Genetic variation in susceptibility of Dothistroma needle blight among provenances and families of Pinus muricata.
The susceptibility of Pinus muricata to needle blight caused by Dothistroma septospora var. septospora [Mycosphaerella pini] was investigated in 3 provenance tests and a progeny test in New South Wales, Australia, and compared with that in Pinus radiata, the major commercially planted species. Provenances of P. muricata from Humboldt, Mendocino and Sonoma counties in N. California were moderately susceptible for the first 3 or 4 years but then developed a high degree of field resistance. There was a general trend to increasing susceptibility in provenances progressively southwards along the California coast with one exception, Santa Rosa Island provenance, which appeared to be resistant. All provenances from Monterey southwards, except Santa Rosa Island, were at least as susceptible as P. radiata. The Mendocino County provenance of P. muricata is suggested as an alternative to P. radiata for planting on sites where there is a high risk of severe needle blight, as it possesses a combination of high disease resistance, rapid growth rate, and desirable wood properties. A 6-yr-old progeny test of trees of Sonoma County provenance yielded a heritability estimate of 0.29 for percentage of needles infected by needle blight, which is very similar to estimates for the same trait in P. radiata. This indicates that although this population is already substantially less susceptible than the most resistant provenance of P. radiata, there is still a high degree of heritable variation, and that resistance could be improved even further by within-provenance selection.