Evaluation of phosphite to protect a South African Proteaceae from Phytophthora root rot

Abstract

Phytophthora cinnamomi is a globally recognised invasive plant pathogen, affecting approximately 5000 host species. In South Africa, previous research has linked P. cinnamomi to root rot in endemic Proteaceae, including Leucadendron argenteum. Recent observations have noted high mortality rates in L. argenteum, with P. cinnamomi readily isolated from the roots and collars of dying trees. Phosphite is commonly used to control Phytophthora diseases, but its efficacy in protecting native South African flora remains uncertain. To address this, trials were conducted to evaluate phosphite’s effectiveness against P. cinnamomi infection in L. argenteum. In the glasshouse trial, four-month-old seedlings were treated with 5 g/L phosphite and then inoculated with P. cinnamomi. Non-treated inoculated seedlings exhibited rapid wilting within three weeks, while treated seedlings showed significant disease reduction, with no difference in root weight and seedling height compared to non-inoculated controls. In the field trial, L. argenteum trees treated with 40 g/L and 50 g/L phosphite injections over 30 months showed no significant difference in survival rates compared to untreated controls. The lack of phosphite efficacy in the field trial was attributed to the presence of Armillaria spp. The study underscores the threat P. cinnamomi poses to L. argenteum populations and highlights the additional risk from Armillaria. While phosphite has demonstrated effectiveness against Phytophthora root rot, further investigation is needed to determine if P. cinnamomi and Armillaria spp. have synergistic effects on L. argenteum mortality. Additionally, exploring phosphite’s potential to protect plants from A. mellea at the seedling stage is warranted.