Macadamia is an economically important tree nut that is native to Australia and currently commercially cultivated in many tropical and subtropical regions in Africa, Asia, and North and South America. Among the four species of macadamia, Macadamia integrifolia and M. tetraphylla and their hybrids are commercially produced for their edible cream-coloured kernels. Botryosphaeria branch dieback caused by Botryosphaeriaceae fungi is an emerging threat to the macadamia industry. The dearth of knowledge on the biology and ecology of the causative pathogens often impairs branch dieback management in macadamia orchards. An improved understanding of etiology of the Botryosphaeria branch dieback and the factors that influence the spread of the pathogens and promote infection would aid disease control. The specific aims of this study were to (i) determine the diversity and pathogenicity of Botryosphaeriaceae fungi associated with branch dieback disease of macadamia in Australia, (ii) examine the seasonal dynamics of conidial dispersal in macadamia orchards, (iii) determine the influence of soil moisture conditions on the severity of Botryosphaeriaceae infection and impact on plant growth, and (iv) examine the pathogenic variation of Botryosphaeriaceae species on major macadamia cultivars. To identify and determine the causal agents of Botryosphaeria branch dieback, various plant parts were collected from 59 commercial macadamia orchards between 2019 and 2021. The samples included branches (>15 mm in diam.), twigs (0.8) between the sap flow rates and disease severity under the three watering regimes was established, which suggests that soil moisture is a critical factor that influences the severity of Botryosphaeriaceae disease development in macadamia (Chapter 5). This study showed that regardless of the plant part from which isolates were initially derived, significant pathogenic variation exists among Botryosphaeriaceae isolates within the same species on six main macadamia cultivars. The results grouped the most aggressive species (L. iraniensis and N. luteum), together with the less susceptible cultivars (‘Beaumont’, ‘HAES 246’, ‘HAES 816’ and ‘Variety P’) and partitioned the most prevalent species, L. pseudotheobromae, separately from all the six cultivars. The most susceptible cultivars (‘A268’ and ‘HAES 344’) clustered with the moderately aggressive species, L. theobromae (Chapter 6). This study has established the etiology and disease cycle of Botryosphaeria branch dieback in macadamia, regarding the sources and timing of inoculum of the pathogens. In addition, the knowledge of the optimal sap flow rates for reducing disease severity provide a crucial information required to manage the risk of infection and the severity of the disease after infection. Inference from this research suggests that winter pruning is less likely to pose a risk of Botryosphaeriaceae infection compared to summer or autumn pruning practices in macadamia orchards.