Management Strategies for Macadamia Branch Dieback in South Africa

Abstract

Macadamia trees are potentially at risk of many diseases especially those caused by latent pathogens. The Botryosphaeriaceae and Diaporthaceae species are among the most important latent pathogens, with widespread distribution across many climate zones and regions. Due to many recent reports of branch dieback in macadamia trees in South Africa this study aimed to: (i) identify the causal fungal species; (ii) examine the aggressiveness of the species obtained; (iii) evaluate the reaction of macadamia cultivars against the most aggressive species; (iv) evaluate the effectiveness of fungicides on disease development and (v) understand pathogens distribution in nurseries as well as common woody plants surrounding macadamia orchards. The samples were collected from trees showing symptoms in 12 orchards and the obtained isolates were identified by DNA sequence data for the rDNA-ITS and TEF1-α loci. Seven Botryosphaeriaceae species (Diplodia eriobotryicola, D. pseudoseriata, Lasiodiplodia theobromae, L. pseudotheobromae, Neofusicoccum parvum, N. kwambonambiense and Pseudofusicoccum olivaceum) and two Diaporthaceae species (Diaporthe ilicicola and Di. macadamiae) were identified. While L. theobromae was the most aggressive species, it produced significantly shorter lesions on the cultivars A4 and Nelmak2 than on Beaumont when their tolerance and susceptibility were evaluated. Out of the 14 commercial tested fungicides in the laboratory, greenhouse and field, five fungicides were confirmed to be effective against L. theobromae. In the survey of pathogen distribution, one species of Diaporthaceae was found to be common between trees in orchards and those in the nursery, while three species of Botryosphaeriaceae were common between trees in orchards and surrounding trees. This study highlights the risks posed by Botryosphaeriaceae and Diaporthaceae species to the macadamia crop. The evaluation of cultivar tolerance and efficacy testing of fungicides provide valuable insights for disease management and mitigation strategies in macadamia orchards, emphasising the need for proactive measures in preserving tree health and productivity.