Biphenyl and dibenzofuran phytoalexins differentially inhibit root-associated microbiota in apple, including fungal and oomycetal replant disease pathogens.

Apple replant disease (ARD) is a serious soilborne disease in apple nurseries and orchards worldwide. ARD is the result of an unbalanced soil microbiome in which multiple soil-borne plant pathogenic fungi, oomycetes and nematodes form a disease complex. Biphenyl and dibenzofuran phytoalexins are found in greater quantities in the roots of apple plants grown in ARD soil compared to disinfected ARD soil. However, the contribution of these compounds to plant health or disease is not yet understood. Here, the antimicrobial activity of fourteen chemically synthesized biphenyl and dibenzofuran phytoalexins was tested against eight selected microorganisms isolated from either rhizosphere soils or apple roots. These included five potentially beneficial bacteria (Rhodococcus pseudokoreensis strain R79^T, Rhodococcus koreensis strain R85, Streptomyces pulveraceus strain ES16, Streptomyces ciscaucasicus GS2, Priestia megaterium strain B1), two ARD fungal pathogens (Ilyonectria robusta H131 and Dactylonectria torresensis N3) and one oomycete (Globisporangium terrestre). Two phytoalexin mixtures reflecting the percentages of the individual compounds inside the roots (Mixture A) and the root exudate (Mixture B) were also tested. The two phytoalexin mixtures demonstrated a higher antimicrobial activity than the individual phytoalexins, suggesting a synergistic effect. The minimum inhibitory concentration (MIC) and the half maximal effective concentration (EC₅₀) values determined to be active against the eight microbes were within a range of 2.5-fold the ecologically relevant phytoalexin concentration (approximately 33 and 24 µg ml^-1 in roots and exudates, respectively). The results contribute to our understanding of the impact of apple root phytoalexins on ARD and suggest potential strategies for disease management.