RESULTS AT AGE 15 YEARS FROM A HALF-DIALLEL CROSS AMONG 10 LOBLOLLY PINES SELECTED FOR RESISTANCE TO FUSIFORM RUST (CRONARTIUM-QUERCUUM F-SP FUSIFORME)

Ten loblolly pine seed orchard clones selected for their high resistance to the southern fusiform rust fungus were crossed in a half-diallel mating scheme. The 45 crosses were outplanted in a progeny test in 1974 in Houston County, Georgia, an area of very high rust hazard. Fifteenth-year data showed that the 10 clones differed at the 0.01 level in general combining ability for height and for percentage free of fusiform rust infection at 15 or cumulative percentage free to 15. Also, specific combining abilities of the 10 clones varied at the 0.01 level for rust-free percentage cumulative to age 15. Heritability of percentage rust-free was 0.69 based on 15th-year data and 0.52 based on cumulative records. Heritability of tree height was 0.48 on a family or 0.28 on an individual-tree basis. The 10 clones divided sharply into half with moderate resistance and half with low resistance to rust, even though all had shown high resistance in earlier tests. Mortality percentages of trees with infected stems were not significantly correlated with stem infection percentages, suggesting little or no linkage between susceptibility to stem infection and tolerance of stem cankers. The 3 stages of selection produced a genetic gain of about 32%; that is, 32% more trees were rust-free at age 15.