Rain Shelter Cultivation Reduces Root Rot Incidence of Panax notoginseng by Altering Root Exudates and Bacterial Communities under Micro-Irrigation and Fertilization

Panax notoginseng is an important medicinal crop in China. The high incidence of root rot in P. notoginseng during the rainy season has restricted the development of the industry. It is believed that frequent rainfall and a warm soil environment are important factors that promote root rot incidence. However, there is still a significant knowledge gap in the relationship between rainfall and root rot incidence. To understand the effects of rainfall and fertilizer on root exudates, the soil bacterial structure, and root rot in P. notoginseng, four treatments were chosen for both field and pot experiments. These treatments included DW (rain shelter and no fertilizer), RW (no rain shelter and no fertilizer), DWF (rain shelter and fertilizer), and RWF (no rain shelter and fertilizer). The results showed that both factors (rain shelter and fertilizer) significantly affected root rot incidence and several other parameters. Among them, the effect of a rain shelter is more significant than that of fertilizer, and the combination of the two further improves the effect. DW and DWF treatments significantly reduced the soil moisture, phenolic acid, and root rot incidence, while significantly increasing the soil temperature and enzyme activities compared to RW or RWF. Seven phenolic acids secreted by P. notoginseng roots were all positively correlated with root rot incidence. Root rot was also positively correlated with Planctomycetota, Acidobacteriota, and Gemmatimonadota and negatively correlated with Firmicutes, Proteobacteria, Patescibacteria, and Nitrospirota. DWF treatment decreased the soil moisture and the concentration of p-hydroxybenzoic acid, syringic acid, phthalic acid, and vanillic acid and promoted the growth of Firmicutes and Proteobacteria, leading to the lowest incidence of root rot.