Understanding Host-Pathogen Interactions in the Sphaerulina musiva-Populus Pathosystem

Graduation date: 2016Host-parasite interactions between the pathogen, Sphaerulina musiva, and its host, Populus spp., were examined in a series of studies. In the first study, variation in resistance of Populus nigra from 7 locations in Europe was differentiated based on the parameters cankers per cm and disease severity score. The analysis evaluated if location, isolate, genotype, and their interactions significantly affected variation in these parameters. In addition, the correlation in disease severity score between single isolate and bulk isolate inoculations was examined. Location (72%; P < 0.001) explained the majority of variation in resistance for cankers per cm followed by genotype(location) (28%; P < 0.001). In contrast, genotype(location) (51%; P < 0.001) explained the majority of variation in disease severity score followed by location (26%; P = 0.025). A significant isolate effect (P = 0.034) and genotype(location)*isolate interaction (P = 0.004) were also present in the disease severity score model. When correlating single isolate to bulk isolate inoculations the correlation coefficients for disease severity score indicated a significant range of correlations (r = 0.871 to r = 0.952) with significant P-values (P < 0.05). The second study determined whether or not there is a correlation between resistance to stem infection and resistance to leaf infection. The results indicated no correlation between leaf spot severity and stem canker severity (r = 0.452; P = 0.068). Older leaves showed significantly lower disease severity compared to younger leaves (leaf 1 vs. leaf 5) when averaged across all genotypes in both experiments. The lack of a consistent correlation among leaves and stems may have implications for disease management and deployment of resistant genotypes and suggests that there may be different mechanisms controlling resistance in the stem compared to the leaves. The third study examined the genetic basis of the interaction between S. musiva and P. trichocarpa. RNA-seq was used to identify potential S. musiva effectors involved in stem canker formation at 2 and 3 weeks post-inoculation. The total reads at 2 weeks were 4,198,267; at 3 weeks reads increased to 4,915,453 compared to 3,125,004 for the control. Approximately 20% of the total reads were aligned to the S. musiva reference genome and the remaining reads were aligned to P. trichocarpa. Analysis of the fungal reads identified 70 genes at 2 weeks post-inoculation and 110 genes at 3 weeks post-inoculation that were differentially expressed between the inoculated trees and the control. The most highly expressed gene at 2 weeks and second at 3 weeks had homology to Extracellular protein 2 (Ecp2) produced by Cladosporium fulvum. Sphaerulina musiva Ecp2 (SmEcp2) was infiltrated into P. trichocarpa leaves and caused necrosis on some susceptible genotypes