Biology and pathogenicity of fungi causing husk rot of macadamia in South Africa

Fungal diseases of macadamia fruit in South Africa have the potential to cause notable economic damage in this rapidly growing industry. To improve our understanding of the species involved in husk rot in macadamia orchards, a survey was conducted over two consecutive growing seasons to identify Colletotrichum, Diaporthe and Calonectria spp. that are associated with husk rot-infected macadamia fruits, and to investigate the occurrence of these fungi in asymptomatic and symptomatic fruits at the four stages of fruit development. Of the 425 fungal isolates obtained from the survey, Colletotrichum and Diaporthe were the most frequently isolated genera confirming the important role that these causal agents play in the husk rot epidemics. The detection of Calonectria species was low, only from symptomatic fruits and limited to a few locations in the main macadamia-producing provinces in South Africa. Colletotrichum and Diaporthe species were detected throughout the season at different stages of fruit development and in both symptomatic and asymptomatic fruits. The study confirmed that three fungal pathogens cause husk rot of macadamia in South Africa, with Colletotrichum and Diaporthe species that may have a latent phase in macadamia fruit. Studies of the growth characteristics of the husk rot pathogens revealed varied optimal growth temperatures, which may influence their prevalence in the different provinces in South Africa where macadamia is grown. The significance of the varied prevalence and biology of the causal agents in husk rot epidemics are discussed, which may be helpful management strategies.