Pythium pringsheim biography

Pythium is a genus with over 200 species found worldwide, some of which are residents of terrestrial habitats while others are aquatic. In terms of nutrient acquisition, species within the genus may be saprophytes, plant or animal parasites, or mycoparasites. The genus was established by Pringsheim in 1858 (13) and members were considered as true fungi until recently when they were moved to a new Kingdom, Stramenopila. Within that kingdom, the genus Pythium is in the phylum Heterokonta, class Peronosporomycetes, order Pythiales, family Pythiaceae (7). The order Pythiales also includes the genera Phytophthora and Pythiogeton.

Pythium species are eukaryotes (have true nuclei) that have filamentous (thread-like), coenocytic (non-septate threads lacking cross walls) cell growth. The cell wall of many oomycetes is composed of cellulose and β-1, 3 glucan with minimal amounts of chitin. Chitin is a major component of the walls of true fungi. The asexual or vegetative stage of Pythium produces chlamydospores (thick walled resting spores), sporangia (that germinate directly to produce a hypha or indirectly to give rise to vesicle outside the sporangium, within which zoospores are formed), and hyphal swellings (spherical sporangia-like structures that do not give rise to zoospores). During sexual reproduction, an antheridium fertilizes an oogonium to produce a thick walled oospore. Some Pythium species are heterothallic and require opposite mating types to reproduce sexually but most are homothallic and do not require an opposite mating type. Standard keys (17;6) differentiate species within Pythium based on the shapes, sizes, and locations on the mycelium of the sporangium, antheridium, oogonium, and oospore.

Not all species or isolates within some species form zoospores. Zoospores are asexual, bi-flagellated, thin walled spores that move toward host cells where they encyst. The cysts then produce a germ tube and an appressorium that aids in the penetr

J. VAN DER PLAATS-NITERINK

Centraalbureau voor Schimmelcultures, Baarn

Summary

This revision of the species of Pythium Pringsh. is mainly based on living cultures preserved at the Centraalbureau voor Schimmelcultures. Eighty five species are recognized and described in alphabetical sequence; sixty four of these are available as living strains. In addition, two highly specialized cellulolytic species are treated in an appendix. Pythiumbuismaniae and P. macrosporum are described as new species. These species and some incompletely known ones are keyed out dichotomously. Seven species are heterothallic and produce sexual organs only in combinations of compatible isolates, whilst others that fail to produce oogonia can generally not be identified at species level. Sixty five taxa are listed as excluded, incompletely known or doubtful.
The species are illustrated by line drawings, a few light micrographs and scanning electron micrographs of omamented oogonia. For each species a compilation of literature data on occurrence and pathogenicity is given, reference being made to 1133 pubfications.

I. Introduction

Butler’s (1907), Matthews’s (1931), Sideris’s (1931, 1931) and Frezzi’s (1956) treatments of Pythium dealt with limited numbers of species. Middleton’s work (1943) comprised all the species known at that time. Waterhouse (1967, 1968a) compiled the diagnoses and descriptions of more than 180 species of Pythium and provided a key to 89 recognized species. As several new species were subsequently described, a new monograph was required to cover all present-day knowledge of the genus and to evaluate the available taxonomic criteria. Most of the recognized species are represented by living strains preserved at the Centraalbureau voor Schimmelcultures (CBS).
Pythium species are often pathogenic to many plants and to animals such as fishes or Crustacea. They can

Genus of single-celled organisms

For the ancient city named Pythium, see Pythion.

Pythium is a genus of parasiticoomycetes. They were formerly classified as fungi. Most species are plantparasites, but Pythium insidiosum is an important pathogen of animals, causing pythiosis. The feet of the fungus gnat are frequently a vector for their transmission.

Morphology

Hyphae

Pythium species, like others in the family Pythiaceae, are usually characterized by their production of coenocytichyphae without septations.

Oogonia

Generally contain a single oospore.

Antheridia

Contain an elongated and club-shaped antheridium.

Ecological importance

Pythium-induced root rot is a common crop disease. When the organism kills newly emerged or emerging seedlings, it is known as damping off, and is a very common problem in fields and greenhouses. Thus there is tremendous interest in genetic host resistance, but no crop has ever developed adequate resistance to Pythium. This disease complex usually involves other pathogens such as Phytophthora and Rhizoctonia. Pythium wilt is caused by zoospore infection of older plants, leading to biotrophic infections that become necrotrophic in response to colonization/reinfection pressures or environmental stress, leading to minor or severe wilting caused by impeded root functioning.

Many Pythium species, along with their close relatives Phytophthora, are plant pathogens of economic importance in agriculture. Pythium spp. tend to be very generalistic and unspecific in their large range of hosts, while Phytophthora spp. are generally more host-specific.

For this reason, Pythium spp. are more devastating in the root rot they cause in crops, because crop rotation alone often does not eradicate the pathogen as Pythium spp. are also good saprotrophs, and survive for a long time on