Initial steps towards biocontrol in hops: Successful colonization and plant growth promotion by four bacterial biocontrol agents

Verticillium wilt, caused by Verticillium nonalfalfae and V. dahliae, is a devastating disease in hops that can cause considerable economic crop losses. The perennial use of hops combined with the long persistence of the pathogen in soil make it difficult to suppress the disease with conventional measures. Biological control agents (BCA) are the basis of an environmentally friendly plant protection strategy that uses plant promotion and antagonistic effects of microorganisms. We evaluated the effect of four selected beneficial bacterial strains, Burkholderia terricola ZR2-12, Pseudomonas poae RE* 1-1-14, Serratia plymuthica 3Re4-18, and Stenotrophomonas rhizophila DSM14405 T for their use in hops. All strains were shown to be both rhizosphere and endorhiza competent, and their abundances ranged from log 10 3.0 to log 10 6.2 CFU g− 1 root fresh weight in the endorhiza and from log 10 2.9 to log 10 4.7 CFU g− 1 root fresh weight in the rhizosphere with B. terricola ZR2-12 showing the highest overall cell densities. Microscopic visualization of DsRed-labeled transformants with confocal laser scanning microscopy showed different colonization patterns and confirmed the rhizosphere competence. Growth promoting effects on seedlings treated with bacteria were found for S. plymuthica 3Re4-18 and S. rhizophila DSM14405 T. Competent colonization and plant growth promoting effects are the most important prerequisites towards efficient biocontrol