Insecticidal activity of 23 essential oils and their major compounds against adult Lipaphis pseudobrassicae (Davis) (Aphididae: Homoptera)

Essential oils from 23 species of plants comprising 14 genera and 4 plant families were obtained by Clevenger-type water distillation. The major compounds in these essential oils were identified with GC-MS and their insecticidal activity against adult turnip aphids, Lipaphis pseudobrassicae (Davis), tested with dosage-mortality bioassays. We examined mortality only for viviparous adults because sizeable aphid populations on crucifer (Brassicaceae) hosts are largely produced by these wingless, parthenogenic females. Twenty-two of the oils were directly applied to aphid females in randomized blocks at concentrations of 0.0, 1.0, 2.5, 5.0 and 10.0 mg ml(-1). Essential oils mixed with a non-toxic emulsifying agent, dimethyl sulfoxide (DMSO), more easily penetrated the waxy insect cuticle. Probit analysis and LC(50) at three different exposures showed aphids were quickly incapacitated and killed by aliphatic aldehydes, phenols and monocyclic terpenes contained in Bifora and Satureja oils and at applied concentrations as low as 0.3 to 1.0 mg ml(-1). Only enough Pimpinella isaurica oil and its three phenylpropanoid fractions were available for testing at a single concentration of 10 mg ml(-1). We could not spare any additional P. isaurica oil for testing at other concentrations. Phenylpropanoids isolated from P. isaurica oil when recombined or left naturally blended in the oil were highly bioactive against L. pseudobrassicae at 10 mg ml(-1).     

Usefulness of nonhost plants in managing Plasmodiophora brassicae

Germination of resting spores of Plasmodiophora brassicae , causal agent of clubroot in crucifers, may be stimulated by certain nonhost plants. Without a host plant to infect, such germination would lead to a reduced persistence of resting spores in the soil. The effect of four nonhost plants on P. brassicae was investigated in a 3-year field experiment and a 14-month glasshouse experiment. Three of the plant species used, leek ( Allium porrum ), winter rye ( Secale cereale ) and perennial ryegrass ( Lolium perenne ), have been reported to stimulate resting spore germination, while the fourth, red clover ( Trifolium pratense ), does not. In the field experiment, none of the plant species reduced the concentration of P. brassicae in soils when tested with bioassay plants (Chinese cabbage, Brassica rapa var. pekinensis ). In the glasshouse experiment, there was a lower disease level in all plant treatments compared with the plant-free control following incorporation and decomposition of plant roots. At this time, pH in the soils with plant treatments was higher than that in the control soil. There were no indications of a species-specific interaction between any of the nonhost plants investigated and P. brassicae , and it cannot be concluded that any of them would be useful in the sanitation of P. brassicae -infested soils within short time periods.     

Cytoplasmic male sterility in Brassicaceae crops

Brassicaceae crops display strong hybrid vigor, and have long been subject to F₁ hybrid breeding. Because the most reliable system of F₁ seed production is based on cytoplasmic male sterility (CMS), various types of CMS have been developed and adopted in practice to breed Brassicaceae oil seed and vegetable crops. CMS is a maternally inherited trait encoded in the mitochondrial genome, and the male sterile phenotype arises as a result of interaction of a mitochondrial CMS gene and a nuclear fertility restoring (Rf) gene. Therefore, CMS has been intensively investigated for gaining basic insights into molecular aspects of nuclear-mitochondrial genome interactions and for practical applications in plant breeding. Several CMS genes have been identified by molecular genetic studies, including Ogura CMS from Japanese radish, which is the most extensively studied and most widely used. In this review, we discuss Ogura CMS, and other CMS systems, and the causal mitochondrial genes for CMS. Studies on nuclear Rf genes and the cytoplasmic effects of alien cytoplasm on general crop performance are also reviewed. Finally, some of the unresolved questions about CMS are highlighted.     

菘蓝生殖方式的初步研究

对十字花科菘蓝属植物菘蓝的生殖方式进行观察,观察结果表明:(1)菘蓝花两性,四强雄蕊,花粉较多,主要靠昆虫传粉;(2)P/O(花粉/胚珠)值为9 426~28 280,属于异交繁育系统,其传粉过程观察、自异交试验及花粉在柱头上萌发的显微观察结果也证明菘蓝主要以异花传粉的方式进行交配;(3)菘蓝花期自交结实率只有7.8%,具有较强自交不亲和性,自交不亲和的作用部位是柱头表面,属于孢子体自交不亲和型。     

Determinants of parasitoid assemblages of the diamondback moth,Plutella xylostella,in cabbage farmer fields in Senegal

Conservation biological control, which fosters the optimal use of indigenous natural enemies, is a promising way for reducing pesticide reliance in horticultural systems. A two-year field survey was conducted in the main cabbage-producing area in Senegal (Niayes) to assess the potential of indigenous parasitoids to control populations of Plutella xylostella (Lepidoptera, Plutellidae). Results showed an overall low level of parasitism (11.7%) which was independent of host abundance, but was highly variable among fields (0–50%). Parasitism was predominant in the late part of dry season. Insecticide use, mostly relying on broad-spectrum insecticides, had a negative effect on the overall parasitism rate. Observations conducted throughout the cabbage crop cycle showed that parasitism unexpectedly decreased with crop aging (from 41 to 60 days post transplanting), likely due to repeated insecticide applications. Four main parasitoid species including Oomyzus sokolowskii (Kurdjumov) (Eulophidae) (48.8%), Apanteles litae Nixon (Braconidae) (32.5%), Brachymeria sp. (Chalcididae) (11.3%), and Cotesia vestalis Haliday (Braconidae) (7.3%) were identified. Parasitism due to O. sokolowskii was greater during the first part of the dry season whereas parasitism due to A. litae was greater during the second part of the dry season. Parasitism due to Brachymeria sp. was not affected by time of season but was greater in the Centre and North than in the South of Niayes. Parasitism due to C. vestalis was equal in the three zones but was higher in the late part of the dry season. The diversity of parasitoids was constant across zones but was greater during the second part of dry season. A positive relationship between diversity (Shannon diversity index H′) and parasitism rate was observed, suggesting a positive effect of parasitoid diversity on natural pest control. Parasitoids have a promising role to play as biocontrol agents of P. xylostella populations in Senegal, provided significant changes to current insecticide use are made. Better knowledge of their resource requirements including crop and non-crop habitats, and provision of these in and around crops is also needed.     

Can winter cover crops influence weed density and diversity in a reduced tillage vegetable system?

Weeds are a major constraint for organic crop production. Previous research has found that cover crops in reduced tillage systems can provide weed interference, subsequently reducing inputs and improving crop yield. However, questions remain about effects of cover crop species identity and cover crop biomass on weed suppression and crop yield. This four-year study investigated how winter cover crops grown alone or in mixture influenced weed presence and crop yield in a reduced tillage organic vegetable system. Treatments were barley (Hordeum vulgare L.), crimson clover (Trifolium incarnatum L.), mixed barley + crimson clover, and a no-cover crop control. Plots were flail-mowed and strip-tilled prior to planting main crops (2011 and 2012: broccoli Brassica oleracea L.; 2013 and 2014: crookneck squash Cucurbita pepo L.). We measured density, diversity, and community composition of weeds and viable weed seeds, changes in weed percent cover within growing seasons, and crop yield. We found that the presence of barley, crimson clover, or barley + crimson clover reduced weed density by 50% relative to the control. Cover crop biomass negatively influenced weed density and weed seed diversity, and positively influenced squash yield. Weed percent cover within growing seasons did not respond differentially to cover crop treatment. Cover crop treatment and cover crop biomass had no influence on weed or weed seed community composition. These results suggest that reduced tillage winter cover crops in mixture or monoculture can similarly suppress weeds and improve yield, primarily due to biomass effects.     

Monitoring expression of selected Plasmodiophora brassicae genes during clubroot development in Arabidopsis thaliana

The expression of 12 cDNAs from Plasmodiophora brassicae , among them two novel sequences, was determined during clubroot development on Arabidopsis thaliana . The aim was to find cDNAs expressed at distinct stages of pathogenesis. The relative amount of infection with active plasmodia could be estimated using Pb Actin cDNA as an internal standard. Two cDNAs, Pb Brip9 and Pb CC249, were strongly expressed at stages of disease development corresponding to the occurrence of sporulating plasmodia. Therefore, it should be possible in the future to find more cDNAs which could be used as markers for certain stages of clubroot development.     

芸薹属重要作物的比较基因组研究进展

从比较基因组学的概念和内容、芸薹属重要作物的比较遗传作图、比较QTL定位等方面阐述了各物种间基因组的共线性关系、染色体内和染色体间的同源性、重要性状基因在QTL区域的映射,对芸薹属作物分子育种改良等方面的研究进行了展望.     

Weeds that can do both tricks: vegetative versus generative regeneration of the short-lived root-sprouting herbs Rorippa palustris and Barbarea vulgaris

Weeds of arable land have two strategies for coping with severe disturbance: they have either a very short life cycle and survive disturbance events (ploughing) as seeds or they rely on an underground bud bank and a large regeneration capacity from fragmented roots or stems. Representatives of the respective strategies differ in their investments: annual weeds invest in generative structures and production of easily dispersible or durable seeds, whereas perennial weeds invest preferentially to underground storage organs bearing buds which serve for vegetative propagation. Even when perennial weeds may also produce seeds under favourable conditions, these may serve for further field infestation and spreading. However, the ability of some short‐lived annual weeds to regenerate from roots is often overlooked in studies on mechanisms for disturbance survival. Here, we show that short‐lived weeds capable of adventitious sprouting from roots may be very successful in vegetative regeneration from root fragments. Using a pot experiment, short‐lived root sprouters were found to have higher (Rorippa palustris) or the same (Barbarea vulgaris) fitness when regenerating from root fragments as when regenerating from seed. Even though this finding needs to be tested on other species and in different experimental settings, the results indicate the potential importance of adventitious sprouting from roots in short‐lived plants. Better knowledge of this phenomenon is crucial for understanding both the population dynamics of short‐lived root‐sprouters in disturbed habitats and the ruderal strategy of plants generally.     

Orchard floor management effects on nitrogen fertility and soil biological activity in a newly established organic apple orchard

This study addresses the often-competing goals of organic fertility and weed control by evaluating alternative orchard floor management strategies for their impact on N cycling, tree performance, and soil biological activity in a newly established apple (Malus domestica Borkh.) orchard. The standard tillage weed control practice resulted in satisfactory tree growth with desirable levels of leaf N and most other nutrients; however, soil biological activity did not improve. Maintenance of a living cover understory increased soil N concentration and availability and improved soil biological activity; however, tree growth was less than in other treatments likely in response to competition with the living cover understory for space and water. Application of wood chip mulch resulted in exceptional tree growth which may have resulted from greater water availability, but available soil N was lower, and consequently, tree leaf N concentration was low; in addition, soil biological activity was not improved. Clove oil organic herbicide provided poor weed control resulting in lower leaf N and tree growth and did not improve soil biological activity. Brassicaceae seed meal applications enhanced N availability and soil nematode abundance, but leaf N and many other nutrients were below desirable levels, and additional research is needed to optimize this treatment. We conclude that meeting the multiple objectives of weed control, optimal tree health, and increased soil biological activity may require employment of different orchard floor management strategies at different times during the life of the orchard.