Characterisation of Macadamia flower blight in South Africa and efforts towards chemical control

Abstract

Macadamia flower blight diseases have the ability to severely damage the yield of commercial macadamia farms, leading to financial loss for the industry. Understanding the species diversity and prevalence of flower blight fungi may provide critical knowledge in developing more effective control methods as well as optimising application timing for control methods in South African macadamia orchards. The fungal species that cause macadamia flower blight in South Africa have not been determined, and their pathogenicity status towards macadamia flowers are also unknown. Thus, the aim of this study was to identify the fungal species present on diseased macadamia flowers and racemes in South Africa. Known macadamia flower blight pathogens, first described in Australia, were identified in this study in addition to eight potentially novel Neopestalotiopsis species. Potentially novel species of Pestalotiopsis and Cladosporium were also identified. Neopestalotiopsis was found to be the most prevalent flower blight fungus on diseased macadamia flowers in South Africa, with N. macadamiae being the most prevalent species to be identified. The HAES 788 cultivar was the most susceptible to infection by Neopestalotiopsis. The pathogenicity of N. macadamiae towards two commonly planted macadamia cultivars was confirmed through field pathogenicity trials employing Koch’s Postulates. Although commercial farms are slowly moving away from chemical control, due to the hazard these chemicals hold to both humans and the environment, it remains an important aspect of an integrated disease control strategy. Therefore, the in vitro efficacy of 12 fungicides against the mycelial growth and conidial germination of N. macadamiae and C. cladosporioides was evaluated. The EC50 metric was used to compare relative efficacies of different fungicides. All tested fungicides had an inhibitory effect on mycelial growth for both species. However, for conidial germination, copper showed no inhibitory effect on either species with fluxapyroxad that exhibited no effect on N. macadamiae. The most effective fungicide of both pathogens was statistically found to be pydiflumetofen (FRAC 7) based on mycelial growth and conidial growth inhibition. Other fungicides from FRAC groups 3 (difenoconazole), 7 (boscalid), 12 (fludioxonil) and M05 (chlorothalonil) can also be regarded as additional candidates to use in control. The use of fungicides from different FRAC groups is important to manage development of resistance due to repeated exposure.