Changes in Fungal and Oomycete Community Composition Following Irrigation Reductions Aimed at Increasing Water Use Efficiency in a Containerized Nursery Crop

Abstract

Ornamental production in the United States spans an estimated 267,000 ha and uses approximately 169,000 m3 of water yearly. Ornamental growers, particularly those with air-closed systems, are interested in implementing reduced irrigation technologies such as sensor-controlled irrigation systems to improve water use efficiency in response to water scarcity. To evaluate phytobiome impacts, we utilized an Illumina metabarcoding approach to characterize changes in fungal and oomycete rhizosphere communities of commercially grown poinsettia, complementing previous studies of single-pathogen impacts. Minor reductions (from 54 to 50% volumetric water content [VWC]) did not have large effects on rhizosphere fungal community structure but shifts in plant health status (symptomatic to asymptomatic) correlated with shifts in fungal diversity and evenness, with an increased abundance of several taxa, including Plectosphaerella cucumerina and Fusarium spp., in symptomatic plants. Further reductions in irrigation (45, 35, and 25% VWC) did not affect overall fungal community composition but the lowest VWC increased the abundance of potential root rot pathogens such as Phoma multirostrata and Plectosphaerella cucumerina. Reduced irrigation volumes decreased oomycete diversity and abundance of several known pathogens in the Phytophthora genus. Addition of a bacterial product (Companion) with putative pathogen-suppressive and drought-tolerance enhancing effects had no influence on fungal community composition or abundance but did increase abundance of some oomycete species at 45% VWC. This study shows that adoption of reduced irrigation may have greater effects on fungal versus oomycete taxa in the system, with consistent patterns of fungal enhancement, including putative fungal pathogens and beneficial organisms, and oomycete suppression.