Paenarthrobacter nitroguajacolicus can promote cucumber growth and control cucumber corynespora leaf spot.

Currently, the disease control in cucumber mainly depends on agrochemicals, which is not an environmentally benign strategy. Biocontrol bacteria not only resist plant pathogens but also promote plant growth, which is ecofriendly and sustainable option. A biocontrol bacterial strain BJ-5 was screened using Corynespora cassiicola as the target pathogen, and BJ-5 was determined to be Paenarthrobacter nitroguajacolicus by morphological and molecular methods. The effect of BJ-5 on C. cassiicola was studied, including the spore germination, cell membrane permeability and infected cucumbers. BJ-5 inhibited the germination of C. cassiicola spores in vitro and led to atrophy and deformation of the C. cassiicola budding tubes. BJ-5 caused the relative extracellular conductivity of C. cassiicola mycelia to increase compared with the control. Additionally, BJ-5 reduced the severity of cucumber corynespora leaf spot of cucumber infected with C. cassiicola. The inhibition efficacy of BJ-5 suspension as a foliar spray against cucumber corynespora leaf spot reached 63% inhibition, which is higher than a 5000-fold dilution of Luna-Son SC fungicide. In addition, BJ-5 was tested on the emergence of cucumber seedlings, recording the biomass and photosynthesis of cucumber during the growth period. BJ-5 at 1.5 x 105 CFU·mL-1 promoted the germination of cucumber seeds and increased biomass and photosynthesis at the adult plant stage. Also, the secondary metabolites of BJ-5 were determined. BJ-5 could produce chitinases, siderophore, cellulase, amylase and protease in the respective medium. Finally, adaptation assay of BJ-5 showed good salt tolerance and good adaptability in alkaline conditions, and that BJ-5 retains inhibition of fungi activity at higher temperatures. This is the first report of the biocontrol by P. nitroguajacolicus with antagonism to C. cassiicola and promote cucumber growth. This study indicates that P. nitroguajacolicus may serve as potential biocontrol agents against cucumber corynespora leaf spot fungus. [ABSTRACT FROM AUTHOR]