Genetic Variation in Growth and Blister-Rust Resistance in a Pinus strobus x P. wallichiana Hybrid Population

This experiment consists in a controlled crossing according to a factorial design performed between 7 female trees of P. strobus and 4 male trees of P. wallichiana to combine the rapid growth of former species with high resistance to Cronartium ribicola of the latter one. The hybrid families were artificially inoculated at age 2, and field planted at age 6. Blister rust resistance (BRR), tree survival (TS), total height growth, (H) annual height growth (h), diameter (D), basal area (BA), stem volume (V), stems straightness (SS) and branch thickness (BT) were the traits measured at age 17. Statistical analysis produced the results presented below. Significant (p<0.05) and highly significant (p<0.01; p<0.001) differences were found among hybrid families. Differences among female effects were highly significant (p<0.001) for all tested traits including BRR, suggesting that nuclear additive genes controlled these traits. Significant differences were found among male parents for H but no significant differences for BRR; therefore, all four male parents transmitted a similar level of resistance. The ratio σ²_GCA/σ²_SCA variance accounted for 8.1 for BRR, 8.5 for H, 3.5 for V, 9.3 for SS and 1.9 for BT. Similarly, the ratios of σ²_GCA-F /σ²_GCA-M variance due to female parents were 70.5 for BRR, 23.6 for H, 1.0 for V, 0.4 for SS and 1.0 for BT, were found. Narrow-sense heritabilities, at individual level, were low to moderately high, ranging between 0.085 for BT and 0.421 for BRR. By comparison with the mean of P. strobus parent species, the BRR heterosis was highly positive, but negative for all growth traits. If the hybrids will be used in operational planting programs, a significant genetic gain for BRR and growth traits could be achieved.