Discovery of young, isolated planetary mass objects in the final sigma Orionis star cluster

We present the discovery by optical and near-infrared imaging of an extremely red, low-luminosity population of isolated objects in the young, nearby stellar cluster around the multiple, massive star final sigma Orionis. The proximity (352 parsecs), youth (1 million to 5 million years), and low internal extinction make this cluster an ideal site to explore the substellar domain from the hydrogen mass limit down to a few Jupiter masses. Optical and near-infrared low-resolution spectroscopy of three of these objects confirms the very cool spectral energy distribution (atmospheric effective temperatures of 1700 to 2200 kelvin) expected for cluster members with masses in the range 5 to 15 times that of Jupiter. Like the planets of the solar system, these objects are unable to sustain stable nuclear burning in their interiors, but in contrast they are not bound to stars. This new kind of isolated giant planet, which apparently forms on time scales of less than a few million years, offers a challenge to our understanding of the formation processes of planetary mass objects.     

The effects of host diversity and other management components on epidemics of potato late blight in the humid highland tropics

ABSTRACT A field study at three highland sites near Quito, Ecuador, was conducted to determine whether host-diversity effects on potato late blight would be as important as recently found in studies conducted in temperate areas. We compared three potato mixtures and use of mixtures in combination with different planting densities and two fungicide regimes. Treatment comparisons were made by absolute and relative measures of host-diversity effects and incorporating a truncated area under the disease progress curve as a means of standardizing comparisons across sites. Potato-faba intercrops consisting of only 10% potato provided an estimate of the effects of dilution of susceptible host tissue. Host-diversity effects were very different across study sites, with a large host-diversity effect for reduced disease only at the site most distant from commercial potato production. Planting density had little influence on host-diversity effects or on late blight in single-genotype stands. Fungicide use in combination with potato mixtures enhanced a host-diversity effect for reduced late blight. Potato-faba intercrops produced only a small decrease in potato late blight. Effects of host diversity on yield were variable, with the greatest increase in yield for mixtures treated with fungicides at the site most distant from commercial potato production. The effects of host diversity on late blight severity may be less consistent in the tropical highlands than in the temperate zone, but can contribute to integrated disease management.     

Very virulent infectious bursal disease virus in southeastern Europe

Clinical outbreaks of severe acute infectious burial disease (IBD) were recorded since the mid- and late 1990s in several countries in the southeastern part of Europe. Epidemiologic data showed that both infectious bursal disease virus (IBDV)-vaccinated and IBDV-nonvaccinated chickens were affected with acute IBD and mortality up to 50% independent of the IBDV vaccination status of the appropriate parent flocks. For investigation of the causative agent of acute IBD, the variable region of VP2 was amplified, cloned, and sequenced. Nucleotide sequence analysis of polymerase chain reaction fragments showed several silent nucleotide exchanges in comparison with the sequence of the very virulent (vv) IBDV strain UK661. Also, restriction enzyme cleavage sites proposed specific for vvIBDV were present in all investigated strains. On the basis of clinical signs in affected flocks, recorded epidemiologic data, and sequence analysis, it is very likely the IBD-causing strains were of the vv phenotype.     

Epidemiology in mixed host populations

ABSTRACT Although plant disease epidemiology has focused on populations in which all host plants have the same genotype, mixtures of host genotypes are more typical of natural populations and offer promising options for deployment of resistance genes in agriculture. In this review, we discuss Leonard's classic model of the effects of host genotype diversity on disease and its predictions of disease level based on the proportion of susceptible host tissue. As a refinement to Leonard's model, the spatial structure of host and pathogen population can be taken into account by considering factors such as autoinfection, interaction between host size and pathogen dispersal gradients, lesion expansion, and host carrying capacity for disease. The genetic composition of the host population also can be taken into account by considering differences in race-specific resistance among host genotypes, compensation, plant competition, and competitive interactions among pathogen genotypes. The magnitude of host-diversity effects for particular host-pathogen systems can be predicted by considering how the inherent characteristics of a system causes it to differ from the assumptions of the classic model. Because of the limited number of studies comparing host-diversity effects in different systems, it is difficult at this point to make more than qualitative predictions. Environmental conditions and management decisions also influence host-diversity effects on disease through their effect on factors such as host density and epidemic length and intensity.     

Frequency of Sexual Reproduction by Mycosphaerella graminicola on Partially Resistant Wheat Cultivars

ABSTRACT The frequency of sexual reproduction has a profound effect on the population structure and the adaptive potential of a facultatively sexual parasite. Little is known about the relationship of quantitative host resistance to the frequency of sex in pathogens. We sampled over 5,000 fungal fruiting bodies from eight different wheat cultivars over a 3-year period. The cultivars possessed varying degrees of susceptibility to Mycosphaerella graminicola, a facultatively sexual pathogen that is hetero-thallic and bipolar. The fruiting bodies were classified as M. graminicola pycnidia or ascocarps (asexual and sexual fruiting bodies, respectively), other identifiable fungi, or unidentified. In all 3 years, area under the disease progress curve (AUDPC) explained a significant proportion of the variation in ascocarps as a percentage of M. graminicola fruiting bodies (P < 0.0005). The mean percentage of M. graminicola ascocarps from all cultivars was 63% in 1998, when the epidemic was intense, and 14% in 1999, a year of low disease levels. In 2000, samples were taken at 7-day intervals from 6 June to 27 June from two cultivars with substantially different AUDPCs (788 and 2,185 percentage-days). The less diseased cultivar yielded its first M. graminicola ascocarps 1 week later than the more diseased cultivar, and respective means of ascocarps as a percentage of M. graminicola fruiting bodies across sampling dates were 20.2 and 59.3%. The ratio of sexual to asexual reproduction by M. graminicola is likely to be strongly conditioned by infection density.     

Canine Isosporosis – Epidemiology of Field and Experimental Infections

Isospora spp. are the causative agents of canine isosporosis. Of the 3590 diagnostic samples from Austrian dogs (≤2 years old), 8.7% contained Isospora oocysts, 78% of which from dogs up to 4 months of age. Non‐haemorrhagic and haemorrhagic diarrhoea were significantly more prevalent in Isospora‐infected animals than in coccidia‐negative ones. Twelve of 15 litters from a large commercial dog breeding unit (examined from the third to the 10th week of life) also excreted Isospora (average prevalence: 36.4%) in intensities from 333 to 35 000 oocysts per gram of faeces (opg). In experimental trials 26 3‐week‐old Beagle puppies were infected with low (600–6000), medium (10 000) or high (20 000) dose of Isospora ohioensis‐group or Isospora canis field isolates. Additionally 21 puppies were infected as above and treated with a symmetrical triazintrione. Parasitological and clinical parameters were examined. The two Isospora species differed significantly concerning intensity and duration of excretion. The pre‐patent period was 6–7 days for I. ohioensis and 10–12 days for I. canis. The latter species showed significantly longer excretion and higher opg. This was not influenced by simultaneous infections with both species. Individual patterns of faecal consistency were very variable, irrespective of the infection dose. Treatment significantly reduced both the intensity and the duration of oocysts excretion as well as diarrhoea in comparison with the infected, untreated group and thus proved to be effective against coccidiosis in experimental infections.     

Estimation of rates of recombination and migration in populations of plant pathogens-a reply

ABSTRACT We find that the maximum likelihood method proposed by J. K. M. Brown has deficiencies that limit its usefulness for actual data sets. We propose two alternative statistical methods based on maximum likelihood that could be used to quantify rates of recombination and immigration in fungal populations. We also show that minor modification of our original method, which was based upon posterior probabilities, leads to a result that is identical to one of the maximum likelihood methods. Our previous estimates of the relative contributions of sexual reproduction, asexual reproduction, and immigration to the genetic structure of a Mycosphaerella graminicola population did not change significantly following reanalysis of our data with these new methods.     

The role of selection on the genetic structure of pathogen populations: Evidence from field experiments with Mycosphaerella graminicola on wheat

Summary Coevolution refers to reciprocal genetic changes that occur in two or more ecologically interacting species. In agricultural ecosystems, we are especially concerned with the genetic response of pathogen populations to resistant cultivars produced by plant breeding programs. It would be useful to be able to predict whether disease resistance is likely to be durable or ephemeral before a cultivar is widely grown. Though it may not be possible to predict durability in advance, knowledge of the genetic structure of pathogen populations may prove useful for making predictions about the rate at which pathogens adapt to resistant varieties. Much has been learned about the genetic structure of populations of obligate fungal pathogens such as rusts and mildews, which have become paradigms for plant pathology. We have focused our effort on the population genetics of the less known, non-specialized, necrotrophic pathogens, such as the Septorias of small grains. Our approach has been to use DNA fingerprinting and RFLP analysis to conduct field experiments that elucidate how populations of fungal pathogens adapt in agroecosystems. Our results suggest that mating system may have a greater impact than natural selection on the genetic structure of populations of Mycosphaerella graminicola (anamorph Septoria tritici).     

Aquaculture: The legal framework: Bruce H. Wildsmith. Emond-Montgomery Ltd., Toronto,Canada, 1982, 313 pp. U.S. $ 45.00, ISBN 0-920-722-08-3

In the present communication the gross symptoms of severe riboflavin deficiency are reported as observed in six experiments conducted with four independent stocks of rainbow trout fry and fingerlings. Unlike other reports on salmonids, ocular opacities were infrequently observed, and neither their rate of incidence nor their severity was related to dietary riboflavin content. Compared with both satiety-fed and pair-fed controls, riboflavin-deficient trout exhibited lethargy, apparent muscular weakness, reduced appetite and growth rate, severe fin erosion (all fins), rapid opercular movements, light greenish-yellow skin coloration and high mortality rate. These symptoms, although due to riboflavin deficiency per se, are nonspecific. As the cataracts produced by others in riboflavin-deficient trout appear to be a specific, diagnostic symptom, it is important to identify those conditions which can preclude their development in riboflavin deficiency.     

仔猪球虫病的防制

集约化养猪生产会使一些病原体突破其生态壁龛而造成严重的问题。大多数产房都具有促使猪等孢球虫等微生物大量孳生的优良条件 ,比如温度、湿度以及高度敏感的宿主群体。在这样的环境中 ,猪等孢球虫会乘机迅速生长 ,适应这一特殊的猪场环境 ,直到它们最终在猪群内引发病症。仔猪球虫病是仔猪断奶前 (主要在 7～ 1 4日龄期间 )下痢最常见的原因 ,能够从若干方面对养猪业造成危害 (详见表 1 ) ,所以具有相当重要的经济意义。但人们对于经济损失的大小缺乏准确的估计 ,这是因为人们对这一疾病还缺乏认识 ,诊断率过低 ,并且对此病还缺乏具有代表…