Involvement of Dactylonectria and Ilyonectria spp. in tree decline affecting multi-generation apple orchards

Abstract

This study started from typical replant disease symptoms limited to specific foci within three multi-generation orchards showing homogeneous growth. A plant growth assay was conducted using soil from symptomatic and asymptomatic tree root zones along planted rows and from strip-rows. Root colonizing fungal communities were investigated, then the study turned to pathogenicity of Cylindrocarpon-like fungi (Dactylonectria and Ilyonectria spp) and their extracellular exudates. Growth of apple rootstock plantlets in soil from symptomatic trees was significantly lower than in those observed in soil from asymptomatic trees and from strip-rows. Among the main group of endophytic filamentous fungi isolated from roots, Cylindrocapon-like fungi (Dactylonectria torresensis and, to a lesser extent, Ilyonectria robusta), along with binucleate Rhizoctonia spp., prevailed mostly in plantlets grown in soil collected from planted rows. On the other hand, Fusarium spp. prevailed in plantlets grown in soil from the strip-rows. Cylindrocarpon-like fungi was found to be the most negatively correlated with plant growth. As findings of artificial inoculation with main root colonizing fungal species were not in line with what was observed in native soils, a further investigation was performed on secondary metabolites through which Dactylonectria torresensis exerts pathogenicity; this analysis revealed that both phytotoxins (tentoxin, HC toxin and zearalenone) and cytotoxic compounds (rabelomycin and nidulin) may be involved. Findings suggest that extracellular compounds released by D. torresensis may have contributed to the severe growth reduction associated with replant disease-like symptoms.