Pseudomonas Exacerbates Apple Replant Disease Induced by Fusarium

Abstract

Apple replant disease (ARD) causes substantial economic losses worldwide, China included. Analyzing the causes of this replant disease from the perspective of rhizosphere microecology is therefore crucial. In this study, rhizosphere soils from apple trees continuously cropped were examined. The mechanism by which ARD occurs was elucidated from the soil microbiome, via its high-throughput sequencing and co-occurrence network, NetShift, and correlation analyses. Core bacterial microbes were isolated, and their roles in altering the microecological environment were verified through reinoculation experiments. The results indicated that the disease indices for apple seedlings cultivated increased in continuously cropped soils. Bacterial diversity decreased in continuously cropped apple orchards for 10 years (R10) and 15 years (R15), but the relative abundance of Pseudomonas increased. In contrast, fungal diversity increased, with the relative abundance of Fusarium also increasing. With the increase of planting years, the co-occurrence network of rhizosphere microorganisms shifted from a “bacterial type” to a “fungal type”. As a dominant genus, Pseudomonas nevertheless exhibited significant network variation after 10 years of consecutive cultivation, suggesting that this soil microorganism could be key to ARD’s occurrence. Further, the correlation analysis revealed, for the first time, that Pseudomonas is negatively correlated with bacterial diversity but positively correlated with the relative abundance of Fusarium, indicating a close relationship between Pseudomonas and Fusarium in replant soil. Four key Pseudomonas amplicon sequence variants (ASVs) strains were isolated from the continuously cropped rhizosphere soil of apple trees, and reinoculation experiments verified that adding Pseudomonas did indeed exacerbate the occurrence of replant diseases in both strawberry and apple, with a significantly higher disease index than obtained for a single Fusarium inoculation. The findings of this study provide new and timely insights into the mechanism underlying the occurrence of ARD.

Keywords: Apple Replant Disease, Microbial diversity, Pseudomonas, Fusarium