The current outbreak of stemphylium leaf blight of onion in New Zealand – identification of cause and review of possible risk factors associated with the disease

During the 2017–18 growing season, significant outbreaks of leaf blight occurred in Pukekohe, Hawke’s Bay and Canterbury commercial onion fields. It was unknown if the causal agent was Stemphylium vesicarium, a pathogen already present in New Zealand that causes stemphylium leaf blight (SLB), or a new introduction of another Stemphylium species. Morphological and molecular characterisation methods were used to identify the pathogen present on diseased onion leaves. The possibility that climate may have been a contributor to the outbreak was evaluated using hourly temperature and relative humidity data, and comparing the 2017–18 growing season with the previous four seasons in these regions when no disease was observed. Our research indicates that the recent leaf blight outbreak in New Zealand was caused by S. vesicarium, and not the introduction of a novel species of Stemphylium. The warm, and wet summer of 2017–18 possibly contributed to the SLB outbreak.