Abstract
Botrytis blight, Cladosporium blight, Pestalotiopsis blight, and Phytophthora blight are distinct diseases of macadamia flowers. Rachis tip dieback (RTD) is part of the symptoms associated with all these flower blights. However, some racemes show flower blight without RTD, while others exhibit RTD but maintain healthy flowers. RTD has also been associated with physiological and environmental stresses. This study investigated the prevalence and etiology of RTD in 20 commercial macadamia orchards in Australia at flowering seasons in 2023 and 2024. From 307 raceme samples with RTD and 54 asymptomatic samples, 20 fungal genera were identified by DNA sequencing.
Neopestalotiopsis
(27.5%),
Epicoccum
(20.4%),
Diaporthe
(19.1%),
Cladosporium
(7.8%),
Alternaria
(5.6%), and
Fusarium
(5.6%) were the most frequently isolated genera from RTD samples. Pathogenicity assays with representative isolates and flower blight pathogens showed that
N. maddoxii
,
N. macadamiae
, and
E. italicum
were the primary pathogens associated with RTD. In addition, established flower blight pathogens were capable of inducing RTD symptoms, and species of
Diaporthe
and
Epicoccum
are reported here as novel pathogens of macadamia racemes. The high diversity of fungal species obtained revealed that multiple species can infect the same racemes and co-occur in the same tree expressing RTD symptoms. This study revealed that RTD has a biotic cause and is part of the flower blight disease complex in macadamia. The contributions of RTD to yield losses and a potential tool for predicting flower blight severity to support management decisions are discussed.