Organic mulching was widely adopted in agricultural cultivation for a long time. The general effect of mulching on soils has also been fully recognized . However, in terms of the influence of organic mulching on the control of disease and the composition of microorganisms in soils, the depth of understanding is far from enough .
The results of the present study indicated that organic mulching application led to a significant decline in the disease incidence of litchi downy blight, along with an increase in the abundance of soil bacterial and fungal community. The findings were in line with numerous studies on other crops that practices of bio-organic fertilizer application raised soil microbial diversity and enhance plant disease suppression [24, 25]. Therefore, the modification of soil microbial community composition and functional potentials might contribute to the increase of disease suppression on litchi downy blight under organic mulching application.
The structure and functions of soil microbial community are closely related to soil quality and ecosystem stability and sustainability, which are crucial for plant health and productivity . Various studies demonstrated the alteration of soil microbial community was closely associated with soil suppressiveness to soilborne pathogenic fungi [27, 28]. Wu et al. highlighted the close association between replant disease and the variations in structure and potential functions of rhizosphere bacterial community . Yang et al. indicated that soil microbial diversity had a strong effect on tobacco wilt disease level . In addition, any modifications in soil microbial community assemblages will have a cascade of effects on soil structure and nutrient cycling, including soil aggregate stability and decomposition processes [31, 32].
Soil microbiota is considered to be a critical factor to regulate soil quality and sustainability , its community and diversity are significant involved with soil-plant health via triggering different functional roles, which including decomposing organic matter, ecosystem regulators, and biological antagonism [34, 35]. In this study, we found that organic mulching application could increase the soil bacterial community diversity and enhance the populations of beneficial microorganisms. Similarly, the phyla Proteobacteria, Actinobacteria, Acidobacteria and Chloroflexi, which are consistently predominant bacteria in tillage soil , are dominant bacteria in our study. Proteobacteria plays an important role in straw decomposition and soil nutrient uptake , and Acidobacteria is considered to have extensive metabolic and genetic functions . Therefore, the relative abundance of phyla Proteobacteria and Acidobacteria increased after organic mulching application might be attribute to organic matter incorporation. The fungal taxonomic composition and richness of functional fungi in the soils under organic mulching methods also significantly higher than that in bare soils with conventional tillage methods. The dominant Aspergillus and Thermomyces genera were observed under organic mulching application. Certain affiliated genera including Thermomyces spp. are the key contributors to the hemicellulose hydrolysis during root-surrounding decomposition, which supply nutrients to microbial system in the soils .
Litchi downy blight caused by P. litchii is one of the most destructive diseases in litchi planation. As the important origin of primary infection, residual pathogens on dropped-blossoms and fruits play crucial role in epidemic of litchi downy blight . Our findings indicated that organic mulching application reduced disease incidence and manipulated the soil microbial community structure, especially the increasing bacterial families such as Burkholderia, Bacillus, and Paenibacillus, which have been proved to possess antagonistic activities and contribute to disease suppressiveness [41–43]. Moreover, the frequent occurrence of specific beneficial fungal species, such as Trichoderma sp. and Penicillium sp., reflect the survival of P. litchii in planting system. Several studies revealed the excellent beneficial effects of Trichoderma, Aspergillus and Penicillium for pathogen control [44–47]. Their fungal abundance may be also related to the suppression of litchi downy blight through mycoparasitism fabrication of lytic enzymes and antimicrobial chemicals and competing for space and food, which was consistent with mentioned results of the reduction of litchi downy blight in organic mulching soils. On the other hand, organic mulching application led to a significant decrease in the relative abundance of the genus Fusarium, which is the major crop pathogens in the world and attribute to many soilborne disease through colonizing root surfaces, producing some plant irritant and some secondary metabolite during growth and metabolism , exhibited lower abundance in organic mulching soils in comparison with control soils. Therefore, organic mulching might have exerted positive feedback effects on litchi planation and resistance through inhibiting potential soil borne pathogens and fostering plant-beneficial microbes which targeting the causal agent P. litchii.
In conclusion, the findings in this study revealed that the litchi downy blight was closely associated with dysbiosis of soil microbiota. The organic mulching application could delay this disease mainly depend on reshaping the soil microbial community and modifying the potential functions microbes that harbor antagonistic activities against P. litchii and contribute to soil suppressiveness. Our results reinforce the influence of organic mulching on disease control and soil microbial diversity in litchi plantation from the aspects of microbial structure and ecological function. Further work is needed to investigate the survival of causal agent in the presence of beneficial microorganisms and to link the potential functions to organic mulching application.