Classical and molecular methodologies were used to determine the inheritance of Phytophthora root rot (PRR) resistance in red raspberry. The varieties 'Latham' and 'Titan,' resistant and susceptible, respectively, were used to create F(1), F(2), B(1), B(2), and S(1) populations for analysis. Generational means analysis was used to calculate the components of genetic variation and estimates of narrow and broad sense heritability for the plant disease index and the incidence of petiole lesions. The plant disease index showed additive genetic variation with additional significant interactions, but the incidence of petiole lesions was non-additive. A dominant, two-gene model was shown to be the best fit for the observed segregation ratios when classification for resistance was based on a combination of all criteria measured. Molecular linkage maps were generated from the segregating B(2) population. Linkage maps of both parents were constructed from amplified fragment length polymorphism (AFLP), Random amplified polymorphic DNA (RAPD), and uncharacterized resistant gene analog polymorphism (RGAP) markers with seven linkage groups each totaling 440 and 370 cM of genetic distance, respectively. An analysis of the distributional extremes of the B(2) population identified several RAPD markers clustered on two linkage groups associated with PRR resistance. QTL analysis identified two similar genomic regions on each map that explained significant percentages of phenotypic variation observed for the disease assessment criteria. Genetic mapping supports the dominant two-gene model developed from generational means analysis. The results reconcile conflicting reports on inheritance of PRR resistance, provide a basis for further investigation of durable resistance to Phytophthora caused diseases, and indicates that recurrent selection is the appropriate approach for the development of new resistant cultivars.

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As part of a program to develop forward and reverse genetics platforms in the diploid strawberry [Fragaria vesca L.; (2n = 2x = 14)] we have generated insertional mutant lines by T-DNA mutagenesis using pCAMBIA vectors. To characterize the T-DNA insertion sites of a population of 108 unique single copy mutants, we utilized thermal asymmetric interlaced PCR (hiTAIL-PCR) to amplify the flanking region surrounding either the left or right border of the T-DNA. Bioinformatics analysis of flanking sequences revealed little preference for insertion site with regard to G/C content; left borders tended to retain more of the plasmid backbone than right borders. Primers were developed from F. vesca flanking sequences to attempt to amplify products from both parents of the reference F. vesca 815 x F. bucharica 601 mapping population. Polymorphism occurred as: presence/absence of an amplification product for 16 primer pairs and different size products for 12 primer pairs, For 46 mutants, where polymorphism was not found by PCR, the amplification products were sequenced to reveal SNP polymorphism. A cleaved amplified polymorphic sequence/derived cleaved amplified polymorphism sequence (CAPS/dCAPS) strategy was then applied to find restriction endonuclease recognition sites in one of the parental lines to map the SNP position of 74 of the T-DNA insertion lines. BLAST search of flanking regions against GenBank revealed that 46 of 108 flanking sequences were close to presumed strawberry genes related to annotated genes from other plants.

A hydroponic method was developed and tested for screening red raspberry genotypes for resistance to Phytophthora fragariae var. rubi, the most common causal agent of Phytophthora root rot in raspberry. Plants of `Titan' and `Encore' exhibited typical disease symptoms, with the latter developing significantly smaller stem lesions and fewer petiole lesions. The resistant cultivar, `Latham', regenerated healthy root tissue from the crown and older-order roots after initial infection and necrosis of young roots and exhibited no other symptoms beyond minor leaf chlorosis. This component of the resistance reaction has not been documented previously. A segregating F1 population from the cross of `Latham' × `Titan' had a survival rate of 56% with 42% classified as resistant, exhibited minimal symptoms, and produced varying amounts of healthy root tissue. This screening method allows multiple observations of all plant tissues, including roots, under repeatable and definable growth chamber conditions. It should be useful for classifying the phenotype of individuals in segregating red raspberry populations to investigate the inheritance of Phytophthora root rot resistance using molecular markers.