Application of the combination of 1,3-dichloropropene and dimethyl disulfide by soil injection or chemigation: effects against soilborne pests in cucumber in China

The combination of 1,3-dichloropropene＋dimethyl disulfide （1,3-D＋DMDS）, which forms a pre-plant soil fumigant, can provide a substitute for the environmentally unfriendly methyl bromide （MB）. Three greenhouse trials were performed to evaluate the root-knot nematode and soilborne fungi control efficacy in the suburbs of Beijing in China in 2010-2014. Randomized blocks with three replicates were designed in each trial. The combination of 1,3-D＋DMDS （10＋30 g m-2） significantly controlled Meloidogyne incognita, effectively suppressed the infestation of Fusarium oxysporum and Phytophthora spp., and successfully provided high commercial fruit yields （equal to MB but higher than 1,3-D or DMDS）. The fumigant soil treatments were significantly better than the untreated controls. These results indicate that 1,3-D＋DMDS soil treatments can be applied by soil injection or chemigation as a promising MB alternative against soilborne pests in cucumber in China.

Anaerobic soil disinfestation:A chemical-independent approach to pre-plant control of plant pathogens

Due to increasing regulations and restrictions, there is an urgent need to develop effective alternatives to chemical-dependent fumigation control of soilborne pests and pathogens. Anaerobic soil disinfestation （ASD） is one such alternative showing great promise for use in the control of soilborne pathogens and pests. This method involves the application of a carbon source, irrigation to field capacity, and covering the soil with a plastic tarp. While the mechanisms of ASD are not completely understood, they appear to be a combination of changes in the soil microbial community composition, production of volatile organic compounds, and the generation of lethal anaerobic conditions. The variety of materials and options for ASD application, including carbon sources, soil temperature, and plastic tarp type, influence the efficacy of pathogen sup- pression and disease control. Currently, both dry （e.g., rice bran） and liquid （e.g., ethanol） carbon sources are commonly used, but with different results depending on environmental conditions. While solarization is not an essential component of ASD, it can enhance efficacy. Understanding the mechanisms that mediate biological changes occurring in the soil during ASD will facilitate our ability to increase ASD efficacy while enhancing its commercial viability.

Identification of candidate soil microbes responsible for small-scale heterogeneity in strawberry plant vigour

Studies were conducted to identify candidate soil microbes responsible for observed differences in strawberry vigour at a small spatial scale, which was not associated with visual disease symptoms. Samples were obtained from the soils close to the rhizosphere of ‘big＇ and ‘small＇ plants from small plots which exhibited large local heterogeneity in plant vigour. A metabarcoding approach was used to profile bacterial and fungal compositions, using two primer pairs for 16 S ribosomal RNA genes（16S r DNA） and one for the fungal internal transcribed spacer（ITS） region. Of the two 16 S r DNA primer sets, the 341F/805 R resulted in sequences of better quality. A total 28 operational taxonomic units（OTUs） had differential relative abundance between samples from ‘big＇ and ‘small＇ plants. However, plausible biological explanation was only possible for three fungal OTUs. Two were possible phytopathogens： Verticillium spp. and Alternaria alternata although the latter has never been considered as a main pathogen of strawberry in the UK. For samples from ‘small＇ plants, the abundance of these OTUs was much greater than from ‘big＇ plants. The opposite was true for a mycorrhizal OTU. These results suggest that soil microbes related to crop production can be identified using metabarcoding technique. Further research is needed to assess whether A. alternata and Verticillium spp. could affect strawberry growth in the field.

Effects of soil moisture and length of irrigation on soil wetting to deliver fumigants through microirrigation lines in sandy spodosols

Soil fumigant delivery through microirrigation (drip) lines has the potential to replace direct soil injection into planting beds. However, wetting coverage in these Spodosols must be improved to increase soilborne pest and weed control. Field trials were carried out to determine the impact of soil moisture on the extent of wetting cross-sectional areas through varying irrigation times. Soil moisture contents were: a) 7% moisture (field capacity), and b) 20% (saturation), along with 2, 4, 6, 8, and 10 h of irrigation. Pressed beds had 70 cm tops. Drip lines had emitters spaced 30 cm apart delivering 0.056 L·min–1 per m of row at 55 kPa, and two drip lines were buried at 2.5 cm below the surface and 30 cm apart from each other. Water was mixed with a blue marking dye to analyze the water distribution patterns. Beds were opened at the emitters and high-resolution digital pictures were taken for each treatment. Resulting images were adjusted using photographic software and covered areas across the beds were determined. Regression analysis showed significant quadratic equations for both soil moisture situations, with saturated soils obtaining the highest cross section coverage (90 and 94% after 8 and 10 h). In field capacity beds, the maximum cross section coverage obtained was 82%. Within each soil moisture situation, there were no differences between 8 and 10 h of irrigation.

The Concentration of 2-Propenyl Glucosinolate in Biofumigant Crops Influences Their Anti-Fungal Activity (In-Vitro) against Soil-Borne Pathogens

This study investigated the biofumigation potential of nine Brassica species/cultivars by determining the levels of 2-propenyl glucosinolate in their roots and shoots, and their in-vitro suppression of four major soil-borne pathogens of vegetable crops. Hydrolysis of 2-propenyl GSL produces volatile isothiocyanate (ITC) compounds which are known to have anti-fungal activity. HPLC results showed that 2-propenyl GSL only occurred in root and shoot residue of flowering plants of four Brassica cultivars developed for green manuring (Caliente 199?, Mustclean?, Nemfix? and BQ Mulch?) and in the standard (mustard seed meal) treatment Fumafert?. Levels of 2-propenyl GSL varied several fold within the four Brassica cultivars, with 77-88% of the total concentrations recorded in the shoot tissues. In in vitro assays, the level of fungal suppression by volatiles emitted by hydrated shoot and root residues related to their content of 2-propenyl GSL, and the dose of residue applied to five soilborne test pathogens (S. minor, Rhizoctonia solani, Fusarium oxysporum, Pythium dissotocum and Rhizoctonia solani). The variation in 2-propenyl GLS levels found in the Brassica green manure crops tested provides scope for selecting cultivars with greater potential for biofumigation, and to control multiple soil-borne disease problems in vegetable farms.

Interspecific plant interaction via root exudates structures the disease suppressiveness of rhizosphere microbiomes

Terrestrial plants can affect the growth and health of adjacent plants via interspecific interaction.Here,we studied the mechanism by which plant root exudates affect the recruitment of the rhizosphere microbiome in adjacent plants—with implications for plant protection—using a tomato(Solanum lycopersicum)–potatoonion(Allium cepa var.agrogatum)intercropping system.First,we showed that the intercropping system results in a disease-suppressive rhizosphere microbiome that protects tomato plants against Verticillium wilt disease caused by the soilborne pathogen Verticillium dahliae.Second,16S rRNA gene sequencing revealed that intercropping with potatoonion altered the composition of the tomato rhizosphere microbiome by promoting the colonization of specific Bacillus sp.This taxon was isolated and shown to inhibit V.dahliae growth and induce systemic resistance in tomato plants.Third,a belowground segregation experiment found that root exudates mediated the interspecific interaction between potatoonion and tomato.Moreover,experiments using split-root tomato plants found that root exudates from potatoonion,especially taxifolin—a flavonoid compound—stimulate tomato plants to recruit plant-beneficial bacteria,such as Bacillus sp.Lastly,ultra-high-pressure liquid chromatography–mass spectrometry analysis found that taxifolin alters tomato root exudate chemistry;thus,this compound acts indirectly in modulating root colonization by Bacillus sp.Our results revealed that this intercropping system can improve tomato plant fitness by changing rhizosphere microbiome recruitment via the use of signaling chemicals released by root exudates of potatoonion.This study revealed a novel mechanism by which interspecific plant interaction modulates the establishment of a disease-suppressive microbiome,thus opening up new avenues of research for precision plant microbiome manipulations.

Responses of soil nematode community to monoculture or mixed culture of a grass and a legume forage species in China

Mixed cultivation of fast-growing grasses and nitrogen(N)-fixing legumes for forage production is widely considered effective for obtaining sustained high forage yields without depleting soil N levels.However,the effects of monoculture and mixed culture of these species on soil food webs are poorly understood.In this study,soil nematode communities were examined as indicators of the soil food web structure of monoculture and mixed culture of grass and legume at three N levels,i.e.,338(low),450(moderate),and 675(high)kg N ha-1 year-1,across 2 years in wet and dry seasons,using the grass Paspalum wetsfeteini and the legume Medicago sativa(alfalfa),both commonly cultivated worldwide.Repeated-measures analysis of covariance showed that compared with grass monoculture,legume monoculture and grass-legume mixture increased abundances of herbivorous,bacterivorous,and fungivorous nematodes in the soil food web under the low and moderate N fertilization levels.Principal response curve results showed that the abundance of Helicotylenchus,a plant parasite,was significantly higher under legume monoculture than other planting systems at the low N fertilization level.Structural equation model analysis indicated that the legume increased bacterivore abundance,while increasing N fertilization decreased omnivore abundance.The legume might increase the quantity and quality of food resources for soil biota,resulting in the bottom-up control of soil nematode communities.Our results indicate that targeted control of a soilborne pathogen,Helicotylenchus,is required in alfalfa-based planting systems.In addition,high inorganic N application,which is detrimental to legume-rhizobia symbiosis,nullified the otherwise positive effects of legumes on soil nematodes.

Emerging role of roots in plant responses to aboveground insect herbivory

Plants have evolved complex biochemical mechanisms to counter threats from insect herbivory. Recent research has revealed an important role of roots in plant responses to above ground herbivory （AGH）. The involvement of roots is integral to plant resistance and tolerance mechanisms. Roots not only play an active role in plant defenses by acting as sites for biosynthesis of various toxins and but also contribute to tolerance by storing photoassimilates to enable future regrowth. The interaction of roots with beneficial soil- borne microorganisms also influences the outcome of the interaction between plant and insect herbivores. Shoot-to-root communication signals are critical for plant response to AGH. A better understanding of the role of roots in plant response to AGH is essential in order to develop a comprehensive picture of plant-insect interactions. Here, we summarize the current status of research on the role of roots in plant response to AGH and also discuss possible signals involved in shoot-to-root communication.