Effects of Colletotrichum gloeosporioides and Poplar Secondary Metabolites on the Composition of Poplar Phyllosphere Microbial Communities.

Poplar anthracnose caused by Colletotrichum gloeosporioides is a common disease affecting poplars globally that causes the destruction and alteration of poplar phyllosphere microbial communities; however, few studies have investigated these communities. Therefore, in this study, three species of poplar with different resistances were investigated to explore the effects of Colletotrichum gloeosporioides and poplar secondary metabolites on the composition of poplar phyllosphere microbial communities. Evaluation of the phyllosphere microbial communities before and after inoculation of the poplars with C. gloeosporioides revealed that both bacterial and fungal OTUs decreased after inoculation. Among bacteria, the most abundant genera were Bacillus, Plesiomonas, Pseudomonas, Rhizobium, Cetobacterium, Streptococcus, Massilia, and Shigella for all poplar species. Among fungi, the most abundant genera before inoculation were Cladosporium, Aspergillus, Fusarium, Mortierella, and Colletotrichum, while Colletotrichum was the main genus after inoculation. The inoculation of pathogens may regulate the phyllosphere microorganisms by affecting the secondary metabolites of plants. We investigated metabolite contents in the phyllosphere before and after the inoculation of the three poplar species, as well as the effects of flavonoids, organic acids, coumarins, and indoles on poplar phyllosphere microbial communities. We speculated that coumarin had the greatest recruitment effect on phyllosphere microorganisms, followed by organic acids through regression analysis. Overall, our results provide a foundation for subsequent screening of antagonistic bacteria and fungi against poplar anthracnose and investigations of the mechanism by which poplar phyllosphere microorganisms are recruited. IMPORTANCE Our findings revealed that the inoculation of Colletotrichum gloeosporioides has a greater effect on the fungal community than the bacterial community. In addition, coumarins, organic acids, and flavonoids may have recruitment effects on phyllosphere microorganisms, while indoles may have inhibitory effects on these organisms. These findings may provide the theoretical basis for the prevention and control of poplar anthracnose.

Verification of the Protective Effects of Poplar Phenolic Compounds Against Poplar Anthracnose.

Poplar anthracnose caused by Colletotrichum gloeosporioides is one of the most important diseases widely distributed in poplar-growing areas in China, causing serious economic and ecological losses. In this study, three poplar species showed different resistance to poplar anthracnose: Populus × canadensis was resistant, Populus tomentosa was susceptible, and P. × beijingensis showed intermediate resistance. However, it remains uncertain whether phenolic compounds in poplar are involved in this resistance. Therefore, we determined the concentrations of phenolic compounds and their antifungal activity. Before and after the C. gloeosporioides inoculation, 20 phenolic compounds were detected in P. × canadensis and the number increased from 12 to 14 in P. × beijingensis but decreased from seven to four in P. tomentosa. Thus, phenolic compounds may be positively correlated with the degree of disease resistance. We selected seven phenolic compounds for further analysis, which varied considerably in content after inoculation with C. gloeosporioides. These seven compounds were salicin, arbutin, benzoic acid, salicylic acid, chlorogenic acid, ferulic acid, and naringenin, which helped poplar trees to limit the growth of C. gloeosporioides and differed in their antifungal effects, with phenolic acids having the strongest inhibitory effect. In addition, the optimal concentrations of different substances varied. We demonstrate that these phenolic compounds produced by poplar do play a certain role in the process of poplar resistance to anthracnose. These findings lay a foundation for future research into the antifungal mechanism of poplar trees and may be useful for enhancing the prevention and control of poplar anthracnose.