Phytotoxicity of Dactylonectria spp. exudates on kiwifruit vine and profile of secondary metabolites for understanding their relationship with the host plant

Kiwifruit vine decline syndrome (KVDS), which is responsible for a progressive decline in yields in Italy, is linked to a non-specific reduction in root development, the biotic or abiotic causes of which are still unclear. Dactylonectria is one of the most common soil-borne fungi associated with kiwifruit vines, However, its role in KVDS has not yet been clarified; therefore, Dactylonectria functional relationship with kiwifruit vine at rhizosphere level was investigated focusing solely on its exudates. Six isolates, four from kiwifruit-bearing soils belonging to D. torresensis. D. ecuadoriensis, D. novozelandica and D. pauciseptata, and two D. torresensis from a virgin soil, was tested for phytotoxicity of exudates by growing kiwifruit in vitro plants on MS media amended at 50% with sterile fungal filtrates. Parallelly, low molecular weight (LMW) metabolic profile of fungal filtrates was evaluated with untargeted metabolomics approach using LC-MS/TOF. The fungal isolates from kiwifruit bearing soil were much richer in LMW metabolites than those from virgin soil. Non host specific mycotoxins such as Aflaxoxin B2, Alternariol and Alterlactone were found in these isolates along with a number of metabolites most of which were characterised by antimicrobial properties. D. torresensis isolates were clearly discriminated from the other three species based on metabolic profile. A negative Pearson correlation between the growth index of kiwifruit vitro-plants and LMW metabolite count per isolate (richness) showed that the latter were associated with phytotoxicity, but the degree of correlation (r = −0.639) indicated that they were not the only component responsible. The results suggest that kiwifruit vines may alter the composition of the Dactylonectria spp. community in the root zone, which in turn may directly or indirectly contribute to kiwifruit vine decline syndrome.