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Ecology and Epidemiology

Seedborne Cercospora beticola Can Initiate Cercospora Leaf Spot from Sugar Beet (Beta vulgaris) Fruit Tissue

    Affiliations
    Authors and Affiliations
    • Rebecca Spanner1 2
    • Jonathan Neubauer1
    • Thies M. Heick3
    • Michael A. Grusak1
    • Olivia Hamilton1 2
    • Viviana Rivera-Varas2
    • Ronnie de Jonge4
    • Sarah Pethybridge5
    • Kimberley M. Webb6
    • Gerhard Leubner-Metzger7
    • Gary A. Secor2
    • Melvin D. Bolton1 2
    1. 1Edward T. Schafer Agricultural Research Center, United States Department of Agriculture–Agricultural Research Service, Fargo, ND, U.S.A.
    2. 2Department of Plant Pathology, North Dakota State University, Fargo, ND, U.S.A.
    3. 3Institute for Agroecology, Aarhus University, Slagelse, Denmark
    4. 4Plant-Microbe Interactions, Department of Biology, Science4Life, Utrecht University, Utrecht, The Netherlands
    5. 5Plant Pathology & Plant-Microbe Biology Section, School of Integrative Plant Science, Cornell AgriTech, Cornell University, Geneva, NY, U.S.A.
    6. 6Soil Management and Sugar Beet Research Unit, United States Department of Agriculture–Agricultural Research Service, Fort Collins, CO, U.S.A.
    7. 7Department of Biological Sciences, Royal Holloway University of London, Egham, U.K.

    Cercospora leaf spot (CLS) is a globally important disease of sugar beet (Beta vulgaris) caused by the fungus Cercospora beticola. Long-distance movement of C. beticola has been indirectly evidenced in recent population genetic studies, suggesting potential dispersal via seed. Commercial sugar beet “seed” consists of the reproductive fruit (true seed surrounded by maternal pericarp tissue) coated in artificial pellet material. In this study, we confirmed the presence of viable C. beticola in sugar beet fruit for 10 of 37 tested seed lots. All isolates harbored the G143A mutation associated with quinone outside inhibitor resistance, and 32 of 38 isolates had reduced demethylation inhibitor sensitivity (EC50 > 1 µg/ml). Planting of commercial sugar beet seed demonstrated the ability of seedborne inoculum to initiate CLS in sugar beet. C. beticola DNA was detected in DNA isolated from xylem sap, suggesting the vascular system is used to systemically colonize the host. We established nuclear ribosomal internal transcribed spacer region amplicon sequencing using the MinION platform to detect fungi in sugar beet fruit. Fungal sequences from 19 different genera were identified from 11 different sugar beet seed lots, but Fusarium, Alternaria, and Cercospora were consistently the three most dominant taxa, comprising an average of 93% relative read abundance over 11 seed lots. We also present evidence that C. beticola resides in the pericarp of sugar beet fruit rather than the true seed. The presence of seedborne inoculum should be considered when implementing integrated disease management strategies for CLS of sugar beet in the future.

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