: A Novel Killer Pathogen of Plane () Causing Canker Stain and Root and Collar Rot.

Decline symptoms associated with lethal stem and branch canker stain along with root and collar rots were observed on 5- to 7-year-old roadside oriental plane trees () in Diyarbakır, Turkey. Above-ground symptoms included leaf necrosis, leaf curling, extensive bluish or blackish staining of shoots, branches, stem bark, and wood surfaces, as well as stem cankers and exfoliation of branch bark scales. A general decline of the trees was distinctly visible from a distance. A /-like oomycete species with globose to ovoid, often papillate and internally proliferating sporangia was consistently isolated from the fine and coarse roots and stained branch parts and shoots. The pathogen was identified as based on several morphological features. Partial DNA sequences of three loci, including nuclear rDNA internal transcribed spacer (ITS) and the large ribosomal subunit (LSU), and mitochondrial cytochrome c oxidase subunit II (II) confirmed the morphological identification. All isolates were homothallic, developing gametangia, ornamented oogonia with elongate to lobate antheridia. Pathogenicity of . was tested by inoculation on excised shoots and by root inoculation on seedlings. produced large lesions and blights on shoots in just 5 days and killed 100% of the seedlings in a month. This paper presents the first confirmed report of as an important pathogen on a plant species causing branch and stem cankers, and root and collar rot, in and on , resulting in a rapid decline of trees and suggesting a threat to plane.