Incidence, pathogenicity and diversity of Alternaria spp. associated with alternaria leaf spot of canola (Brassica napus) in Australia

Abstract

Studies were undertaken to determine Alternaria spp. associated with leaf spot symptoms on canola (Brassica napus) in two cropping seasons (2015, 2016) across southern Australia. Major allergen Alt a1 and plasma membrane ATPase genes were used to identify Alternaria spp. In 2015, 112 isolates of seven Alternaria spp. were obtained, with A. metachromatica predominating. In 2016, 251 isolates of 12 Alternaria spp. were obtained, with A. infectoria predominating. Alternaria spp. isolates were morphologically and phylogenetically identified and studies to determine their pathogenicity on both B. napus (cv. Thunder TT) and B. juncea (cv. Dune) confirmed 10 species (A. alternata, A. arborescens, A. brassicae, A. ethzedia, A. hordeicola, A. infectoria, A. japonica, A. malvae, A. metachromatica and A. tenuissima) as pathogenic on both Brassica species. Alternaria ethzedia, A. hordeicola and A. malvae were recorded for the first time in Australia on any host and the record of A. arborescens was the first for New South Wales (NSW) and South Australia (SA). Other first records included A. infectoria on B. napus in NSW; A. japonica on B. napus in NSW and Western Australia (WA); A. metachromatica on any host in NSW, Victoria (VIC), WA and SA; and A. tenuissima on B. napus in NSW, SA and WA. It is evident that alternaria leaf spot on canola across southern Australia is not solely caused by A. brassicae, but that a range of other Alternaria spp. are also involved to varying degrees, depending upon the year and the geographic locality.

Virulence variability across the Alternaria spp. population determines incidence and severity of alternaria leaf spot on rapeseed

“…This species has been frequently reported as a pathogen on Brassicaceae in Australia—for example on rapeseed and mustard (Al‐lami et al . , ); on Chinese cabbage, hoary stock ( Matthiola incana ), wild radish (Australian Plant Pest Database, )—and is considered a significant pathogen in Italy on wild and cultivated rocket (Garibaldi et al . , ; Gilardi et al .…”

“…29 and 135 isolates in 2015 and 2016 cropping seasons, respectively (Al‐lami et al . , ). The number of isolates used for each species was: 9 for A. alternata , 2 for A. brassicae , 8 for A. conjuncta , 19 for A. ethzedia , 11 for A. eureka , 10 for A. hordeicola , 14 for A. infectoria , 4 for A. japonica , 18 for A. metachromatica , and 6 for A. tenuissima .…”

“…A single susceptible genotype, B. napus ‘Thunder TT’, was used for virulence tests (Al‐lami et al . , ). Ten replicate pots of each cultivar were used for each isolate, each pot sown with four seeds (pots 6 × 6 × 9 cm) and the subsequent seedlings were thinned to one plant per pot.…”

“…had previously been phylogenetically identified (Al‐lami et al . , ) using sequencing of two loci, major allergen Alt a 1 (Hong et al . , ) and plasma membrane ATPase (Lawrence et al .…”

“…associated in one location (Al‐lami et al . , ). In association with 2015–2016 surveys of rapeseed crops across southern Australia (authors’ unpublished data), 363 isolates of Alternaria spp.…”