Fruit abscission in macadamia due to husk spot disease

Abstract

Macadamia is indigenous to Australia. Only two species; M. integrifolia and M. tetraphylla and their interspecific hybrids are considered edible and they constitute the commercial macadamia industry. A fungal pathogen, Pseudocercospora macadamiae, causes husk spot in Australia, resulting in premature fruit abscission giving rise to nuts with low oil content. P. macadamiae is believed to have co-evolved with macadamia and the fungus infects the fruit pericarp (husk) via open stomata and grows intercellularly throughout the parenchymatous tissue where it causes accelerated abscission of immature and mature fruit. Depending on the time of infection, environmental conditions, and cultivar, the time to fruit abscission at the onset of husk spot symptoms may take up to 18 weeks. Factors that break the latency and influence fruit abscission due to P. macadamiae infection are unknown and may be involved in the production of ethylene by the husk tissue and/or abscisic acid by the fungus. Comparison of 18 macadamia genotypes revealed variations to accelerated fruit abscission from husk spot lesions on abscised fruit. This may be related to variation in stomatal abundance in the genotypes. The relationship between kernel maturation, in terms of oil content, fruit abscission in diseased and healthy husk is discussed.