Swiss Needle Cast and the Foliar Mycobiome of Coastal Douglas-fir (Pseudotsuga menziesii)
Date
2024
Authors
Hayward, Emma
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Abstract
Fungal pathogens of trees are an essential component of forest ecosystems as an ecological driver of diversity and natural selection; however, they can also have devastating effects. My research aims to understand better Nothophaeocryptopus gaeumannii, the causal agent of Swiss Needle Cast (SNC), a disease affecting Douglas-fir (Pseudotsuga menziesii). This pathogen infects the needles of its host, is associated with defoliation, and is endemic throughout the Douglas-fir range. In the Pacific Northwest of the US and Canada, a rise in the incidence and severity of SNC, which is thought to be linked to climatic changes and forestry practices, has been observed in the past few decades. There is a genetic component to SNC tolerance, enabling the selection of SNC resilient genotypes for reforestation. However, fungal load is not always correlated with needle loss, suggesting a more complex relationship. My thesis work collected SNC symptom severity data (measured as stomatal occlusion severity, needle loss severity, and relative growth rate) on a general combining ability population with a high incidence of SNC from the provincial Douglas-fir breeding program. Host phenotyping revealed a similar lack of correlation between disease signs and symptoms. Stomatal occlusion severity and relative growth rate were found to be low-moderately heritable traits within this breeding population (h2 = 0.19-0.34 and 0.28, respectively), whereas needle loss severity had low heritability (h2 = 0.11-0.12), and considerable variation within families and across locations in the plot. I hypothesize that this is due to this study site's endemic level of infection. SNC severity at endemic levels is likely not strong enough to drown out other biotic and abiotic factors contributing to needle loss. I propose stomatal occlusion incidence as a reasonable trait to breed for resistance under endemic-level conditions. The second part of my research investigated fungal community composition within Douglas-fir needles. Community analysis was done to determine if specific communities or taxa determine SNC symptom response or drive this lack of correlation between SNC signs and symptoms. I used ITS amplicon metagenomics supplemented by fungal culturing to characterize the foliar mycobiome of a subset of individuals from the same Douglas-fir breeding population. The Douglas-fir foliar mycobiome was diverse, including mostly rare OTUs and a few highly abundant OTUs, such as N. gaeumannii (64.5% of reads). N. gaeumannii was found to have a negative correlation with the second most abundant OTU, belonging to the genus Rhizosphaera, identifying it as a direct competitor with N. gaeumannii in the foliar environment for some host genotypes. The genus Rhizosphaera exhibited associations with particular families within the breeding population, suggesting some level of genetic determination. Foliar mycobiome community assemblages differed significantly among families but not between disease severity levels or groupings. The foliar mycobiome's composition, therefore, has implications for breeding programs in the context of synthetic communities ('SynComs') as seen in the context of white spruce and its pathogen spruce budworm. Despite no clear patterns in fungal community composition based on SNC symptom severity, fungal taxa that are known foliar pathogens, as well as taxa that are known to produce beneficial bio-active compounds (i.e., mycoparasitic taxa), were found within my dataset. This could explain some of the variation associated with needle loss, which was not correlated with the fungal load of N. gaeumannii at this site.
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Keywords
Microbiome, Douglas-fir, Genetics, Molecular, Fungi