Identification of Leptosphaeria species causing stem canker and dry rot disease of Brassica species in New Zealand

Abstract

Leptosphaeria maculans and L. biglobosa are major pathogens of brassica crops, including canola (oilseed rape) and swede. Selection of canola cultivars with resistance genes to correspond with the avirulence genes of the local fungal population is an approach used overseas for controlling the disease. This study aimed to determine the main causal agent(s) in New Zealand, and, for a subset of 30 L. maculans isolates, the presence of 7 avirulence alleles (AvLm1, AvLm2, AvLm4, AvLm5, AvLm6, AvLm7, and AvLm11). The pathogenicity of these isolates on canola ‘Flash’ and swede ‘Highlander’ was also determined. Leptosphaeria maculans was the predominant species (n=127) recovered from leaf lesions, stem canker and dry rot tissue on canola, swede and turnip plants, with only 4 isolates identified as L. biglobosa recovered from stem cankers on canola, cauliflower and kale. All avirulence alleles were present in the New Zealand L. maculans population, with AvLm5 and AvLm6 amplified from all 30 isolates. The most common allele structure was avLm1-avLm2-avLm4-AvLm5-AvLm6- AvLm7-avLm11 (n=15, Avr refers to avirulence and avr refers to virulence allele) representing isolates recovered from canola, swede and turnip across New Zealand, followed by avLm1-avLm2-AvLm4- AvLm5-AvLm6-AvLm7-AvLm11 (n=7) only recovered from canola from Canterbury. L. maculans isolates were more pathogenic on swede ‘Highlander’ than canola ‘Flash’, forming more extensive stem cankers on swede after 65 days. There was no correlation between pathogenicity of the isolates and crop origin, or avirulence groupings observed. This study provides a base line evaluation of the avirulence allele frequencies in the New Zealand L. maculans population.