MARKER-ASSISTED HOP (HUMULUS-LUPULUS L.) BREEDING

The resistance to hop damson aphid (Phorodon humuli Schrank) is relate d to a combination of three markers in the hop essential oil: alpha-pi nene, beta-pinene and the undefined peak 92, The accession is resistan t if the oil contains the compound 92 and the index ratios of alpha-pi nene to peak 92 and beta-pinene to peak 92 are sufficiently low (criti cal value is 4), If one of the ratios is above these critical values, the accession is susceptible. The resistance to downy mildew (Pseudope ronospora humuli Miyabe et Takah) is related to 9 independent markers present in the essential oil if at least one of them is present in amo unt above the critical index. The critical index is the threshold abov e which the marker becomes responsible for susceptibility of the acces sion to downy mildew. In non European accessions 4 markers (peak 33, i so-amyliso-butyrate, trans-ocimene, geraniol) and in European accessio ns 5 markers (methyl caprinate, alpha-humulene, peak 68, methyl-deca-4 -enoate and peak 51) have been observed. The accession is resistant to downy mildew if alpha-humulene is present below the critical index an d the indexes for the rest of the markers are below the detection limi t or are relatively low.