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Do reduced irrigation practices alter opportunistic pathogen dynamics in nursery systems?

Johanna Del Castillo Munera: University of California

<div>Deficit irrigation allows nurseries to adapt to water shortages and improve water use efficiency, but may enhance opportunistic root and crown pathogens. To better understand and manage effects of deficit irrigation on opportunistic pathogen abundance and diversity in a commercial setting, we looked at both the effect of a single pathogen (<em>Pythium aphanidermatum</em>) and shifts in fungal and oomycete community assemblages (using Illumina MiSeq analysis of ITS1-2 and ITS 6-7 respectively) in poinsettia (<em>Euphorbia pulcherrima).</em> When poinsettias were inoculated with<em> P. aphanidermatum</em> under moderate deficit irrigation (25% VWC), disease incidence was double that of plants under mild deficit irrigation (35% VWC) or saturated irrigation (45% VWC) treatments, but the effect was not significant (<em>P</em>= 0.146). There was a negative effect of pathogen treatment on shoot growth under the lowest VWC treatment (P<0.05), but not in the well-watered or mildly stressed plants. In preliminary community analyses, 157 fungal and 8 oomycete genera were present in poinsettia roots, and fungal communities differed between commercial greenhouse and controlled studies. Single organism studies indicate a potential risk for irrigation deficit to enhance losses from opportunistic pathogens; expanded community analyses will more broadly elucidate risks on pathogen recruitment. These studies can be used to develop water-pathogen co-management strategies, to enable water use adaptation.</div>