Concentration and intervals of hydrogen dioxide applications to control Puccinia hemerocallidis on daylily

Hydrogen dioxide (H2O2) is a disinfestant used to kill fungal spores, such as urediniospores of Puccinia hemerocallidis, on plant and production surfaces. Excised sections of daylily leaves with sporulating rust pustules were sprayed with concentrations between 19 and 270 g active ingredient (a.i.) H2O2 l−1. Treated spores were rubbed onto the surface of PDA, and germination of >100 spores per replication per treatment was determined 24 h later. An exponential decay model was fitted to the data and a lethal dose (LD) of 57 and 114 g a.i. H2O2 l−1 solution was predicted to cause 90 and 99% urediniospore mortality, respectively. In an irrigation pad study, one label concentration (2.7 g a.i. H2O2 l−1) and approximate LD90 and LD99 concentrations (54 and 108 g a.i. H2O2 l−1) were sprayed 1 or 2 times per week on healthy daylily plants exposed to naturally dispersing inoculum. Disease incidence and severity decreased with increasing concentration and number of applications of H2O2 per week. H2O2, at 108 g a.i. l−1 applied once per week, provided control equal to a fungicide treatment (azoxystrobin and chlorothalonil plus thiophanate-methyl rotation), but was also significantly phytotoxic. In a greenhouse study, two concentrations (2.0 and 2.7 g a.i. H2O2 l−1) registered for application on plants and one concentration (3.4 g a.i. H2O2 l−1) registered for application on production surfaces were sprayed 2, 3, and 5 times per week on healthy daylily plants exposed to naturally dispersed inoculum. In greenhouse experiment I where the maximum mean incidence was 59%, 2.7 and 3.4 g a.i. H2O2 l−1 applied 2–5 times per week provided control equal to the fungicide treatment. In greenhouse experiment II where the maximum mean incidence was 89%, 5 applications of H2O2 at 2.0–3.4 g a.i. H2O2 l−1 provided control that was better than the water treatment but not as good as the fungicide treatment. Higher label concentrations (2.7 g a.i. H2O2 l−1) and multiple applications per week (≥5) can provide disease control equal to a fungicide when disease pressure is low, but may not be effective when disease pressure is high.