Marker assisted selection for resistance to Fusarium oxysporum in the greenhouse carnation

Losses of up to 40%, caused by the pathogen Fusarium oxysporum f.sp. dianthi, race 2, have been reported in carnation crops. As a consequence growers often decide to switch to a different crop. The genetic mechanism behind resistance to Fusarium in the greenhouse carnation (Dianthus caryophyllus) has not been thoroughly investigated to date. Nevertheless it is an important prerequisite to any attempt to tag the major genes involved. On this basis, we first concentrated on analyzing the inheritance of resistance and only then on tagging major resistance loci. Using a segregating F2 family derived from a cross between a resistant and a sensitive parent, a field test was performed which enabled phenotypic scoring following natural inoculation of the progeny. From the results, we extrapolated a genetic model whereby resistance is determined by two major genes which interact additively. Based on the hypothesized model and by focusing on the extreme segregants, one of the major genes for resistance was tagged using random oligonucleotide primers. Linkage was established by examining the entire F2 family (consisting of 107 segregants) using a standard t-test. F3 populations derived from selected F2 segregants were used to verify linkage and to identify F2 segregants homozygous for the marker. The RAPD fragment was sequenced and used to synthesize sequence-specific primers in an attempt to establish a PCR-based, SCAR marker analysis for resistance. The same fragment was also used as a probe for a Southern blot analysis and an RFLP marker was established that co-segregated with the resistant progeny. By exploiting the existing F2 family, containing the tagged locus, it should now be possible to develop an additional marker. This work thus paves the way toward tagging the loci involved in resistance, thereby enabling efficient selection for resistance at the genotype level in carnation.