State of the art on the Kiwifruit Vine Decline Syndrome in Italy

Kiwifruit Vine Decline Syndrome (KVDS) is dramatically impacting kiwifruit industry in Italy. KVDS causes kiwi vine root rots with the consequent collapse of plants within a few months after the appearance of the first symptoms. Once symptoms appear on the canopy, the root system is already compromised. After over 10 years from the first reports, the mechanisms behind KVDS are not yet fully understood, although its biotic origin has been demonstrated and the climate change is likely to play an important role in promoting the syndrome. Several putative pathogens including Fungi and Bacteria have been associated to KVDS, but only Oomycete species have been constantly detected on symptomatic plants in different geographic areas. Here we summarize results of investigations conducted in northwest, northeast and southern Italy to characterize the microorganisms associated to KVDS by analysing roots, rhizosphere and soils using conventional isolations, baiting and metagenomic analyses. Taken together, our results support the role of oomycetes, belonging to different genera, as causal agents of KVDS. In northwest and southern Italy Phytopythium vexans is the most frequently detected species and it is significantly more abundant in both soil, rhizosphere, and root samples collected from symptomatic plants, although, genetically different populations seem to be associated to different geographic areas. Several other oomycete species have also been detected including Phytopythium litorale, P. chamaehyphon, Phytophthora spp., Globisporangium spp., and Pythium spp.. In particular, a Phytophthora taxon closely related to P. sojae/asiatica (clade 7) seems to play a relevant role in northeast Italy. Even if pathogenicity of different isolates has been verified through Koch’s postulates fulfilment, further work is needed to conclusively elucidate the role of P. vexans and other oomycetes (mainly Phytophthora spp.) in the wider context of plant-microbiome-environment interactions.