Towards developing an integrated approach for the treatment of white root rot in commercial avocado orchards

In 2016, Dematophora necatrix, the causal agent of white root rot (WRR), was identified in an avocado orchard in South Africa. Since then, this pathogen has spread to all avocado-growing regions in the country. This study aimed to evaluate the in-field efficacy of the biological control product EXTRASOL (Bacillus subtilis and Bacillus amyloquefaciens subsp. plantarum) and the fungicide fluazinam against WRR in two commercial avocado orchards in Tzaneen, Limpopo, South Africa. Over two growing seasons (2022 and 2023), disease severity, the number of D. necatrix-positive baits, and leaf area index (LAI) were monitored. Results showed that fluazinam significantly improved the health of WRR-symptomatic trees. At the start of the trial, symptomatic trees exhibited an average disease severity score of 4, which significantly declined to near 0 after two years, with symptoms largely absent. LAI of symptomatic trees treated with Fluazinam was significantly higher (> 4) in both orchards after two years. The combination of fluazinam and EXTRASOL performed similarly to fluazinam alone, while EXTRASOL alone provided minor benefits. Fluazinam residues also fell below detectable limits in the fruit of treated trees as soon as 2 weeks following treatment. These findings support fluazinam as a safe and effective treatment for WRR in South African avocado orchards.