Root and stem rot (RSR) is a very detrimental disease of vanilla worldwide. Fusarium oxysporum is frequently associated with the disease but other Fusarium species are also reported. In this international study, 52 vanilla plots were surveyed in three of the most important vanilla producing countries (Madagascar, Reunion Island and French Polynesia) in order to determine the aetiology of RSR disease. Subsets from the 377 single‐spored Fusarium isolates recovered from rotten roots and stems in the surveys were characterized by molecular genotyping (EF1α and IGS gene sequences) and pathogenicity assays on Vanilla planifolia and V. ×tahitensis, the two commercially grown vanilla species. Fusarium oxysporum was shown to be the principal species responsible for the disease, representing 79% of the isolates recovered from the RSR tissues, 40% of which induced severe symptoms on inoculated plantlets. Fusarium oxysporum isolates were highly polyphyletic regardless of geographic origin or pathogenicity. Fusarium solani, found in 15% of the samples and inducing only mild symptoms on plantlets, was considered a secondary pathogen of vanilla. Three additional Fusarium species were occasionally isolated in the study (F. proliferatum, F. concentricum and F. mangiferae) but were nonpathogenic. Histopathological preparations observed in wide field and multiphoton microscopy showed that F. oxysporum penetrated the root hair region of roots, then invaded the cortical cells where it induced necrosis in both V. planifolia and V. ×tahitensis. The hyphae never invaded the root vascular system up to 9 days post‐inoculation. As a whole, the data demonstrated that RSR of vanilla is present worldwide and that its causal agent should be named F. oxysporum f. sp. radicis‐vanillae.
Jullien Nicolas et al., « L'engagement dans des collectifs de production de connaissance en ligne » Le cas GeoRezo,
Background and Aims Oil palms showing exceptional vigour and dubbed as “giant palms” were identified in some progenies in breeding. A panel of phenotypical traits were studied to characterize these trees. Hypothesis that gigantism and other anomalies might be linked to polyploidy was investigated. Methods Twenty sib pairs of palms from different crosses, each comprising of a giant and a normal oil palm, were studied by flow cytometry with rice Nipponbare as standard reference. Parallelly, palms were assessed in the field using eleven phenotypic traits. A principal component analysis (PCA) was conducted to define relationships between these phenotypical traits and a linear discriminant analysis (LDA) to predict ploidy level and giant classification. Finally, a co-dominant molecular marker study was implemented to highlight the sexual process leading to the formation of 2n gametes. Key Results A first group of oil palms presented an oil palm/rice peak ratio around 4.8 corresponding to diploid oil palms, whereas the second group presented a ratio around 7, classifying these plants as triploid. The PCA let to classify the plant in 3 classes: 21 were normal diploid palms, 10 were giant diploid palms while 11 were giant triploid palms. The LDA revealed three predictors for ploidy classification: phyllotaxy, petiole size and circumference of the plant but surprisingly not height. The molecular study revealed that triploid palms resulted from 2n gametes issued of the second division restitution of meiosis in parents. Conclusions This study confirms and details the process of sexual polyploidization in oil palm. It also identifies three phenotypical traits to assess ploidy level of the giant oil palms in the field. In practical, our results provide a cheap scientific method to identify polyploids palms in the field.
Basal stem rot caused by Ganoderma boninense is the major threat to oil palm cultivation in South-East Asia, which accounts for 80% of palm oil production worldwide, and this disease is increasing in Africa. The use of resistant planting material as part of an integrated pest management of this disease is one sustainable solution. However, breeding for Ganoderma resistance requires long-term and costly research, which could greatly benefit from marker assisted selection (MAS). In this study, we evaluated the effectiveness of an in silico genetic mapping approach that took advantage of extensive data recorded in an ongoing breeding program. A pedigree-based QTL mapping approach applied to more than 10 years’ worth of data collected during pre-nursery tests revealed the quantitative nature of Ganoderma resistance and identified underlying loci segregating in genetic diversity that is directly relevant for the breeding program supporting the study. To assess the consistency of QTL effects between pre-nursery and field environments, information was collected on the disease status of the genitors planted in genealogical gardens and modeled with pre-nursery-based QTL genotypes. In the field, individuals were less likely to be infected with Ganoderma when they carried more favorable alleles at the pre-nursery QTL. Our results pave the way for a MAS of Ganoderma resistant and high yielding planting material and the provided proof-of-concept of this efficient and cost-effective approach could motivate similar studies based on diverse breeding programs.
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