Alleviation of apple replant disease in sandy soils by clay amendments increasing plant available silicon

Abstract Aims The production of apple fruits in orchards or plants in tree nurseries is negatively affected by apple replant disease (ARD), worldwide. Our objective was to develop a method to counteract ARD without applying chemical soil disinfection. We tested if (i) an addition of clays with high release of plant available silicon reduces ARD symptoms and (ii) a biochemical effect of silicon on the plant defense reaction occurs. Methods In a greenhouse experiment, apple rootstocks ‘M26’ were grown for 8 weeks in a sandy replant soil, a heat disinfected control and a grassland soil with and without amendment by bentonite and the clay blend Florisol®TM Profi (6 and 18%). The micromorphology of silica bodies (phytoliths) in roots and the phytoalexin concentration as a measure of the defense reaction of roots were analysed. In a second approach, the gene expression of ARD biomarkers involved in phytoalexin biosynthesis was determined after two weeks. Results Normalised gene expression was significantly higher for the ARD soil with clay amendments compared to the ARD variant whereas phytoalexin concentrations were similar. Significantly lower phytoalexin levels were found in the heat disinfected and Grass variants, indicating less defense reaction than in the ARD variant. Clay amendment increased soluble silicon in the substrates and consequently the epidermis and vascular bundles, which indicates high silicon accumulation. Overall, plants grown in ARD and Grass soils with amendments showed improved growth compared to plants grown in the respective soils without amendments. Conclusion The amendment by clays rich in plant-available silicon may be a measure to counteract ARD symptoms. However, these results need to be reproduced in future experiments with ARD soils of other origins.