Phylogenetic analysis of sugarcane rusts based on sequences of ITS and the 5.8 S rDNA revealed two highly divergent ITS groups
among isolates of Puccinia sp. sensu Muta, 1987 and P. kuehnii specimens. Although there is sufficient divergence (exceeding normal intraspecific variation) between the ITS regions of the
two groups to support separation into different species, unusually high homology of the ITS group I sequences with those of
members of Cronartium and identical sequences of the D1/D2 regions of the LSU rDNA for all the isolates of “Puccinia sp.” and P. kuehnii that otherwise exhibited different ITS sequences, suggest that the two highly divergent sequences may have resulted from abnormal
genetic events leading to non-orthologous, intraspeciflc polymorphisms. The other sugarcane rust, P. melanocephala and the grass rusts, P. miscanthi and P. rufipes, were separated from “Puccinia sp.” and P. kuehnii and from each other in D1/D2 region analyses, indicating that D1/D2 region sequences may more correctly reflect phylogenetic
relationships in these rusts than do the ITS regions. Further studies to examine differences in patho-genicity or finer morphological
features within P. kuehnii that may be correlated with the high divergence in ITS sequences and experiments to determine if these two sequence types
represent intraspeciflc polymorphism are necessary.
Received 11 October 2000/ Accepted in revised form 24 November 2000 相似文献
Size and composition of the weed seedbank was assessed after 12 years of application of four tillage systems in two crop rotations. Mouldboard and chisel ploughing at 45 cm, minimum tillage at 15 cm and no tillage were compared in continuous winter wheat and a pigeon bean/winter wheat 2-year rotation. Weed control was based upon post-emergence herbicide application. Weed seedling emergence from soil samples taken at 0–15, 15–30 and 30–45 cm depths was assessed in a non-heated glasshouse for 12 months. The tillage system influenced weed seedbank size and composition to a much greater extent than crop rotation. Total weed seedling density was higher in no tillage, minimum tillage and chisel ploughing plots in the 0–15, 15–30 and 30–45 cm layers respectively. Density in the whole (0–45 cm) layer did not differ significantly among tillage systems. With no tillage, more than 60% of the total seedlings emerged from the surface layer, compared with an average 43% in the other tillage systems. Crop rotation did not influence either weed seedbank size or seedling distribution among soil layers, and only had a small influence on major species abundance. The weed seedbank was dominated (>66%) by Conyza canadensis (L.) Cronq. and Amaranthus retroflexus (L.), which thrived in chisel ploughing and no tillage respectively. Results suggested that crop rotation and substitution of mouldboard ploughing by non-inversion tillage (especially by minimum tillage) would not result in increased weed problems, whereas use of no tillage might increase weed infestations because of higher seedling recruitment from the topsoil. 相似文献
The biological activity of a series of N-(pyrid-3-yl)thioureas and -carbodiimides, analogues of the insecticide/acaricide diafenthiuron, towards the carmine spider mite (Tetranychus cinnabarinus Boisd.) and the two-spotted spider mite (T. urticae Koch) was analysed using QSAR methodology. A canonical correlation analysis allowed for the establishment of a prediction model and the identification of outliers within that model. The chemodynamic behaviour of certain compounds in the two series, including these outliers, determined by using photochemical experiments in the laboratory as well as in the glasshouse, was shown to be responsible for anomalous results obtained in the biological tests. It was found from the above study that the biological activity towards both spider mite species is extremely sensitive to the kinetics of formation of the carbodiimides from the corresponding thioureas and to the photostability of the former. The results obtained with a thiourea which underwent no appreciable photochemical transformation into the corresponding carbodiimide supported the hypothesis that the thioureas are also in-vivo propesticides of the acaricidal carbodiimides. 相似文献
The aim of this analysis was to characterise the temporal pattern of infection during the 1997/98 classical swine fever (CSF) epidemic in The Netherlands and hence identify and quantify risk factors for infection in different enterprise types and areas. Survival analysis and Cox proportional hazards regression were used to describe the epidemic. Substantial differences in temporal survival patterns (herd breakdown rate) were found between areas where different control policies operated. Factors with a significant influence on the infection hazard of individual herds included: sow numbers as a percentage of total sows and fatteners (HR = 3.38 for mixed herds (0.1–60% sows) vs. fattening herds (0% sows) and HR = 2.74 for breeding herds (60–100% sows) vs. fattening herds), the number of ‘transport contacts per month’ (>0.3 vs. <0.3; HR = 4.11), pig density (pigs/km2) in the area (HR1000 pigs 1.48) and herd size (HR100 pigs = 1.01).
Pre-emptive slaughter in an area appeared to be associated with lower subsequent disease levels. Higher frequency of transport contacts for welfare slaughter during the epidemic, however, well regulated and controlled, was associated with a substantially higher risk of becoming infected. The positive association of a higher pig density with CSF indicates the potential importance of local spread as a factor in disease transmission and emphasizes that dilution of the pig population can contribute to reduction in CSF occurrence. This analysis suggests however, that if pre-emptive slaughter can promptly be applied effectively in an area after initial diagnosis, pig density is then not a significant factor. Mixed and breeding herds had a higher probability of becoming infected than fattening herds, possibly due to different types and frequencies of inter-herd contacts. These contacts continue to some extent during the epidemic, despite the standstill of animal movements. 相似文献