Bacillus sp. A8a reduces leaf wilting by Phytophthora and modifies tannin accumulation in avocado

The objective was to assess the biocontrol capacity of Bacillus sp. A8a in avocado (Persea americana) plants infected by Phytophthora cinnamomi. A greenhouse experiment was implemented with four treatments: 1) control plants; 2) plants infected with P. cinnamomi; 3) plants inoculated with Bacillus sp. A8a; 4) plants infected with P. cinnamomi and inoculated with Bacillus sp. A8a. We evaluated several morpho-physiological variables during the experiment, which lasted 25 days after infection (dai). Moreover, we analyzed tannin density in stems at 25 dai to determine the plant defense response against the disease. Inoculation with strain A8a reduced wilting symptoms by 49 % at 25 dai, compared with non-inoculated plants. No differences were detected in morphophysiological variables between treatments. However, a greater tannin accumulation was registered in the xylem of infected plants, whilst plants inoculated with strain A8a displayed a larger tannin density in the cortex. Our results confirm the biocontrol activity of Bacillus sp. A8a in avocado plants and suggest that tannin differential accumulation in the cortex of plants inoculated with the bacteria may contribute to the enhanced tolerance of avocado plants against Phytophthora root rot.