(3ξ,4ξ,5ξ,6ξ,7ξ,11ξ)-3,6-Dihydroxy-8-oxo-9-eremophilene-12-oic Acid, a New Phytotoxin of <i>Alternaria alternata</i> ssp. <i>tenuissima</i> Isolates Associated with Fruit Spots on Apple (<i>Malus</i> × <i>domestica</i> Borkh.)

Abstract

<i>Alternaria</i> sp. infections on apple (<i>Malus</i> × <i>domestica</i> Borkh.) lead to impaired fruit quality and yield losses by leaf blotches and fruit spots, caused by host-specific toxins (HSTs) of the <i>Alternaria</i> 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 <i>in vitro</i> 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 <i>Alternaria</i> phytotoxins and a new putative toxin, (3ξ,4ξ,5ξ,6ξ,7ξ,11ξ)-3,6-dihydroxy-8-oxo-9-eremophilene-12-oic acid (<b>1</b>). The constitution and the relative configuration of the ring stereocenters of <b>1</b> were elucidated by NMR spectroscopy, revealing unique structural features among <i>Alternaria</i> phytotoxins. Indeed, molecular analysis revealed the lack of the toxin-related genes <i>AMT1</i>, <i>AMT4</i>, and <i>AMT14</i> in all isolates from the region, suggesting that Alternaria apple blotch in the area was associated with another metabolite (<b>1)</b>.