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is an important root pathogen of several legume and forage crops worldwide. Disease management is challenging due to resilient oospores that can persist in the soil for long periods of time. Several research groups are working towards incorporating disease resistance into crop cultivars involving large-scale disease resistance screening and, for some crops, the use of molecular markers associated with quantitative trait loci for resistance. The refinement of laboratory protocols for efficient pathogen isolation, reliable and simple zoospore production, and a repeatable indoor screening assay are important tools needed to further research in this area. The standardization of these protocols among research groups would provide an efficient platform for research collaboration. The effect of different media and sporulation induction methods on zoospore production was assessed. The most effective and repeatable method of A. euteiches zoospore production involved culturing and incubation for 4 days on autoclaved wheat leaf segments placed on corn meal-yeast extract-phosphate buffer agar. Colonized wheat leaves were transferred into 100 mL of distilled water in 250 mL flasks and incubated at 100 rpm for 18 h at 24ºC to induce zoospore production. Five zoospore concentrations ranging from 100 to 10 000 zoospores mL−1 were evaluated using susceptible and partially resistant pea and lentil genotypes to optimize zoospore concentration for germplasm screening. The best differentiation of levels of resistance among lentil and pea genotypes was achieved with 1000 zoospores mL−1.

Impatiens downy mildew is caused by Plasmopara obducens, a pathogen known in the United States for over a hundred years, but newly attacking ornamental Impatiens walleriana in production and in the landscape. Little is known about the life cycle of P. obducens; thus, in this study an attempt was made to determine whether the pathogen is homothallic or heterothallic. Fourteen single-sporangium isolates and three single-zoospore isolates were used in single and dual inoculations of stem tissue to see whether the pathogen was homothallic or heterothallic; all isolates tested were able to produce oospores when inoculated singly, suggesting homothally. Cold treatment at 0 C for at least 1 mo induced oospores to germinate and produce primary sporangia. Inoculation of plant tissue with germinating oospores resulted in infection. Other incubation temperatures (-10, 10, and 20 C) did not induce germination, but fluctuating temperatures (between -10 and 0 C, or 0 and 10 C) induced some germination. Spores incubated at -10 C had significantly thicker walls than spores incubated at other temperatures. Evidence suggests that oospores can serve as an overwintering stage.

A new species of Vaucheria was isolated from the Santa Gertrudis River in Oaxaca, Mexico. This taxon was distinguished from the other six species in the Pseudogeminata group of section Racemosae by wider siphonous filaments and antheridia of a larger diameter, lack of septa in the gametophore, broader oogonial beak, erect oogonia and oospores that lacked yellowish spots. This species was named Vaucheria zapotecana sp. nov. The phylogenetic relationship of this species was investigated by analyzing a 2022-base-pair sequence that included the rbcL gene; results revealed a close relationship among this species, V. pseudogeminata and other species of the section Racemosae; although, the sequence for V. zaptecana is distinct.

Red raspberry (Rubus idaeus) production is a vital component of northwestern Washington's agriculture. The main objectives of this study were to document the occurrence of soilborne pathogens Phytophthora rubi and Pratylenchus penetrans in early stage production fields, relate this information to soil properties, and better understand the individual and combined effect of P. rubi and P. penetrans on raspberry root health. P. rubi was found at each field and P. penetrans population densities were variable (0 to ∼8000 nematodes/g dry root) across locations. In controlled greenhouse studies, P. rubi was very pathogenic to red raspberry ‘Meeker’ at densities >10 oospore/gram soil and there was no interaction between P. rubi and P. penetrans. P. rubi is endemic to raspberry production in this region, and is an aggressive pathogen on raspberry. However, the chronic damage to roots caused by P. penetrans should not be ignored.

Matured botanical compost of Croton sparsiflorus and Azadirachta indica were evaluated for promotion of sorghum downy mildew at the rate of 0.5% (w/w). Exceptionally high promotional rates of 26.9% (C. sparsiflorus) and 30.1% (A. indica) were observed as compared to negative control (8.8%). The enhancement of disease was higher than that obtained by the supply of recommended dose of urea (13.3%). As the pathogen (Peronosclerospora sorghi) is a biotroph and cannot multiply by using organics as nutrient source in the absence of a host, the mechanism of disease enhancement is inferred to be through stimulation of oospore germination by organic compounds.

A new species of Pythium isolated from soybean in Ohio is described. Pythium delawarii sp. nov. is characterized by globose internally proliferating sporangia, aplerotic oospores and diclinous antheridia that make broad lengthwise contact. Sporangia produce conspicuous papilla and germinate indirectly by producing zoospores via a vesicle and proliferate internally or the sporangia germinate directly with either one or more germ tubes. Pythium delawarii is pathogenic on soybean causing damping-off of seedlings. This oomycete can grow at 10-34 C with an optimum of 28 C. The sequence of the ITS1, 5.8S and ITS2 region of the rDNA did not match the sequence of any known Pythium species but was similar to P. citrinum, P. litorale and P. sterilum. P. delawarii can be distinguished from these three species based on the presence of aplerotic oospores and diclinous antheridia and the absence of hypogynous antheridia. Therefore biological, morphological and molecular data support the recognition of a new species.

Leaf blight and corm rot caused by Phytophthora colocasiae are the most devastating diseases of taro. Oospores of P. colocasiae have not been considered important in the survival in natural soil because the pathogen is heterothallic and there is essentially no chance for the presence of both A1 and A2 mating types in the same host tissue. During our recent survey of the mating type distribution of P. colocasiae in Taiwan seven homothallic isolates of Phytophthora were obtained from diseased taro leaves at Tsu Chi in central Taiwan. These organisms were identified as P. colocasiae based on morphological characteristics, ITS sequence homology and pathogenicity to taro plants. The homothallic isolates of P. colocasiae segregated into A1 and A2 types in addition to the original A1A2 type during asexual reproduction and vegetative growth. The homothallic isolate and the mixture of its A1 and A2 segregants produced abundant oospores in live tissue of taro petioles on or away from soil, indicating the possibility of oospores as a survival structure and the source of genetic variation in certain areas in nature.

Oospore characteristics are described for charophyte species in New Zealand and a simple key to species is presented, based on consistent oospore features. Oospore dimensions measured using a light microscope and an image analysis system showed that some taxa could be distinguished on size. The ornamentation on the outer wall of oospores was described from both light microscopy and scanning electron microscopy and was particularly useful for distinguishing Nitella species, while striae number and basal features were helpful in identifying the remaining genera. Species oospore ornamentation is compared with the findings of previous workers and the need for revision of some species in New Zealand is raised. Taxonomic uncertainties involve New Zealand records for Nitella cristata, whereas intraspecific variations in oospore ornamentation within N. hookeri and N. pseudoflabellata indicate that additional taxa should be recognised.

The taxonomic status ofNitella comptonii (Charales, Characeae) is not well understood because this species has not been recorded since the early 20th century, and detailed examinations of fully mature individuals have not been performed. Recently, we collected fully matureN. comptonii from the Okinawa Islands in Japan. These exhibited a distinctive fertile spherical head in the axils, and their vegetative and reproductive structures were consistent with those in the original description of the species. In 1965 in his ‘A revision of the Characeae’, R. D. Wood assignedN. comptonii as a subspecies ofN. pseudoflabellata. In our study, scanning electron microscopic analysis ofN. comptonii oospores revealed that external and internal oospore morphology ofN. comptonii was clearly different from that ofN. pseudoflabellata. In addition, multiple DNA phylogeny using the concatenated sequences of the chloroplastrbcL,atpB, andpsaB genes, and nuclear 5.8S ribosomal RNA gene and internal transcribed spacer reg...

A new species, Pythiogeton zizaniae, was isolated from diseased water bamboo (Zizania latifo- lia) in central Taiwan. The organism formed a colony with scanty mycelia and mycelial aggregates on rye- water bamboo medium. Special treatments were required for production of sporangia which were terminal, noncaducous and mostly ovoid. Chlamydo- spores were absent. The fungus was homothallic. Oogonia produced on V-8 water bamboo medium in water were mostly globose to subglobose and each was attached with a club-shaped, monoclinous antherid- ium by the base of the oogonium stalk. Oospores were plerotic and globose to subglobose. Py. zizaniae caused death of water bamboo suckers but did not infect seedlings of corn, rice, wheat, sorghum, cucumber, tomato, soybean or water spinach. It also did not affect cucumber and tomato fruit, carrot roots or potato tubers.

A damping-off disease of wheat was shown in a wheat field in Kidwan village, El-Minia city, Egypt, during December, 2000. Pythium diclinum was the causal agent of such disease and this is the first reported work of its isolation as a disease to wheat. Wheat seedlings collected from that field showed browning lesions at the basal part and wilting followed by damping-off. Examination of root pieces and other infected parts yielded only Pythium diclinum. The pathogen was characterized by its typically filamentous zoosporangia, diclinous antheridia and aplerotic thick-walled oospores. Pathogenicity of this fungus was determined on wheat under greenhouse conditions and P. diclinum was proved to be pathogenic on wheat. Two isolates of each of Gliocladium roseum and Trichoderma harzianum were tested for their bio-control activity against damping-off disease of wheat caused by P. diclinum. Incorporation of G. roseum or T. harzianum isolates into carboxymethylcellulose seed coating successfully eliminated pre-emer...

Six taxa of Nitella (Charales, Charophyceae), collected from Asia, were investigated using light and scanning electron microscopy (SEM) for the oospores, and sequencing of the gene encoding the lar...

Brevilegnia ensenadensis sp. nov. is described from litter (floating twigs, leaves, and roots) in a man-made, polluted channel near a petroleum refinery, in Buenos Aires Province, Argentina. The species is illustrated and compared with other species of the genus; it can be identified by the preponderance of smooth or papillate oogonia, with a distinctive irregular inner wall, borne in bent or coiled oogonial stalks, which are terminal, lateral or of the intercalary position. The oogonia often are irregular in shape, and the oospheres mostly are immature. The single oospore is eccentric inside the great oogonia, and the species develops mainly diclinous antheridial branches and has fewer monoclinous and androgynous ones.

Cauliflower (Brassica oleracea var. botrytis) with a root rot disease were found in a field in Kidwan village, 3 km south of El‐Minia city, Egypt, during January 2001. Cauliflower plants showed wilting soon after transplanting. In addition, cauliflower seedlings collected from the same field showed browning lesions at the basal part of the stem and the root system and eventually led to wilting. Examination of root pieces and other infected parts yielded only one type of fungus characteried as Pythium ultimum var. ultimum by its typically saclike monoclinous and diclinous antheridia and aplerotic oospores. P. ultimum var. ultimum was shown to be pathogenic on cauliflower under glasshouse conditions. This is the first report of P. ultimum var. ultimum causing disease to cauliflower in Egypt. Bacteria producing antibiotics against the root‐rot fungus Pythium ultimum var. ultimum were selected. Agar discs from rhizosphere soil of cauliflower containing colonies were transferred onto water agar plate ...

A new species of Pythium isolated from wheat and apple roots in eastern Washington is described. Pythium abappressorium sp. nov. is characterized by abundant appressoria. Plerotic oospores and sporangia are formed from the appressoria and remnants of the appressoria remain attached to the base of sporangia at maturity. Smaller appressorial swellings, reminiscent of hyphal swellings, are also formed within the appressoria. Pythium abappressorium is pathogenic to wheat, causing damping-off and stunting, but is not pathogenic to apples. The fungus can grow in the temperature range 5 to 30 C, with an optimum of 20 C. The sequence of the ITS1 region of the rDNA did not match the sequences from a worldwide collection of over 1200 isolates, including types and neotypes, suggesting that this species has not been previously described.

Complex and defined media have been previously proposed for production of oospores of Pythium oligandrum , a fungal mycoparasite of several disease-causing fungi. However, oospore production in synthetic media requires long periods of incubation and yields lower oospore numbers than in complex media. Moreover, although complex media produce high oospore yields, these yields are not reproducible because of the variability in the composition of complex nutrient sources. In the present study, the average composition of molasses reported in the literature was utilized as a base to develop a new defined medium for P. oligandrum oospore production. Forty-two substrates defined in nine stock solutions: carbohydrates, vitamins, sterols, non nitrogenous acids, amino acids, minerals, nucleic acid bases, CaCl 2 and MgSO 4 were tested at two levels (present or absent) in two fractional factorial designs. Each of these nine variables had a significant main effect on oospore production. Furthermore, the effect of each ...

Achlya fuegiana sp. nov., occurring on lit- ter (floating dead leaves and twigs) in Laguna Verde stream, in Tierra del Fuego Province (Argentina) is described, illustrated and compared with similar spe- cies. The species produces principally papillate oo- gonia and androgynous antheridial branches. Im- mature oospheres, oogonial wall and oospores are distinctly yellowish. This Argentine species differs from other known ones with centric-subcentric oo

Achlya ambispora sp. nov. occurring on litter (floating dead leaves and twigs) in Las Cotofla stream, Tierra del Fuego Province, Argentina, is described and illusbated and cornpared with similar species. The species produces principally androgynous and monoclinous, rarely diclinous antheridial branches; oogonial wall and oospores are distinctly yellowish at maturity and differ from other species in having also 1–6(-22) centric-subcentric oospores, in oogonia somethes apiculate, that are frequently intercalary or proliferate in chains., Instituto de Botánica "Dr. Carlos Spegazzini"

The sediment depth limits to germling emergence were determined for five common New Zealand submerged plants. A significant trend was observed of declining emergence with increasing burial depth. Emergence from small oospores (13–16 μg) of Nitella pseudoflabellata A. Br. and Nitella leptostachys A. Br. was limited to the surface sediment (

Aphanomyces euteiches is a phenotypically diverse pathogen, but the origins of this diversity are not known. To determine if cross-fertilization occurred between pea and bean pathotypes of A. euteiches, a technique to outcross isolates of A. euteiches was developed. Random amplified polymorphic DNA (RAPD) markers and several phenotypic traits were used to identify F, hybrids. Following 16 matings among isolates of different pathotypes, 4 of 82 germinated oospore isolates were inter-pathotype F, hybrids. Two of the four hybrids were infertile, but two were fertile and used to analyze the inheritance of virulence phenotype, host specificity (pathotype), hyphal growth rate, size of the aplerotic zone, and RAPD markers. As compared with parental pea and bean pathotype isolates, F, hybrids exhibited a pea pathotype and intermediate phenotypes in hyphal growth rate and size of the aplerotic zone. Subsequent self-fertilization of F1 hybrids yielded an F2 population that demonstrated segregation of virulence phen...

Variation in virulence, host specificity, hyphal growth rate, and size of the aplerotic zone were studied in Aphanomyces euteiches using successive single-zoospore progeny (asexual reproduction) and single-oospore progeny from selling (sexual reproduction). No variation in virulence or host specificity to pea or bean was found in generations of single-zoospore or single-oospore isolates derived from four field isolates. In each isolate, hyphal growth rate and size of the aplerotic zone were relatively stable traits among six generations of single-spore progeny; the phenotypes of progeny remained similar to parental phenotypes. The traits were found to be the same within, but different between, two pea and two bean pathotype isolates. Stable randomly amplified polymorphic DNA (RAPD) markers associated with different isolates were selected in successive generations of zoospore and selfed oospore progeny and could be used to distinguish the four isolates. Results suggest that the genes involved in determinin...

Development of sorghum downy mildew, incited by Peronoscleospora sorghi (Weston and Uppal) C.G. Shaw, on maize, sorghum and Johnson grass was investigated at two locations in Uganda during three seasons (1994 and 1995). More sorghum downy mildew developed on the Johnson grass and sorghum than on the maize at all locations and in all seasons. No significant differences were observed in sporulation of P. soghi on the three hosts. Leaf shredding occurred on the three hosts but was the least on maize. Cross-inoculation with both conidia and oospores was achieved on the three hosts. Since the fungal population formed oospores and sporulated readily on the three hosts, which is typical of the sorghum strain, the disease in Uganda is attributed to the sorghum strain.

Studies were conducted in potato fields on Prince Edward Island (PE) and New Brunswick (NB) to determine the overwintering viability, germination, and infectivity of oospores of Phytophthora infestans. Oospore viability was determined using plasmolysis (4 M NaCl) and tetrazolium bromide (MTT) tests. After seven months exposure to winter conditions in potato fields, the viability of oospores buried on PE and NB ranged from 2.5 to 13% and from 1 to 28%, respectively, with the plasmolysis test and from 13 to 44% and from 2.5 to 40%, respectively, with the MTT test. In other studies on PE, oospore viability ranged from 5 to 15% 12 months after burial. In laboratory studies, oospores exposed to temperatures of -20 and -50°C for up to 90 days had viabilities greater than 50%, whereas exposure to 0, 4, or 15°C resulted in average viabilities from 23 to 36%. The lowest viability, 22%, was recorded at the highest temperature tested, 36°C. Oospore germination was low, ranging from 6 to 19%. However, this level was ...

Isolates of Phytophthora infestans were collected from British Columbia at various times during 1993-1997 and from eight other provinces in Canada during 1993-1995 and characterized for mating type (either Al or A2), growth response to metalaxyl (either sensitive-MS or insensitive-MI), and random amplified polymorphic DNA (RAPD) patterns. Among the isolates tested, 312 were from diseased potato cultivars, while 14 originated from tomato or hairy nightshade plants with blight symptoms. A composite of RAPD patterns generated by nine primers following PCR amplification was used to identify novel genotypes based on presence of polymorphic bands. The occurrence in Canada of genotypes that corresponded to US I, US 6, US 7, and US 8 was confirmed during this study. The RAPD primers also identified 15 novel genotypes from British Columbia (BC) and 4 from New Brunswick, regions in which both Al and A2 mating types of P. infestans have previously been found and oospore production reported. In 1993, genotypes US 6 a...

Pythium mastophorum, not previously recorded in New Zealand, was isolated from soil from two Auckland strawberry‐growing sites. The oospores have characteristic regular conical mammiform spines, readily distinguishing it from Pythium spp. other than P. polymastum. A brief study of these isolates was carried out to determine how the pathogenicity to vegetable seedlings of P. mastophorum compared with that of other Pythium spp. They were found to be moderately pathogenic to a range of these seedlings.

Members of the Characeae produce a resistant, thick-walled zygote called the oospore. Oospore characters have been used in taxonomic treatments of species of the two most diverse genera in the family, Nitella C.A. Agardh and Chara Linnaeus. However, the taxonomic value of oospore characters depends upon their morphological constancy within species. In order to determine the amount of oospore variation within species and populations of Chara, oospores of Chara australis R. Brown, C. fibrosa C.A. Agardh ex Bruzelius sensu Wood (1962), and C. muelleri A. Braun were examined using scanning electron microscopy. Chara australis, a dioecious perennial species, displayed a large amount of intrapopulation variation in the length, width, and number of striae of its oospores, but the ultrastructure of the oospore wall was consistent within and among populations. Chara muelleri, a monoecious species of temporary habitats, exhibited less within-population variation, and the ultrastructure of the oospore wall ...

The constraints upon pearl millet ( Pennisetum glaucum (L.) R. Br.) cultivation were investigated in four regions in southwest Niger. Drought and low soil fertility constitute the principal problems according to farmers. Pearl millet downy mildew ( Sclerospora graminicola (Sacc.) Schroet.) is one of the possible biotic constraints. Its occurrence and epidemiology were studied in two regions in southwest Niger with different mean annual rainfalls. Disease incidence ranged from 7.6% to 42.0% in the area with highest rainfall and from 0% to 16.4% in the drier region. Disease severity ranged from 4.3% to 24.8% and from 0% to 8.5%, respectively, in the two regions, which gave a good indication of estimated crop loss. Oospore density in the soil was highly correlated with disease incidence at 90 days after sowing ( r = 0.83, 18df) indicating the important role of oospores in disease epidemiology. Soil organic C content, pH(H2O) and loam fraction showed significant correlations with disease incidence ( r = 0.70,...

Asexual and sexual reproductive structures of Peronospora viciae, which causes downy mildew of pea (Pisum sativum), were studied using low temperature scanning electron microscopy. Sporangiophores ...

Pythium plurisporium sp. nov. was isolated from diseased roots and crowns of bentgrass (Agrostis palustris) from two golf courses in North Carolina. The species is distinguished from other Pythium ...

A population of Cakile maritima (Cruciferae) growing locally in Baltim and Rashid areas at the northern part of Nile Delta, Egypt was observed to be infected with the downy mildew fungus, Peronospora parasitica. This is the first record of this host as new to the infection of Peronospora parasitica in Egypt. The shape and mode of branching of conidiophores, measurements of conidia, oospores and ultrastructure of this fungus are criteria for its identification. The fine structure of intercellular hyphae, haustoria, and invaded host cells was described. Extensive lomasomes were observed in haustorial mother cell and haustoria particulary during initial penetration to host cell. Special modifications of the host cytoplasm in the vicinity of haustoria were described. The interaction between the fungus and its new host was also discussed.

Twenty-two central European stonewort species and two species groups belonging to the genera Chara, Lychnothamnus, Nitella, Nitellopsis and Tolypella may be identified by oospores using a dichotomous determination key based on binocular and light microscope features. Size, shape, colour, membrane decoration and ridge number are used as identification criteria. A detailed oospore description for each central European charophyte species allows a reliable identification to the species or species group level. Because of the importance and wide distribution of stoneworts in aquatic plant communities, the identification of oospores may give information on ecological parameters such as water temperature, chemistry and light regimes in charophyte stands. Ecological changes due to climatic or anthropogenic impacts may therefore be elucidated for past and present freshwater ecosystems.

Oospores from a cross between (+) and (-) mating types of Pythium splendens had a germination rate of only 2–3% on nutrient media. When oospores from 20-day-old cultures were treated with 0.25% KMn...

Sensitivities of various physiological pro? cesses to elevated temperature were investigated to determine the mechanisms of inhibition of oospore production by high temperature in Phytophthora. The production of a hormones and their receptors by A1 and A2 isolates of Phytophthora parasitica was inhibited at high temperature. However, after being produced, a hormones and their receptors were stable at high temperature. No oospores were produced when A1 and A2 cultures were exposed to high temperature during hormone reception or oospore formation after hormone induction. Results suggest that the inhibition by high temperature of sexual reproduction in P. par? asitica was through inhibition of hormone production, hormone reception, receptor formation, and oospore formation after hormone induction of sexual repro? duction.

Fourteen strawberry genotypes (Fragaria x ananassa Duch.) were evaluated in the field for yield and fruit size. 'Honeoye' was found to be the highest yielding cultivar, followed by 'Kent,' Chambly,' 'Glooscap,' and 'Oka.' The fourteen genotypes were inoculated with individual North American (NA) eastern races (A-1 to A-6) and NA west coast races (A-7 to A-10) of Phyfophthora fragariae or a mixture of them which are the cause of red stele. A disease severity index (DSI), based on the number of oospores in the root stele and the percent root length with stela reddening, was used to quantify genotype susceptibility. 'Jewel,' 'Elvira,' 'Veestar,' 'Annapolis,' and Midway' were found to be more resistant than the other genotypes to the mixture of the ten races (Al-AlO) of P.fragariae. Inoculation of the fourteen genotypes with individual races of American red stele showed that 'Annapolis' and 'Sparkle' were resistant to most races of P.fragariae, whereas 'Honeoye' and 'Bounty' were the most susceptible.

Several native grasses, sorghum lines and maize genotypes were tested for susceptibility to Peronosclerospora sorghi (Weston and Uppal) C. G. Shaw at two locations in Nigeria. In the southwestern location, neither the native grasses nor the sorghum varieties were infected, but most maize varieties were susceptible to downy mildew. In the north‐central location, differential sorghum lines T x 412, T x 430, FFBL, L1499, T x 415 and most maize varieties were systemically infected. Among the native grasses, only Dactylactenium aegyptium (L.) P. Beauv. and Roettboelia cochinchinensis (Louv.) Clayton were partially susceptible to downy mildew. Oospores from diseased sorghum and conidia from both infected sorghum and maize genotypes were found to be involved in the downy mildew disease cycle in the north‐central location, while only conidia from infected maize were detected and caused disease on maize in the southwestern location. This suggests the presence of a sorghum pathotype of downy mildew which i...

Epidemics on vine leaves of anthracnose, caused by Elsinoe ampelina Shear, and of downy mildew, caused by Plasmopara viticola (Berkeley & Curtis ex de Bary) Berlese & de Toni, were studied in an Auckland vineyard during 11 seasons,1971/72/1981/82. Primary infection by E. ampelina, from conidia on cane lesions, occurred in late September—early October, within 4 weeks from budburst. An epidemic failed to develop in only one season, 1972/73, in which rainfall was unusually low. Leaves that escaped infection in their first 4 weeks generally remained free from anthracnose thereafter, and the most severe epidemics occurred in seasons in which the intervals between rain periods were short. Primary infection by P. viticola, presumed to be from oospores, occurred in November–early January, corresponding to the period from 2 weeks before anthesis to fruit set. Primary infections resulted in only small numbers of lesions. In 8 of the 11 seasons there was no downy mildew at the end of December on canes selec...

Oospore ornamentation in three species of Nitella (Charophyta) is investigated with reference to developmental stage. Australian populations of N. stuartii A. Braun possess characteristics identical to those reported for Indian material. Oospore characteristics of N. cristata A. Braun and N. pseudoflabellata A. Braun are described for the first time using SEM. Variation between oospores at different developmental stages on the same plant is greater than variation between mature oospores from different populations of the same species. The timing of development of the highly resistant oospore wall may be related to autecological similarities rather than to taxonomic affiliations. SEM may be used to confirm species determinations in spore-bank studies, as mature oospore wall characteristics were not significantly changed by residence in soil.

The morphology and ultrastructure of the spores of the enigmatic Lower Devonian plant Parka decipiens are briefly described and previous interpretations of the plant are discussed. It is suggested that the alete spores of Parka could have been haploid or diploid. The possibility that some early plant spores were diploid may be viewed as supporting the interpolation theory for the origin of alternation of generations in land plants. The spores of Parka may lie (in morphological and developmental terms) somewhere between the oospores produced by algae and the triradiate haploid spores of the bryophytes.

The Japanese endemic charalean species Nitella gracilens Morioka (=N. furcata subsp. furcata var. sieberi f. gracilens) was thought to be extinct until it was rediscovered at the type locality, Lake Ashinoko, Kanagawa Prefecture. Uni-charalean cultures of N. gracilens were established and sexual organs developed and matured. The morphology of this alga, examined by light microscopy, corresponds closely with the original description by Morioka, particularly with regard to the oospores. The mature oospore walls of N. gracilens are orange or yellowish-brown and the fossa wall of the oospore is smooth or has a finely granulate ornamentation. In addition, scanning electronic microscopic examination demonstrated that the fossa walls have minute granulate structures that are compactly arranged. These observations contrast with a previous report that the oospore wall ornamentation of this taxon is reticulate. Since oospore wall ornamentation and the form of the dactyls in N. gracilens are essentially different fr...

Two new species of Couchia are described as parasites in the eggs of midges. Couchia amphora sp. nov. typically produces terminal flask-shaped zoo- sporangia with occasional secondary zoosporangia arising by basipetalous succession. Oogonia contain oospores, which usually fill the oogonium. Couchia limnophila sp. nov. typically produces broadly ellip- soidal zoosporangia which are single and terminal, but sympodial renewal may rarely occur. Oospores of C. limnophila do not usually fill the oogonium. The new species are contrasted with the previously de- scribed C. circumplexa which, like C. limnophila, typ- ically produces broadly ellipsoidal zoosporangia and which, like C. amphora, usually produces oogonia whose oospores fill the oogonium. Couchia circum- plexa differs from both new species in renewing its zoosporangia by internal proliferation. Typical mor- phological forms and configurations of appressorial complexes produced by each of the three species are useful in species determination, however such use- fulness is limited by the intergradation of forms among the three species. Drawings and full descrip- tions are included to aid in identification of the new species.

Viability of Phytophthora oospores was determined by morphology, ability to plasmolyze in osmotica and by rose-colored staining with tetrazolium bromide. Morphologically, normal oospores contained ...

Reliable methods for oospore production by Phytophthora syringae were developed using low temperatures and media amended with vegetable oil or made from the leaves of stone fruit trees. All of the fourteen isolates tested individually produced oospores in media after 4 weeks at 5 and 9 C but most did not produce oospores at temperatures above 12 C. Similarly, oospores were observed only in almond leaves kept at 12 C and below. Oospore production was greatly increased when corn oil, linseed oil, or wheat germ oil was added to various base media and the number of oospores produced increased with the amount of wheat germ oil added (up to 48 ml/L medium). The addition of /-sitosterol resulted in a slight but significant increase in oospores. Abundant oospores were produced by an almond isolate of P. syringae in almond leaf medium (75 g fresh leaves per liter medium). Oospores were also produced in media made from the leaves of other stone fruit trees, but not in apple leaf medium.

One isolate of water mold from field soil in Shikoku Island, Japan, was Plectospira myriandra, rediscovered 59 years after its original description by Drechsler in 1927. During a survey of pythiaceous fungi in Japanese soils, an interesting fungus was isolated from a field in which bamboo had been grown in Nangoku City of Shikoku District (Island), Japan. This fungus was very similar to Aphanomcyes, but unlike Aphanomyces, it featured lobulate sporangia with uniseriate production of zoospores inside long evacuation tubes. Also, there were numerous antheridial elements per oogonium, and oospores and parthenospores borne in oogonia with and without antheridia, respectively. The morphology and taxonomy of the fungus identified as Plectospira myriandra Drechsler is described in this report. A preliminary report has been published (Watanabe, 1986).

SummaryThe ability of P. cactorum to produce oospores in various apple tree tissues inoculated with zoospores was investigated. Bud scales, petals, green leaves and pre-abscission leaves yielded ca. 105 oospores g−1 f.wt. while fruitlets picked in mid-July gave ca. 103 oospores g−1 f.wt. Because of their bulk and time of availability fallen fruitlets are probably the most important of these substrates for regeneration of P. cactorum in modern orchards, although other detached organs may be sources of inoculum in exceptional years.

Oospore morphology and germination were studied using 17 isolates representing three morphological forms (MF) of Phytophthora palmivora from cacao. Oospore production was most abundant on carrot ag...

The structure and ornamentation on the outer organic wall (ectosporotine) of the oospores of 22 species of Nitella are described based on scanning electron microscope (SEM) observations of type or ...

Growing Brassica napus in Aphanomyces euteiches infested soils for four months reduced the disease severity index (DSI) from 58 to 27, 72 to 53, and 81 to 56 for each of three soils tested. Over the same period there was no change in DSI when Triticum aestivum, Trifolium repens, or no plants were grown in the same soils. When dried leaves and stems of five cruciferous species were incorporated (0.5% of the soil dry weight) into infested soil, there were significant reductions in DSI and oospore numbers. The reductions were greater after six weeks of incorporation compared to three weeks. With soils of DSI 50, 75, and 100, Brassica napus, Raphanus sativus, and Sinapis alba were equally effective, reducing DSI by 32 – 38% and oospore numbers by 66 – 72%. When the same five species were grown for six months in infested soils and the roots alone were incorporated in the soils for six weeks, there was an average reduction of 41% in DSI and 56070 in oospore numbers by Brassica oleracea, Brassica napus,...

Growth of Phytophthora arecae, P. cactorum, P. cambivora, P. capsici, P. cinnamomi, P. citricola, P. citrophthora, P. colocasiae, P. cryptogea, P. drechsleri, P. erythroseptica, P. fragariae, P. he...

Soil, mummified cocoa pods on trees, and pods from husk piles were examined for species of Phytophthora in several experimental cocoa field plots in Brazil. Phytophthora capsici and P. citrophthora were recovered from soil and from husk piles. Compatibility types Al and A2 of P. capsici were isolated from a single pod. Plerotic oospores of Phytophthora with amphigynous antheridia still attached to the oogonial stalk were found in pods from husk piles. Fifteen percent ofthe oospores removed from pods germinated. Twenty single-oospore isolates were compared with respect to colony and sporangia morphology, sporangium production, stalk lengths, and compatibility type. Most cultures from ger? minated oospores resembled P. capsici but some had unique characteristics that included noncaducous sporangia and large (> 50 /nm in length) sporangia. The majority of single-zoospore isolates from singleoospore cultures were similar to their respective parent cultures. However, some differed with respect to size of sporangia, caducity of sporangia, and compatibility type.