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

Alternaria sp. infections on apple (Malus x 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, (3 xi,4 xi,5 xi,6 xi,7 xi,11 xi)-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).

Automated summary

Infections of Alternaria sp. on apples lead to fruit quality decline and yield losses due to toxins produced by the Alternaria apple pathotype. Pathogenic isolates provoking necrotic lesions on apple leaves and fruits were identified and tested through in vitro assays. The lack of toxin-related genes in the isolates from the region suggests that Alternaria apple blotch in the area may be associated with a different metabolite.