Biological control of boxwood blight by Pseudomonas protegens recovered from recycling irrigation systems

Boxwood blight caused by Calonectria pseudonaviculata (Cps) is an emerging disease in the United States. It has caused significant loss to the ornamental nursery and landscape industries. In this study, 1547 bacterial strains recovered from recycling irrigation systems were evaluated in vitro and in planta for their biological control efficacy against Cps. Three most potent stains 13A3, 14D5, and 34B6 identified all belonged to Pseudomonas protegens. Strains 13A3 and 14D5 reduced Cps mycelial growth by 95 to 100% in two sets of in vitro experiments including a 48-well plate assay and a dual culture assay. Strain 34B6 also reduced Cps growth by 93% in the 48-well plate assay and 66 to 72% in dual culture plates. In planta, strain 14D5 reduced Cps infection by more than 50%, while strains 13D3 and 34B6 by 40 to 46%. Pseudomonas protegens strains produced antifungal secondary metabolites that reduced Cps growth and infection. Other possible modes of action against boxwood blight are discussed.