Studies on the aetiology of kiwifruit decline: interaction between soil-borne pathogens and waterlogging

Abstract

Aims In 2012, Italian kiwifruit orchards were hit by a serious root disease of unknown aetiology (kiwifruit decline, KD) that still causes extensive damage to the sector. While waterlogging was soon observed to be associated with its outbreak, the putative role of soil microbiota remains unknown. This work investigates the role of these two factors in the onset of the disease. Methods Historical rainfall data were analysed to identify changes that might explain KD outbreak and mimic the flooding conditions required to reproduce the disease in a controlled environment. A greenhouse experiment was thus designed, and vines were grown in either unsterilized (U) or sterilized (S) soil collected from KD-affected orchards, and subjected (F) or not (N) to artificial flooding. Treatments were compared in terms of mortality rate, growth, and tissue modifications. Results KD symptoms were only displayed by FU-treated vines, with an incidence of 90%. Ultrastructural observations detected tyloses and fibrils in the xylem vessels of all plants, irrespective of the treatment. Phytopythium vexans and Phytopythium chamaehyphon, isolated from roots of FU plants, emerged as the associated microorganisms. Conclusions We succeeded in reproducing KD under controlled conditions and confirmed its association with both waterlogging and soil-borne microorganism(s).

Pathogenicity of Phytopythium chamaehyphon: A New Player in Kiwifruit Vine Decline Syndrome of Actinidia chinensis var. deliciosa ‘Hayward’ in Italy

“…P. chamaehyphon strain KD-15 from FVG (ITS region sequence under the accession number MN535819; Savian et al 2020a), further identified based on sequencing the LSU and COI regions (accession numbers MW431329 and MW430118), was selected for the pathogenicity test.…”

“…Both trials were kept in a greenhouse at 28±5°C. After transplanting, water content was maintained proximal to field capacity for 3 weeks for trial 1 and 4 weeks for trial 2, before applying three rounds of flooding and drainage to induce symptoms (Savian et al 2020a). Wilting appearance was recorded after 25, 35, and 50 days post inoculation (dpi) in trial 1 and every 2-3 days in trial 2.…”

“…Cooccurrence of P. chamaehyphon strain KD-15 and Phytopythium vexans was indeed observed in with a frequency of 55%. However, although the concept of a "pathobiome" associated with KVDS is well accepted (Savian et al 2020a;Donati et al 2020;Prencipe et al 2020;Spigaglia et al 2020), we do not fully know yet the composition of the KVDS microbial community and the interactions occurring between these pathogens.…”

“…Phytophthora genera) seem to play a major role in KVDS (Savian et al 2020a;Donati et al 2020;Prencipe et al 2020), however, also fungi (genus Desarmillaria) and bacteria (genus Clostridium) have been associated with the disease (Donati et al 2020;Spigaglia et al 2020).…”

“…The role of the interaction between soil-borne pathogens and waterlogging in the development of KVDS was demonstrated using soils from infected orchards of Friuli Venetia Giulia (FVG, NE Italy; Savian et al 2020a). Two species, belonging to Phytopythium genus, were isolated and emerged as potential pathogens: Phytopythium vexans and Phytopythium chamaehyphon.…”