Previous studies have described the development of control methods against bacterial wilt diseases caused by Ralstonia solanacearum. This review focused on recent advances in control measures, such as biological, physical, chemical, cultural, and integral measures, as well as biocontrol efficacy and suppression mechanisms. Biological control agents (BCAs) have been dominated by bacteria (90%) and fungi (10%). Avirulent strains of R. solanacearum, Pseudomonas spp., Bacillus spp., and Streptomyces spp. are well-known BCAs. New or uncommon BCAs have also been identified such as Acinetobacter sp., Burkholderia sp., and Paenibacillus sp. Inoculation methods for BCAs affect biocontrol efficacy, such as pouring or drenching soil, dipping of roots, and seed coatings. The amendment of different organic matter, such as plant residue, animal waste, and simple organic compounds, have frequently been reported to suppress bacterial wilt diseases. The combined application of BCAs and their substrates was shown to more effectively suppress bacterial wilt in the tomato. Suppression mechanisms are typically attributed to the antibacterial metabolites produced by BCAs or those present in natural products; however, the number of studies related to host resistance to the pathogen is increasing. Enhanced/modified soil microbial communities are also indirectly involved in disease suppression. New promising types of control measures include biological soil disinfection using substrates that release volatile compounds. This review described recent advances in different control measures. We focused on the importance of integrated pest management (IPM) for bacterial wilt diseases.

https://www.jstage.jst.go.jp/article/jsme2/advpub/0/advpub_ME14144/_pdf

Recent trends in control methods for bacterial wilt diseases caused by Ralstonia solanacearum

Soil microbes play important roles in plant growth and health. Little is known about the differences of soil microbes between healthy and bacterial wilt infected soils with Ralstonia solanacearum. By Illumina-MiSeq sequencing of 16S rRNA and 18S rRNA gene amplicons, we found the soil microbial composition and diversity were distinct between healthy and bacterial wilt infected soils. Soil microbial community varied at different plant growth stages due to changes of root exudates composition and soil pH. Healthy soils exhibited higher microbial diversity than the bacterial wilt infected soils. More abundant beneficial microbes including Bacillus, Agromyces, Micromonospora, Pseudonocardia, Acremonium, Lysobacter, Mesorhizobium, Microvirga, Bradyrhizobium, Acremonium and Chaetomium were found in the healthy soils rather than the bacterial wilt infected soils. Compared to bacterial wilt infected soils, the activities of catalase, invertase and urease, as well as soil pH, available phosphorous and potassium content, were all significantly increased in the healthy soils. In a conclusion, the higher abundance of beneficial microbes are positively related the higher soil quality, including better plant growth, lower disease incidence, and higher nutrient contents, soil enzyme activities and soil pH.

/pdf/microbial-community-composition-is-related-to-soil-3oq7zhwkh0.pdf

Microbial community composition is related to soil biological and chemical properties and bacterial wilt outbreak.

Summary
Healthy soils that contain an active microbiome and food web are critical to sustainably produce food for a growing global human population. Many studies have focussed on the role of microbial species diversity and the presence of key functional groups as important controls on the many functions that a sustainable food system relies on.
Here, we synthesise recent ecological empirical evidence and theory to propose that the interactions between organisms in the soil food web are the critical determinant of soil function.
We propose the Rhizosphere Interactions for Sustainable Agriculture Model, in which crop roots recruit small, modular, highly connected soil rhizosphere networks from large, static, relatively unconnected and diverse bulk soil networks. We argue that conventional agricultural management disrupts the connections between rhizosphere and bulk soil networks.
Synthesis. We identify future research directions for optimising ecological connections between roots and rhizosphere microbial and faunal networks, and between rhizosphere networks and bulk soil networks in agricultural production systems. Knowledge on these connections can be applied in agricultural systems to sustainability produce food for a growing global population.

/pdf/below-ground-connections-underlying-above-ground-food-17uc1vosm8.pdf

Below-ground connections underlying above-ground food production: a framework for optimising ecological connections in the rhizosphere

Agricultural waste recycling in horticultural intensive farming systems by on-farm composting and compost-based tea application improves soil quality and plant health: A review under the perspective of a circular economy.

Root exudates play an important role in ecosystem response to climate change, but the functional consequences of drought‐induced changes in the quality of root exudates are unknown. Here, we addressed this knowledge gap in a unique experimental approach. We subjected two common grassland species that differ widely in their growth strategies and root systems, the grass Holcus lanatus and the forb Rumex acetosa, to 2 wk of drought. We collected root exudates and soils at the end of the drought and after 2 wk of recovery and readded all root exudates to all soils in a fully reciprocal set‐up to measure root‐exudate‐induced respiration. We found that soil treatment was unimportant for determining root‐exudate‐induced respiration. By contrast, root exudates collected from plants that had experienced drought clearly triggered more soil respiration than exudates from undroughted plants. Importantly, this increased respiration compensated for the lower rates of root exudation in droughted plants. Our findings reveal a novel mechanism through which drought can continue to affect ecosystem carbon cycling, and a potential plant strategy to facilitate regrowth through stimulating microbial activity. These findings have important implications for understanding plant and ecosystem response to drought.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/nph.16001

Changes in root-exudate-induced respiration reveal a novel mechanism through which drought affects ecosystem carbon cycling.

Bacterial wilt caused by Ralstonia solanacearum is one of the most serious tobacco diseases worldwide. Brevibacillus brevis (L-25) and Streptomyces rochei (L-9) with strong inhibitory effects on R. solanacearum in vitro were isolated from the rhizosphere of a healthy tobacco plant in a severely wilt-diseased field. Pot and field experiments were conducted to evaluate the biocontrol effect of the isolated antagonists alone and in combination with organic fertilizer. In pot experiment, the control efficacy was 92.3–100 % in the treatments applied with L-25 and L-9 alone or together with organic fertilizers. When bioorganic fertilizer containing L-9 and L-25 was applied to the soil in field condition, the control efficacies were 95.4 and 30.0 in the Anhui and Guizhou field plots, respectively. The counts of bacteria and actinomycetes in rhizosphere soil were significantly increased (p ≤ 0.05) under all antagonist applications compared with CK (PR). In contrast, fungal and R. solanacearum densities in the rhizosphere soil applied with antagonists were much lower than the CK (PR) rhizosphere. Combined application of the two antagonists had better effect than single antagonist treatments. The antagonists were more effective when they were combined with organic fertilizer as compared with the antagonistic strains only. These results allow us to conclude that a combination of the biocontrol agents, L-25 and L-9, together with organic fertilizers can effectively control bacterial wilt by affecting soil microbial structure.

Tobacco bacterial wilt can be biologically controlled by the application of antagonistic strains in combination with organic fertilizer

Muskmelon (Cucumis melo L.) wilt caused by Fusarium oxysporum f. sp. melonis leads to severe economic losses. A bio-organic fertilizer (BIO) fortified with an antagonistic strain of Bacillus subtilis Y-IVI was used to control this disease. Pot experiments were carried out to investigate the efficacy and to elucidate biocontrol mechanisms for the disease. BIO significantly reduced the disease incidence. Population of F. oxysporum in plant shoots of the BIO treatment were about 1000-fold lower than the control. Population of Y-IVI remained high in muskmelon rhizosphere of the BIO treatment during the experiment. Concentration of antifungal lipopeptides, iturin A, in the BIO treatment was significantly higher than other treatments. Ten days after transplantation, the salicylic acid content in BIO-treated plant leaves was significantly higher than control. In conclusion, BIO effectively controlled muskmelon wilt, possibly because the antagonistic microbes effectively colonize the plant rhizosphere and shoots to preclude pathogen invasion. Furthermore, Y-IVI produces antifungal lipopeptides in the rhizosphere.

Biocontrol of Fusarium wilt disease in muskmelon with Bacillus subtilis Y-IVI

Identification of benzoic acid and 3-phenylpropanoic acid in tobacco root exudates and their role in the growth of rhizosphere microorganisms

The invention discloses a distilled grain organic fertilizer and a production method of the distilled grain organic fertilizer. A fermentation material of the distilled grain organic fertilizer comprises the following components in parts by mass: 100 parts of distilled grains, 3-5 parts of straw, 1-2 parts of wood sawdust, 2-2.5 parts of quick lime and 0.2-0.3 part of a strain. The production method comprises the following steps: inoculating the fermentation material with the strain, uniformly mixing to construct a strip pile, and performing fermentation; heating the strip pile to 60-65 DEG C, and turning the pile; regulating the pile turning frequency under a principle of maintaining the temperature of the strip pile to be not more than 65 DEG C; naturally ventilating for 1-2 months after fermenting; further crushing and screening to obtain a distilled grain organic fertilizer final product. The method for producing the distilled grain organic fertilizer, disclosed by the invention, is short in fermentation time and controllable in fermentation degree; the obtained product comprises the following components in percentage by weigh: 76% of an organic matter, 4% of N, 2.18% of P2O5, 1.29% of K2O, 7.74% of total nutrients and 29% of moisture; the pH value of the product is 6.2; indicators of the mortality of roundworm eggs and the number of fecal coliforms of the product can meet the requirements of NY884.

Distilled grain organic fertilizer and production method thereof

A pot experiment was conducted using different ratios of triple superphosphate (TSP) to pig manure (PM) to determine the best ratio for combining inorganic and organic fertilisers to attain optimum crop yields, and investigate the mechanisms affecting P availability in a wheat (Triticum aestivum L.)–soybean (Glycine max L.) rotation in eastern China. The TSP/PM treatment ratios used were: T1, 0/0 as a control; T2, 100/0; T3, 90/10; T4, 80/20; T5, 70/30; T6, 0/100. All treatments except T1 received the same amount of P. Treatment T4 gave significant yield increases of 25.6% in 2008 and 16.8% in 2009 compared with T2. Regarding the various forms of organic P (Po) under different fertilisation treatments, the labile Po of treatment T4 was 60.0% higher than that of T2. The contents of moderately labile Po were in the order T6 > T5 = T4 > T3 > T2 > T1. The addition of pig manure decreased the contents of iron (Fe)-P and calcium-P and occluded-P more than the content of aluminium-P. Compared with the T2 treatment, soil inorganic P decreased by 5.3%, 12.1%, and 15.0% when the ratios of TSP/PM in fertiliser were T3, T4, and T5, respectively. Microbial biomass and phytase activity of treatment T4 were not significantly different from those of T5 and T6, but significantly increased compared with T2. When the amount of leachate was 50 mL, only 1.6%, 8.4%, and 9.8% of total 32P leached from the T1, T2, and T4 soils, respectively. Treatment T4 resulted in higher grain yield, increased labile Po and moderately labile Po, and decreased Fe-P and occluded-P, and promoted microbial biomass P and phytase activity, which were better outcomes for a wheat–soybean rotation. Fertilising soil with combined organic and inorganic P sources could lower P fixation and enhance P mobility.

Li Xiang

Papers

Tobacco bacterial wilt can be biologically controlled by the application of antagonistic strains in combination with organic fertilizer

Biocontrol of Fusarium wilt disease in muskmelon with Bacillus subtilis Y-IVI

Identification of benzoic acid and 3-phenylpropanoic acid in tobacco root exudates and their role in the growth of rhizosphere microorganisms

Distilled grain organic fertilizer and production method thereof

Interactive effects from combining inorganic and organic fertilisers on phosphorus availability