Brain, P. | Search bioRxiv

bioRxiv2018

Quantitative Trait Loci Controlling Phytophthora cactorum Resistance in the Cultivated Octoploid Strawberry (Fragaria x ananassa)

The cultivated strawberry, Fragaria x ananassa (Fragaria spp.) is the most economically important global soft fruit. Phytophthora cactorum, a water-borne oomycete causes economic losses in strawberry production globally. A bi-parental cross of octoploid cultivated strawberry segregating for resistance to P. cactorum, the causative agent of crown rot disease, was screened using artificial inoculation. Multiple resistance quantitative trait loci (QTL) were identified and mapped. Three major effect QTL (FaRPc6C, FaRPc6D and FaRPc7D) explained 36% of the variation observed and in total, the detected QTL explained 86% of the variation observed. There were no epistatic interactions detected between the three major QTLs. Testing a subset of the mapping population progeny against a range of P. cactorum isolates revealed no major differences in host response, however, some lines showed higher susceptibility than predicted, indicating that additional undetected factors may affect the expression of some quantitative resistance loci. Using historic crown rot disease score data from strawberry accessions, a preliminary genome-wide association study of 114 individuals revealed additional loci associated with resistance to P. cactorum. Mining of Fragaria vesca Hawaii 4 v1.1 genome revealed candidate resistance genes in the QTL regions.

plant biology↗

bioRxiv2017

Characterisation of the pathogenicity of strains of Pseudomonas syringae towards cherry and plum

Bacterial canker is a major disease of cherry and other stone fruits caused by several pathovars of Pseudomonas syringae. These are P.s pv. morsprunorum race 1 (Psm R1), P.s pv. morsprunorum race 2 (Psm R2) and P.s pv. syringae (Pss). Psm R1 and R2 were originally designated as races of the same pathovar, however phylogenetic analysis has revealed them to be distantly related. This study characterised the pathogenicity of P. syringae on cherry and plum, in the field and the laboratory. The field experiment identified variation in host cultivar susceptibility to the different pathogen clades. The cherry cultivar Merton Glory exhibited a broad resistance to all clades, whilst cultivar Van showed race-specific resistance. Psm R1 may be divided into a race structure with some strains pathogenic to both cherry and plum and others only pathogenic to plum. The results of laboratory-based pathogenicity tests were compared to results obtained on whole-trees. Only cut shoot inoculations were found to be sensitive enough to detect cultivar variation in susceptibility. Measuring population growth of bacteria in detached leaves reliably discriminated pathogens from non-pathogens. In addition, symptom appearance discriminated Psm races from non-pathogens which triggered a rapid hypersensitive response (HR). The pathogen Pss rapidly induced disease lesions and therefore may exhibit a more necrotrophic lifestyle than hemi-biotrophic Psm races. This in-depth study of pathogenic interactions, identification of host resistance and optimisation of laboratory assays, will provide a framework for future genetic dissection of virulence and host resistance mechanisms.

plant biology↗