Characterisation of husk rot in macadamia

Abstract

The causal agent of husk rot of macadamia is often attributed to Colletotrichum gloeosporioides sensu lato. However, in recent husk rot outbreaks, the characteristic concentric ring of pycnidia of C. gloeosporioides that is associated with the disease was often absent. Due to its sporadic occurrence, the importance of husk rot is often underrated and attributed to environmental and physiological factors. In order to determine the significance, prevalence and factors that influence husk rot in macadamia, this study examined the aetiology of husk rot in Australia. The relative incidence and severity of husk rot was evaluated in several macadamia orchards over eight consecutive years. Pathogenicity assays were developed to confirm the identity of the causal agent. A range of fungi from several genera including: Diaporthe, Lasiodiplodia, Colletotrichum, Pestalotiopsis, Aspergillus, Alternaria, Nigrospora and Epicoccum were isolated from samples of macadamia pericarps with husk rot symptoms from different orchards. Fungi in the genus Diaporthe were most frequently isolated, often from symptomatic fruit. Results from pathogenicity trials showed the characteristic soft or spongy black lesions characteristic of husk rot symptoms in wounded fruits that were incubated with the diseased fruit or inoculated with a conidial suspension of Diaporthe spp. Our results suggest that injury to the macadamia fruit pericarp not only predisposes the pericarp to pathogen infection but it is a prerequisite for infection. Large variations in husk rot severity were observed over years. Husk rot severity was linked to days after anthesis and was associated with mean weekly relative humidity and minimum temperatures. This study confirmed that Diaporthe species cause husk rot in macadamia, hence, a rationale for adopting Phomopsis husk rot as the name of the disease is discussed.