The root rot pathogen Phytophthora cinnamomi: a long-overlooked threat to the Cape Floristic Region of South Africa

Abstract The globally important plant pathogen Phytophthora cinnamomi was first reported in South Africa in 1931, where it caused substantial damage to avocado orchards. Surprisingly, 40 years passed before the pathogen was recognised as a significant threat to South Africa’s natural ecosystems. This first became evident when P. cinnamomi caused a “quick decline” of the iconic silver trees ( Leucadendron argenteum ) in the Cape Floristic Region (CFR) of the Western Cape Province. Subsequent research has underscored the role of P. cinnamomi as a major root rot pathogen affecting numerous native species. Despite these findings, there has been limited research on the extent of the threat P. cinnamomi poses to Cape flora, leaving the risk of extinction for many species largely unknown. A recent observation of P. cinnamomi causing rapid mortality in Sorocephalus imbricatus, a Critically Endangered Proteaceae, underscores the urgent need for a comprehensive evaluation of this pathogen’s impact on Cape flora and the associated extinction risks. Given the high number of rare and threatened species in the CFR, many of which belong to families known to be vulnerable to P. cinnamomi , there is a pressing need to initiate an intensive local research programme to fill this critical gap. To address this, we propose a structured research programme that will guide targeted mitigation efforts against P. cinnamomi . Enhancing our understanding of P. cinnamomi ’s threat to the CFR, a global biodiversity hotspot, will be essential to inform conservation strategies and to set restoration priorities in the region.