Monitoring virulence and sexual compatibility in Brazilian Bremia lactucae populations

During the winter, there is a high occurrence of downy mildew on lettuce caused by Bremia lactucae. This oomycete shows variability in virulence, so understanding the genetic structure of the pathogen population becomes essential for obtaining resistant cultivars. Thus, the objective of this study was to determine sexual compatibility in Brazilian populations of B. lactucae and investigate the occurrence of sexual reproduction of the pathogen on lettuce (Lactuca sativa) and prickly lettuce (Lactuca serriola). Leaf samples were collected in 33 municipalities in seven Brazilian states. The virulence structure of the populations was monitored using the EU-C sextet code. B. lactucae populations from the states of São Paulo, Paraná, Rio de Janeiro, and Rio Grande do Sul shared six of the 15 virulence factors evaluated. Twenty-five virulence phenotypes (v-phenotypes) were found, with the sextet codes 31-00-02, 31-16-02, 31-24-02, and 31-01-02 being more frequent. The predominance of some v- phenotypes indicates that clonal reproduction is still the main form of B. lactucae propagation. The genes and resistance factors of the cultivars Argelès (Dm38), Balesta, and Bartoli are recommended as suitable sources of lettuce resistance in Brazil. Natural occurrence of oospores was detected in most sampled locations, in lettuce and prickly lettuce plants. Virulence variability of Brazilian isolates is the result of the pathogen's ability to reproduce both sexually and asexually, with a prevalence of homothallic isolates; although the majority were of the predominant B₂ mating type, there was a high incidence of predominant B₁ in addition to B₁ = B₂.     

Phylogenetic relationships of some spirurine nematodes (Nematoda: Chromadorea: Rhabditida: Spirurina) parasitic in fishes inferred from SSU rRNA gene sequences

Abstract: Small subunit rRNA sequences were obtained from 38 representatives mainly of the nematode orders Spirurida (Camallanidae, Cystidicolidae, Daniconematidae, Philometridae, Physalopteridae, Rhabdochonidae, Skrjabillanidae) and, in part, Ascaridida (Anisakidae, Cucullanidae, Quimperiidae). The examined nematodes are predominantly parasites of fishes. Their analyses provided well-supported trees allowing the study ofphylogenetic relationships among some spirurine nematodes. The present results support the placement of Cucullanidae at the base of the suborder Spirurina and, based on the position of the genus Philonema (subfamily Philoneminae) forming a sister group to Skrjabillanidae (thus Philoneminae should be elevated to Philonemidae), the paraphyly of the Philometridae. Comparison of a large number of sequences of representatives of the latter family supports the paraphyly of the genera Philometra, Philometroides and Dentiphilometra. The validity of the newly included genera Afrophilometra and Caranginema is not supported. These results indicate geographical isolation has not been the cause of speciation in this parasite group and no coevolution with fish hosts is apparent. On the contrary, the group of South-American species ofAlinema, Nilonema and Rumai is placed in an independent branch, thus markedly separated from other family members. Molecular data indicate that the skrjabillanid subfamily Esocineminae (represented by Esocinema bohemicum) should be either elevated to the rank of an independent family or Daniconematidae (Mexiconema africanum) should be decreased to Daniconematinae and transferred to the family Skrjabillanidae. Camallanid genera Camallanus and Procamallanus, as well as the subgenera Procamallanus and Spirocamallanus are confirmed to be paraphyletic. Paraphyly has also been found within Filarioidea, Habronematoidea and Thelazioidea and in Cystidicolidae, Physalopteridae and Thelaziidae. The results of the analyses also show that Neoascarophis, Spinitectus and Rhabdochona are monophyletic, in contrast to the paraphyletic genus Ascarophis. They further confirm the independence of two subgenera, Rhabdochona and Globochona, in the genus Rhabdochona. The necessity of further studies of fish-parasitizing representatives of additional nematode families not yet studied by molecular methods, such as Guyanemidae, Lucionematidae or Tetanonematidae, is underscored.     

Diversity of defence mechanisms in plant–oomycete interactions: a case study of <Emphasis Type="Italic">Lactuca</Emphasis> spp. and <Emphasis Type="Italic">Bremia lactucae</Emphasis>

Plant pathogenic oomycetes, including biotrophic downy mildews and hemibiotrophs/necrotrophs such as Phytophthora and Pythium, cause enormous economic losses on cultivated crops. Lettuce breeders and growers face the threat of Bremia lactucae, the causal agent of lettuce downy mildew. This pathogen damages leaf tissues and lettuce heads and is also frequent on wild Asteraceae plants. The interactions of Lactuca spp. with B. lactucae (abbr. lettuce–Bremia) display extreme variability, due to a long co-evolutionary history. For this reason, during the last 30 years, the lettuce–Bremia pathosystem has been used as a model for many studies at the population, individual, organ, tissue, cellular, physiological and molecular levels, as well as on genetic variability and the genetics of host–parasite interactions. The first part of this review summarizes recent data on host–parasite specificity, host variability, resistance mechanisms and genetics of lettuce–Bremia interactions. The second part focuses on the development infection structures. Phenotypic expression of infection, behaviour of B. lactucae on leaf surfaces, the process of penetration, development of primary infection structures, hyphae and haustoria are discussed in relation to different resistance mechanisms. In the third part, the components of host resistance and the variability of defence responses are analysed. The role of reactive oxygen species (ROS), antioxidant enzymes, nitric oxide (NO), phenolic compounds, reorganization of cytoskeleton, electrolyte leakage, membrane damage, cell wall disruption, hypersensitive reaction and plant energetics are discussed in relation to defence responses. In general, the extreme variability of interactions between lettuce and Bremia, and their phenotypic expression, results from diversity of the genetic background. Different mechanisms of resistance are conditioned by an orchestra of defence responses at the tissue, cell, and molecular levels. The various events responsible for defence involve a complex interaction of the processes and reactions mentioned above. This review also provides an overview on the timing of pathogen development, host pathological anatomy, cytology and physiology of lettuce–Bremia associations. The significance of these factors on the expression of different resistance mechanisms (non-host and host resistance, race-specific and race non-specific resistance, field resistance) is discussed.     

Screening of wildcucumis species against downy mildew (Pseudoperonospora cubensis) isolates from cucumbers

Under controlled inoculation, a set of 56 accessions belonging to 19 wild species of the genusCucumis was studied for resistance to seven isolates of cucurbit downy mildew (Pseudoperonospora cubensis (Berk. et Curt.) Rostow.) from cucumber. No resistance toP. cubensis was detected in theseCucumis accessions. In three host accession/pathogen isolate combinations, limited sporulation was observed. Nine newCucumis species are described as hosts forP. cubensis: C. africanus, C. ficifolius, C. figarei, C. meeusii, C. metuliferus, C. myriocarpus, C. leptodermis, C. sagittatus andC. zeyheri. Results are discussed in relation to the origin and evolution ofCucumis species.     

Effects of selective thinning on growth and development of beech (Fagus sylvatica L.) forest stands in south-eastern Slovenia

We studied the effects of two types of selective thinning on beech stands formed by a shelterwood cut in 1910 — with lower number of crop trees and higher thinning intensity (T1) and higher number of crop trees with lower thinning intensity (T2). The stands were thinned in 1980, 1991 and 2001. Despite a lower stand density after thinning, the annual basal area increments of thinned stands in both thinning periods (1980–1991 and 1991–2002) were around 20% higher compared to those of the control (unthinned) stands. The mean annual basal area increment of dominant trees was 30–56% larger in the thinned plots compared to the control plots. Of 176 initial crop trees in the T1, 72% were chosen again during the last thinning. In the T2, 258 crop trees were chosen in the first thinning, and only 62% of these trees were chosen again during the last thinning. Only crown suppression and diameter classes of crop trees significantly influenced their basal area increment when diameter classes, crown size, crown suppression, and social status were tested. In the thinned stands, the dominant trees are more uniformly distributed if compared to the dominant trees in the control plots. Finally, the herbaceous cover and the species diversity were higher in the thinned plots.     

Antimicrobial susceptibility of streptococci from various indications of swine, horses, dogs and cats as determined in the BfT-GermVet monitoring program 2004-2006

A total of 500 streptococci from two indications of swine (beta-haemolytic streptococci from infections of the urinary/genital tract including strains from the mastitis metritis agalactia syndrome as well as S. suis from infections of the central nervous system and the musculoskeletal system), two indications of horses (S. equi from respiratory tract infections and beta-haemolytic streptococci from infections of the genital tract), as well as three indications of dogs and cats (beta-haemolytic streptococci from infections of the respiratory tract, the urinary/genital tract, and skin/ear/mouth) were investigated for their susceptibility to antimicrobial agents. Regardless of the animal origin and indication, the most frequently detected resistance properties were resistances against sulfamethoxazole (20-78%), tetracycline (17-93%) as well as gentamicin (14-79%). Resistance to penicillins or cephalosporins was very rarely detected - if at all.     

Antimicrobial susceptibility of Pseudomonas aeruginosa from dogs and cats as well as Arcanobacterium pyogenes from cattle and swine as determined in the BfT-GermVet monitoring program 2004-2006

During the BfT-GermVet monitoring program, Pseudomonas (P) aeruginosa from dogs and cats (n = 99) as well as Arcanobacterium (A.) pyogenes from cattle and swine (n = 90) were examined for their antimicrobial susceptibility. In general, P. aeruginosa is known to be resistant against many antimicrobial agents whereas A. pyogenes is thought to be susceptible to most agents in-vitro. However, representative and actual minimum inhibitory concentration (MIC) values are missing for both veterinary pathogens. In the present study, MIC values were determined and categorized according to the recommendations given in the Clinical and Laboratory Standards Institute (CLSI) documents M31-A2 and M31-S1. For susceptibility testing of A. pyogenes, the CLSI methodology was slightly modified. Specific breakpoints were not available for most of the antimicrobial agents tested. P. aeruginosa isolates from infections of the skin, ear and mouth as well as the urinary and genital tract of dogs and cats were either resistant or exhibited high MIC values to most antimicrobial agents tested. However, gentamicin resistant isolates were observed in only 27% and 11% (intermediate isolates 29% and 39%), respectively. For the same bacterium/host animal/organ system combinations, enrofloxacin resistance was detected in only 24% and 11% of the isolates (intermediate isolates 49% and 61%). For A. pyogenes, resistance was most prevalent against tetracycline (33%-56%, bovine and porcine isolates) and sulfonamides (26%-40%, bovine isolates).