Phytotoxic Effects of Hypochlorous Acid, Chloramines, and Chlorine Dioxide in Irrigation Water Applied to Bedding and Vegetable Plants

Abstract

Many producers sanitize recycled irrigation water before reapplication to crops in order to avoid transmission of waterborne pathogens. Previous research and grower experience has shown the potential for phytotoxicity when sanitizing agents including hypochlorous acid, chlorine dioxide, and chloramines are applied in irrigation water. The objective of this study was to measure and photograph phytotoxic responses when 0 to 100 mg·L^-1 of these three sanitizers were applied to 39 species of container-grown ornamental and vegetable plants. In one trial, 0, 1, 2, 4, 8, or 16 mg·L^-1 was applied once daily as an overhead irrigation at 15 mL/plant to seed Pelargonium xhortorum L.H. Bailey ‘Ringo 2000 Deep Red’, Gomphrena sp. ‘Fireworks’, and Viola xwittrockiana Gams ‘Panola XP Baby Boy Mixture’ in 144-count plug trays over three weeks. No phytotoxic or growth suppression effects were observed. In a second trial, 39 species received either no sanitizing agent or 8 mg·L^-1 of chloramine, hypochlorous acid and chlorine dioxide applied five times a week for 6 weeks in both seedling plug trays and after transplant into 10-cm-diameter (4-inch) pots. Ocimum basilicum L. ‘Genovese’ basil, Begonia obliqua L. ‘Baby Wing White’ begonia, Dianthus chinensis xbarbatus L. ‘Floral Lace Purple’ dianthus, Lactuca sativa L. ‘Vulcan’, and ‘Green Star’ lettuce and Lobularia maritima (L.) Desv. ‘Clear Crystal White’ alyssum showed phytotoxicity symptoms (leaf bronzing and chlorosis) from hypochlorous acid or chlorine dioxide. Ocimum basilicum L. ‘Genovese’ basil also showed some phytotoxic responses to chloramineas well. In a third trial, the 39 species received two applications of 0 or 100 mg·L^-1 of the sanitizers, resulting in widespread damage on most (64%) plant species from chlorine dioxide, but no phytotoxicity from hypochlorous acid or chloramines except on Angelonia angustifolia Benth. ‘Serena Purple’ angelonia, Ocimum basilicum L. ‘Genovese’ basil, or Salvia splendens Sellow ex Nees ‘Vista Red’ salvia. Given that typical applied concentrations of hypochlorous acid or chlorine dioxide are below 2 mg·L^-1, results indicate that at this level phytotoxicity is not likely when solutions are applied to foliage once daily under rapid drying conditions. However, under other conditions, such as when chemical injector equipment malfunctions, or a mixing error occurs, sanitizers can rapidly cause phytotoxicity.