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.