A review of the use of phosphonates in the management of Phytophthora nicotianae in citrus in South Africa

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.