Pythium undulatum, cause of root rot of Abies procera Christmas trees and Pseudotsuga menziesii in Northern Germany

Abstract

A devastating disease of Abies procera grown as Christmas trees in a forest situation in Northern Germany is reported. Disease symptoms began as a root rot with disintegration of the cortex of lateral and main roots, followed by rapid chlorosis and necrosis of the foliage which had a striking copper colour on dead and dying trees. All disease symptoms were reproducible in pathogenicity tests with potted A. procera and Pseudotsuga menziesii plants infected with the isolated pathogen. The symptoms were indistinguishable from those reported for Phytophthora spp., but the pathogen was identified by microscopy and ITS sequence analysis as Pythium undulatum, which has not previously been described as a cause of root rot of coniferous trees in outdoor situations. This fungus was isolated also from roots of A. amabilis, A. grandis and P. menziesii showing similar below-and above-ground symptoms. Trees of varying ages (6–22 years) were affected. Abies nordmanniana appeared not to be susceptible to P. undulatum even when in direct root contact with diseased A. procera or A. amabilis. The establishment and spread of the disease between 1999 and 2002 appeared to be correlated with poor soil drainage following a series of unusually wet summers.