The opportunistic pathogen,Neofusicoccum australe, is responsible for crown dieback of peppermint (Agonis flexuosa) in Western Australia

Abstract

Recently, severe dieback symptoms have been observed in Agonis flexuosa, a tree native to the south-west of Western Australia. It was suspected that these symptoms were caused by an opportunistic pathogen and a sampling strategy was designed to investigate the causal agent of the cankers in symptomatic trees and the presence (or absence) of this pathogen as an endophyte of asymptomatic trees. Neofusicoccum australe was the most commonly isolated species from cankers of A. flexuosa. It was also the only species isolated which was a known canker pathogen. Agreater diversity of fungi was isolated as wood endophytes of asymptomatic A. flexuosa, but N. australe was again the most common associate. In several cases the same vegetative compatibility group of N. australe was found in cankered and asymptomatic trees. The pathogenicity of isolates of N. australe from cankered and asymptomatic trees towards seedlings of A. flexosa was similar. These data support the hypothesis that the recent dieback of A. flexuosa is caused by a common fungal endophyte, which is capable of causing disease in a stressed host. The disease is not caused by an introduced pathogen. The inciting factors leading to the decline are still unknown.