Identification and characterization of the pathogens associated with tea red rust in India and their biocontrol under field conditions

• Cephaleuros virescens and C. parasiticus from tea red rust were identified. • Stable 7-month liquid formulations from B. paramycoides and M. barkeri were made. • Both formulations effectively controlled tea red rust in lab and field tests. • Both formulations induced defense-related enzymes against the pathogens. • B. paramycoides and M. barkeri are promising agents for tea red rust control. Red rust of tea, caused by Cephaleuros spp., is an algal disease that can lead to substantial yield losses in major tea-growing countries, including India. This study first identifies and characterizes the causal agents associated with algal red rust disease and then focuses on their field management through a 3-year field trial (2021 to 2023) across three experimental sites. Micro-morphological, host invasion, and phylogenetic (18S SSU) analysis revealed the association of Cephaleuros virescens (subcuticular) and C. parasiticus (intercellular) with the disease. Liquid bioformulations of two bacterial antagonists, namely Bacillus paramycoides BOK01 and Microbacterium barkeri BPATH02, were prepared and tested against the pathogens under laboratory and field conditions. The efficacy of the biocontrol formulations was compared with a copper-based chemical (i.e., Copper oxychloride), commonly used in India. Under controlled conditions, a 20 % suspension concentrate liquid formulation (1 × 10 8 CFU/ml) of both bacterial antagonists reduced over 66 % spore viability of Cephaleuros species at 2 % concentration. Field trials of the two bacterial-based bioformulations showed over 60 % disease reduction at 2 % and 5 % concentrations compared to the control. Both bioformulations were viable for up to seven months of storage, exhibiting a high population density (i.e., >1 × 10 8 CFU/ml). Copper oxychloride provided significantly higher disease reduction (>68 %) than the bacterial bioformulations across all locations and years, but no significant yield difference occurred between the two treatments. Taken together, we conclude that B. paramycoides BOK01 and M. barkeri BPATH02-based liquid bioformulations have good potential in the integrated management of algal red rust in tea gardens.