Plasma-Activated Water Priming Enhances Field Lettuce Yield, Uniformity and Disease Resilience Across Variable Nitrogen Strategies

Plasma-Activated Water (PAW) promotes plant growth, but whether it should replace conventional nitrogen, supplement it, or prime the crop early in the season is unclear. We compared nine nitrogen strategies differing in source (urea vs. PAW), rate (full, 1/5, 1/10, and zero), and timing on field-grown romaine lettuce in two trials. In Trial 1 (late summer), no treatment effect was significant, although the 80/20 urea/PAW blend and PAW priming co-led in yield and uniformity. Trial 2, grown in cooler autumn–winter conditions with ten times the rainfall and naturally occurring downy mildew on an estimated 62.5% of plants, produced 19–41% lower fresh weights and significant treatment effects. PAW priming (1/5 N as PAW first, then full-rate urea; about 73% of the full seasonal nitrogen) ranked in the top two in both trials, significantly exceeded full-rate urea in Trial 2, and produced 62.5% of heads above a 700 g marketable threshold against 9.4% for urea. The 50/50 blend and PAW priming declined least between trials, while the 80/20 blend and pure urea declined most; reduced-rate treatments from either source were indistinguishable from the control. Yield loss fell disproportionately on the wrapper leaves, consistent with the observed disease. PAW priming maintained yield and improved uniformity across two contrasting seasons and merits further evaluation. “Disease resilience” here denotes yield maintained in a disease-present season, not treatment-driven resistance: disease was not rated plot by plot.