Diatomophthoraceae – a new family of olpidiopsis-like diatom parasitoids largely unrelated to Ectrogella

The oomycete genus Ectrogella currently comprises a rather heterogeneous group of obligate endoparasitoids, mostly of diatoms and algae. Despite their widespread occurrence, little is known regarding the phylogenetic affinities of these bizarre organisms. Traditionally, the genus was included within the Saprolegniales, based on zoospore diplanetism and a saprolegnia/achlya-like zoospore discharge. The genus has undergone multiple re-definitions in the past, and has often been used largely indiscriminately for oomycetes forming sausage-like thalli in diatoms. While the phylogenetic affinity of the polyphyletic genus Olpidiopsis has recently been partially resolved, taxonomic placement of the genus Ectrogella remained unresolved, as no sequence data were available for species of this genus. In this study, we report the phylogenetic placement of Ectrogella bacillariacearum infecting the freshwater diatom Nitzschia sigmoidea. The phylogenetic reconstruction shows that Ectrogella bacillariacearum is grouped among the early diverging lineages of the Saprolegniomycetes with high support, and is unrelated to the monophyletic diatom-infecting olpidiopsis-like species. As these species are neither related to Ectrogella, nor to the early diverging lineages of Olpidiopsis s. str. and Miracula, they are placed in a new genus, Diatomophthora, in the present study.

Detection and Characterization of QoI-Resistant Phytophthora capsici Causing Pepper Phytophthora Blight in China

Phytophthora capsici is a highly destructive plant pathogen that has spread worldwide. To date, the quinone outside inhibitor (QoI) azoxystrobin has been the choice of farmers for managing this oomycete. In this study, the sensitivity of 90 P. capsici isolates collected from Yunnan, Fujian, Jiangxi, Zhejiang, and Guangdong in southern China to azoxystrobin was assessed based on mycelial growth, sporangia formation, and zoospore discharge. Furthermore, the mitochondrial cytochrome b (cytb) gene from azoxystrobin-sensitive and -resistant P. capsici isolates was compared to investigate the mechanism of QoI resistance. The high values for effective concentration to inhibit 50% of mycelial growth and large variation factor obtained provide strong support for the existence of azoxystrobin-resistant subpopulations in wild populations. The resistance frequency of P. capsici to azoxystrobin was greater than 40%. Sensitive P. capsici isolates were strongly suppressed on V8 medium plates containing azoxystrobin supplemented with salicylhydroxamic acid at 50 µg ml−1, whereas resistant isolates grew well under these conditions. Multiple alignment analysis revealed a missense mutation in the cytb gene that alters codon 137 (GGA to AGA), causing an amino acid substitution of glycine to arginine (G137R). The fitness of the azoxystrobin-sensitive isolate is similar to that of the G137R mutant. Additionally, the P. capsici isolates used in this study exhibited decreased sensitivity to two other QoI fungicides (pyraclostrobin and famoxadone). Necessary measures should be taken to control this trend of resistance to QoI that has developed in P. capsici in southern China.

New insights into Plectospira genus (Oomycetes, Straminipila): morphological and molecular analyses

We analyzed the morphology and phylogenetic placement of three isolates of Plectospira belonging to Plectospira gemmifera and P. myriandra that were isolated from soil, water bodies and reservoir sediment. The molecular data are from the partial large subunit and the complete internal transcribed regions of the ribosomal DNA. The genus is characterized by the presence of lobulated zoosporangia and mode of zoospore discharge forming an evanescent cluster in the exit orifice. Our isolates clustered together with the single isolate of the genus available in GenBank, however, our phylogenetic analysis and morphological characterization showed that the isolate identified as Plectospira myriandra CBS 523.87 is possibly another taxon. In addition, P. gemmifera is recorded for the first time in Brazil and its sequences were inserted for the first time in a phylogenetic reconstruction.

Achlya catenulata sp. nov., a new Saprolegniales (Oomycetes, Straminipila) from Brazilian mangrove swamp

Achlya catenulata sp. nov. was collected from water samples in a mangrove swamp of the “Parque Estadual da Ilha do Cardoso”, São Paulo State, Brazil. This new species is characterized by the presence of achlyoid type of zoospore discharge from both primary and secondary sporangia, catenulate smooth-walled oogonia in chains of up to 11 oogonia, diclinous antheridial branches and eccentric oospores, which generally failed to mature. Maximum likelihood phylogenetic analyses based on the ITS and LSU regions (rDNA) placed this species within the Achlya sensu stricto clade.

Survival of Phytophthora colocasiae in Field Soil at Various Temperatures and Water Matric Potentials

A large number of the zoosporangia of Phytophthora colocasiae incorporated into moist soils germinated by zoospore discharge, and/or lysed in the soil during the first 5 days of incubation, decreasing the initial number of colony-forming units from 1 × 104 to 1 × 102 per g of soil in all treatments. Eighteen days after incorporation, the viable zoosporangia present in moist soils had thickened their cell walls and germinated only directly, often producing smaller zoosporangia. A few thick-walled chlamydospores were observed, and they germinated only directly. Zoosporangia in soils at -1,500 J/kg matric potential survived longer than 107 days, and the amount of viable Zoosporangia present at that time was approximately 0.1 × 102 CFU/g of soil. Apparently the great majority of the thin-walled zoosporangia produced on V8 agar, when incorporated into moist soil, germinated indirectly in the first 5 days of incubation. Zoosporangia that did not germinate became resting zoosporangia by increasing their wall thickness or by producing chlamydospores. These enabled the pathogen to survive in soil at -1,500 J/kg matric potential for more than 3 months. However, in the absence of the host, the pathogen is predicted to survive less than 1 year due to its lack of saprophytic ability to colonize nonhost tissues.

Rhizosphere Microorganisms From the Jarrah Forest of Western Australia and Their Effects on Vegetative Growth and Sporulation in Phytophthora cinnamomi Rands

Soil dilution plate techniques were used to compare the numbers of bacteria, actinomycetes and fungi in the rhizospheres of Acacia pulchella, Banksia grandis and Eucalyptus marginata (jarrah). The most frequently isolated microorganisms and those detected in significantly different numbers in the rhizospheres of the three species were tested for their effects on sporangium production, zoospore discharge, zoospore germination and mycelial growth of the jarrah dieback pathogen Phytophthora cinnamomi. The total population of fungi in rhizosphere soil from B. grandis was much greater than that found in the rhizospheres of the other two species while the convesse was true for bacteria and actinomycetes, of which the largest populations were associated with A. pulchella. Penicillium spinulosum outnum- bered the combined population of other fungi in the Banksia rhizosphere but formed a much smaller proportion of the jarrah and Acacia rhizosphere microfloras, particularly the latter. P. spinulosum had no effect on mycelial growth or zoospore discharge in P. cinnamomi; it had some ability to stimulete sporangium production and, although it partly suppressed spore germination, the inhibitory effect was less pronounced than that noted for most other microorganisms. In contrast, microorganisms which strongly inhibited mycelial growth, zoospore discharge and germination represented a greater proportion of the Acacia rhizosphere microflora compared with the other microfloras, especially that of B. grandis. While some actinornycetes and fungi produced antibiotics that inhibited vegetative growth of P. cinnamomi in dual cultures, mycelial inhibition was often attributable to nutrient depletion of agar media by the test microorganisms. Similarly, nutrient deprivation resulting from microbial competition for substrates was also considered to be the stimulus for sporangium production in liquid media. The results are discussed in relation to previously reported suppression of P. cinnamomi in forest soils beneath stands of A. pulchella and the associated implications of this for biological control of jarrah dieback.

Indirect Effect of Floristic Composition on Production of Sporangia by Phytophthora cinnamomi in Jarrah Forest Soils

Production of sporangia and zoospore discharge by Phytophthora cinnamomi were investigated in jarrah forest soils collected beneath Acacia pulchella, Banksia grandis and Eucalyptus marginata and incubated under conditions which excluded a direct effect of these species on the soil physical environment. Sporangium production was always significantly greater in soils from B. grandis than in soils from A. pulchella; soils from E. marginata gave intermediate results. There was also evidence that discharge of zoospores was suppressed in soils from A. pulchella. Although three isolates of P. cinnamomi differed in their abilities to sporulate per se, they responded similarly to different treatments.