Determination and characterization of the causal agent of Pinus radiata needle blight in Chile

Abstract

The leaf and shoot blight disease known locally as Daño Foliar del Pino (DFP) is the most important problem affecting Chilean Pinus radiata plantations. The disease is typified by needle infections, exudation of resin at the bases of needle brachyblasts and necrotic lesions in the cambium of younger trees, which eventually girdle branches. The disease causes death of seedlings young trees and mature trees can also succumb after a few years of successive infection. DFP emerged very recently and its cause was unknown. Studies that make up this thesis led to the identification of the causal agent of DFP, which was described as the new species Phytophthora pinifolia A. Durán, Gryzenh. & M.J. Wingf. This is the first discovery of a Phytophthora sp. infecting needles and shoots of a Pinus sp. anywhere in the world. Two diagnostic procedures were developed to screen large numbers of samples rapidly and cost effectively for the pathogen. Population genetic studies showed that the P. pinifolia population in Chile is a single clonal lineage that exists in the absence of sexual recombination. This suggests strongly that P. pinifolia is an introduced pathogen in Chile although its origin and natural host-range is uknown. In order to promote effective management of P. pinifolia, three fungicides were screened in vitro and in planta. Mefenoxam was shown to be the most effective compound, also providing the most consistent results. DFP is recognized as one of the most important disease problems to strike non-native pine plantation forestry in recent times. Studies in this thesis represent the first ever on the disease and its causal agent and they provide a foundation for the many studies that must follow to ensure that P. pinifolia does not lead to disease disasters elsewhere in the world.