Virulence and Host Range of Fungi Associated With the Invasive Plant Ageratina adenophora

Abstract

To determine whether disease-mediated invasion of exotic plants can occur and whether this increases the risk of disease transmission in local ecosystems, it is necessary to characterize the species composition and host range of pathogens accumulated in invasive plants. In this study, we found that Didymellaceae, a family containing economically important plant fungal pathogens, is commonly associated with the invasive plant Ageratina adenophora. Accordingly, we characterized its phylogenetic position through multi-locus phylogenetic analysis, as well as its environmental distribution, virulence, and host range. The results indicated that 213 fungal collections were from 11 genera in Didymellaceae, ten of which are known, and one is potentially new. Didymella, Epicoccum, Remotididymella, and Mesophoma were the dominant genera, accounting for 93% of total isolates. The virulence and host ranges of these fungi were related to their phylogenetic relationship. Boeremia exigua, Epicoccum latusicollum, and E. sorghinum were found to be strongly virulent toward all tested native plants as well as toward A. adenophora; M. speciosa and M. ageratinae were weakly virulent toward native plants but strongly virulent toward A. adenophora, thus displaying a narrow host range. Co-evolution analysis showed no strong phylogenetical signal between Didymellaceae and host plants. Isolates S188 and Y122 (belonging to M. speciosa and M. ageratinae, respectively) showed strong virulence toward A. adenophora relative to native plants, highlighting their potential as biocontrol agents for A. adenophora invasion. This study provides new insights into the understanding of the long-term ecological consequences of disease transmission driven by plant invasion.

Keywords: Didymellaceae; biocontrol agent; disease transmission; pathogen accumulation; plant–pathogen co-evolution.

Figure 1

Phylogenetic tree and source of the Didymellaceae isolates. (A) Phylogenetic tree of Didymellaceae. Genera names in red represent the distribution of the isolates collected in this study and the numbers in brackets represent the number of strains isolated. For additional details, please see Supplementary Figures 1, 2. (B) Source of the Didymellaceae isolates.

Figure 2

Evaluating the virulence of 131 Didymellaceae toward Ageratina adenophora and 14 native plants. (A) Heatmap of leaf spot sizes transformed by log₂(x + 1). (B) Average spot size of genera (≥3 isolates) on native plants and A. adenophora. (C) Percent pathogen distribution in the 15 different hosts. ALL, Allophoma; BOE, Boeremia; DID, Didymella; EPI, Epicoccum; LEP, Leptosphaerulina; MA, Mesophoma ageratinae; MS, M. speciosa; NEO, Neoascochyta; NOT, Nothophoma; PAR, Paraboeremia; REM, Remotididymella; and STA, Stagonosporopsis. Error bars represent 1 SE.

Figure 3

The Epicoccum clade in the phylogenetic tree. Red circles represent strongly virulent isolates, black circles represent weakly virulent isolates, blue circles represent isolates for which virulence was not evaluated in this study, and nodes without symbols represent the standard sequences downloaded from GenBank.

Figure 4

Co-evolution analysis of Didymellaceae pathogens and host plants. The tree on the right shows the fungal phylogenetics and the tree on the left shows the plant phylogenetics. The lines in the middle represent the pathogenic relationships between fungi and host plants.

Figure 5

Evaluating the virulence of 18 Didymellaceae against Ageratina adenophora and 29 native plants. (A) Heatmap of leaf spot sizes transformed by log₂(x + 1). (B) Host percentage and average spot size of typical isolates on native plants and A. adenophora. AA, Ageratina adenophora; AB, Achyranthes bidentata; AD, Ampelopsis delavayana; AE, Amphicarpaea edgeworthii; AH, Arthraxon hispidus; AM, Abelmoschus moschatus; AN, Alnus nepalensis; BE, Bambusa emeiensis; CG, Cyclobalanopsis glauca; CO, Cynanchum otophyllum; CT, Celtis tetrandra; FM, Fallopia multiflora; HT, Hypoestes triflora; LC, Lindera communis; MF, Michelia figo; MR, Myrica rubra; OF, Oreocnide frutescens; PH, Petunia hybrida; PP, Pueraria peduncularis; PT, Parthenocissus tricuspidata; QV, Quercus variabilis; RC, Rubia cordifolia; RI, Reinwardtia indica; RP, Rubus parvifolius; RX, Rosa xanthina; SE, Sechium edule; SS, Smilax scobinicaulis; UL, Urena lobata; ZB, Zanthoxylum bungeanum; and ZM, Zehneria maysorensis. Isolate names in red represent fungi that are highly virulent toward A. adenophora. Plant names in red represent the tested hosts from Figure 2.

Figure 6

Virulence evaluation for potential biocontrol strains. (A) The course of disease development on injured leaves. (B) Comparison of lesion sizes at day 4 post-inoculation between injured and non-injured leaves from Ageratina adenophora with different invasion histories. There were significant differences in leaf spot size among the isolates of the injured leaves of A. adenophora with an 80-year invasion history (p = 0.030, Kruskal-Wallis test). Error bars represent 1 SE.