Impact of phytoplasm (Candidatus Phytoplasma australiense) on cabbage trees (Cordyline australis (Forst.f.) Endl.) throughout New Zealand, 1990–2014

Abstract

ABSTRACT Worldwide, tree species sometimes fall victim to epidemics of microbial pathogens, usually spread by sap-sucking insects. One such example is the recent devastation of cabbage trees (Cordyline australis) in New Zealand by a damaging phytoplasm (‘Candidatus Phytoplasma australiense’). Cabbage trees are among New Zealand’s most emblematic plant species, playing an iconic role in many landscapes. They were first reported dying in Northland in 1987, of a condition given the name ‘sudden decline’. The small hemipteran Zeoliarus oppositus is now known to be the main vector spreading the disease. Here I document the establishment and spread of the disease throughout the North Island and upper South Island of New Zealand over a 24-year period. In this study, some 31,000 living and dead cabbage trees were counted on roadsides in the North Island and northern South Island, in 1990, 1995, 2000, 2006, and between 2012 and 2014. Infected trees lost their leaves, leaving dead white trunks standing until they rotted away over two or three years. Large, mature trees were particularly susceptible to infection and were all but wiped out in many parts of the North Island districts of Northland, Bay of Plenty, the Waikato, King Country, and Hawkes Bay, transforming some landscapes. The South Island was almost free of the disease, with the exception of Marlborough and Nelson. The percentage of dead trees peaked between 1990 and 2006 but declined by 2012–2014 as susceptible large trees became rarer. Differences in the insect vector’s regional abundance are thought to account for the national pattern of deaths.