First Report of Septoria sp. Infecting Stevia rebaudiana in France and Screening of Stevia rebaudiana Genotypes for Host Resistance

Stevia rebaudiana, for which cultivation is on the increase worldwide, accumulates acaloric intense sweeteners called steviol glycosides (SGs) in its leaves. Yields can be affected by Septoria leaf spot (SLS) caused by Septoria spp. The objectives of the research were (1) to morphologically and genetically characterize five isolates of Septoria sp. found for the first time from outbreaks of Septoria in stevia fields in Southwestern France and (2) to screen S. rebaudiana germplasm from diverse origins through an automated inoculation method using one of the isolates. Multilocus sequence typing grouped the five isolates obtained from symptomatic plants, closely related to Septoria lycopersici and Septoria apiicola. The response to Septoria sp. of 10 genotypes from different origins was assessed for disease severity (DS), either by visually scoring the symptomatic portion of the whole plants or the portion of symptomatic foliar area (PLSA) determined by image analysis, and the area under the disease progress curve (AUDPC) calculated on the basis of the disease severity rating taken 12, 15, 18, and 21 days after inoculation. No genotypes with complete resistance were identified. Moderately susceptible genotypes “Gawi” and “Esplac1” exhibited only 10 to 15% of symptomatic part on whole plant and the slowest disease development. They could be distinguished from highly susceptible ones “E8”, “C”, and “E161718” exhibiting up to 40% of symptomatic part on whole plant. The variability of response to Septoria sp. that exists in S. rebaudiana opens up the field of breeding strategies for the development of new cultivars for sustainable and organic S. rebaudiana production.