Rice sheath blight, a fungal disease caused by Rhizoctonia solani, seriously threatens rice production. Some of the volatile organic compounds (VOCs) produced by microbes are inhibitory to the growth of the plant pathogen, and hence may have the potential as environmentally friendly antifungal substances. However, information on the inhibitory effect of VOCs released by rice rhizosphere bacteria on R. solani is scarce. In this study, bacteria from the rice rhizosphere capable of inhibiting the growth of R. solani via releasing VOCs were screened using a double Petri dish assay. Headspace solid phase microextraction and gas chromatography mass spectrometry (GC/MS) were used to identify and quantify the VOCs. The contributions of VOCs to the inhibition of the growth of R. solani were estimated by constructing a random forest model, and were verified using pure compounds. Nine strains (i.e., Pseudomonas sp. No. 3, Enterobacter sp. No. 26, Enterobacter sp. No. 34, Pseudomonas sp. No. 35, Ralstonia sp. No. 50, Bacillus sp. No. 62, Arthrobacter sp. No. 146, Brevibacillus sp. No. 2–18, and Paenisporosarcina sp. No. 2–60) showed various inhibition on R. solani growth via VOCs. The inhibitory effect ranged from 7.84% to 100%, with Ralstonia sp. No. 50 completely inhibiting the growth of R. solani. Five VOCs (i.e., benzoic acid ethyl ester, 3-methyl-butanoic acid, 2-ethyl-1-hexanol, 3-methyl-1-butanol, and 6-methyl-5-hepten-2-one) identified by random forest model were confirmed to be toxic to R. solani when applied as a pure chemical compound. In particular, benzoic acid ethyl ester, 3-methyl-butanoic acid, and 2-ethyl-1-hexanol were lethal to R. solani. In summary, the rice rhizosphere bacteria (Ralstonia sp. No. 50) and VOCs (benzoic acid ethyl ester, 3-methyl-butanoic acid, and 2-ethyl-1-hexanol) showed potential to be used as new resources for biological control of rice sheath blight.

Background: Engineering root microbiomes holds great promise to enhance plant health. Enhanced plant resistance via breeding or genetic modification can promote recruiting beneficial microbes, but is challenging to achieve. Results: Here we showed that calcium (Ca) addition caused significant changes in tomato physiology, resulting in enhanced immunity towards the pathogen Ralstonia solanacearum along with increased levels of salicylic acid (SA), sugar content, and defense enzyme activities in roots. High Ca levels significantly altered the root microbiomes, enriching sixteen bacterial genera, including Dyella japonica , Rhodanobacter glycinis , Paenibacillus polymyxa , and Pseudomonas aeruginosa , with the mostly enriched genus showing a 16.5-fold increase in the relative abundance compared to no Ca addition. Associated with the enhancement of these bacterial genera, tomato wilt incidence was reduced from 80 to 0%. The growth of these four bacterial strains was promoted to varying extent by the addition of SA, sucrose, trehalose, and Ca in vitro. All the four bacterial strains had antagonistic ability against R. solanacearum . A synthetic microbial community (SynCom) of these four strains significantly inhibited R. solanacearum growth in whole root exudates treated with Ca, and the combination of SynCom and Ca addition resulted in nearly eradication of the tomato bacterial wilt. Conclusions: Our findings revealed an effective and simple method for enriching beneficial microbiomes to protect plants against pathogens. Our results also suggested that combination of immunity boosters and beneficial microbes offers a novel strategy to combat soil-borne diseases.

There are many kinds of soil amendments that consist of different materials. The soil amendment is usually of benefit to plant health. However, the effects of the soil amendments on plant disease have rarely been compared and the involved mechanisms are largely unknown. In the present study, we investigated the influences of five contrasting soil amendments (i.e., potassium silicate (PS), calcium silicate (CS), biochar (BC), calcium silicate humic acid (SCHA), and bio-organic fertilizer (BOF)) on tomato bacterial wilt. In addition, we dissected the mechanism with high-throughput sequencing. The results showed that BC, SCHA, and BOF significantly reduced the incidence and delayed the disease, while BOF significantly reduced the incidence of bacterial wilt disease in the whole tomato growing period. In the early stage of the disease, BC, SCHA, and BOF significantly reduced the soil pH compared to CK. However, the contents of soil NH₄⁺-N and NO₃⁻-N were significantly increased. Some beneficial bacteria genera (Burkholderia, Mortierella, and Trichoderma) had a certain correlation with the incidence. Burkholderia and Mortierella were negatively associated with morbidity, but Trichoderma was positively associated with morbidity. Particularly, the Spearman correlation and the least partial squares path analysis indicated that Trichoderma was significantly positively correlated with the disease incidence, the soil physicochemical properties, and the numbers of soil pathogens (NSP) were significantly positively correlated with the number of root pathogens (NRP) and the physicochemical properties of plants were negatively correlated with the disease incidence. Moreover, BOF had better inhibitory effect on the occurrence of tomato bacterial wilt. Our results have implications for the better integrated management of tomato bacterial wilt.