Characterization of Phytopythium Species Involved in the Establishment and Development of Kiwifruit Vine Decline Syndrome

Abstract

Since 2012, the kiwifruit vine decline syndrome (KVDS) has progressively compromised Italian kiwifruit orchards. Different abiotic and biotic factors have been associated with the establishment and development of KVDS. During monitoring of orchards affected by KVDS in north-western Italy during 2016–2019, 71 Phytopythium spp. were isolated. Based on maximum likelihood concatenated phylogeny on the ITS1-5.8S-ITS2 region of the rDNA, large subunit rDNA, and cytochrome oxidase I, isolates were identified as P. vexans (52), P. littorale (10), P. chamaehyphon (7) and P. helicoides (2). Phytopythium litorale and P. helicoides are reported for the first time as agents of KVDS in Italy. To demonstrate pathogenicity and fulfil Koch’s postulates, representative isolates of P. vexans, P. littorale, P. chamaehyphon and P. helicoides were inoculated in potted plants. In these trials, waterlogging was applied to stress plant with a temporary anoxia and to favour the production of infective zoospores by the oomycetes. In experiments in vitro, the four species showed the highest growth at 25–30 °C, depending on the media used. P. helicoides was able to grow also at 40 °C. The four species were able to grow in vitro at a pH ranging from 5.0 to 8.0, showing that pH had less effect on growth than temperature. The present study suggests a strong role of different species of Phytopythium in the establishment and development of KVDS. Phytopythium spp. could be favoured by the average increase in soil temperatures during summer, associated with global warming.

Soil, rhizosphere, and root microbiome in kiwifruit vine decline, an emerging multifactorial disease

“…The results are listed in Supplementary Table S4 . Isolates of P. vexans were constantly reisolated from diseased plant roots (sampled from diseased orchards) and identified by using macro- and micromorphological analysis and sequencing, as described in Prencipe et al (2023) . Five representative isolates were deposited in the Turin University Culture Collection (TUCC; Supplementary Table S5 ).…”

“…A previous study in Northwestern Italy, which included the orchards considered in this study, revealed a high occurrence of Phytopythium spp. isolated from the root tissues of kiwi plants showing clear symptoms of KVDS ( Prencipe et al, 2023 ). These findings complement the metabarcoding results of this study.…”

“…Macro and micromorphological analyses were performed on the isolates. In addition, molecular identification was performed, as described by Prencipe et al (2023) .…”

“…The main pathogens associated with disease development, based on isolation techniques, are mainly soil-borne oomycetes belonging to the genera Phytopythium and Phytophthora , although they cannot be considered as the exclusive causal factors ( Tacconi et al, 2015 ; Bardi, 2020 ). Both oomycete genera are characterized by a lifestyle that requires water for zoospore spread and shows optimal growth at high temperature ( Brasier et al, 2022 ; Prencipe et al, 2023 ); therefore, waterlogging and climate change are considered as co-factors for KVDS development. Kiwifruit has been reported to be sensitive to both root anoxia caused by flooding events and high transpiration demand, together with high soil temperatures ( Save and Serrano, 1986 ; Smith et al, 1989 ).…”

“…Similarly, P. chamaehyphon pathogenicity was confirmed by Savian et al (2021) on kiwifruit. In addition, oomycetes were isolated from KVDS-symptomatic plants in the northwestern area of Italy in 2016, 2017, and 2019 ( Prencipe et al, 2023 ). Oomycetes belonging to the species P. vexans , P. chamaehyphon , Phytopythium helicoides , and Phytopythium litorale were recovered at the highest rates.…”