Rhabdocline needle cast — most recent findings of the occurrence of Rhabdocline pseudotsugae in Douglas-fir seeds

Abstract

Rhabdocline pseudotsugae Syd. is one of the major fungal pathogens in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). To date, macroscopic and microscopic analyses of disease symptoms have resulted in the description of the Rhabdocline needle cast pathogen as a highly specialized needle parasite. In practice, the parasite has traditionally been identified by its fruiting bodies on infected needles. But nevertheless, it is not possible to detect the fungus if typical morphological traits are absent. Polymerase chain reaction (PCR) based techniques facilitate the detection of even small quantities of fungal DNA, regardless of the existence of visible fungal structures. Recent investigations into paths of infection and distribution using PCR have identified R. pseudotsugae in various types of Douglas-fir tissue including embryonic tissue which did not exhibit any symptoms. Taking these findings as a basis, the present study systematically tested seeds from five German, 13 North American, and two Ukrainian areas of origin for infection with R. pseudotsugae. The fungus was definitively detected in samples from five German and seven North American areas of origin. Nineteen percent of the tested seeds were infected. This indicates that infected seeds might represent another potential source of infection in addition to the ascospore-based distribution of R. pseudotsugae.