Biological control of hairy root (Rhizobium rhizogenes) in apple nurseries through Rhizobium radiobacter antagonists (strain K-84 and native strain UHFBA-218)

Hairy root caused by Rhizobium rhizogenes is one of the major hindrances in raising quality nursery apple plants. Based on the survey analysis, carried out in the present study, disease incidence ranging from 3.00 to 21.05% was recorded in different apple nurseries of Himachal Pradesh. The plant pathogen isolated from rhizosphere soil of hairy root-infested apple seedling rootstock, in the present study, identified as Rhizobium rhizogenes strain UHF P10 (NCBI Gen Bank accession no: MG745303.1) shared 100% homology with R. rhizogenes and belonged to biovar 2. Application of Rhizobium radiobacter antagonists (Strain K-84; native strain UHFBA-218 and their streptomycin and rifampicin-resistant mutants) as seed treatment completely inhibited hairy root disease. The application of strain UHFBA-218 as root dip reduced hairy root incidence to 3.33% whereas, application of K-84 as root dip minimized incidence to 4.99 %. The hairy root incidence in untreated plants during the first and second year of field trials was 18.57% and 20.23%, respectively. Seed and root dip application of both strains of R. radiobacter (K-84; native strain UHFBA-218) along with their antibiotic-resistant counterparts also promoted the growth of apple plants and increased population of antagonists.