Emergence of a novel Alternaria species infecting invasive garlic mustard (Alliaria petiolata) in the United States.

Abstract

The Brassicaceae invasive weed, garlic mustard (Alliaria petiolata), is a Eurasian biennial herb that has rapidly spread across North America infesting forests and field borders, negatively impacting plant biodiversity and agroecosystem health. In 2022, a severe garlic mustard dieback event occurred in a limited section of a large, forested garlic mustard population in Maryland, United States. Diseased plants were heavily defoliated with remaining intact leaves having irregular-shaped necrotic and chlorotic lesions. Two isolates of an unknown fungal pathogen were collected, sequenced and identified as Alternaria, and confirmed to be pathogenic to garlic mustard. All inoculated garlic mustard plants rapidly developed severe symptoms within 72-hours, mimicking the symptoms observed in the field. A multilocus sequence analysis identified the two strains as a distinct species that appears to be a new monotypic sister lineage to Alternaria section Sonchi and most closely related to the Japanese Apiaceae pathogen A. triangularis. This study reports the first documentation of a novel, pathogenic Alternaria species identified from the introduced range of the invasive weed garlic mustard. In addition to its potential use as a garlic mustard bioherbicide, future studies will provide critical insights in the role non-native invasive weeds play in harboring and selecting for novel pathogenic microbes, as well as biosecurity risks to U.S. agriculture.