Introduction

Apple replant disease (ARD) causes reduced growth and fruit yield and affects orchards and tree nurseries worldwide. A number of pathogens have been consistently identified as causal agents of ARD; however factors affecting disease-severity are not fully understood.

Aims

We examined five soils from German tree nurseries and apple orchards featuring different soil characteristics and replant histories. We aimed to link the plant-soil interaction to replant disease severity.

Methods

In a greenhouse experiment, young apple plants were grown for eight weeks on untreated and disinfected (control) soils. Growth parameters were recorded to evaluate the severity of ARD. The defence response of the plants was examined by expression analysis of ARD indicator genes (BIS3, B4H and ERF1B) and GC–MS-based detection of phytoalexins. The fungal and bacterial rhizosphere communities were investigated by ITS and 16S rRNA amplicon sequencing, respectively.

Results

After eight weeks, ARD symptoms were observed on all soils. Growth depression was highest on soils that had faced intensive apple cultivation and lowest on a soil with only one year of apple cultivation prior to the experiment. These results correlated with increases in the BIS3 expression level and the phytoalexin content in the roots. No bacteria and fungi commonly found in increased abundance in ARD soils were consistently detected in all soils.

Conclusions

Replant history influenced disease severity more than soil characteristics. ARD symptoms correlated with BIS3 expression and phytoalexin (PA) formation. PA exudation increased the relative abundance of bacterial genera with the potential ability to degrade phenolic compounds.

中文翻译：

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将土壤特性、根际微生物组组成和植物防御反应与苹果再种植病害的严重程度联系起来

 介绍

苹果再植病 （ARD） 会导致生长和果实产量下降，并影响全世界的果园和树木苗圃。许多病原体一直被确定为 ARD 的病原体;然而，影响疾病严重程度的因素尚不完全清楚。

 目标

我们检查了来自德国苗圃和苹果园的五种土壤，这些土壤具有不同的土壤特性和再种植历史。我们旨在将植物-土壤相互作用与再种植疾病的严重程度联系起来。

 方法

在温室实验中，苹果幼苗在未经处理和消毒的（对照）土壤上生长八周。记录生长参数以评估 ARD 的严重程度。通过 ARD 指示基因 （BIS3 、 B4H 和 ERF1B） 的表达分析和基于 GC-MS 的植物抗毒素检测来检查植物的防御反应。分别通过 ITS 和 16S rRNA 扩增子测序研究真菌和细菌根际群落。

 结果

八周后，在所有土壤上观察到 ARD 症状。在面临密集苹果种植的土壤上，生长抑制最高，在实验前仅种植一年苹果的土壤上最低。这些结果与 BIS3 表达水平和根中植物抗毒素含量的增加相关。在所有土壤中均未持续检测到 ARD 土壤中丰度增加的常见细菌和真菌。

 结论

再种植历史对疾病严重程度的影响大于土壤特性。ARD 症状与 BIS3 表达和植物抗毒素 （PA） 形成相关。PA 渗出增加了细菌属的相对丰度，具有降解酚类化合物的潜在能力。