Molecular identification of Stemphylium spp. from leaf blight of onions in New Zealand
During the 2017-2018 onion growing season, significant outbreaks of leaf blight occurred in the Pukekohe, Hawkes Bay and Canterbury commercial onion fields. With the use of sequencing data and observation of spores, the causal organism responsible for the outbreak was identified to species level. The results from the sequence data indicate that the outbreak of leaf blight observed in the 2017-2018 onion growing season was caused by Stemphylium vesicarium. Comparisons of the sequence data of these isolates with those of two New Zealand isolates from 1979 and 1983 revealed that the earlier isolates were also S. vesicarium. These results show that the recent outbreak of leaf blight in onions in New Zealand was not the result of the introduction of a novel species of Stemphylium. Instead, the likely cause of the outbreak of Stemphylium leaf blight (SLB) was environmental. It is likely that the warm, wet, humid summer of 2017-18, in conjunction with several storms with high winds, resulted in unusually severe SLB.