Characterising the diversity of oomycetes in a multi-use landscape in Aotearoa New Zealand

Abstract

Te Kaha, rohe of Te Whānau-ā-Apanu, is a multi-use landscape that in recent years has invested in kiwifruit orchard expansion. This transition to a more horticulturally intensive crop will create a greater need for irrigation, and current schemes propose diverting water from the nearby Kereu River. Landowners and kaitiakitanga are cognisant that untreated irrigation water from natural bodies can lead to the dissemination of water-borne plant pathogen propagules, and thus, this research sought to characterise the diversity of oomycetes present in both aquatic and terrestrial sites in the Te Kaha catchment, with a focus on Phytophthora. In January 2023, water samples were collected and filtered at nine positions along a 9.3 km stretch of the Kereu River. Soil samples were also collected from nine sites that featured varying land usage — kiwifruit, maize, and ngahere | native forest. Oomycete specific ITS1 amplicons were generated from water and soil eDNA and sequenced using Illumina MiSeq. Kereu River sites were also baited in situ using a selection of native, exotic, and agriculturally relevant plant leaves. Exotic plant baits were more successful in yielding cultures with morphologies congruent with Phytophthora, whereas native plant baits had a higher incidence of producing isolates with sporangial exit tubes, a feature associated with Pythium. The exact identities of these Phytophthora isolates are being confirmed with Sanger sequencing. Similarity in Oomycota communities between river and soil samples was compared using ANOSIM analysis. Oomycete communities from water were almost entirely distinct from those present in soil (stress = 0.065, R = 0.99, p=0.001). The differences in Oomycota diversity in soils between land usage was also compared, which found significant differences with some overlap in taxa present (stress = 0.074, R = 0.42, p = 0.03). Of the 804 ASV produced, approximately 2% belonged to the Phytophthora genus. Only a third of terrestrial sites had Phytophthora present, whereas Phytophthora were detected in all locations along the Kereu River. Phytophthora spp. belonging to clades 2b and 12 were unique to soils, whereas species belonging to clades 6, 7, and 8 were exclusive to the Kereu River. Only one ASV, belonging to the Clade 2c ‘P. citricola complex’, was found in both terrestrial and aquatic landscapes. While there is currently no evidence of Phytophthora ‘spill-over’ between terrestrial and aquatic systems in Te Kaha, our results suggest that continued monitoring of this catchment could prove beneficial as these neighbouring landscapes increasingly interact in the future.