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.
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