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During the past two years the integrity of Albugo candida as the only species of Albugo parasitic to Brassicaceae has been challenged. The existence of two distinct species parasitic to Brassicaceae has been confirmed, to which a third species was added. For the purpose of further exploring the diversity of the A. candida complex, eight Albugo specimens on Draba lasiocarpa, D. nemorosa, and D. verna (Brassicaceae) were morphologically and molecularly compared with other Albugo species. Based on sequence comparisons and thorough investigation of the characteristics of the oospores, especially surface ornamentation, Albugo voglmayrii sp. nov., parasitic to Draba nemorosa, is described from five specimens collected in Korea and China. It differs from the previously described species, A. candida, A. koreana, and A. lepidii, by its oospore wall ornamentation. The morphological discrepancy is supported by high genetic distances to other species of Albugo in ITS rDNA and cox2 mtDNA. Albugo specimens from D. lasiocarpa and D. verna were grouped with A. candida, revealing that two distinct species may cause white blister rust on the genus Draba. Therefore, the paradigms that: (1) there is only a single species parasitic to Brassicaceae, that (2) oospore morphology is useful only for distinguishing between largely unrelated species; and (3) in general only one species of Albugo may occur on a single host genus need to be discarded.

The oomycete ( Peronosporomycete ) Chlamydomyzium oviparasiticum , previously recorded as a parasite of rotifer eggs, was found infecting Rhabditis nematodes in a sample of rotting garden compost. For the first time C. oviparasiticum was cultured in liquid media, which enabled more detailed studies of zoospore behaviour and facilitated the use of confocal microscopy. Rhabditis nematodes were successfully re-infected from liquid-cultured inoculum. Light (including video) microscopy and transmission electron microscopy were used to document details of thallus development, zoospore release and resting spore morphology to enable comparison with other oomycete species. This species showed several significant saprolegnialian characters such as the ‘achlyoid’ pattern of spore formation, centrifugal cleavage and structured encystment vesicles. In contrast, spore release into a transient vesicle was a peronosporalean characteristic. The thick-walled resting spores showed relatively poor cytoplasmic preservation and had a thick multi-layered wall. It was still not possible to unequivocably decide whether these were chlamydospores or parthenogenically formed oospores. The phylogenetic significance of these observations is discussed.

Pythium attrantheridium sp. nov. is a new species isolated from cavity spot lesions of carrots as well as apple and cherry seedlings from various locations widely distributed in Canada and the USA. This fungus is closely related to the heterothallic P. intermedium, but is distinguished by: (1) unique molecular characteristics; (2) unique morphological characteristics; and (3) mating incompatibility with P. intermedium. The ITS region of the nuclear rDNA of all strains of P. attrantheridium studied is different from that of all other known Pythium spp. The oogonia attract a large number of antheridia when compatible mating types contact each other. The positive mating type produces zoospores unlike those of P. intermedium. Thus, biological, morphological and molecular data support the recognition of a new species.

Analysis of 18S rDNA sequence and morphological data indicate that Achlya s. lat. is polyphyletic. Aplanopsis spinosa and Achlya species with subcentric oospores and laterally applied antheridia are transferred to Newbya gen. nov. The relationship of Calyptralegnia and Aplanes androgynus to Newbya is discussed. Molecular data support morphologically based conclusions concerning the relatedness of Pythiopsis and Aplanopsis terrestris. Achlya truncatiformis nom. nov., is provided for material previously identified as Achlya recurva. The following new combinations within Newbya are made: N. pascuicola (type species), N. apiculata, N. braziliensis, N. curvicollis, N. megasperma, N. oblongata, N. oblongata var. gigantica, N. oligacantha, N. polyandra, N. recurva, N. spinosa, and N. stellata combs nov.

In several studies of oak decline in Europe, one semi-papillate (Phytophthora psychrophila sp. nov.) and two non-papillate homothallic Phytophthora species (P. europaea and P. uliginosa spp. nov.) were isolated, together with other Phytophthora species, from rhizosphere soil samples which could not be assigned to existing taxa. P. psychrophila differs from other semi-papillate species of Waterhouse's morphological Group IV, like P. ilicis and P. hibernalis, by its uniform, dome-shaped and cottonwool-like colony growth pattern on V8 juice agar and malt extract agar, the occurrence of sympodially branched primary hyphae, the high variation in size and shape of the sporangia, shorter pedicels, lower optimum temperature for growth, and ITS sequences. P. europaea is distinguished from related non-papillate Group V and VI species, namely P. fragariae, P. cambivora, and the ‘alder phytophthora’, by producing oogonia with tapered bases, irregular walls and exclusively paragynous antheridia, its cardinal temperatures for growth, and ITS sequences. P. uliginosa differs from related Group V and VI species by its large oogonia with exclusively paragynous antheridia, the predominant occurrence of ellipsoid sporangia with markedly wide exit pores, its slow growth, low cardinal temperatures, its colony growth patterns, and ITS sequences. P. uliginosa is separated from P. europaea by its larger oogonia without tapering bases, lower cardinal temperatures and growth rates, different colony growth patterns, and greater aggressiveness on Q. robur.

Evidence that the biometric characters of the oogonial unit of Pythium are of significant taxonomic value, allowing for a good species separation, has been presented by other workers, based on 40 species, each represented by fairly few isolates. Since broad intraspecific variation and interspecific overlapping of biometric parameters was observed in isolates of four Pythium species from Denmark, it was suspected that a such wide variability would weaken the conclusions obtained by previous biometric approaches. Species to be studied should be represented by higher numbers of isolates to reflect intraspecific variation. In the present biometric study 49 isolates were used to test the robustness of the biometric approach and to examine the taxonomic value of biometric characters from a practical perspective. Good species separation was obtained despite intraspecific variation. The biometric parameter set for the oogonium, consisting of oogonial, oospore and ooplast diameters, supplemented with the thickness of the oospore wall or the protoplast diameter and with derived indices of volume or linear ratios of the measured parameters were all of taxonomic value for the separation of species, as shown from canonical variate plots after linear discriminant analysis. More detailed information on species separation, isolate-species or isolate—isolate associations could be obtained by separation based on the Mahalanobis distance function in the linear discriminant analysis. Both the derived volume and linear indices contributed significantly to species separation and appeared to be equally satisfactory. The substrates used for oogonium production may influence the biometric parameters to an extent which may seriously affect the results of the biometric approach to taxonomy. These results stress the importance of standardizing substrates in morphological taxonomy and identification work.

The analysis of random amplified microsatellite (RAMS) markers of Phytophthora cactorum from strawberry showed that leather rot of strawberry fruits and crown rot are not caused by genetically different strains of this species. UPGMA-analysis showed that the populations of P. cactorum on strawberry differ between North America and Europe and that no clear genetic separation between isolates from different plants can be made among these populations. Slight morphological variation exists in the diameters of oogonia and oospores among the isolates but is insufficient for the identification of genetic groups or host specificity of P. cactorum isolates. Pathogenicity experiments proved that strains show a tendency towards host specialization.

The morphology of gametangia was examined in 43 pairs of isolates (mating types A1‹A2; 11 A1 and 24 A2 isolates; five isozyme}electrophoretic types) of Phytophthora cinnamomi. An amphigynous antheridium always formed with each oogonium. However, in 41 of the crosses a proportion (39 had 0‐2‐10% and two had " 30%) of oogonia also consistently had single or multiple paragynous antheridia. Single or multiple paragynous antheridia formed concurrently with amphigynous ones during the period of gametangial production in paired colonies. Where there were multiple paragynous antheridia associated with an oogonium, sometimes additional antheridia formed after fertilization or even after oospores were visible. Developmental studies showed that when meiosis in amphigynous and paragynous antheridia was simultaneous, fertilization tubes developed synchronously from both. However, cytological examination indicated that either a nucleus from an amphigynous or a paragynous antheridium fertilized the oosphere. Observations of paragynous and amphigynous, and amphigynous-only associations suggested that fertilization from either type of antheridium only occurred when meiosis in the oogonium was nearly synchronous with that of the antheridium. Asynchronous meiosis between oogonia and antheridia may contribute to failed fertilization and aborted oospore development. This appears to be the first description of paragynous antheridia in P. cinnamomi and the second observation of oogonia with both paragynous and amphigynous antheridia in a heterothallic Phytophthora species. Moreover, the development of both paragynous and amphigynous antheridia with an oogonium is rare in Phytophthora, as is the development of multiple antheridia. Antheridial variation is a characteristic to be taken into account in isolate identification. Nuclei from paragynous antheridia appear able to fertilize oospheres and therefore, have a role in sexual reproduction.

After purification by thin-layer chromatography (tlc) several fatty acids stimulated oospore formation in Phytophthora cactorum but not in Phytophthora parasitica. Palmitoleic acid was the most effective and was the only fatty acid tested that promoted oospore formation in P. parasitica. Purification of palmitoleic acid by Florisil column chromatography resulted in a two-fold increase in activity over that purified by tlc. When 0.01 to 100 ng of cholesterol was added to basal medium containing palmitoleic acid, the added cholesterol did not significantly change oospore production by P. cactorum. Oospore formation in P. cactorum was slightly stimulated by tlc-treated aliphatic hydrocarbons, or derivatives of hydrocarbons, but was strongly stimulated by geraniol and squalene. Vitamin A and vitamin A esters stimulated oospore formation in both P. cactorum and P. parasitica. tlc treatment also significantly increased the activity of cholesterol and β-sitosterol on oospore formation by P. cactorum. Both compounds did not stimulate oospore formation by P. parasitica even after tlc treatment.

Polyclonal antibodies were produced to purified oospores of Aphanomyces euteiches. The serum primarily reacted with plant pathogens of the genus Aphanomyces. The antiserum was used to detect oospores from naturally infested soils.

Stability of the phenotypic characteristics of growth rate, colony type, sensitivity to streptomycin and sensitivity to chloramphenicol of the conidium progeny of selfed oospore cultures of Pythium splendens was investigated. Progeny were divided into parental A type, D type for those deviated from A type but with uniform pattern and M type for those with irregular growth pattern or with dissimilar appearance on two replicate plates. Conidium progeny of A type isolate 167F and its A type oospore cultures showed the same colony type and displayed little variation in growth rate, and sensitivity to streptomycin and chloramphenicol. The D and M type oospore cultures displayed considerable variation in all phenotypic characteristics measured. Similar variation in conidium progeny following selfing was observed in isolate 167F-9, an A type oospore progeny of isolate 167F. Following sexual reproduction conidium progeny continued to vary in second and third conidium generations tested. The phenomenon of continuing variation in successive asexual generations induced by sexual reproduction may plan an important role in evolution, survival and development of pathogenicity and fungicide resistance of an organism by increasing the diversity and extent of the constitutive variation resulting from gene recombination during sexual reproduction.

Mycoparasitism by Pythium periplocum, a species with spiny oogonia, was evaluated and compared on several host fungi with parasitism by P. oligandrum and P. acanthicum on the same fungi. These species were found to be aggressive mycoparasites with a wide host range. Differential reaction among some of the hosts tested were found in relation to each mycoparasitic species. Thirty isolates of each of these species, obtained from diverse agricultural soils in California were further studied. Pythium ultimum, one of the most susceptible hosts for the three species, in an in vitro test of mycoparasite virulence, was slightly susceptible, moderately susceptible, and highly susceptible to most of the isolates of P. acanthicum, P. oligandrum and P. periplocum tested, respectively. In one test, sugar beet seedlings were protected from damping-off due to P. ultimum by co-inoculation of seedlings with P. periplocum or P. oligandrum. In the same in vitro test P. acanthicum gave no protection against P. ultimum. The degree of disease protection varied among isolates within each mycoparasitic species studied. As part of the study each mycoparasitic isolate was critically characterized by the morphology of oogonia, oospores, oogonial spines, sporangia and the rate of daily growth of mycelium at 25 °C. Pythium periplocum was readily separated from the other two species by its lobate sporangia and size of the oospore. Pythium oligandrum could be separated from P. acanthium by its acutate oogonial spine apex, longer spine and faster growth rate.

The relationships between isolates of the complex species Phytophthora megasperma from chickpea in Spain and from chickpea and lucerne in Australia were investigated. In the isolates from Spain > 90% of the sporangia had flattened apices, whereas in the Australian isolates only 5·6% of sporangia had conspicuously flattened apices, the remainder being convex. No differences were found between the sex organs of the two groups, except that oospore walls were thinner in the Spanish isolates (&- x = 2·2 ± 0·6 μm) than in the Australian collection (&- x = 3·4 ± 1·0 μm). The isolates from Spain had an optimum temperature for growth of ca 24°C and a maximum temperature range of 30–33°, whereas the Australian isolates had a higher optimum growth temperature (26°) and a higher maximum temperature range (33–35°). RAPD analysis of total DNA revealed a genetic dissimilarity of ca 80% between the two groups of isolates. The study therefore differentiated two genetically distinct groups among isolates causing chickpea root and stem rot in Spain and Australia. However, it is suggested that designation of a new taxon for the Spanish isolates should be deferred until all extant representatives of the P. megasperma complex have been fully molecularly and phylogenetically analysed.

A worldwide collection of 77 isolates of Phytophthora citrophthora from 30 different host species was investigated for variation in morphology and isozyme patterns. All isolates produced non-caducous, papillate sporangia. Only those isolated from cacao ( Theobroma cacao ) in Brazil produced chlamydospores, whereas cacao isolates from Indonesia did not produce chlamydospores. Over 70% of the isolates were sterile; the remainder could be designated as the A2 mating type based on pairings with P. capsici , though P. citrophthora itself did not produce oospores. The species was separated into three distinct isozyme subgroups: CTR1 containing isolates from widely distributed geographical locations on a broad range of hosts, CTR2 composed exclusively of Brazilian isolates from cacao, and a new subgroup CTR3 consisting of Indonesian isolates also isolated from cacao. A total of 14 enzymes and 17 putative loci were used to compare the isolates and seven loci were polymorphic. Peptidase was diagnostic for CTR1, malate dehydrogenase (MDH1) for CTR2 while the migration patterns for both glucose-6-phosphate dehydrogenase and hexokinase (HEX2) differentiated CTR3. A redescription of R. citrophthora is presented based on all 77 isolates including physiological, morphological and isozyme characters that can assist identification.

Mature (three-week) and immature (five-day) sexual progeny were raised from two matings between isolates of Phytophthora infestans with 2C-DNA content (and therefore presumed diploids) but which differed for isozyme genotype and restriction fragment length polymorphisms (RFLPs) in their mitochondrial DNA (mtDNA) and nuclear DNA (nDNA). Approximately 90% of the progeny which were derived from mature oospores, but approximately 50% of the progeny from immature oospores, were classed as hybrids on the basis of their Gpi 1 genotype. All single-oospore cultures, whether derived from a mature or an immature oospore, contained a single mtDNA type from either the A1 or the A2 parent. All single-zoospore lines derived from a single germ sporangium of a germinating hybrid oospore also had the same mtDNA type, Gpi 1 genotype and mating type. These data suggest that mtDNA is inherited uniparentally in P. infestans with no evidence for segregation, elimination or recombination of types. Mating type segregated differently in hybrid offspring; those with type II mtDNA (from the A1 parent) had a preponderance of A1 mating type. Some nRFLPs also segregated unexpectedly in the population with type II mtDNA, suggesting that any bias in the segregation of mating type is caused by viability disturbance rather than linkage between mating type gene(s) and mtDNA.

Worldwide collections of Pythium arrhenomanes and P. graminicola were studied morphologically and by examining variation in PCR-amplified rDNAs. Restriction patterns of the internal transcribed spacer and portions of the nuclear large-subunit rDNA were diagnostic for the two species. Isolates of P. graminicola shared identical restriction patterns. Most isolates of P. arrhenomanes exhibited identical restriction patterns; however, two genetic subpopulations, each with multiple isolates from a single geographic location, were detected. A phylogenetic analysis of the molecular data, which included two additional species, indicated that P. aphanidermatum was an outgroup and P. myriotylum was more closely related to P. arrhenomanes than was P. graminicola . No restriction site variation was detected among the four species in the nuclear small-subunit rDNA. Isolates of P. arrhenomanes , including both subgroups, differed from the other three species by one restriction site in the mitochondrial large-subunit rDNA. A mean of less than five antheridia per oogonium characterized isolates of P. graminicola , whereas five or more per oogonium characterized most isolates of P. arrhenomanes . The ranges of isolate means for oogonium/oospore diameter overlapped between the two species. Discriminant function analysis suggested that the number of antheridia per oogonium is more important than oogonium/oospore diameter in separating the two species. The rDNA results indicate that P. arrhenomanes and P. graminicola are distinct species. However, due to similar morphology and intraspecific variation, misidentifications of isolates of the two species have occurred.

Severe root rot of raspberry is caused in Europe and North America by a homothallic, non-papillate Phytophthora sp., which has been identified by different investigators as P. erythroseptica, P. fragariae or as a highly pathogenic variant of P. megasperma . Two collections of such highly pathogenic raspberry isolates from Europe and North America were compared with recognized specimens of the three above named species, which were originally isolated from potato, loganberry and strawberry, and a variety of perennial host plants, respectively. All such raspberry isolates, regardless of previous identification, formed an essentially homogeneous group with respect to colony and growth characteristics, the production and morphology of gametangia, the morphology and dimensions of sporangia, electrophoretic banding patterns of mycelial proteins and pathogenicity. They were readily distinguishable from recognized isolates of P. erythroseptica and P. megasperma with respect to cultural, morphological, and electrophoretic criteria. In contrast, they were very similar to isolates of P. fragariae from strawberry (and a single isolate from loganberry) although the two groups could be separated by differences in growth rate on some agar media, production of oospores in culture, and small differences in electrophoretic banding patterns, as well as in pathogenicity. It was concluded that the raspberry isolates should be assigned to P. fragariae , but that they should be given a subspecific epithet at the varietal level to distinguish them from strawberry isolates of the species i.e. P. fragariae var. rubi .

When mature 30-d-old oospores of Phytopthora cactorum were tested for germination immediately after harvesting from culture, 30% germinated in sterile glass distilled water and 90% in soil extract; however, after oospores had been stored at 2–4 °C in sterile distilled water for 2 or more weeks, germination was less than 1% in water, but increased to 80% in filter-sterilized soil extract, oospores exudate, mycelial exudate, and alfalfa root exudate. When similarily-treated oospores were incubated in solutions of amino acids, sugars, organic acids, various salts, urea, and adenine, oospores germinated (35–75%) only in the amino acid solutions. The maximum and minimum concentrations of alanine and glycine for stimulation of oospore germination were 10 and 0·1 m m , respectively. About 30–80% activation (erosion of the inner oospore wall without germination) occurred in solutions of sugars, nitrate salts, urea and in distilled water, but germination did not occur. When either sucrose or a mineral salt solution was added to alanine at 1 m m , the percent germination of oospores was higher than with alanine alone. Activated oospores in sucrose (10 m m ) germinated only when sucrose was replaced by alanine (10 m m ). The germination of oospores in undiluted and diluted soil extract (1:10) was 75% but in undiluted soil extract from combusted soil, germination was reduced to 56·6%, and in dilutions of 1:10 and 1:100 none of the oospores in combusted soil extract germinated.

Decreases in osmotic and matric potential caused a reduction in mycelial growth and oospore germination of Pythium oligandrum . The response of oospore germination to osmotic potential was different from that of growth, with maximum germination maintained over −0·5 to −2·0 MPa, ceasing between −3·0 and −3·5 MPa, whereas mycelial extension decreased below −0·5 MPa and ceased between −2·5 and −3·0 MPa. Reductions in growth were related to osmoticum in the order of glycerol > NaCl > KCl. In media where the matric potential was adjusted with polyethylene glycol (PEG 6000) both growth and oospore germination ceased at −2·0 MPa.

Some field isolates of Phytophthora infestans from England and Wales had a flat, adpressed, waxy appearance when grown in agar culture. These isolates formed very little aerial mycelium and few sporangia, but abundant oospores were formed on or below the agar surface. When these cultures were propagated following single-sporangium and single-hyphal tip isolation, white, fluffy colonies with aerial mycelium and abundant sporangia were formed. These propagations were usually self-sterile of either A1 or A2 mating type and did not form oospores. Occasionally a propagation from a single protoplasmic unit was self-fertile. When grown together self-sterile propagations of A1 and A2 mating type reformed self-fertile cultures. Self-fertile cultures have also been synthesised from A1 and A2 field isolates following co-cultivation on agar. Self-fertile isolates would therefore appear to be intimate mixtures of A1 and A2 hyphae. However, results also indicate that heterokaryons/heteroplasmons occurred at a low frequency. Other work on self-fertility reports less strict inhibition of sporangiogenesis and more frequent transmission of the character to asexual progeny.

Sexual progeny were raised from matings between 2C (diploid) × 4C (tetraploid), 2C × 2C and 4C × 4C isolates of Phytophthora infestans. The percentage germination of oospores and establishment of colonies were generally lower in the 2C × 4C and 4C × 4C crosses than in the 2C × 2C cross. Isoenzyme analysis of progeny from some matings revealed that most were hybrids but that the proportion of selfs was generally higher in 2C × 4C crosses. The fluorochrome DAPI was used to determine DNA contents in samples of all progenies. Hybrids from the 2C × 2C cross were all 2C whereas those from 2C × 4C crosses ranged from 2C to > 3C. Progenies from the 4C × 4C crosses ranged from 2C to 4C.

When different mating types of Pythium splendens were paired on opposite sides of a polycarbonate membrane, numerous oospores were produced by (+) but not (−) isolates, indicating that sexual reproduction of this fungus is a result of selfing of the (+) isolate in response to the stimulation by a hormone produced by the (−) isolate. Hormonal regulation of sexual reproduction in P. splendens was greatly affected by culture age and different isolates also differed in ability to produce hormone and to form oospores in response to the hormone. Four single-conidium subcultures of (+) isolate 461 of Pythium splendens produced oospores in sectors on fresh V-8 agar after being stored in sterile distilled water for 48–93 months. Single-spore cultures obtained from conidia produced on oospore sectors consisted of 293 (+), 3 (−) and 3 (±) type. All the 36 single-propagule colonies obtained from a stored culture originated from spherical structures smaller than conidia produced on V-8 agar. The mating type distribution of these colonies were 33 (+), 1 (−) and 2 (±). Single-conidium cultures from the ± type behaved either as (+) or (−). Hormone production and reception of the (−) variant were characteristic of the (−) wild type. Results suggested that oospore formation in single cultures by aged P. splendens is due to the appearance of the opposite mating type during the storage period and the (±) type is a transitional state in the process of mating type change.

In a cytological study, some isolates of Phytophthora meadii were found to be predominantly polyploid and to show normal oospore development, while others were predominantly diploid. The diploids produced mainly small oogonia, which degenerated during meiosis with the production of many empty oogonia; but also some larger oogonia with polyploid nuclei and normal oospore development. It is therefore suggested that P. meadii is derived from a species hybrid in which meiosis is upset by the genetical imbalance; and that chromosomal doubling during vegetative growth, followed by selection, leads to the production of tetraploid forms which can undergo normal meiosis. No differences were found in the soluble protein patterns of diploid and polyploid isolates. While both A1 and A2 compatibility types were found in each ploidy group, the polyploid A2's (presumed compatibility type heterozygotes) tended to be weakly self-fertile, and one of them gave rise to a self-sterile polyploid A1 culture. This demonstrates the difficulty of maintaining the heterozygous condition during vegetative growth in a polyploid. The possible role of polyploidy in the apparent decline of the A2 type in Sri Lankan P. meadii populations is discussed.

One hundred and sixty-two isolates representing worldwide sources of Phytophthora cambivora, P. cinnamomi and P. cactorum were compared using 18 isozyme loci separated by starch-gel electrophoresis. Based on isozyme analysis, the three species were clearly separated and each was further subdivided into electrophoretic types (ETs). No evidence suggests a close relationship between P. cambivora and P. cinnamomi. P. cambivora separated into eight ETs. One ET was the most common worldwide; there were four unique ETs from Australia, one from Japan and two from the U.S.A. No distinguishing morphological features were associated with the different ETs. P. cinnamomi also divided into eight ETs. The two ETs, which represented all the A2 isolates used in the study, could be distinguished from the majority of the A1 isolates based on the migration pattern of isocitrate dehydrogenase. Only one exception to this was seen in an isolate which was reported to have switched mating types from A2 to A1. P. cactorum was divided into two ETs, one common and the other represented by two isolates from strawberry from New York. These two isolates also differed in their sporangial dimensions and oospore pigmentation. The potential of cellulose acetate electrophoresis was evaluated as a rapid diagnostic method. Three enzymes — phosphoglucose isomerase, malate dehydrogenase and lactate dehydrogenase — were found to be diagnostic, thus allowing clear differentiation of the three species.

Isolates of Phytophthora infestans of A1 mating type from central Mexico and Wales, when mated to a range of A2 isolates from Russian and Mexico, produced a proportion of abnormal oogonia with several oospheres. Similar oogonia were observed in self-fertile lines resulting from treatment of a Russian A1 isolate with a chemical mutagen followed by the fungicide, metalaxyl.

Pythium oligandrum was grown in static and aerated culture in a liquid medium containing 30 g l −1 cane molasses. A switch from vegetative growth to sexual reproduction occurred 7 d after inoculation leading to mass production of oospores at the expense of existing mycelium, so that after 18–21 d biomass mainly consisted of oospores. These survived air-dried at 5–30 °C for at least 16 weeks. The relevance of these findings to commercial oospore production using waste materials is discussed.

This paper provides the first evidence of A2 type 1 and type 2 isolates of Phytophthora cinnamomi producing selfed oospores in planta in an Australian soil and in a potting mix. Oospores were observed in infected lupin (Lupinus angustifolius) roots incubated for 7d either in the substrate under potted Acacia pulchella plants, or in soils collected from under and near varieties of A. pulchella in jarrah (Eucalyptus marginata) forest. The A2 type isolates varied in their ability to produce selfed oospores and none were produced by A1 isolates. The gametangial association was amphigynous and spores were predominantly spherical with diameters from 13-40 microm. The oospores were viable but dormant. Two A2 type isolates produced small numbers of selfed oospores with amphigynous antheridia axenically in Ribeiro's liquid medium within 30 d, and one A2 type 2 isolate produced oospores after mating with an A1 strain. Evidence is presented that the presence of roots of Acacia pulchella, and particularly A. pulchella var. glaberrima and var. goadbyi, enhances the production of oospores.

Vegetative growth rate, and size of sporangia, chlamydospores and oospores from 94 P. ramorum isolates were measured and the isolates were paired in vitro with four different heterothallic Phytophthora species isolated from infected nursery plants in Germany. P. ramorum isolates originated from different European countries and from Canada and the USA. 66 of the 67 European isolates were determined as mating type Al; only one isolate was of mating type A2. Of the 27 North American isolates tested, seven (all from nurseries) were determined to be the Al mating type and 17 to be the A2 mating type. Three isolates did not produce gametangia during the incubation period. Discriminant analysis of all data allowed a grouping based on the vegetative growth rate. The two groups corresponded with the mating type no matter whether the isolates originated from Europe or North America. The Al isolates were much more homogeneous in their morphology than the A2 isolates. They grew faster, had larger chlamydospores and did not produce gametangia with P. cambivoro Within the A2 group, the single European isolate of mating type A2 (BBA 16/02) and three US isolates showed intermediate characters and were classified with the discriminant function into that of the opposite mating type. The morphological characters and the mating behaviour of the isolates will be discussed.

Reliable production of infective spores is an essential requirement in the screening of putative biological control agents. We report the development of a novel and simple medium for the production of viable oospores of Aphanomyces cochlioides, one of the principal causes of damping-off of sugar beet. The semi-solid medium consists of 5 g l−1 corn meal agar and 1 g l−1 mycological peptone and supports the development of 2·25 × 105 oospores cm−2 in 5 d. In a plant bioassay for post-emergence damping-off, the oospores had a consistent pathogenic effect with 50 ± 5·7% damping-off induced by 170 oospores g−1 soil (− 15 kPa matric potential) after 20 d. Higher numbers (1700 g−1 soil) produced 88·8 ± 9·1% disease in emerged plants after 20 d.

A Phytophthora pathogen of trees and shrubs previously designated Phytophthora sp. O-group is formally named as P. inundata sp. nov. P. inundata falls within the P. gonapodyides-P. megasperma major ITS Clade 6, its present nearest known relative being P. humicola. It has non-papillate sporangia, fairly large oogonia (average ca 40 microns) with thick walled oospores, amphigynous antheridia, a distinctive colony type, a high optimum temperature for growth of 28-30 degrees C, fast growth at the optimum, and a high upper temperature limit for growth of ca 35-37 degrees. A study of the breeding system of eight P. inundata isolates showed them to be classically heterothallic with A1 and A2 compatibility types. However some P. inundata A1 x A2 combinations failed to mate even though the same isolates mated successfully with P. drechsleri testers. Others were 'silent' A1s or A2s, unable to produce their own gametangia but able to induce gametangial formation in the opposite sexual compatibility type of another species. This indicates a partial breakdown of the sexual mechanism in the species. Two isolates (one A1 and one A2) were unpredictably and chimaerically self-fertile, suggesting A1 + A2 chromosomal heteroploidy. The association of P. inundata with ponds and rivers and with root and collar roots of trees and shrubs after flooding is discussed.

The lenticular-shaped bodies about 4–8 m diam, which have been called pellucid spots (bodies) in oospores, were clearly differentiated in Phytophthora capsici by light microscopy with the nuclear stains iron haematoxylin, acetocarmine and methyl green-pyronin Y, and by phase contrast microscopy of unstained oospores treated with Herr's clearing agents and fixative. Two nuclei in oospores were observed to fuse which indicated that apparent karyogamy had occurred. In three homothallic taxa, Phytophthora megasperma f.sp. medicaginis, P. megasperma f.sp. glycinea and P. cactorum , oospores germinated only after nuclei had fused. In young oospores (less than 19-d-old) the percentage of oospores in which nuclei had fused was lower than in older oospores (more than 25 d). Also, the percentage of germination of young oospores was much lower than of older oospores. Fusion of nuclei in P. megasperma f.sp. glycinea oospores occurred much later than in P. megasperma f.sp. medicaginis and P. cactorum . This was correlated with a lower percentage of oospore germination of P. megasperma f.sp. glycinea compared to that of P. megasperma f.sp. medicaginis and P. cactorum . Nuclei in oospores of P. megasperma f.sp. glycinea were shown in vivo by time lapse photomicrography to migrate toward each other, make contact, and fuse prior to germination which indicated karyogamy. Use of the nuclear stain, aceto-orcein, on 40-d-old oospores showed that the numbers of nuclei per oospore varied from zero to eight.

The ultrastructural architecture of the oospore-oogonium walls of Phytophthora megasperma was studied in the soybean (f. sp. glycinea ) and alfalfa (f. sp. medicaginis ) isolates with the major objective of elucidating changes that occur during oospore germination. For comparison, ultrastructural changes in isolated oogonium-oospore walls brought about by digestion with various β-glucanases were also examined. Major emphasis was placed on determining the structure of the conspicuous inner oospore wall (IOW). This thick (2–3 μm) layer showed little staining by any of the procedures used, including staining for polysaccharides by the Thiery procedure. There were only minor differences in electron density across the IOW. Observations on sections of whole oospores and mechanically fractured, or partially digested, walls indicated that the IOW contains microfibrils embedded in an amorphous matrix which tend to be oriented parallel to the plasma membrane. During oospore germination, the IOW is extensively eroded in a characteristic ‘scallop’ pattern that creates numerous cavities separated by radial strands of undigested wall. The electron dense outer oospore wall (OOW) remained intact and unaltered and probably confines the activity of the wall digesting enzymes to the IOW. In the digested cavities, removal of the wall matrix exposed the network of underlying microfibrils together with some electron dense globular material. Treatment of isolated oogonium-oospore walls with exo-β-1,3 glucanase or a mixture of endo-β-1,3-glucanases caused substantial digestion of the oogonium wall matrix and the IOW. Following enzymatic digestion, both the IOW and, in particular, the oogonium walls showed greater reactivity with the Thiery procedure. The observed patterns of wall digestion indicate that the amorphous β-1,3-glucan component of the IOW is extensively consumed during germination and is probably the main supply of carbon and energy for germination. No evidence of a pre-formed germination wall layer in the oospore was found. As the innermost layers of the oospore wall became digested, a new wall was produced internal to the IOW.

Thirty-three laboratory isolates of Peronospora parasitica , 27 from the U.K. and the others from the U.S.A. and China, were examined for ability to produce oospores in monoculture. Oospores were found in host tissue infected by 1 of the 5 B. campestris isolates, 2 of the 12 B. oleracea isolates, and all 14 of the B. napus isolates, but not by the sole B. juncea isolate. The predominantly homothallic nature of the B. napus isolates was unexpected on the basis of earlier surveys. Repeatedly single sporing a B. napus isolate did not resolve separate mating types. This isolate produced oospores in abundance in B. napus hosts, some in B. campestris and few in B. oleracea hosts. Homothallic behaviour was not induced in self-sterile B. oleracea isolates of P. parasitica when grown on B. napus hosts. A survey of three field crops of oilseed rape found oospores throughout the 1985–6 season. Self-sterile isolates fell into one of two mating-type compatibility groups, provisionally designated P1 and P2.

Growth and sexual reproduction of Phytophthora cactorum were studied in medium with a fixed amount of sucrose (10 g l −1 ) but varied amounts of several nitrogen sources. As compared to glutamate and ammonium (buffered with succinate), growth with glycine was delayed, but the growth rate and total biomass eventually produced were similar with these three nitrogen sources. Oospore production was similar with glutamate asparagine and glycine, declining as nitrogen increased from 2·5 to 20 m m ; no oospores were produced with 40 m m nitrogen. Oospores were produced on medium with 40 m m ammonium nitrogen when the initial pH was 6·5, but none with 20 m m at pH 4·5. Nitrogen was exhausted from the medium before the sugar, when the initial nitrogen concentration was 10 m m or less. Oogonium induction occurred on the exhaustion of nitrogen, and the presence of excess sugar in the medium at this time favoured oospore production. Medium-replacement experiments (mycelium grown with 30 m m nitrogen and transferred to N-free solutions) showed that solutions with salts (including Ca 2+ ), sucrose and succinate were especially favourable to oospore production. Mycelium increased in dry weight in replacement solutions containing sucrose, but succinate did not contribute to dry weight, although labelled succinate was shown to be metabolized to CO 2 . Oospore development thus depends on the presence at the time of induction of medium components which contribute to dry weight and supply energy. Sporangia of P. cactorum became numerous in cultures in the dark only after the sugar was exhausted from the medium, and their production was inhibited in replacement solutions containing sucrose or glucose.