced during the pathogen's life cycle and swim through soil water to reach susceptible plant tissues. Upon contact with roots, zoospores encyst and produce germ tubes that penetrate the epidermis directly or through wounds, facilitated by enzymes that degrade plant cell walls. This process is highly favored by cool, wet soil conditions (typically 10–20°C for species like Pythium ultimum), although some species thrive in warmer temperatures up to 35°C. Once inside, the pathogen colonizes cortical tissues, leading to rapid tissue necrosis. Secondary invasion by bacteria often exacerbates the damage, turning initial water-soaked lesions into advanced rots.^[53]^[43]^[54] Symptoms of Pythium diseases typically begin with water-soaked, dark brown to black lesions on roots or lower stems, progressing to wilting, yellowing, and stunting of foliage as the root system deteriorates. Infected roots often appear shriveled or "rat-tailed" due to cortex loss, with feeder roots absent or necrotic. Above-ground signs include sudden collapse in seedlings or gradual decline in mature plants, sometimes accompanied by secondary bacterial soft rots that produce foul odors. These symptoms are particularly acute in overwatered or poorly drained soils, where the pathogen's spread is unchecked.^[53]^[43] Pythium has a broad host range, affecting over 90 plant families, including cereals like corn, vegetables such as tomatoes and cucurbits, and ornamentals like chrysanthemums. This wide susceptibility makes it a major concern across diverse agricultural systems. Economically, Pythium diseases cause substantial losses, particularly in seedling production; for example, they account for approximately US$25 million in annual yield reductions in U.S. corn production alone, with total losses exceeding US$357 million from 2016 to 2019. In cucurbits and solanaceous crops like peppers, significant yield reductions occur in affected fields, especially in nurseries where replanting costs amplify damages.^[55]^[54]^[43]

Diseases in Animals

Pythium species, particularly P. insidiosum, cause pythiosis, a rare but severe granulomatous infection in mammals, manifesting primarily as subcutaneous, gastrointestinal, or vascular lesions.^[56] In horses, the most commonly affected species, cutaneous pythiosis presents as non-healing ulcers, nodules, or proliferative masses on the limbs or trunk, often following exposure to contaminated water through wounds.^[57] Dogs typically develop gastrointestinal forms, characterized by vomiting, diarrhea, weight loss, and abdominal masses due to intestinal wall invasion, with cutaneous lesions also reported in large-breed males under 3 years old exposed to wetlands. Sheep and goats experience ulcerative dermatitis on the limbs, while rare cases occur in cattle, cats, and even marine mammals like harbour porpoises.^[58]^[59] Pathogenesis involves motile zoospores encysting on damaged skin or mucosal surfaces, followed by germ tube penetration into tissues, leading to chronic inflammation and vascular occlusion.^[60] In vascular forms, hyphae-like structures invade arterial walls, causing aneurysms and thrombosis, which exacerbate tissue necrosis and complicate treatment due to poor immune response penetration.^[61] This results in granulomatous reactions with eosinophilic infiltrates, often forming "kunkers"—yellowish, coral-like masses of fungal elements.^[62] Epidemiologically, pythiosis predominates in tropical and subtropical regions, such as Southeast Asia, the Americas, and Australia, where warm, stagnant water harbors the pathogen; recent studies indicate an increase in cases in more northern areas of North America, potentially linked to climate change.^[63] Zoonotic transmission to humans is infrequent but documented in immunocompromised individuals, typically via cutaneous exposure, with low overall potential due to host specificity.^[64] In aquatic animals, various Pythium species cause opportunistic infections, including skin and gill lesions in fish such as salmonids and channel catfish, often in stressed aquaculture settings.^[65] For instance, P. catenulatum and related species invade fin and gill tissues, leading to ulcerative dermatitis or respiratory distress, though less commonly than Saprolegnia. Infections in crustaceans like shrimp (P. insidiosum) result in gill blackening and mortality in hatcheries. Diagnosis relies on serology (e.g., ELISA detecting antibodies to P. insidiosum antigens), culture on selective media, and histopathology revealing broad, sparsely septate hyphae.^[66] Molecular PCR confirms species identity.^[67] Untreated mortality reaches 95% in equine cases, while overall mortality in dogs is 84% (exceeding 90% for gastrointestinal forms), underscoring the disease's severity.^[68]^[69]

Management

Prevention

Preventing Pythium infections requires proactive cultural and environmental practices that minimize conditions favorable to the pathogen's survival and spread, such as excess moisture and poor soil aeration. Improving soil drainage is a foundational strategy, achieved through raised beds, tiling, or selecting well-drained sites to reduce waterlogging that promotes oospore germination and zoospore motility.^[8] Avoiding overwatering, particularly in greenhouses or nurseries, further limits pathogen activity by maintaining soil moisture levels below the threshold for Pythium proliferation, typically around field capacity rather than saturation.^[8] Soil solarization offers an effective non-chemical method to reduce Pythium populations in the topsoil layers. This technique involves covering moist soil with clear plastic sheeting during warm months to trap solar heat, raising temperatures to 40-50°C for 4-6 weeks, which kills oospores and other propagules while enhancing beneficial microbial activity.^[70] Studies at nursery sites have demonstrated significant reductions in Pythium spp. viability following solarization, with soil temperatures at 5 cm depth reaching up to 55°C under optimal conditions.^[71] Sanitation practices are essential to prevent the introduction and dissemination of Pythium inoculum. Tools, pots, and trays should be sterilized using a 10% bleach solution or steam to eliminate clinging zoospores or mycelium fragments before reuse.^[72] Seed treatment with hot water or approved disinfectants can reduce surface contamination, while sourcing certified disease-free seeds and transplants minimizes initial infection risk.^[5] Crop rotation with non-host plants, such as grasses, can help reduce inoculum levels over multiple years, though oospores may persist longer.^[73]^[74] Although no fully resistant soybean varieties exist, cultivars differ in susceptibility to Pythium damping-off, with more tolerant ones showing reduced stand losses compared to susceptible lines.^[75] Using certified disease-free transplants for vegetables and ornamentals further prevents inadvertent introduction of the pathogen.^[5] Regular monitoring through soil testing allows early intervention before symptoms appear. Assays for oospores involve plating soil dilutions on selective media like PARP to quantify viable propagules, with levels above 10-20 oospores per gram indicating high risk. Baiting techniques, such as floating leaf disks or needles in soil-water suspensions, detect motile zoospores within 24-48 hours, enabling site-specific preventive adjustments like targeted drainage improvements.

Control Methods

Control of established Pythium infections primarily relies on chemical fungicides, with phenylamides such as metalaxyl and its active isomer mefenoxam being widely used due to their systemic action against oomycete pathogens. These fungicides inhibit RNA polymerase I, disrupting nucleic acid synthesis in Pythium species and effectively suppressing diseases like root rot and damping-off. Typical application rates range from 0.5 to 1 kg/ha, depending on the crop and formulation, with soil drench or foliar spray methods ensuring uptake. However, resistance has emerged in field isolates, with up to 59% of Pythium populations showing resistance to mefenoxam in some surveys as reported in a 2001 study in Florida, necessitating rotation with fungicides from different mode-of-action groups to maintain efficacy.^[76]^[77] Biological control offers an environmentally friendly alternative, employing antagonists like Trichoderma spp. and Pseudomonas fluorescens to suppress Pythium through mechanisms such as competition for nutrients, production of antifungal compounds, and direct parasitism. Trichoderma species, for instance, exhibit mycoparasitic activity by coiling around Pythium hyphae and degrading cell walls, while P. fluorescens produces siderophores and antibiotics that inhibit pathogen growth in the rhizosphere. Mycoviruses have also shown promise in biocontrol; certain double-stranded RNA viruses infecting Pythium-like oomycetes, such as Globisporangium ultimum (formerly Pythium ultimum), induce hypovirulence, reducing sporulation and pathogenicity without harming host plants. Field applications of these agents, often as seed treatments or soil amendments, have demonstrated consistent suppression of Pythium-induced diseases in crops like cucumber and pea.^[78]^[79] Integrated approaches combine chemical, biological, and cultural practices to enhance control while minimizing resistance risks and environmental impact. For example, pairing low-dose fungicides with antagonists like Trichoderma and cultural amendments such as gypsum (calcium sulfate) to adjust soil pH toward neutrality (around 7.0) has proven effective, as higher pH levels inhibit Pythium zoospore motility and encystment. Efficacy studies report 70-90% reduction in disease severity and pathogen colonization when these methods are integrated, particularly in greenhouse and field settings for susceptible crops like chrysanthemum and soybean. Rotation of antagonists with fungicides and monitoring soil conditions further sustains long-term suppression.^[80]^[81] Emerging strategies focus on molecular interventions, including RNA interference (RNAi)-based silencing of pathogenicity genes in Pythium. For instance, targeting the Puf4 gene, which regulates RNA-binding proteins essential for oomycete development, via host-induced gene silencing (HIGS) or spray-induced gene silencing (SIGS) has reduced Pythium aphanidermatum virulence in host plants by disrupting sporangia formation and hyphal growth. Additionally, phosphonate treatments, such as potassium phosphite, induce systemic resistance in plants by activating defense pathways like jasmonic acid signaling, providing curative control against Pythium root rots with efficacy comparable to traditional fungicides when applied post-infection. These approaches are gaining traction for their specificity and low resistance potential, though field optimization remains ongoing.^[82]^[83]^[84]

References

[1]
What is Pythium?
Pythium species are eukaryotes (have true nuclei) that have filamentous (thread-like), coenocytic (non-septate threads lacking cross walls) cell growth. The ...Missing: biology | Show results with:biology
[2]
Genome of Pythium myriotylum Uncovers an Extensive ... - NIH
The Pythium genus includes plant pathogens, animal pathogens, and mycoparasites (e.g., P. oligandrum) as well as parasites of other Pythium species. In spite of ...
[3]
Molecular Genetics of Pathogenic Oomycetes - PMC
Other important pathogens include more than a hundred species of the genus Pythium, which are abundantly present in water and soil habitats and cause a ...Oomycete Biology · Gene Structure · Molecular And Genomic...
[4]
Globisporangium and Pythium Species Associated with Yield ... - NIH
Mar 17, 2023 · This is the first report of Globisporangium and Pythium species causing disease in pyrethrum globally and suggests that oomycete species in the family ...
[5]
[PDF] Pythium Root Rot of Herbaceous Plants - Purdue Extension
This publication describes Pythium root rot, the symptoms of the disease, the disease cycle, and how to effectively manage the problem. The three most commonly ...
[6]
Volatile Organic Compounds from Pythium oligandrum Play a Role ...
Feb 6, 2023 · The oomycete Pythium oligandrum is a soil-inhabiting parasite and predator of both fungi and oomycetes, and uses hydrolytic enzymes ...
[7]
[PDF] Diseases of Agronomic and Vegetable Crops caused by Pythium
The most common species of Pythium that cause important plant diseases in Florida are Pythium myriotylum and P. aphanidermatum. Other species of Pythium that ...
[8]
Pythium Root Rot / Floriculture and Ornamental Nurseries / Agriculture
The pathogens that are responsible for Pythium root rot, also known as water molds, are present in practically all cultivated soils and attack plant roots ...
[9]
Biochemical and genetic analyses of the oomycete Pythium ... - NIH
Jun 5, 2018 · The oomycete microorganism, Pythium insidiosum, causes the life-threatening infectious condition, pythiosis, in humans and animals worldwide.
[10]
Pathogenicity and Host Range of Pythium kashmirense—A Soil ...
Jun 12, 2021 · The host range includes insects, fish, mammals or algae, but the majority of Pythium spp. are pathogens of plants occurring in water or soil.2.2. Host Range And... · 3. Results · 3.2. Host Range And...
[11]
Pythium (1PYTHG)[Overview] - EPPO Global Database
Basic information: Taxonomy: Kingdom Chromista ( 1CHROK ) Phylum Oomycota ( 1PSDFP ) Class Oomycetes ( 1OOMYC ) Order Pythiales ( 1PYTHO ) Family Pythiaceae ( ...<|control11|><|separator|>
[12]
An assessment of the species diversity and disease potential of ...
Sep 27, 2024 · ... species are soil-borne plant pathogens. Taxonomically, Pythium s.l. is classified in the domain Eukarya, the kingdom Chromista, the ...
[13]
Cell biology of plant–oomycete interactions - Wiley Online Library
Oct 12, 2006 · Oomycetes have cellulosic cell walls, they form coenocytic hyphae and they are diploid during most of their life cycle. A variety of biochemical ...Reaching A Favourable... · Penetration Of The Host... · The Plant Response To...
[14]
The taxonomy and biology of Phytophthora and Pythium
Feb 12, 2018 · The genera Phytophthora and Pythium include many economically important species which have been placed in Kingdom Chromista or Kingdom Straminipila, distinct ...
[15]
Phylogeny of the genus Pythium and description of new genera
The phylogenetic trees show that the genus Pythium is a highly divergent group and divided into five well- or moderately supported monophyletic clades.
[16]
(PDF) Phylogeny of the genus Pythium and description of new genera
Aug 5, 2025 · The phylogenetic trees show that the genus Pythium is a highly divergent group and divided into five well- or moderately supported monophyletic clades.
[17]
Phylogenetic relationships of Pythium and Phytophthora species ...
Analysis of ITS, cox II, and beta-tubulin genes revealed four clades. Some Pythium species are proposed as intermediate in the Pythium-to-Phytophthora line.
[18]
Observations on the Genus Pythium (Pringsh.)
... Pythium, founded by Pringsheim in 1858 as a group subordinate to the Saprolegniæ. During some recent investigations which I have lately made for the purpose ...
[19]
Molecular phylogeny and taxonomy of the genus Pythium
The phylogeny of 116 species and varieties of Pythium was studied using parsimony and phenetic analysis of the ITS region of the nuclear ribosomal DNA.Missing: nomenclature shift
[20]
Fifty years of oomycetes—from consolidation to evolutionary and ...
Aug 13, 2011 · Packer and Clay (2000) caused a major paradigm shift by demonstrating that a Pythium sp. colonizing mature black cherry trees (Prunus ...
[21]
Introduction to Oomycetes - American Phytopathological Society
Jan 1, 2010 · There are many features distinguishing oomycetes from fungi. Septa (cell walls) in the hyphae are rare, resulting in a multinucleate condition ...
[22]
FINE STRUCTURE OF FUNGI AS REVEALED BY ELECTRON ...
The electron microscope has contributed to our knowledge of growth and development of fungal structures. It has extended previous knowledge of the process of ...
[23]
Phytopythium: molecular phylogeny and systematics - PMC - NIH
The genus Pythium as defined by Pringsheim in 1858 was divided by Lévesque & de Cock (2004) into 11 clades based on molecular systematic analyses. These clades ...
[24]
Molecular phylogeny and taxonomy of the genus Pythium - PubMed
The phylogeny of 116 species and varieties of Pythium was studied using parsimony and phenetic analysis of the ITS region of the nuclear ribosomal DNA.
[25]
Full article: Resolving the Globisporangium ultimum (Pythium ...
Globisporangium ultimum (formerly Pythium ultimum) is one of the most common plant pathogens found in soil and also one of the most pathogenic Globisporangium ...
[26]
Pythium - an overview | ScienceDirect Topics
However, as per Kirk et al. (2008), Pythium is coming under the Kingdom Chromista, phylum Oomycota, class Oomycetes, order Pythiales, and family Pythiaceae.
[27]
Monograph of the genus Pythium - Studies in Mycology
Dec 22, 1981 · This revision of the species of Pythium Pringsh. is mainly based on living cultures preserved at the Centraalbureau voor Schimmelcultures.
[28]
Identification based on morphology — Pythium
Pythium identification involves looking for sporangia, oogonia, antheridia, zoospores, non-septate hyphae, and thick-walled oospores. Cultures are observed ...Missing: review | Show results with:review
[29]
An ultrastructural study of zoosporogenesis inPythium proliferum de ...
Zoosporogenesis in the oomycete,Pythium ... Greenwood, 1966: Structural aspects of zoospore production inSaprolegnia ferax with particular reference to the cell ...
[30]
[PDF] Genetic and Cytological Evidence for a Diploid Life Cycle in Pythium ...
Ultrastructure aphanidermatum increased germination to 94% and also of mitosis in the aquatic fungus Catenaria anguillulae. eliminated propagules such as ...<|separator|>
[31]
Pythium Life Cycle - Root Rot Task Force
Pythium's life cycle starts with oospores, which germinate into hyphae, then produce zoospores and new oospores, allowing for both asexual and sexual ...
[32]
Pythium aphanidermatum (damping-off) | CABI Compendium
Temperature optimum for growth is 34°C, with a minimum at 10°C and maximum above 43°C. On water agar the hyphae are thinner than some of the other common ...
[33]
Molecular Detection and Quantification of Pythium Species
On the phylogenetic side, genes commonly used in oomycete taxonomy (ITS, cox1, cox2, and β-tubulin) can give conflicting or confusing phylogenies at the species ...
[34]
https://extension.psu.edu/pythium
[35]
[PDF] Tropical and Subtropical Agroecosystems
The genus Pythium comprises of 120 described species which vary in their pathogenicity and crop hosts. The species that infect plants are ubiquitous in soil.Missing: habitat aquatic
[36]
PYTHIUMS AS PLANT PATHOGENS - Annual Reviews
The genus Pythium includes a number of readily recognized species with wide distributions and host ranges. The taxonomic position of the genus and.
[37]
[PDF] Germination In Vivo of Pythium aphanidermatum - Oospores and ...
Thick-walled oospores, with few exceptions (9), are commonly considered the primary propagule of. Pythium spp. capable of long-term survival in soil (7,. 8).
[38]
Root Rot Pathogen: Pythium | PT Growers and Consumers
Feb 6, 2017 · Pythium diseases are less prevalent when the growing medium's pH is below 5.5. Pythium has been shown to be vectored by fungus gnats and shore ...Missing: optimal | Show results with:optimal
[39]
Cellulose decomposition by Pythium and its relevance to substrate ...
The cellulolytic activity of some Pythium spp. removes them from the substrate-group, 'sugar fungi'. It is suggested that an index linking cellulolysis to ...Missing: breakdown | Show results with:breakdown
[40]
Carbohydrate-Active Enzymes in Pythium and Their Role in Plant ...
When infecting plants, Pythium rapidly degrades simple carbohydrates within plant cells and then allocates its metabolic resources to reproductive and survival ...
[41]
Pythium Damping-Off and Root Rot of Capsicum annuum L. - NIH
Apr 13, 2021 · The Pythium genus contains species that are pathogenic to plants [39,40,41], animals [42], algae [43], and other fungi [44]. More than 330 ...<|control11|><|separator|>
[42]
Pythium oligandrum in plant protection and growth promotion
In Pythium oligandrum, scientists again recognise its remarkable ability to not only directly protect plants through mycoparasitism but also sensitise plants to ...
[43]
The mycoparasite Pythium oligandrum induces legume pathogen ...
Oct 19, 2023 · We found that P. oligandrum promotes plant growth in these two species and protects them against infection by the oomycete Aphanomyces euteiches.
[44]
Pythium oligandrum - microbewiki
Apr 24, 2011 · They have a positive effect on nutrient cycling through their key role in the decomposition and recycling of organic matter (8). Interestingly, ...
[45]
Predators of Soil Bacteria in Plant and Human Health - APS Journals
Jul 26, 2022 · In this review, we compare the modes of predation of predators of soil bacteria, describe their influence on soil bacteria communities, and ...
[46]
Chemical Detection of Microbial Prey by Bacterial Predators
This informa- tion suggests that the Pythium predator may be attracted either to autolytic or lytic products of the fungal cell wall. Inhibition by ethanol. An ...
[47]
Protozoan-Induced Regulation of Cyclic Lipopeptide Biosynthesis Is ...
The grazing activity of protozoan predators significantly impacts the dynamics, diversification, and evolution of bacterial communities in soil ecosystems. To ...Missing: biodiversity | Show results with:biodiversity
[48]
Pythium spp. Associated with Root Rot and Stunting of Winter Wheat ...
Mar 15, 2021 · In eastern North Carolina, mild to severe stunting and root rot have reduced yields of winter wheat, especially during years with abundant rainfall.<|separator|>
[49]
Soil Suppressiveness Against Pythium ultimum and Rhizoctonia ...
Mar 31, 2023 · In the present study, we applied eleven soil health treatments combined with conventional and organic agricultural management in a long-term field experiment.
[50]
Approaches for the discrimination of suppressive soils for Pythium ...
In this study, we look at different soil suppressiveness values against Pythium irregulare and link these responses to the main parameters.
[51]
Disease Caused by Pythium
The pathogen may proliferate and move from the root to other parts of the host plant. If the plant survives, Pythium may colonize the stem and cause stem rot.
[52]
Review of Pythium Species Causing Damping-Off in Corn
Oct 11, 2021 · Taxonomically, Pythium now resides in the domain Eukarya, kingdom Chromista, phylum Oomycota, class Oomycetes, order Pythiales, and family ...
[53]
Pythium, a garden pathogen? - Plant diseases - RHS
Over the four year study, confirmed Pythium cases were associated with 90 host families encompassing 175 genera. The top host plant record was Taxus, and was ...
[54]
Pythium insidiosum: an overview - PubMed
Nov 20, 2010 · Pythium insidiosum is an oomycete pathogenic in mammals. The infection occurs mainly in tropical and subtropical areas, particularly in horses, dogs and humans.
[55]
Horse Owner Alert: Pythiosis in Water - UF Large Animal Hospital
Nov 18, 2024 · Pythiosis is a life-threatening infectious disease caused by the pathogen Pythium insidiosum, which can lead to painful and debilitating skin ...
[56]
Diseases caused by Pythium insidiosum in sheep and goats: a review
Pythiosis is characterized most commonly by ulcerative dermatitis, mainly in the limbs of sheep and occasionally of goats.
[57]
Infection with Pythium flevoense in a harbour porpoise (Phocoena ...
Nov 2, 2023 · Pythiosis is typically associated with exposure to tropical or subtropical freshwater conditions, and typically caused by Pythium insidiosum.Pathology · Genetics And Microbiology · Discussion
[58]
Life cycle of the human and animal oomycete pathogen Pythium ...
1. Light photomicrographs show the different steps required by P. insidiosum to complete its asexual cycle on a grass leaf. Note the beginning of sporangial ...
[59]
Pythiosis - WSAVA2002 - VIN
After encystment, the zoospores develop germ tubes in the direction of the affected tissue, and these structures allowed for penetration and invasion tissues6.
[60]
Pythiosis in Dogs
Pythiosis is an infectious disease caused by a fungus-like organism, Pythiuminsidiosum, that naturally inhabits wetlands, ponds, and swamps.
[61]
Geographic distribution of Pythium insidiosum infections in US
Dec 27, 2021 · Pythiosis in animals is common in tropical and subtropical areas of the world. Recently, pythiosis in the US has been diagnosed ...
[62]
The Zoonotic Potential of Fungal Pathogens - PubMed Central - NIH
Sep 15, 2024 · Pythiosis is caused by Pythium insidiosum. Pythiosis has been seen in horses and dogs, as the animal hosts primarily affected [73,74]. The ...
[63]
[Mycoses in fish] - PubMed
Fungal infections were correlated with findings of Saprolegnia, Branchiomyces, Pythium, Ichtyophonus and sometimes Cladosporium. ... MeSH terms. Animals; Fish ...
[64]
Pythium Serology & Mucormycosis Serology
Pythium and Mucormycosis Serology services provide veterinary diagnostic testing for Pythium insidiosum and Apophysomyces spp. infections.<|control11|><|separator|>
[65]
Cutaneous Pythiosis in 2 Dogs, Italy - CDC
Jun 12, 2023 · Pythiosis is a granulomatous disease caused by oomycete organisms affecting mainly horses, humans, and dogs, and is historically associated with ...
[66]
Epidemiological Survey of Equine Pythiosis in the Brazilian ...
Generally, the average prevalence of equine pythiosis in both regions was 5%, with mortality and lethality rates of 1.3% and 23.1%, respectively, in the ...
[67]
[PDF] Soil Solarization - UC Vegetable Research & Information Center
The maximum temperature of soil solarized in the field is usually from 108° to 131°F (42° to 55°C) at a depth of 2 inches (5 cm) and from 90° to 99°F (32° to ...
[68]
Soil solar heating for reductions of populations of Pythium spp ...
Soil solar heating for reductions of populations of Pythium spp., Fusarium spp., and weeds at the Colorado State Forest Service Nursery, Fort Collins, ...
[69]
[PDF] Pythium Damping-off & Root Rot in Tobacco Float Systems
Pythium species. (spp.) are one of several pathogen groups to cause pre-emergent damping off. This earliest occurrence of Pythium disease results in a low ...
[70]
Pythium Root and Crown Rot of Industrial Hemp
Aug 8, 2019 · Many Pythium species have a wide host range, which can limit management strategies, including crop rotation.
[71]
Pythium Damping-off of Soybean | NDSU Agriculture
Damping-off, the rotting and death of seeds and seedlings, can be a devastating disease and is of great economic importance to soybean production.Missing: impact | Show results with:impact
[72]
On the antifungal mode of action of metalaxyl, an inhibitor of nucleic ...
Growth inhibition of mycelial pellets of Pythium splendens by the acylalanine metalaxyl (CGA 48988) started 4 hr after addition of 0.1 and 0.3 μg/ml of this ...
[73]
Resistance to Mefenoxam and Metalaxyl Among Field Isolates of ...
Among isolates from all locations, 30% were classified as sensitive, 10% as intermediate, and 59% were resistant to mefenoxam. Mefenoxam-resistant isolates were ...
[74]
Biological control of Pythium species - Taylor & Francis Online
Sep 20, 2008 · Both bacterial and fungal antagonists are known to affect Pythium species by producing antibiotics, competing for space or nutrients or by direct parasitism.
[75]
Biocontrol of Pythium in the pea rhizosphere by antifungal ...
Mar 15, 2002 · Aims: Four well-described strains of Pseudomonas fluorescens were assessed for their effect on pea growth and their antagonistic activity ...<|control11|><|separator|>
[76]
(PDF) Biological control of Pythium root rot of chrysanthemum in ...
Aug 6, 2025 · ... controls, and AUDPC values were reduced by 61% and 65% (Table. 3). For P. dissotocum, the respective strains reduced colonization by 70–90%.
[77]
[PDF] Influence of Gypsum Application in Disease Management of Onion ...
Sep 9, 2021 · The application of gypsum into the soil is usually recommended for changing the pH of the soil. As a result, it facilitates disease management.
[78]
A non-classical PUF family protein in oomycetes functions as a pre ...
Jul 31, 2025 · To explore whether Puf4 can be used as a RNAi target for the control of Pythium, the Puf4 gene of P. aphanidermatum, PaPuf4, was cloned and ...
[79]
https://enviromicro-journals.onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-2672.2001.01260.x
[80]
Inhibition of Pythium spp. and Suppression of Pythium Blight of ...
Pythium diseases. Potassium phosphate was applied at 9.6 kg/ha phosphoric acid, and mefenoxam was applied at 0.76 kg/ha. The experimental design was a split ...