Finding DNA markers linked to disease resistance and other economically advantageous genes in apple

An AFLP marker for scab resistance was converted into a SCAR, which performed well when tested on a seedling population from a Priscilla X Anna cross, segregating for scab resistance. The progeny of a “Northern Spy” and “Cox’s Orange Pippin” apple cross were tested to determine if a DNA marker linked to resistance towards the crown, collar and root rot pathogen, Phytophthora cactorum, could be identified. “Northern Spy” and “Cox’s Orange Pippin” are resistant and susceptible, respectively. The resultant progeny were established in tissue culture either by germinating the seeds in vitro or via explants grown from the seedlings first germinated in soil. An in vitro method was developed to screen the progeny for resistance to P. cactorum as follows: The plants were transferred to root-inducing medium, challenged with the pathogen in vitro, and classified as susceptible or resistant. The reproducibility of the in vitro pathogen challenge method was tested. Not all the plant lines exhibited consistent phenotypes in separate pathogen challenge tests. A number of factors, most notably plant size, appear to have influenced the results. DNA from susceptible and resistant seedlings was bulked and was screened with 234 RAPD primers for polymorphisms. The polymorphic bands were converted to SCAR markers. However, these markers were not informative since they were detected in both resistant and susceptible plants. In addition to RAPDs, polymorphism between the parental plants was investigated using SSRs. However, results with the primers used were not consistent. Hence the progeny was not screened with those SSRs.