DISCUSSION
Much research has focused on the relative macroscopic behavior of phyllosphere microorganisms, and on the behavior of phyllosphere microorganisms when non-specific parasitic bacteria infest plants. However, there are no reports on obligate parasitic bacteria. In work by Liu et al. (2022) on the community composition of phyllosphere microorganisms of female and male ash trees (Fraxinus ), the core phyllosphere microbiota was consistent, with the dominant phyla beingCyanobacteria and Proteobacteria ; however, some differences were identified at the genus level, with the genusArsenophonus present in female ash only. As in the present study, Guo (2009) collected samples from several sites and found that the phyllosphere microorganisms of Populus tremula mainly consisted of bacteria, followed by actinomycetes and fungi; however, the populations and numbers of phyllosphere bacteria differed between sites. Recently, by analyzing the structure of the phyllosphere fungal communities of dominant tree species in broadleaf red pine forests in Changbai Mountains, Wang et al. (2022) found that the dominant phyla were Ascomycota and Basidiomycota and the dominant classes were Dothideomycetes and Taphrinomycetes , and that the canopy phyllosphere fungal communities of the six dominant tree species differed significantly in composition. Some researchers have also explored the complexity in host–microbe interactions and suggested that the rational design and use of microbial tools, such as phyllosphere probes, have broad potential for applications in economic production activities, such as agriculture, viticulture, forestry, and biofuels, and even for improving human health (Whitaker et al., 2019).
Sampling was based on the fact that geographical location and its relative environment are decisive factors in the community composition of interstitial microorganisms. Therefore, samples were collected from four representative areas in Hainan Province, China: WN in eastern Hainan, WZS and BS in central Hainan, and DZ in western Hainan. Sample WZS_3 from Wuzhishan was markedly different when compared with sample WZS_1 or WZS_2, and the sequencing results of its bacterial community composition showed that a portion of the microorganisms came from the soil, which might be related to the changeable climate in the field. It was difficult to distinguish whether the samples were affected by the nearby environment before sampling.
Only Level 3-infected leaves were collected because under field conditions, Level 3 infections are the most common, Level 4 (entire leaf covered with spots) infections mostly occur in greenhouse conditions, while Level 2 (scattered spots on the surface) infections normally occur on older leaves, or when the powdery mildew spots have faded, and only form at the beginning or at the end of the powdery mildew period, when the leaves age faster under higher temperatures. In addition, there were differences in the relative abundances of the communities of Level 3-infected leaves in each region, which might be associated with the number of environmental influences in the field. As a result, the mean values were taken for comparison with uninfected healthy leaves (Level 0).
In this study, state-of-the-art molecular biotechnology was used to investigate the phyllosphere microorganisms of rubber trees with powdery mildew, to identify their dominant populations at the phylum and genus levels, and to determine whether the incidence of powdery mildew disease affected the phyllosphere microbial community structure and the relative abundances of their taxa members. Because one plantation site was sampled per region, the following inferences about regional differences were made with caution.