| Melon is one of the most important fruit crops in China,and melon planting has become the dominant industry in many counties and districts in Shaanxi Province.Facility agriculture has high controllabilitis and intensiveness.Similarly,facility planting of melon has become the main melon planting mode,which was accompanied by a continuous cropping obstacle(CMO).After continuous cropping,the pests and diseases are serious,soil fertility declines,yield and quality decline,soil microbial community structure is unbalanced.Soil phenolic acid accumulation,soil microbial community deterioration and root-knot nematode are considered to be the main causes of continuous cropping obstacles.Adding biocontrol bacteria can effectively alleviate continuous cropping.Coumaric Acid(CA)is a major phenolic acid in melon root exudates.The purpose of the current study was to evaluate the elimination of CA by biocontrol bacteria K3,the effect of K3 on soil microbes and its effect on controlling melon root knot nematode(RKN).The degradation of CA by K3 was qualitatively and quantitatively determined by single carbon source culture method and High Performance Liquid Chromatography(HPLC).The effects of K3 on microbial community in rhizosphere soil of melon seedling were studied by dilution coating plate method and Illumina Mi Seq sequencing method.The effects of CA and K3 on the growth of melon seedlings were measured under potting conditions.The changes of soil enzymes and their effects on the quality of melon fruits were studied by greenhouse experiments.In addition,pot experiment and greenhouse test were conducted to study the effect of K3 on the control of melon root knot nematode.The main findings are as follows:1.The main phenolic acid CA secreted by melon roots could be eliminated by K3.(1)Co-culture test results show: i.Co-culture of the bacterial suspension of soil(the soil didn’t plant melon before.)and K3 fermentation supernatant(compared with pure medium co-culture),the number of soil culturable bacteria increased by 33.06%.The number of fungi decreased by 57.98%,and the difference reached a significant level(P < 0.05).ii.After the bacterial suspension of soil was co-cultured with CA,the number of soil fungi was significantly higher than that of the control(P < 0.05).)13.12%,resulting in a significant decrease in the soil bacteria-to-fungi ratio.iii.After applying K3 agent under potting conditions and exogenous CA.The same trend in the number of rhizosphere microorganisms under potted conditions was observed.(2)Single carbon source culture and HPLC measurement results showed that K3 can decompose and utilize CA as a carbon source.K3 could also decompose CA under potting conditions to relieve the inhibitory of CA on melon seedling growth: compared with the normal plants without CA,the melon seedlings planted in soil containing exogenous CA grew slowly,specifically,the shoot fresh weight and root fresh weight were 33.97% and 40.91% lower than that of control,respectively.At the same time,the plant height,the number of leaves and agronomic traits such as root length decreased by31.84%,8.77% and 19.13%,respectively.Compared with the exogenous CA-treated melon seedlings,the hindrance of the seedlings was significantly alleviated after the K3 agent treatment.The high concentration of K3 was more obvious,and the number of leaves and root length increased significantly(P < 0.05)68.30%,40.38% and 69.13%.The difference between ground fresh weight and root fresh weight was significantly(P < 0.05)different from CK,increased by 1.83 times and 4.84 times respectively.In addition,leaf resistance enzymes and root activity of plants decreased when CA was added,and K3 showed the opposite effect after healing.2.K3 agent could optimize continuous microbial community structure and promote melon growth.(1)The melon seedlings cultivated in melon continuous soil showed obvious continuous cropping obstacles compared with which cultivated in healthy soil.The specific manifestations were that the plants were short and weak,and the roots were underdeveloped.After continuous addition of K3 microbial agents to the soil,the continuous cropping obstacles were improved,the plants grew better,the biomass increased,and the number of soil bacteria increased.(2)Soil microbial diversity analysis showed that the bacterial OTUs of continuous cropping soil increased after K3 treatment,and its characteristic bacterial OTUs was 90.00% higher than that of K3-free continuous cropping soil,while the fungal OTUs decreased.In addition,it was shown that the K3 treatment greatly increased the bacterial abundance of continuous cropping soil,and at the same time,it showed that the dominant species had obvious ecological advantages in the bacterial community.From the analysis of microbial community structure,the abundances of bacteria Proteobacteria and Bacteroidetes which positively correlated with soil health were increased.The relative abundance of plant pathogen Fusarium was significantly(P < 0.05)decreased by 84.61% in fungi,and Chaetomium spp.with plant biocontrol effect.It has increased significantly(P < 0.05)by about 6.49 times.Under greenhouse conditions,K3 treatment increased the number of soil bacteria,enhanced soil enzyme activity and improved melon fruit quality.3.K3 agent could significantly(P < 0.05)reduce the infection rate of melon root knot nematode.(1)Under potting conditions,after inoculation of root-knot nematode,the fresh weight and root dry weight of melon seedlings increased significantly(P < 0.05)by 81.01%and 225.00%,respectively;their plant height,number of leaves,first true leaf area and The number of lateral roots decreased significantly(P < 0.05)by 71.89%,47.50%,63.53% and38.32%,respectively.After treatment with K3,the fresh weight and dry weight of the ground increased significantly(P < 0.05)by 4.92 times and 3.68 times,respectively;the fresh weight of plants and the dry weight of plants increased significantly by 3.62 times and 1.97 times,respectively;plant height and first true leaf area and lateral root number significantly(P <0.05)increased by 1.60 times and 2.75 times respectively.The number of leaves and the number of lateral roots increased significantly(P < 0.05)by 37.93% and 69.70%,respectively.(2)Under the conditions of pot culture and greenhouse culture,K3 could significantly(P <0.05)reduce the infection rate of root-knot nematode,and the control effect under greenhouse conditions was more obvious,reaching 33.87%.In addition,K3 treatment can improve the fruit quality and yield of infected melon plants. |
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