Abstract
Apple replant disease (ARD), caused largely by soil-borne fungal pathogens, has seriously hindered the development of the apple industry. The use of antagonistic microorganisms has been confirmed as a low-cost and environmentally friendly means of controlling ARD. In the present study, we assessed the effects of Bacillus amyloliquefaciens QSB-6 on the growth of replanted apple saplings and the soil microbial environment under field conditions, thus providing a theoretical basis for the successful use of microbial biocontrol agents. Four treatments were implemented in three apple orchards: untreated replant soil (CK1), methyl bromide fumigation (CK2), blank carrier treatment (T1), and QSB-6 bacterial fertilizer treatment (T2). The plant height, ground diameter, and branch length of apple saplings treated with T2 in three replanted apple orchards were significantly higher than that of the CK1 treatment. Compared with the other treatments, T2 significantly increased the number of soil bacteria, the proportion of actinomycetes, and the activities of soil enzymes. By contrast, compared with the CK1 treatments, the phenolic acid content, the number of fungi, and the abundance of Fusarium oxysporum, Fusarium moniliforme, Fusarium proliferatum, and Fusarium solani in the soil were significantly reduced. PCoA and cluster analysis showed that soil inoculation with strain QSB-6 significantly decreased the Mcintosh and Brillouin index of soil fungi and increased the diversity of soil bacteria in T2 relative to CK1. The soil bacterial community structure in T2 was different from the other treatments, and the soil fungal communities of T2 and CK2 were similar. In summary, QSB-6 bacterial fertilizer shows promise as a potential bio-inoculum for the control of ARD.
Highlights
In the main apple planting regions of China, many old apple orchards are facing renewal but are limited by land resources, leading to the development of apple replant disease (ARD), which seriously affects the yield and quality of apples [1,2]
Quantitative analysis based on quantitative PCR (qPCR) confirmed the wide distribution of F. oxysporum in replanted soil around the Bohai Gulf region, and F. verticillioides, F. solani, F. oxysporum, and F. proliferatum isolated from apple replant soil around the Bohai Gulf region were highly pathogenic to Malus hupehensis seedlings. These findings demonstrate that Fusarium spp. are among the main soil-borne pathogenic fungi that cause ARD in
A strain of B. amyloliquefaciens QSB-6 was previously isolated from the rhizosphere soil of healthy apple trees in a replanted orchard and demonstrated to have good inhibitory effects on F. oxysporum, F. moniliforme, F. proliferatum, and F. solani [18]
Summary
In the main apple planting regions of China, many old apple orchards are facing renewal but are limited by land resources, leading to the development of apple replant disease (ARD), which seriously affects the yield and quality of apples [1,2]. Previous studies have reported that replant disease does not arise from a single cause, but instead emerges from a combination of biotic and abiotic factors. The abiotic factors include allelopathy, autotoxicity, and soil physicochemical imbalances [5–8]. The biotic factors include the accumulation of soil-borne pathogens such as nematodes (Pratylenchus spp.), fungi (Rhizoctonia solani, Fusarium spp., and Cylindrocarpon spp.) and oomycetes (species of Pythium and Phytophthora). It is generally believed that replant disease results from a combination of biotic and abiotic factors, biotic factors play a leading role in disease development [2,3,8–10]. This has been widely demonstrated in other studies through soil pasteurization and biocide application [11–13]
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