Abstract
Alternaria sp. infections on apple (Malus × domestica Borkh.) lead to impaired fruit quality and yield losses by leaf blotches and fruit spots, caused by host-specific toxins (HSTs) of the Alternaria apple pathotype like AM-toxins. Fungal isolates were obtained during severe outbreaks on cv. Gala, Golden Delicious, and Cripps Pink(cov)/Rosy Glow(cov) in South Tyrol and other regions in northern Italy. The isolates were tested for pathogenicity using in vitro assays with detached apple leaves. Conidial suspensions of pathogenic isolates were shown to provoke necrotic lesions also in apple seedlings and on fruits. Detached-leaf assay-guided fractionation of the isolates' culture supernatant and a high-resolution liquid chromatography-mass spectrometry (LC-MS) analysis tentatively identified 27 known Alternaria phytotoxins and a new putative toxin, (3ξ,4ξ,5ξ,6ξ,7ξ,11ξ)-3,6-dihydroxy-8-oxo-9-eremophilene-12-oic acid (1). The constitution and the relative configuration of the ring stereocenters of 1 were elucidated by NMR spectroscopy, revealing unique structural features among Alternaria phytotoxins. Indeed, molecular analysis revealed the lack of the toxin-related genes AMT1, AMT4, and AMT14 in all isolates from the region, suggesting that Alternaria apple blotch in the area was associated with another metabolite (1).
Highlights
The genus Alternaria encompasses a plethora of saprophytic or pathogenic fungal species, and several of them are known to produce secondary metabolites, including fungal toxins
Ca. 32 host-specific toxins (HSTs) and 34 nonhost-specific toxins (nHSTs) produced by 14 Alternaria species or Alternaria pathotypes have been reported
24 Alternaria sp. isolates obtained from naturally infected fruit or leaves during Alternaria outbreaks in northern Italy in the 2000s were tested for pathogenicity in a detached-leaf bioassay: 20 isolates were from nine sampling sites in the South Tyrol province (Table S1, Supporting Information, entries 01−03 and 08−24) and four from the neighboring Trentino province (Table S1, entries 04−07)
Summary
The genus Alternaria encompasses a plethora of saprophytic or pathogenic fungal species, and several of them are known to produce secondary metabolites, including fungal toxins. The ability to synthesize HSTs was often used to delineate Alternaria species; the taxonomy of the genus is complex and requires a combination of morphological and genetic characteristics: Conidia, the sporulation apparatus, and the sporulation patterns are considered as key taxonomic features for the Alternaria species discrimination but can be influenced by environmental conditions.[5,7−9] Molecular studies on traditional fungal barcoding genes confirmed the morphological delineation of large- and small-spored taxa but these loci often do not allow differentiation on lower taxonomical levels.[10,11] more variable phylogenetic loci have been identified,[10−14] and recently, Armitage et al.[13] proposed three diverging phylogenetic lineages within the A. alternata species group, described as A. alternata ssp. These lineages are in compliance with the recommendation of Tsuge et al.[15] that morphological differences within the HST-producing Alternaria fungi should be considered as intraspecific variability and not as different species.
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