An integrated approach for the evaluation of biological control of the complex Polymyxa betae/Beet Necrotic yellow vein virus, by means of seed inoculants

Rhizomania is an extremely severe sugarbeet disease caused by the complex Polymyxa betae/Beet Necrotic Yellow Vein Virus (BNYVV). A relatively small number of recently introduced sugarbeet cultivars characterized by a high tolerance to rhizomania are available on the market. An integrated approach was therefore developed using Pseudomonas fluorescens biological control agents (BCAs) in order to improve yield performance of cultivars characterized by a medium tolerance to the disease. A genetically modified biological control agent, Pseudomonas fluorescens F113Rif (pCUGP), was developed for enhanced production of the antimicrobial metabolite 2,4-diacetylphloroglucinol (Phl) and lacking an antibiotic resistance marker gene, making the strain suitable for field release. The ability of synthetic Phl and P. fluorescens F113Rif (pCUGP) to antagonize the fungal vector, P betae, was assessed in microcosm trials. Results encouraged the preparation of multiple field trials in a soil naturally infested with P. betae/BNYVV, to determine the biocontrol efficacy of P. fluorescens F1 13Rif (pCUGP) and to assess its impact on sugarbeet yield and quality and on the indigenous microbial population. While the colonization ability of P. fluorescens Fl 13Rif (pCUGP) was satisfactory at sugarbeet emergence (2.5 × 10 6 CFU g -1 root), control of rhizomania was not achieved. Inoculation of sugarbeet with Pseudomonas fluorescens Fl 13Rif (pCUGP) did not affect crop yield and quality nor affect the numbers of selected microbial populations.