GENOTYPIC VARIATION IN SYMPTOMS OF UPPER MID-CROWN YELLOWING AND

In New Zealand, low needle retention in Pinus radiata D.Don is often associated with infection by a needle-cast fungus, Cyclaneusma minus (Butin) DiCosmo et al., and with a condition known as upper mid-crown yellowing (UMCY). Variability in the expression of these two disorders is known to be controlled by both environmental and genetic factors. In order to estimate the degree of genetic control, adjacent seedlings and clonal trees (six clones selected for vigour in the nursery) were assessed at age 22 years for symptoms of UMCY and of C. minus in stands thinned to three final-crop stocking rates. Estimates of genotypic and non-genetic variance and of broad-sense heritability (ratio of genotypic to phenotypic variance) were obtained in two ways: (a) by subtracting the observed within-clone variance from the phenotypic variance of seedling trees, and (b) by directly estimating clone-to-clone variance. Values obtained by these methods agreed well with each other and with earlier estimates made in a nearby seed orchard. Factors related to method of propagation and physiological ageing are therefore unlikely to inflate clonal variation. Clones may be the more cost-effective indicator for UMCY studies. Genotypic variance in resistance to both disorders is considered to be sufficiently high (at least 64 % for UMCY and 44 % for C minus) to warrant consideration in silvicultural practices