Molecular studies of rust on European aspen suggest an autochthonous relationship shaped by genotype

Abstract

Forests are at increasing risk from pathogen outbreak. Climate change for example enhance the risk of local disease outbreaks, and naturalization of exotic pathogens may follow human activities, warranting robust pest surveillance routines to support forest management. Melampsora pinitorqua (pine twisting rust) is of concern in Swedish forestry, and here we evaluate the use of visible rust scores (VRS) on its obligate summer host, European aspen (Populus tremula) as a tool for quantification of the pathogen. With use of species-specific primers, we could detect the native rust, but we failed to detect two exotic rusts (M. medusae and M. larici-populina). We found that aspen genotype determined the presence of fungal genetic markers (amplifying the ITS2 region of the fungal rDNA sequence) as well as DNA sequences specific to M. pinitorqua. We correlated VRS with the amount of fungal DNA in the same leaf, and we related the findings to aspen genotype-specific parameters such as the ability to synthesize and store leaf condensed tannins (CT). At the genotype level both positive and negative relationships were observed between CTs, fungal markers, and rust infestations. However, at the population level, foliar CT concentrations correlated negatively with general fungal- and rust-specific marker abundances. Our results, therefore, do not support the use of VRS to assess Melampsora infestation in Aspen. They do, however, suggest that the relationship between European aspen and rust infestation may be characterized as autochthonous in northern Sweden.

Genotype, Tannin Capacity, and Seasonality Influence the Structure and Function of Symptomless Fungal Communities in Aspen Leaves, Regardless of Historical Nitrogen Addition

“…Visual inspections included: plant height measured in centimeters (from the plant-soil intersection to the tip of the canopy); counting of Harmandiola galls; assessment of symptoms of Venturia sp. as wilted branches relative to the total number of branches per canopy; other biotic stress symptoms, such as leaf hypersensitive response (HR), abundance of eriophyid mite colonies, damage from chewing insects, and rust pustules (Melampsora pinitorqua [47,67]. Leaf damage symptoms were ranked on a 0-3 scale: 0 indicated no symptoms, 1 indicated low damage corresponding to symptoms on some leaves, 2 indicated moderate damage with symptoms on approximately half of the leaves, and 3 indicated a high level of damage with symptoms on more than half of the leaves.…”

“…Hence, while our study suggested a negative impact of condensed tannins on the abundance of foliar-associated mycobiomes, it is premature to infer a simple relationship. Condensed tannins, for example, were reported to have a negative correlation with twig endophyte infection in poplars within a hybrid zone [95] as well as with the abundance of molecular markers of the biotrophic pathogen Melampsora pinitorqua [47], and the symptoms of necrotrophic pathogen Venturia infection in Populus tremula [66]. Moreover, in a review by Bhat et al (1998) [96], degradation primarily by lamentous fungi such as Aspergillus and Penicillium, along with yeasts, was found to degrade condensed tannins, potentially using them as substrate.…”

“…Based on its degree of conservation, the ITS region is divided into subunits, that with adequate primers may be targeted singly or in combination (i.e. ITS1 to ITS4); providing varied opportunities to estimate fungal richness and diversity in a sample [46,47].…”