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.

Phytophthora species are important pathogens of citrus. They cause fibrous root rot, among other diseases, that lead to significant yield losses of economic importance. The management of Phytophthora diseases of citrus relies on chemicals of which phosphonates form an integral part. Phosphonates are unique in their complex, multipronged mode of action that remains poorly understood. Due to this attribute, they are considered to be at low risk of resistance development. Despite this, there have been recent reports of reduced phosphonate sensitivity in various Phytophthora species including those of relevance to citrus. Therefore, resistance management strategies guided by evolutionary principles should be strictly adhered to, to avoid the selection of resistant strains and a concomitant population shift in sensitivity. Knowledge of fitness costs associated with reduced phosphonate sensitivity in Phytophthora is lacking. Therefore, the aim of this review was to compile the available information on phosphonates and their current efficacy against Phytophthora diseases of citrus in South Africa. Resistance management strategies guided by evolutionary principles and the relevance of fitness costs were also investigated.