Mendelian inheritance of rust resistance to Melampsora larici-epitea in crosses between Salix sachalinensis and S. viminalis

Two F(1), two F(2) and two backcross (BC) full-sib families of Salix sachalinensis x S. viminalis were tested for resistance to two pathotypes of Melampsora larici-epitea in leaf-disc inoculation experiments. Two single-pustule isolates, VM and ST, belonging to pathotypes LET1 and LET5, respectively, were used in the tests. Disease was scored based on the number of uredinia, uredinial diameter and inoculum densities. Both F1 families were completely resistant to both VM and ST. Resistance to VM segregated at a 9: 6: 1 ratio in the F2 families and at a 1: 2: 1 ratio in the BC families, suggesting that two independently segregating genes controlled rust resistance, with resistance dominant over susceptibility. This also indicates incomplete dominance of the resistance alleles over the susceptibility to VM. For ST, the equivalent ratios were 3: 1 and 1: 1, showing that a single dominant gene was responsible for rust resistance. The broad sense heritabilities were >0.91 for uredinial diameter and 0.1-0.33 for the number of uredinia. There were significant overall correlations between data from inoculations with VM and those from inoculations with ST in the number of uredinia, uredinial diameter and disease scores (Spearman's rank correlation coefficients = 0.31-0.75).