Influence Mechanism Of Rhizosphere PH Value On Cucumber Fusarium Wilt

Cucumber（Cucumis sativus L.）is an important fruit vegetable in protected cultivation in China,accounting for nearly one-fifth of the total area of protected vegetables.The specialization of vegetable facility production requires continuous planting of the same local main crops in the same facility soil,and cucumber is a vegetable prone to continuous cropping diseases;at the same time,unreasonable fertilization in facility production leads to widespread soil acidification,which not only affects plant growth and development,but also aggravates the occurrence of soil-borne diseases.Fusarium wilt is an important soil-borne disease of cucumber caused by Fusarium oxysporium f.sp.cucumerinum（FOC）.Our previous studies have found that the occurrence of cucumber Fusarium wilt is positively correlated with the number of FOC in soil and matrix.A large number of studies have shown that the occurrence of plant Fusarium wilt is also related to soil pH value.However,the key factors and internal mechanism of rhizosphere pH value affecting the occurrence of cucumber Fusarium wilt are unclear.In this study,H₂SO₄and Na OH are used to adjust pH value,and Na₂SO₄is used to adjust EC value,so that the pH value of each treatment is consistent with the EC value and in the normal growth range of cucumber.The effects of pH 4.5,5.0,5.5,6.0,6.5,7.0 and 7.5 on the key pathogenic factors of FOC,the key factors of cucumber root growth and disease resistance,and the occurrence degree of cucumber Fusarium wilt were systematically explored and compared.Then,the main factors and mechanisms of soil pH affecting the occurrence of cucumber Fusarium wilt were comprehensively analyzed,in order to provide technical support for the maintenance of soil health and the green prevention and control of cucumber Fusarium wilt.The main results are as follows:1.The study of pH value on the pathogenicity of pathogens showed that the content of fusarium acid under weak acidic conditions（pH 5.5～6.0）was higher than that under neutral and weak alkaline conditions（pH 7.0～7.5）,and the content was the highest under pH 5.5treatment.The activities of cell wall degrading enzymes such as with endo-1,4-β-D glucanase（Cx）and exo-1,4-β-D glucanase（C1）,β-glucosidase（βG）,polymethylgalacturonase（PMG）,polygalacturonase（PG）,pectin lyase（PL）and pectin lyase（PNL）showed the highest activity at pH 5.5,similar to that of toxin content at different pH values.FOC was cultured under different pH conditions,and the effects of crude toxin and cell wall degrading enzyme in mycelium filtrate on cucumber root damage were determined.The results showed that the electrolyte permeability of cucumber root reached the highest at pH 5.5,and the damage degree of root was mycelium filtrate>crude toxin>cell wall degrading enzyme.2.The results of pH value on the resistance of cucumber roots showed that the content of resistant substances was the highest at pH 6.5 when host and pathogen interacted,as the highest content of flavonoids was 5.4 mg·g^-1at 3 d after treatment and 1.5 mg·g^-1at pH 5.5.The expression of disease-resistant genes was the highest at pH 6.5 or pH 7.0,and the expression of genes below pH 5.5 was low.Flavonoids and total phenols were significantly negatively correlated with the incidence（P<0.01）.3.The study of pH value on the incidence of cucumber showed that the highest disease index of FOC was 75.0%at pH 5.5 and the lowest disease index was 38.9%at pH 6.5 under the condition of mutual resistance between host and pathogen.At pH 5.5,the colonization rate of pathogens in cucumber stems and roots was fast and the amount of fungi in plant stems was large.Rhizosphere pH affected the occurrence of FOC by affecting the pathogenicity of FOC,cucumber resistance and pathogen infection.Rhizosphere pH affected the key pathogenic factors of FOC:Fusarium acid,cell wall degrading enzyme and pathogenic gene expression.At pH 5.5,the content of Fusarium acid and cell wall degrading enzyme was high,and the expression of pathogenic genes FGA2,CHSV,GAS1,FRP1,SNF1,PG1 and FGA1 was high.Rhizosphere pH affected the key resistance factors of cucumber:lignin,phytoalexin,disease-related protein,callose and resistance genes were highest at pH6.5.Rhizosphere pH affected the infection rate of FOC in cucumber plants.At pH 5.5,FOC colonized rapidly and colonized more in the aboveground part of cucumber.To sum up,rhizosphere pH value affected the key pathogenic factors of Fusarium wilt and the key resistance factors of cucumber plants.The incidence of cucumber Fusarium wilt was the highest at pH 5.5 and the opposite at pH 6.5.