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.

Identification and mapping of resistance gene analogs (RGAs) in Prunus: a resistance map for Prunus

The genetically anchored physical map of peach is a valuable tool for identifying loci controlling economically important traits in Prunus. Breeding for disease resistance is a key component of most breeding programs. The identification of loci for pathogen resistance in peach provides information about resistance loci, the organization of resistance genes throughout the genome, and permits comparison of resistance regions among other genomes in the Rosaceae. This information will facilitate the breeding of resistant species of Prunus. A candidate gene approach was implemented for locating resistance loci in the genome of peach. Candidate genes representing NBS-LRR, kinase, transmembrane domain classes, as well as, pathogen response (PR) proteins and resistance-associated transcription factors were hybridized to a peach BAC library and mapped by using the peach physical map database and the Genome Database for Rosaceae (GDR). A resistance map for Prunus was generated and currently contains 42 map locations for putative resistance regions distributed among 7 of the 8 linkage groups.

Analogues of virus resistance genes map to QTLs for resistance to sharka disease in Prunus davidiana

Plum pox virus (PPV), the causative agent of sharka disease in Prunoideae, is one of the most serious problems affecting stone fruit production in Europe and America. Resistance to PPV was previously described in a Prunus davidiana clone, P1908, and introduced into peach (Prunus persica) genotypes. Genetic resistance to PPV displays a complex pattern of quantitative inheritance. An analysis of quantitative trait loci (QTLs) for resistance was performed on an F1 interspecific peach population obtained from a cross between the susceptible nectarine cultivar Summergrand and P. davidiana. The hybrids were graft-inoculated with PPV in duplicate following a classical procedure. The incidence of infection was evaluated four times, over two vegetative cycles, by symptom observation and enzyme-linked immunoadsorbent assays (ELISA). Restriction of systemic downward movement of the PPV virus was also evaluated by testing the susceptible rootstocks. Using both analysis of variance and non-parametric tests, six genomic regions involved in PPV resistance were detected. Depending on the scoring data considered, between 22 and 51% of the phenotypic variance could be explained by the quantitative model. One QTL, located in the distal region of linkage group 1, maps in a genomic region that is syntenic to the location of a resistance gene previously identified in the apricot cv. Goldrich. Some QTLs appeared to be temporally specific, reflecting the environmental dependence of PPV-resistance scoring. Candidate gene fragments were amplified by PCR, isolated and mapped on the peach interspecific linkage map. We report here the co-localization of three analogues of virus resistance genes with two distinct genomic regions linked to PPV resistance in P. davidiana.