INTRODUCTION
Heveae brasiliensis , or the rubber tree, is a tall perennial native to the Amazon basin. The rubber tree has been introduced to more than 40 countries and regions in tropical and subtropical areas, where it plays a pivotal role in the economic development of plantations (Lieberei, 2007). As a major producer and consumer of natural rubber, China began to plant rubber trees within its territory in 1906. In 1954, Chinese researchers made a breakthrough in the large-scale cultivation of rubber trees in the subtropics (latitude 18°N and above), leading China to become the fourth largest producer of rubber globally, despite previously having no rubber plantations at all. Currently, there are five natural rubber plantation bases in China established in Hainan, Yunnan, Guangdong, Guangxi, and Fujian provinces. The experiment reported in the present study was conducted at plantations in Hainan.
Rubber tree powdery mildew is one of the most damaging and widespread foliar diseases of rubber trees. This disease affects the profit of the natural rubber industry by causing defoliation and reductions in latex yield. In severe cases, rubber production can be reduced by 45% (Liyanage et al., 2016). As the most serious epidemic disease affecting China’s rubber plantations, powdery mildew occurs in various rubber planting areas every year. For example, the Xishuangbanna land reclamation area suffered major damage due to powdery mildew in 2017, delaying rubber tapping by nearly 2 months and reducing rubber production by tens of thousands of tons, resulting in losses totaling hundreds of millions of yuan (Liu et al., 2018). It is known that phenological and meteorological factors can jointly influence powdery mildew disease. Under optimum conditions, the conidia can repeatedly infect the host tree throughout the year, leading to greatly reduced photosynthesis, which results in slower growth and substantially decreased latex yield (Liyanage et al., 2017). At present, plantation managers rely on chlorothalil and sulfur powder for mildew prevention and control. Although the application of these agents is effective to some extent, chlorothalil and sulfur powder cannot fundamentally prevent disease outbreaks, and their use causes environmental pollution.
The pathogen responsible for rubber tree powdery mildew isErysiphe quercicola . E. quercicola is an obligate parasitic fungal pathogen that cannot be cultured in vitro without host tissue. The pathogen spreads easily through the air and causes serious damage to rubber trees, notably their leaves, as well their inflorescences, buds, and other tissues. The conidia and hyphae of powdery mildew form a white powder that appears on infected leaves, buds, and shoots. Later on, light yellow spots appear on infected leaves, causing the leaves to twist, buckle, or otherwise become distorted in shape and fall off.
Phyllosphere microorganisms are intimately related to plant growth and the occurrence of plant diseases (Liu et al., 2022). The phyllosphere microbial community is determined by various factors such as leaf age, canopy position, season, and chemical spraying (Jager et al., 2001). Two important indicators for inferring the phyllosphere’s microecology are its community structure and metabolic functions (Liu et al., 2022). The regulation of phyllosphere microbial communities has opened a new and alternative approach to the biological control of foliar diseases. In this respect, the structural dynamics of microorganisms on leaf surfaces have great significance for shaping the near-surface environment and ecology (Li et al. 2021). Although the phyllosphere provides relatively stable conditions for the growth and reproduction of its microorganisms, the phyllosphere living environment is harsh. To better adapt to this habitat adversity, the phyllosphere microbial community relies on its diversity and complexity as evinced by its viable but non-culturable (VBNC) features. Although the phyllosphere environment is hostile, its microbial communities are still highly diverse, consisting of bacteria, filamentous fungi, yeasts, algae, and protozoans. However, bacteria are the main microorganisms constituting the phyllosphere. The community’s diversity, dispersal, and development on the leaf surface are mainly associated with physiological and biochemical changes in the host leaf, its living environment, and its level of immunity (Liu et al., 2022). Many phyllosphere microorganisms have co-evolved with their hosts or epiphytes, and thus play key roles in plant productivity and health by affecting leaf function and lifespan, seed mass, apical growth, flowering, and fruit development, as well as by assisting in the removal and degradation of environmental pollutants (Stone et al., 2018).
The community structure and diversity of phyllosphere microorganisms influence the characteristics and strength of the ecological functions of leaves. The phyllosphere’s microecological dynamic equilibrium is central to the composition of its microbial community. The impacts of phyllosphere microbes upon their host plants, surrounding atmospheric conditions, and microbial interactions have been studied. The results suggest that global climate change has altered phyllosphere microbial communities, with potential cascading effects on plant function and evolution and ecosystem functioning (Vacher et al., 2016). In contrast, little research has been conducted on smaller scales, especially in regard to the relationship between phyllosphere microbes and their hosts and pathogens following powdery mildew infection in a cash crop such as rubber trees. To address this knowledge gap, the present study employed MiSeq high-throughput sequencing (i) to investigate the similarities and differences in the composition of the phyllosphere microbial community of rubber trees in Hainan between regions where powdery mildew occurs and where it does not; and (ii) to understand the effects of powdery mildew on the phyllosphere microbial community of rubber tree leaves and the changes in each microbial community. The overarching aim of this study was to provide an empirical basis for the biological control of powdery mildew and further elucidate the evolution of powdery mildew in rubber trees.