Characterisation of Macadamia nut diseases in South Africa

Abstract

The aim of this dissertation was to generate information regarding the species of fungi involved in husk rot disease of macadamia fruits namely their identity, prevalence, pathogenicity, and growth response to temperature. Work presented in this dissertation includes an overview of the nut diseases of macadamia focusing on husk rot and factors impacting on disease development. Surveys of the three main producing regions in South Africa, KwaZulu Natal, Limpopo, and Mpumalanga, were conducted over the 2019/2020 – 2021/2022 seasons. Three new Calonectria husk rot pathogens (Calonectria sp. nov. 1 & 2, Ca. pauciramosa) were identified in Limpopo and Mpumalanga (Calonectria husk rot – CHR). Colletotrichum associated with husk rot was identified to a species level for the first time revealing four species, C. alienum, C. fructicola, C. siamense and C. theobromicola (Anthracnose husk rot – AHR). Putative identities were determined for Diaporthe shedding light on the potential diversity of species associated with macadamia with up to ten putative species found in this study (Phomopsis husk rot – PHR). Ten of the total 17 putative species of fungi associated with husk rot, were able to produce lesions in vitro, a number producing severe lesions during the 14-day incubation. Wounding, often considered vital in the development of AHR and PHR, was unnecessary for successful infection by Calonectria spp. Prevalence of different species also revealed intriguing clues to the epidemiology of these diseases (AHR, PHR and CHR). Calonectria spp. were mostly obtained from Limpopo with no accompanying causal agents detected. In Mpumalanga, Diaporthe showed a tendency for higher isolation frequency and species numbers in early-stage fruits, appearing early rather than later in a season. The presence of Colletotrichum on early stages of fruit development (pea-sized) was confirmed however the prevalence of Colletotrichum increased with season progression and more mature developmental stages in this study. Early detection paired with later symptom development is a line of evidence for latent infections for both Diaporthe and Colletotrichum. Improved understanding of trends in prevalence with regards to Diaporthe and Colletotrichum is dependent on a more accurate taxonomic study into Diaporthe spp. as the potential for endophytic species in early-stage fruits is high. Different responses to temperature observed in species across the three genera may play a role in their prevalence. Calonectria spp. grew comparatively well at the lower temperatures as compared to 126 Colletotrichum and Diaporthe but was disproportionately hindered by heat with 35°C proving lethal. Colletotrichum on the other hand had optimal temperatures of both 25°C and 30°C with growth still occurring at 35°C as was also observed for Diaporthe isolates. More extensive sampling of husk rot in Limpopo and KwaZulu Natal will also aid in understanding the impact that environmental factors and different causal agents may have on the incidence of husk rot in South Africa as a whole. This work has added to the knowledge around the pathogens of husk rot in South Africa. Future work with regards to inoculum sources, infection methods and broader pathogenicity testing of isolates is needed for development of integrated strategies for improved management of this disease in the future.