Identification and Characterization of Phytophthora Species Associated with Crown and Root Rot of Pistachio Trees in California

Abstract

Pistachio is one of the most widely cultivated nut crops in California with approximately 115,000 hectares of bearing pistachio trees. In recent years, several orchards were identified with declining trees leading to substantial tree losses. Symptoms included trees with poor vigor, yellowing and wilting of leaves, crown rot and profuse gumming on the lower portion of trunks. Thirty-seven Phytophthora-like isolates were obtained from crown rot tissues in the rootstock of grafted pistachio trees and characterized by means of multi-locus phylogeny comprising ITS rDNA, beta-tubulin and mt cox1 sequence data. The analysis provided strong support for the delineation and identification of three Phytophthora species associated with declining pistachio trees, including Phytophthora niederhauserii, P. mediterranea and P. taxon walnut. Pathogenicity studies in potted UCBI rootstocks (clonal and seeded) confirmed that all three Phytophthora species can cause crown and root rot of pistachio, thus fulfilling Koch’s postulates. The widespread occurrence of Phytophthora crown rot in recently planted pistachio orchards and the high susceptibility of UCBI rootstocks suggest this disease constitute an emerging new threat to the pistachio industry of California. To the best of our knowledge, this study is the first to report P. niederhauserii, P. mediterranea and P. taxon walnut as causal agents of crown and root rots of pistachio.

The Never-Ending Presence of Phytophthora Species in Italian Nurseries

“…Phytophthora mediterranea was isolated from declining potted myrtle seedlings ( Myrtus communis L.) in Italy [ 66 ]. It was previously observed on pistachio ( Pistacia vera L.) in California [ 67 ]. Although phylogenetically, P. mediterranea is closely related to P. cinnamomi , the two species can be easily distinguished on the basis of some morphological differences, such as size of the sporangia, colony growth pattern and cardinal temperature values.…”