SCREENING APPLES FOR OPD20 600 USING SEQUENCE-SPECIFIC PRIMERS/

Apple scab, caused by Venturia inaequalis (Cke.) Wint., is the most se rious disease of apple trees in many areas of the world. Resistance to V. inaequalis, derived from the small-fruited species Malus floribund a 821, is determined by a major dominant gene, Vf. Using random decame r primers, we identified a RAPD marker, OPD20/600, which is linked to the Vf gene. OPD20/600 was then cloned and sequenced. Sequence-specifi c primers based on the marker were used to further screen M. floribund a 821, 7 scab-susceptible apple cultivars, 10 scab-resistant apple cul tivars, and 28 scab-resistant Coop selections. The sequence-specific p rimers allowed identification of polymorphisms of OPD20/600 based on t he presence or absence of a single band. The advantages of sequence-sp ecific primers over decamer primers for developing genetic markers are discussed.