The effects of phosphite treatment against Phytophthora root rot disease on Leucadendron

Abstract

Leucadendron argenteum, commonly known as the silver tree, is a vulnerable Proteaceae species endemic to South Africa's Cape Floristic Region (CFR). Its conservation status is impaired by urbanisation, agricultural expansion, altered fire regimes, invasive alien plants and microorganisms, threatening its survival. Among these challenges, Phytophthora root rot caused by the invasive pathogen Phytophthora cinnamomi poses a significant risk and contributes to mortality in both cultivated and natural populations. Despite these threats, no established protocols exist for managing Phytophthora root-rot in this plant species, and other South Africa Leucadendron, species. This research was aimed at evaluating the potential of phosphite, a systemic fungicide, as a management tool against P. cinnamomi in the natural ecosystems of South Africa, assess the health status of existing L. argenteum populations, and screen natural populations for variations in susceptibility to P. cinnamomi. The study involved field surveys, controlled inoculation experiments and field trials to evaluate phosphite's efficacy as a fungicide. Significant variations were found in population dynamics and disease susceptibility across different Leucadendron argenteum populations. The Rhodes Memorial population exhibited high resistance to Phytophthora cinnamomi, suggesting a potential genetic basis for resistance. Controlled inoculation experiments confirmed phosphite's ability to mitigate disease symptoms in seedlings, although these results are allusive when compared to mature trees in field settings. In the field trials, another pathogen Armillaria mellea found, which impacted the results. In conclusion, this research underscores the urgent need for proactive management strategies to safeguard L. argenteum against the dual threats of habitat loss and pathogen invasion. It proposes phosphite as a potential intervention for the management of P. cinnamomi with considerations for refining application protocols and understanding long-term ecological impacts. Further work is required to also select individuals with natural resistance for a breeding and reestablishment program. The findings contribute valuable insights into the conservation of L. argenteum and provide a foundation for future research on disease management in vulnerable plant species within biodiversity hotspots like the CFR.