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Isozyme-based genetic diversity, morphological characters, and growth rate at different temperatures were compared in a worldwide collection of 125 isolates presumed to be Pythium irregulare Buisman. The isozyme data was analysed with previously published data for Pythium ultimum Trow and Pythium sylvaticum Campbell & Hendrix. UPGMA cluster analysis yielded a dendrogram with four distinct groups: P. ultimum, P. sylvaticum, and two for P. irregulare. Putative P. irregulare isolates were separated into 33 multilocus genotypes defined by 11 isozyme loci: group A contained 116 isolates in 25 genotypes, and group B, 8 isolates in 7 genotypes. One genotype with a single isolate was determined as P. sylvaticum. Based on the isozyme analysis, group B was considered a distinct taxonomic entity from group A, but lacked any unique morphological character. There was a wide range in oogonium and oospore sizes among different isolates of P. irregulare, with those in group B generally being larger. Some isolates in group A had well developed oogonial spines, but others were essentially spineless, whereas all those in group B were spineless. Both groups A and B contained isolates with distinctly aplerotic oospores and others with essentially plerotic oospores. Antheridial number and shape were highly variable both within and among isolates in the two groups. Growth rate over a range of temperatures varied among isolates in both groups and was not a reliable taxonomic criterion. The irregular shape of oogonia and, when present, oogonial spines were the only reliable characters for distinguishing P. irregulare isolates from other taxa. Key words: taxonomy, Oomycetes, Pythiaceae, Pythium ultimum, Pythium sylvaticum.

A collection of 63 heterothallic and asexual (sexually sterile) isolates were compared by isozyme analysis, vegetative characters, and their ability to produce oospores in paired cultures. The collection included isolates identified as Pythium heterothallicum, Pythium intermedium, Pythium macrosporum, Pythium splendens, Pythium sylvaticum, and Pythium tracheiphilum, and several unidentified isolates. Isolates were separated into 41 multilocus genotypes as defined by 11 isozyme loci. The data were analyzed with previously published data for Pythium ultimum, and dendrograms were obtained from unweighted pair-group method, arithmetic average cluster analysis. Forty-six genotypes (including nine of P. ultimum) grouped into seven clusters and three solitary genotypes. Six clusters contained known isolates of the aforementioned species and the seventh contained P. ultimum. Isolates within both the P. heterothallicum and P. sylvaticum clusters produced oospores when grown in paired cultures. Two of three isolates in the P. splendens cluster produced oospores, but none of the isolates in the P. intermedium and P. macrosporum clusters produced oospores when grown in pairs. Genotypes belonging to the P. intermedium cluster separated into two sister groups that had morphologically different vegetative bodies. There were no consistent vegetative characteristics to distinguish the species P. macrosporum, P. heterothallicum, and P. sylvaticum. Isozyme analysis provides an effective means of identifying Pythium species that do not readily produce oospores in culture. Key words: Pythium heterothallicum, Pythium intermedium, Pythium macrosporum, Pythium sylvaticum, Pythium splendens, Pythium tracheiphilum.

A worldwide collection of 60 isolates of Pythium ultimum and 37 asexual (sexually sterile) isolates presumed to be P. ultimum were compared by isozyme analysis, morphological characters, and their growth rate at different temperatures. Isolates were separated into 10 multilocus genotypes as defined by nine isozyme loci. Five genotypes (U1–U5) comprising 85 isolates differed from each other merely by homo- or hetero-zygous states at one and (or) two loci. Four isolates in genotype U6 produced zoospores and were presumed to be var. sporangiiferum. They differed at one locus from genotypes U1–U5. Genotypes U7, U8, U9, and U10 differed from U1–U5 at one, two, three, and five loci, respectively. Sexual isolates occurred in genotypes U1–U3 and U6–U10, and asexual isolates in genotypes U2–U6. Only minor differences were detected in morphology in six isolates (genotypes U8–U10). A single isolate (genotype U10) differed in growth rate from all others. Isozyme analysis is an effective means of distinguishing P. ultimum from nine other species included for comparative purposes and for identifying asexual isolates that cannot be positively identified by morphology. Keywords: Pythium ultimum var. ultimum, Pythium ultimum var. sporangiiferum, isozymes.

Powellomyces (order Spizellomycetales) is erected for soil-inhabiting chytrids previously classified in Entophlyctis. Fischer's narrative and figures indicate that Entophlyctis was originally described to accommodate aquatic, chytridialean species, and consequently this is not an appropriate genus for spizellomycetalean species. Powellomyces hirtus sp.nov. and Powellomyces variabilis nom.nov. both develop exogenously but differ in distribution of rhizoids on germlings and in colour of colonies in culture. Key words: Chytridiales, Entophlyctis, Powellomyces, Spizellomycetales.

The flagellar apparatuses of 14 species of Phytophthora, 2 of Halophytophthora, and 4 of Pythium are compared in the transmission electron microscope. Except for Phytophthora infestans and Phytophthora mirabilis there were no significant differences in fine structure morphology. There are six flagellar roots: a ribbed triplet consisting of three main microtubules and secondary microtubules; an anterior doublet; a multistranded, band-shaped root of five to nine microtubules; a posterior root of two to four microtubules; and roots consisting of arrays of cytoplasmic microtubules and nuclear-associated microtubules. In P. infestans and P. mirabilis the multistranded root is missing, the posterior root contains five or six microtubules, and the anterior ribbed root contains four main microtubules. The transitional zones in all species are similar. The relationship of the Pythiaceae with other Oomycetes is discussed. Key words: taxonomy, phytogeny, cytology, Oomycetes, Pythiaceae.

The Phytophthora infestans zoospore contains five out of six rootlets that are found in other secondary-type oomycetous zoospores. They include (i) an anterior ribbed rootlet (AR4) that runs along the right side of the anterior groove and consists of four main microtubules, an electron-opaque cord, and secondary (rib) microtubules; (ii) an anterior rootlet (A2) that runs on the left side of the anterior groove and consists of two microtubules; (iii) a posterior rootlet (P6) that runs on the left side of the posterior groove and consists of six microtubules; (iv) an array of cytoplasmic microtubules; and (v) an array of nucleus-associated microtubules. There is no multistranded, band-shaped rootlet (MS). There is a concertina-like structure in the core of the transition zone distal to the basal plate. Differences in rootlet morphology may provide a useful means of classifying species of Phytophthora. Key words: flagellar apparatus, zoospore, Phytophthora infestans, Oomycetes.

Lagena radicicola Vanterpool & Ledingham is an obligate parasite inside root hairs and epidermal cells. It was cultured in a unifungal state on wheat in pots. The life cycle was examined by both light and transmission electron microscopy. The thallus developed inside a single host cell and formed either a single sporangium or one to four resting spores. Zoospore cleavage was completed in vesicles outside the root. The resting spores were similar to oospores in their development and cytology, but there was no evidence of cell fusion and sexuality. Virus-like particles were seen in 3- to 12-month-old cultures, and infected cells became degenerate. Key words: Oomycetes, ultrastructure, virus-like particles, biocontrol.

The zoospore fine structure of Lagena radicicola Vanterpool & Ledingham is compared with the primary and secondary zoospores of Saprolegnia diclina Humphrey. The L. radicicola zoospore is broadly pyriform, it has two subapically attached flagella, a pyriform nucleus that has a fine fibrillar attachment to the posterior kinetosome, and a Golgi body next to the nucleus. There are various vesicles including dense body vesicles and several small expulsion vacuoles but there are no U- or K-bodies. It has a well-organized rootlet system comprising anterior and posterior doublet microtubules, a ribbed triplet that probably provides a cytoskeletal frame for the plasmalemma, and a multistranded, band-shaped rootlet of 4 to 10 microtubules. Cytoplasmic microtubules are very sparse. Interpretation of the rootlet systems was aided by computer graphics; 3-D images of the rootlet system were produced as stereo pairs on a Kontron image analyzer. Based on cell shape, position of the flagella, and angle of kinetosomes classification of the L. radicicola zoospore as either a primary or secondary type is imprecise; however, the various kinds of rootlets are similar to those found in the secondary zoospore of S. diclina but differ in structural detail and orientation. The transition zone in the flagellum of L. radicicola is simple and lacks the typical concertina body found in most oomycete flagella. The relationship of L. radicicola to the Oomycetes and various heterokont algal groups, based on comparative zoospore ultrastructure, is discussed.

Isolates of Phytophthora from alfalfa and soybean and the type isolate of Phytophthora megasperma Drechsler from hollyhock were compared morphologically, for temperature response, and pathogenically. Highly pathogenic, host-specific isolates from soybean and alfalfa, with relatively small oogonia, were identified as P. sojae Kaufmann and Gerdemann and divided into two formae speciales: f.sp. glycines and f.sp. medicaginis, respectively. Less pathogenic isolates from alfalfa with large oogonia were identified as P. megasperma and were compared favorably with Drechsler's original isolate and description. Isolates of P. sojae differed in their cardinal temperatures from those of P. megasperma. We consider P. sojae to be distinct from P. megasperma and propose that this taxon be reintroduced to the literature.

By matching serial sections from the flagellum to the kinetosome in selected species of chytrids (Chytridiales), the kinetosome triplets have been assigned numbers that correspond to the numbers given by general convention to the flagellum doublets. The various organelles in the zoospore are located in nine longitudinal segments that lie proximal to the kinetosome. Each segment is delineated by the cartwheel spokes of the kinetosome; segment 1 lies between subfibers A of triplets 1 and 2. In Rhizophydium chlorogonii Jaczewski, R. granulosporum Scherffel, Chytridium confervae (Wille) Minden, and Chytriomyces hyalinus Karling, the rootlet extends from between kinetosome triplets 1 and 2, but at different angles in each species. In Catenochytridium hemicysti Knox the rootlet extends from the triplet 1, and in Allochytridium luteum Barr & Désaulniers between triplets 9 and 1. In all species the center of the rumposome is located in segments 8, 9, or 1, and the nucleus in segments on the opposite side. The nonfunctional centriole is connected by fibers that extend from kinetosome triplets 6–7 in all species and also from triplets 5 and 8 in some species.

The zoospores of Phytophthora sojae Kaufmann and Gerdemann and P. megasperma Drechsler are cytologically similar. Their flagellar apparatus includes six rootlets consisting of an anterior ribbed triplet, a multistranded band-shaped rootlet with seven to eight microtubules, a posterior rootlet with four and three microtubules, respectively, an anterior doublet rootlet, and arrays of cytoplasmic and nuclear associated microtubules. The flagellar apparatus in these species is contrasted with the flagellar apparatus in P. cinnamomi Rands and P. parasitica Dastur, and with other Oomycetes. Key words: three-dimension image analysis, ultrastructure, fungi.

Five eucarpic, epibiotic chytrids with single discharge pores were examined in culture, and morphological variation on different substrates was reported. Species studied include Rhizophydium constantineani Saccardo; Rhizophydium sphaerocarpum (Zopf) Fischer var. sphaerocarpum; Rhizoclosmatium globosum H. E. Petersen; Chytridium lagenaria Schenk, and Chytridium confervae (Wille) Minden. These chytrids could be classified according to four methods of zoospore discharge: (I) inoperculate and non-vesicular; (II) inoperculate and vesicular; (III) operculate and non-vesicular; and (IV) operculate and vesicular. Presence of a membraneless vesicle during discharge was shown to be of as much taxonomic significance as the presence of an operculum. Taxonomic criteria and phylogeny are discussed.

The principal cytological features and their variations are described in zoospores of 38 isolates belonging to seven species of Spizellomyces. This genus is distinguished from others in the Spizellomycetaceae by the orientation of the microtubules and the position of the nucleus relative to the kinetosome in the zoospore. Microtubules run apparently at random into the cytoplasm from a microtubule-organizing center (spur) on one side of the kinetosome. The nucleus is anteriorly or centrally positioned with a heel-like process extending to near the proximal face of the kinetosome, or it can be posteriorly positioned and elongate with one end close to the kinetosome. Differences between species are reflected by the position and shape of the nucleus, the extent of the endoplasmic reticulum which in some species circumscribes lipid globules, the presence or absence of an apparent connection by fibrillar material between the kinetosome and a nonfunctional centriole, and the morphology of the microtubule-organizing center. The zoospores of Spizellomyces punctatus (Koch) D. Barr, S. plurigibbosus (D. Barr) D. Barr, and S. palustris (Gaertner) D. Barr are cytologically similar, but those of S. acuminatus (D. Barr) D. Barr, S. dolichospermus D. Barr, S. lactosolyticus D. Barr and S. pseudodichotomus (Umphlett) D. Barr each have one or more distinctive characteristic. Spizellomyces dolichospermus and S. pseudodichotomus also have some cytological characteristics in common with the genus Kochiomyces.

Four subtypes of zoospores are described, with the use of the transmission electron microscope, for chytridiomycetous fungi that can be classified with light microscopy in the Rhizophlyctis–Karlingia complex. Subtype A has ribosomes dispersed throughout the cytoplasm; a long fibrillar rhizoplast; mitochondria and microbody closely associated with the rhizoplast; and numerous lipid globules that have occasional association with microbody. Subtype B has ribosomes concentrated into the center of the zoospore, and these are partially surrounded by endoplasmic reticulum; a short fibrillar rhizoplast; microbody associated with a basal lip of the nucleus; mitochondria that surround the rhizoplast; and numerous lipid globules that have occasional association with microbody. Subtype C has ribosomes concentrated (as in subtype B); several mitochondria interwound with microbody in the basal area between the kinetosome and nucleus; and numerous lipid globules that have occasional association with microbody. Subtype D has ribosomes concentrated (as in subtypes B and C); a long heel-like segment of the nucleus that extends to a point adjacent to the kinetosome; a close association between the nuclear heel, microbody, and mitochondria; and a large lipid globule which has close association with microbody and is partially surrounded by endoplasmic reticulum. None of the subtypes contain cytoplasmic microtubules. The taxonomic significance of the cytology is discussed. It is concluded that these subtypes of zoospores are more closely related to the Spizellomycetales than the Chytridiales.

The ultrastructure of the Polymyxa graminis Ledingham zoospore is described with emphasis on the flagellar apparatus. The unique characteristics of this zoospore are the two kinds of rootlets which extend from the proximal ends of each kinetosome to the plasmalemma. One rootlet consists of a microtubule pair with electron-opaque backing along part of its length which ends abruptly at the plasmalemma. The other rootlet consists of three microtubules which, on reaching the plasmalemma, curve around sharply and extend ribbonlike along the side of the zoospore adjacent to the plasmalemma. This arrangement of rootlets is replicated on the second kinetosome. The kinetosome and transition zone are structurally similar to those in certain protozoa and unlike those found in other groupings of zoosporic fungi or algae.

Kochiomyces and Triparticalcar are recently described genera. Each has unique ultrastructural characteristics in its zoospore. They are classified in the new order Spizellomycetales, in the class Chytridiomycetes. The zoospores of Kochiomyces have a posteriorly positioned nucleus which is ensheathed by microtubules. These microtubules originate from a compound spur on one side of the kinetosome. Microbodies, and one to five mitochondria surround the nucleus. Lipid globules occur in the anterior but these are not associated with microbodies. The Kochiomyces zoospore resembles a primitive blastocladiaceous zoospore which suggests a phylogenetic connection. The zoospores of Triparticalcar have an anteriorly positioned nucleus anchored to the kinetosome by microtubules and a spurlike structure. The spur is 0.8–2.5 μm long and tripartite in cross section. Microbodies are closely associated with lipid globules in the anterior and center of the zoospore. Mitochondria occur not only in the posterior, but also in the center of the zoospore where they often lie close to lipid globules and microbodies.

An outline is presented for the reclassification of the Chytridiales into two orders based on fundamental differences in zoospore ultrastructure. The new order Spizellomycetales includes species with the Phlyctochytrium–Entophlyctis type of zoospore and also Karlingia, Olpidium, and Rozella. Four new genera, Spizellomyces, Gaertneriomyces, Triparticalcar, and Kochiomyces replace the Phlyctochytrium complex. The revised Chytridiales includes species with the Rhizophydium–Chytridium–Nowakowskiella type of zoospore, as well as Synchytrium

Zoospore ultrastructure of Olpidium cucurbitacearum Barr & Dias is described. Isolates from Ontario and Japan are alike. The elliptical zoospore has an anterior nucleus partially surrounded by lobed and branched mitochondria. In the central part an interconnecting network consisting ofsmooth and rough endoplasmic reticulum, a Golgi complex, microbodies, and multivesicular bodies predominates. Vacuoles are abundant in the posterior. Ribosomes occur throughout the cytoplasm. The flagellar apparatus consists of the following: a short fibrillar rhizoplast connecting both kinetosome and nonfunctional centriole to an electron-opaque bar on the nuclear envelope; an elongated transition zone; and a 9 + 2 flagellum axoneme running much of the length of the zoospore through to the posterior end. Props do not connect the kinetosome to the plasmalemma; however, prop parts connect to the kinetosome, and prop parts connect to the plasmalemma at the posterior end of the zoospore. The taxonomic relationship to other chytrids is discussed; the zoospore is similar to that of O. brassicae (Woronin) Dang, and Rhizophlyctis rosea (deBary &Woronin) Fischer, and somewhat similar to that of Rozella allomycis Foust.

The ultrastructure of the zoospore of Hyphochytrium catenoides Karling is described. The zoospore has a single, anterior, tinsel flagellum. The nucleus is elongate and convoluted with an indentation at the anterior end in which the Golgi cisternae are located. There are large lipid globules in the posterior end of the cell. The ribosomes are loosely enclosed by endoplasmic reticulum, the nuclear envelope, and mitochondria. The mitochondria have tubular cristae in a dense matrix. Microbodies are found appressed to the nuclear envelope and also free in the ribosomal region. Endoplasmic reticulum sheets traverse the ribosome region. The vesiculate cytoplasm has several distinct types of membrane-bound inclusions: (i) multivesicular bodies, (ii) vesicles containing presumptive mastigonemes, (iii) vesicles having an electron-dense cortex with an electron-transparent center, and (iv) electron-opaque vesicles whose contents seem condensed and only partially fill the vesicles. The transition zone from the flagellum to the kinetosome has three segments: a distal set of struts extending from the axonemal doublets into the axoneme core, a midsection of electron-opaque rings, and a distinctive "disclike" terminal plate with a thickened portion between the doublets and the flagellar membrane. The three-part rootlet system has (i) a "ribbed" pair of microtubules on one side of the kinetosome, (ii) a curved "ribbed," single microtubule with electron-opaque backing which originates near the nonfunctional centriole, and (iii) a straight doublet of microtubules without ribs extending from the nonfunctional centriole posteriorly to the midregion of the zoospore.

The zoospore ultrastructure of 12 species of Rhizophydium is described. Species include the following: R. chlorogonii (Serbinow) Jaczewski; R. constantineani Saccardo; R. haynaldii (Schaarschmidt) Fischer; R. capillaceum Barr; two morphologically and cytologically different species, each previously identified as R. sphaerotheca Zopf; R. patellarium Scholz; R. biporosum (Couch) Barr; R. subangulosum (Braun) Rabenhorst; R. laterale (Braun) Rabenhorst; R. sphaerocarpum (Zopf) Fischer var. spirogyrae Barr; and two isolates of R. pollinis-pini (Braun) Zopf. The Rhizophydium zoospore is basically similar to the Chytridium zoospore having (1) the nucleus, a compact cluster of ribosomes, one or more mitochondria, and a microbody – lipid globule complex compartmentalized into the core of the zoospore by a double membrane system and (2) two to five microtubules connecting one side of the kinetosome to the rumposome on the lipid globule surface and thus anchoring the lipid globule in a lateral–posterior position in the zoospore. Rhizophydium patellarium does not have kinetosome-associated microtubules or a rumposome but does have the membrane-bound core area. In all species, a microbody and mitochondrion are associated with the lipid globule. The number of mitochondria varies from 1 in some species to several or to over 30 in other species. In one isolate of R. pollinis-pini, there is 1 large mitochondrion and in the other there were 30–35 small mitochondria. The peripheral cytoplasm of all species contains clusters of vesicles or endoplasmic reticulum which bud from the double membrane system, vesicles of moderate electron density, and vacuoles of various sizes; R. capillaceum, R. patellarium, and R. subangulosum have in addition vesicles which contain very electron-dense material. Rhizophydium capillaceum and R. sphaerocarpum zoospores have virus-like particles and the R. biporosum zoospore contains a paracrystalline body. The taxonomic significance of the observations and the relationship of Rhizophydium to other chytrids are stressed in the Discussion.

The fine structure of zoospores of Entophlyctis confervae-glomeratae, Rhizophydium patellarium, and Catenaria anguillulae has been described and compared. The zoospores of E. confervae-glomeratae contain many posteriorly located mitochondria in a petal-like arrangement. On the inner (proximal) side of a kinetosome are two electron-dense, slightly curved bands with the convex side directed inward towards the eccentrically located nucleus. Microtubules radiate from the convex side of the bands. Two or three lipid bodies are seen usually in the anterior region of the zoospore with microbodies often associated with them. Ribosomes are scattered throughout the cytoplasm. The zoospores of R. patellarium contain a single, bowl-shaped mitochondrion in the posterior region and a centrally located nucleus. Ribosomes are mainly clustered between the mitochondrion and the nucleus and are partially enclosed by a double-membrane envelope. No microtubules are seen associating with the kinetosome and the non-functional centriole lies parallel to the kinetosome. In the anterior region there is a single lipid body as well as two types of microbodies: very electron-dense microbodies un-associated with any other organelle, and a finely granular microbody associated closely with the lipid body. The zoospores of C. anguillulae contain a single basal mitochondrion with the associated side-body complex. From the proximal region of the kinetosome, a set of 27 microtubules radiate anteriorly and surround the base of the cone-shaped nucleus. Ribosomes are enclosed within a distinct nuclear cap. Gamma bodies are present in the cytoplasm. Taxonomic implications resulting from this study are discussed.

The zoospore of Phlyctochytrium plurigibbosum Barr is globular to amoeboid while swimming and posteriorly uniflagellate. Mitochondria are in the posterior and are petal-like in their arrangement. The nucleus and one or more lipid globules are in the centre to anterior part of the cell. Morphologically, microbodies are intimately associated with lipid globules and loosely associated with mitochondria. There is a conspicuous double membrane system of smooth endoplasmic reticulum, and free ribosomes are dispersed throughout the cytoplasm. Microtubules radiate into the zoospore body from the proximal face of the kinetosome.

Developmental stages and morphological variation are described for three species of fungi from the rumen of a Holstein steer. Neocallimastix sp. is monocentric and has endogenous, rarely exogenous, sporangial development. Piromyces communis is monocentric, rarely polycentric, and has both endogenous sporangium development and exogenous sporangia on sporangiophores. Orpinomyces bovis is described for a polycentric fungus with terminal and intercalary sporangiophore complexes that develop from the rhizomycelium, and polyflagellate zoospores. The advantages and limitations of the light microscope and the enigmatic problem of classification of diverse and polymorphic rumen fungi are discussed.

The flagellar apparatus is described in zoospores of Rhizophydinm chlorogonii, R. constantineani, R. haynaldii, R. laterale, R. patellarium. R. pollinis-pini, R. sphaerotheca, R. subangulosnm, Phlyctochytrium plurigibbosum, Chytridium confervae, C. olla, C. lagenaria, Rhizoclosmatium globosum, and Rluzophlyctis rosea, which are all members of the order Chytridiales. In fine structure, the flagellar apparatus is basically similar to that of fungi in other orders of the class Chytridiomycetes. The characteristics of the flagellar apparatus which are unique to the class are: (1) kinetosome props and associated segment of doublets with a curved extension seen as a hook in thin sections; (2) a spiral structure which connects to the A tubules of the doublets and coils in the core of the transition zone. These characteristics distinguish this class of fungi from the Oomycetes, from algae, protozoa, and other eukaryotes. It is further proposed that six modifications to the flagellar apparatus within the Chytridiales are diagnostic characteristics which could be used for describing subgroupings within the order. These modifications are the presence of a fibrous rhizoplast in some species; presence and form of electron-dense inclusions in the basal area; arrangement of kinetosome-associated microtubules; presence of an electron-dense area in the base of the flagellum; the angle of the nonfunctional centriole to the kinetosome and the presence of a veil-like structure on the side of the nonfunctional centriole. Taxonomic implications of these findings are discussed.

A classification is given for those Chytridiomycetes with monocentric and eucarpic growth forms, endogenous sporangium development, and multipapilliated sporangia that are now known in culture. These belong to six genera which are classified on unique ultrastructural characteristics of the zoospore; however, for convenience of routinely distinguishing these genera, a key is given based on characters detectable by light microscopy. Eight species are described for Spizellomyces, which include S. punctatus (Koch) D. Barr; S. plurigibbosus (D. Barr) comb, nov.; S. kniepii (Gaertner) comb, nov.; S. palustris (Gaertner) comb. nov.; S. acuminatus (D. Barr) comb. nov.; S. dolichospermus sp. nov.; S. lactosolyticus sp. nov.; and S. pseudodichotomus sp. nov. There are two species in Gaertneriomyces: G. semiglobiferus (Uebelmesser) D. Barr and G. tenuis sp. nov. Triparticalcar arcticum (D. Barr) D. Barr and Kochiomyces dichotomus (Umphlett) D. Barr are monotypic genera. Criteria for classifying these fungi in culture are discussed. Rhizophydium and Karlingia are included in the generic key but are not treated at the specific level. This new classification is intended strictly for cultures and neither augments nor replaces earlier and more classical systems.

The zoospore type is described for Gaertneriomyces, a genus in the new order Spizellomycetales, Chytridiomycetes. The nucleus lies in the posterior of the zoospore and it has a basal concavity. The kinetosome and nonfunctional centriole are located just distally to the concavity. Microtubulules radiate in the cytoplasm from a presumed microtubule organizing center consisting of two short, electron-opaque bars attached to the kinetosome. Mitochondria are predominantly in the posterior, and aggregates of lipid globules in the anterior of the zoospore. Microbodies are morphologically associated with the lipid and endoplasmic reticulum. They are also found along the sides of the nucleus and near the mitochondria.

The rootlet systems, kinetosomes, and transition zones in zoospores of Phytophthora parasitica, Thraustochytrium aureum, and Rhizidiomyces apophysatus and secondary zoospores of Saprolegnia diclina are compared. Rootlet systems in P. parasitica and S. diclina are very similar and there are only minor differences between these taxa in morphology of their transition zones. In P. parasitica and T. aureum a number of differences exist in the rootlet systems, but the position and orientation of individual rootlets are similar. There are also differences between these taxa in the alignment of their kinetosomes, the morphology of their kinetosome cores, and transition zones. Rhizidiomyces apophysatus has rootlets that correspond in position and orientation to P. parasitica, but there are not as many rootlets and these are morphologically distinctive. However, R. apophysatus and S. diclina have flagellar transition zones that are morphologically alike. Thus, probably the Hyphochytriomycetes are phylogenetically quite closely related to the Saprolegniales and more distantly related to the Peronosporales. In contrast, the Thraustochytriales are distantly related to the Oomycetes and Hyphochytriomycetes, but nonetheless similarities in the flagellar apparatus indicate that all have a common ancestor.

Two varieties of Rhizophydium sphaerocarpum (Zopf) Fischer are proposed: var. sphaerocarpum, saprophytic on algae, nematodes, and pine pollen; and var. spirogyrae, a parasite of Spirogyra spp. The varieties differ morphologically only in colony color and texture. Variety sphaerocarpum grew best at (10–) 15–20 C and used inorganic nitrogen as ammonium chloride, ammonium nitrate, and sodium nitrate when the inoculum contained actively growing thalli, whereas var. spirogyrae grew best at (25–) 30 C and did not use inorganic nitrogen. Results of pH range tests and carbon nutrition were very similar for the two varieties and both required exogenous thiamine.

The morphology and physiology of two pure culture isolates of Rhizophydium patellarium Scholz have been compared with two isolates of R. capillaceum Barr. The species differ in method of zoospore discharge: R. patellarium by dissolution of part of sporangium wall and R. capillaceum by dissolution of papillae. In physiology there are differences between isolates of each species but there is no useful physiological criterion for distinguishing species. It is proposed to reduce R. collapsum Karling as a synonym of R. patellarium.

A physiological comparison was made of five species of soil chytrids. Phlyctochytrium californicum, P. plurigibbosum, and P. acuminatum could be distinguished from each other and from two species of Rhizophydium by their pH optima, their thiamine requirements, and their differential growth on nitrate, on nitrite, and on the carbohydrates fructose, trehalose, cellobiose, glycerol, and lactose. Temperature optima and ranges were as variable among isolates of species as between species. Rhizophydium sphaerotheca and R. capillaceum were physiologically similar and could only be separated on their ability to use nitrate and fructose.

A pure culture of Phlyctochytrium reinboldtae Persiel isolated from soil near Ottawa, Canada, grew best at 20 and 25 C, slightly at 10 C, and not at all at 35 C. It grew best in alkaline media and required no exogenous vitamins. It used ammonium, nitrate, and nitrite nitrogen and each of the 7 amino acids tested. Of 18 carbon sources, it used glucose, fructose, trehalose, maltose, mannose, cellobiose, glycerol, and starch. It is similar physiologically to P. punctatum Koch and P. plurigibbosum Barr and morphologically to P. papillatum Sparrow and P. dichotomum Umphlett.

Studies of pathogenicity and of morphological variation were made with pure cultures of Rhizophydium sphaerocarpum isolated from Spirogyra and of R. karlingii isolated from Ulothrix. R. sphaerocarpum was parasitic, virtually confined to species of Spirogyra amongst which there was a wide range of susceptibility. R. karlingii grew only on non-living algae and on pine pollen. The rhizoidal system of R. sphaerocarpum was quite variable in the extent of its branching. Variations in sporangium size and of the forms of the endobiotic portion of the thallus in R. karlingii grown on different substrates were observed. Such variations have been used in the past as the basis for creation of new taxa. Pathogenicity tests with Paterson's strain of R. sphaerocarpum confirmed that it is saprophytic on many algae and on pine pollen in contrast with the strain isolated from Spirogyra.

Host–parasite interrelationship studies showed that temperature was the most important environmental factor associated with epidemics of Rhizophydium sphaerocarpum (Zopf) Fischer on Spirogyra. Certain highly susceptible spring species of Spirogyra grew poorly or not at all at 30 C, the optimum temperature for R. sphaerocarpum, while resistant summer species grew well at 30 C, and even at 35 C. Both pure-culture growth of R. sphaerocarpum and infection of Spirogyra were optimal at approximately pH 7.0–7.5, falling off markedly below pH 6.0 and above pH 8.5. Spirogyra itself grew over a wider pH range, especially on the alkaline side. Infection of Spirogyra was negligible in darkness but a light intensity of 40 ft-c permitted some infection. No difference in infection occurred over a range of light intensities from 65 to 980 ft-c or in day lengths varying from 8 to 16 hours at a light intensity of 600 ft-c. Light saturation for Spirogyra was 400 ft-c. In contrast to the effect of light upon infection, pure culture growth of R. sphaerocarpum was considerably greater in total darkness than at light intensities of 20 ft-c or higher.

Culture studies of Entophlyctis confervae-glomeratae (Cienkowski) Sparrow showed that this chytrid was saprophytic on a wide range of plant matter including pine pollen, leaves, algae, and fungi. Morphological variation was so marked that many taxonomic criteria proved valueless and several synonyms are therefore suggested. A comparison with other species and genera is made. It is proposed that Entophlyctis include four species which are saprophytic on plant matter (E. confervae-glomeratae, E. texana Karling, E. pygmaea (Serbinow) Sparrow, and E. crenata Karling); a chitinophilic species (E. lobata Willoughby & Townley); and three species parasitic on algae (E. apiculata (Braun) Fischer, E. molesta Canter, and E. reticulospora Cook).

A physiological comparison was made of five species of soil chytrids. Phlyctochytrium californicum, P. plurigibbosum, and P. acuminatum could be distinguished from each other and from two species of Rhizophydium by their pH optima, their thiamine requirements, and their differential growth on nitrate, on nitrite, and on the carbohydrates fructose, trehalose, cellobiose, glycerol, and lactose. Temperature optima and ranges were as variable among isolates of species as between species. Rhizophydium sphaerotheca and R. capillaceum were physiologically similar and could only be separated on their ability to use nitrate and fructose.

The fine structures of Phlyctochytrium arcticum zoospores, from early differentiation in the sporangium until they encyst and germinate, are described. Cytoplasm cleavage and flagellum formation have been studied. A complex rhizoplast–kinetosome structure is present. The Stüben body associating with a nucleus in a differentiating zoospore apparently divides into a number of similar bodies which are observed in the subsequent stages. Mitochondria, which are scattered about in the cytoplasm of maturing zoospores, are clustered at the posterior end of the motile zoospore, surrounding the kinetosome and the rhizoplast. On encystment, the flagellum is retracted, the rhizoplast disappears, the mitochondria again become scattered in the cytoplasm, and a wall is rapidly laid down. The morphology of the P. arcticum zoospore is compared with other chytridiaceous zoospore types.

Olpidium cucurbitacearum sp. n., from roots of Cucumis sativus L. var. Windermoor Wonder, is described. The fungus is distinguished from other Olpidium spp. by its zoospore and resting spore morphology and its restriction to cucurbitaceous hosts, and from species of Pleotrachelus Zopf by the typical occurrence of a single exit tube per sporangium.

A comparison was made of isolates in culture of Rhizophydium granulosporum Scherffel, R. biporosum (Couch) comb. nov., R. chlorogonii (Serbinow) Jaczewski, R. haynaldii (Schaarschmidt) Fischer, R. laterale (Braun) Rabenhorst, and an unidentified species from water. All isolates grew saprophytically on algae and pine pollen and demonstrated great variability in their morphology on different substrates. An evaluation is given of the characteristics currently used for distinguishing species, and criteria of perhaps more discriminating taxonomic value are suggested.

Two isolates of Entophlyctis confervae-glomeratae (Cienkowski) Sparrow grew at 10–25C and in the range pH 6–9. Both isolates used ammonium, nitrate, and nitrite nitrogen. They used glucose, fructose, trehalose, glycerol, sucrose, cellobiose, maltose, and starch but not lactose, xylose, ribose, sorbitol, raffinose, rhamnose, melibiose, N-acetyl-D-glucosamine, or cellulose.