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Background and aims The Kiwifruit Vine Decline Syndrome (KVDS) is a disease that is currently a challenge for kiwifruit production in Italy, and it is spreading in new production areas. However, the causal agent of this syndrome has not been clearly identified, and we still know little about the overall effects of KVDS on the interactions between the host plant and its microbiome. Methods In this study, we combined metabarcoding and targeted isolation (leaf baiting) to characterize the changes in the rhizosphere and root microbiomes associated with symptoms of KVDS. Results Our results suggest that KVDS has little impact on the bacterial, fungal, and oomycete communities associated with soil and roots, and we detected weak signatures of potential dysbiosis. On the other hand, we found a consistent association of the oomycete Phytopythium vexans with samples from plants symptomatic to KVDS, which matches the nucleotide sequences of the isolates obtained through baiting and, partially, the isolates from previous studies. Conclusion While our results support the idea that P. vexans might be the major candidate agent of KVDS, there are still several unanswered questions that need to be addressed before being able to provide effective solutions to this emerging challenge in kiwifruit production.
Background and aims The Kiwifruit Vine Decline Syndrome (KVDS) is a disease that is currently a challenge for kiwifruit production in Italy, and it is spreading in new production areas. However, the causal agent of this syndrome has not been clearly identified, and we still know little about the overall effects of KVDS on the interactions between the host plant and its microbiome. Methods In this study, we combined metabarcoding and targeted isolation (leaf baiting) to characterize the changes in the rhizosphere and root microbiomes associated with symptoms of KVDS. Results Our results suggest that KVDS has little impact on the bacterial, fungal, and oomycete communities associated with soil and roots, and we detected weak signatures of potential dysbiosis. On the other hand, we found a consistent association of the oomycete Phytopythium vexans with samples from plants symptomatic to KVDS, which matches the nucleotide sequences of the isolates obtained through baiting and, partially, the isolates from previous studies. Conclusion While our results support the idea that P. vexans might be the major candidate agent of KVDS, there are still several unanswered questions that need to be addressed before being able to provide effective solutions to this emerging challenge in kiwifruit production.
Kousa dogwood (Cornus kousa) is an economically important woody ornamental crop that exhibits creamy, white, pointed bracts in late spring, and reddish to pink drupe fruits in late summer and fall. It bears shiny dark green leaves that become reddish-purple to scarlet in the fall. In August of 2023, 3-year-old container grown C. kousa var. chinensis plants in a commercial nursery in Warren Co., Tennessee, exhibited severe yellowing, dieback and root rot symptoms (Fig. 1a and 1b). Dark brown to black lesions were observed in the root and crown region of the plants. Disease severity was 40% to 60% of root area affected, and disease incidence was approximately 40% of 1,000 plants. Surface-sterilized (10% NaOCl: 1 min) symptomatic root tissues were plated on V8-PARPH and incubated at 25°C. Sparse aerial mycelium, showing a distinct rosette or faint radiate to chrysanthemum colony pattern, was observed within four days of incubation (Fig. 2). All isolates produced ovoid or subglose, papillate, and proliferating sporangia in grass blade water cultures (Derviş et al. 2020). Sporangia measured as 19.18 to 24.80 µm X 18.08 to 22.16 µm (n = 50) with a length/width ratio of 1.06 to 1.11. Zoospores observed were between 7.07 to 9.98 µm in diameter (n = 50). Oogonia and oospores were not produced. The ribosomal internal transcribed spacer (ITS) and large subunit (LSU), as well as mitochondrial cytochrome oxidase subunit II (COX-II) genetic markers were amplified and sequenced using primer pairs ITS1/ITS4 (White et al. 1990), NL1/NL4 (Baten et al. 2014), and cox2-F/cox2-RC4 (Choi et al. 2015), respectively. The ITS, LSU, and COX-II sequences of isolates FBG6343, FBG6344 (ITS: PP458373 and PP461387; LSU: PP461390 and PP461391; COXII: PP477112 and PP477113) were 100% identical to those of MN306118, HQ643386, and MN206732, respectively. Based on the morphology (Nechwatal and Mendgen 2006) and sequence data, the isolates were identified as Phytopythium litorale (Nechw.) Abad, De Cock, Bala, Robideau, Lodhi & Lévesque. The pathogenicity test was performed on 3-year-old C. kousa var. chinensis plants grown in a 3-gal container to fulfill Koch’s postulates. Kousa dogwood plants were drench inoculated (800 ml/plant) with a pathogen slurry (two plates of 7-day-old culture/liter) of isolates FBG6343 and FBG6364 (five plants per isolate). Control plants were drenched with agar slurry without the pathogen. The study was conducted in a greenhouse maintained at 21 to 23°C and 70% relative humidity with a 16-h photoperiod and irrigated twice a day for 2 min using an overhead irrigation system. Forty-five days after inoculation, plants showed dieback symptoms, and dark brown lesions developed in the roots of all inoculated plants. Isolates with morphology and sequences identical to those of FBG6343 and FBG6364 were recovered from root tissues of all inoculated plants. All control plants remained symptom-free, and P. litorale was not isolated from the root tissue. Previously, P. litorale was reported to cause disease on apple, kiwi, planatus, and rhododendron (Derviş et al. 2020; Li et al. 2021; Mert et al. 2020; Polat et al. 2023). To our knowledge, this is the first report of P. litorale causing root rot of kousa dogwood in Tennessee and the United States. Identification of this pathogen as the causal agent is crucial to developing timely management practices.
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