Article / Screening of Stevia rebaudiana (Bertoni) susceptibility to Septoria steviae through leaf disk inoculation

Le Bihan, Z., Gaudin, J., Rasouli, D., Boutet, S., Laurencon, M., Cosson, P., Hastoy, C., Rolin, D., Schurdi-Levraud, V. (In press) Screening of Stevia rebaudiana (Bertoni) susceptibility to Septoria steviae through leaf disk inoculation. Plant health progress. https://doi.org/10.1094/PHP-04-24-0039-RS

Résumé : Stevia rebaudiana Bertoni is a perennial cultivated plant that produces steviol glycosides, natural acaloric sweet compounds in its leaves. Many pathogens are capable of infecting this species. Among these pathogens, Septoria steviae, the causal agent of Septoria Leaf Spot, is the most widely described on S. rebaudiana. The objective of this study is to validate a leaf disk screening method for S. rebaudiana response to Septoria steviae infection on 10 S. rebaudiana genotypes with contrasting responses to the fungus. Three independent experiments were carried out in which forty leaf discs per genotype were infected with an inoculum at a concentration of 105 conidia / mL or with water. The response of the 10 genotypes was scored through image analysis. Image analysis was conducted at 9, 12, 15, 18 and 21 days after inoculation (DAI), allowing the proportion of symptomatic foliar area (PLSA) and AUDPC to be quantified. The evaluation of symptom speed development was realized through a linear regression between PLSA and time, for each tested genotype. Genotypes exhibited the lowest susceptibility at 21 DAI, with Cult102_SPA and Cult76_GER displaying 3% of the surface symptomatic. Genotypes with the highest susceptibility were Cult33_FRA and Cult75_GER, with 24% severity. These results correlate with visual scoring performed at 21 DAI and with previous inoculation protocols on S. rebaudiana cuttings, by maintaining the ranking of the different genotypes whatever the test. The development of a leaf-disk inoculation method offers new avenues for breeding strategies to develop tolerant or resistant S. rebaudiana cultivars.