In subalpine forests of the European Alps, Norway spruce (Picea abies) is frequently attacked by the pathogen Chrysomyxa rhododendri. The rust fungus overwinters in the leaves of rhododendrons (Rhododendron ferrugineum, R. hirsutum) and infects the current year spruce needles immediately after sprouting in spring. Once entered the needle, it forms an extensive mycelium and causes needle yellowing and thereupon needle loss. Attacked trees show a reduction of chlorophyll content, photosynthesis, biomass accumulation and growth. As a consequence, reduced timber yield and increased seedling dieback were observed. Constant high infection rates during the last years concerned the forestry and ask for a better understanding of the forest disease as well as defence strategies of Norway spruce.
The main objectives of this study were (a) to evaluate the negative impact of infections on subalpine forests, (b) to identify temporal patterns of spore dispersal and needle infection and their correlation with weather conditions as well as coordination with host phenology, (c) to gain insights into the resistance mechanisms of naturally occurring resistant trees and (d) to establish a spruce population with enhanced resistance.
The analyses included a broad literature review and the assessment of available data. Infection dynamics were analysed by collecting spores continuously with a spore trap, identifying and characterising the spores with light and electron microscopy, determining the infection period by a staggered application of fungicide on selected twigs, monitoring the phenological stages during needle development, and recording weather parameters with a climate station. Norway spruce trees with enhanced resistant phenotype were identified in Tyrol and compared with control trees regarding the phenolic needle content (UHPLC-MS) and genetic variants (genome-wide SNPs). Furthermore, this spruce population was clonally reproduced by rooting of sampled twigs and resistance of cuttings was tested by infection with C. rhododendri spores.
Spore dispersal of both basidio- and aeciospores was found to be extended over a long period, enhancing an overlap with the susceptible phenological stages of the host. The time span of needle infection was limited to about three weeks, and favourable weather conditions crucial for the pathogen dispersal. This can partly explain the variable infection intensities from year to year and site to site. The phenolic profile of needles differed between susceptible and resistant trees, especially regarding the seasonal accumulation and reaction to infection. In addition, several significant genetic associations for phenolic compounds were found. Clonal reproduction was complicated by low rooting success due to the advanced age of selected trees, but enhanced resistance was proven by the infection experiment for several clones.
Repeated severe infections cause serious socio-economical as well as ecological problems due to diminished timber yield and missing rejuvenation in high elevation spruce forests. Due to the positive correlation of airborne basidiospore concentrations with long warm periods in spring, global warming may favour the further spread of the pathogen. Efficient counter strategies are needed, and vegetative reproduction of selected clones combined with resistance tests is a promising method for providing resistant plant material. Results indicate that a combination of constitutive and inducible phenolic defence confers enhanced resistance in P. abies to C. rhododendri and rapid de-glycosylation of flavonoids following infection may play a central role. Identified genetic markers associated with stilbene and flavonoid concentrations could be useful for marker-assisted breeding and selection of genotypes adapted to attack by C. rhododendri.