2019 ASHS Annual Conference

Interest in using runoff water for irrigation of ornamental crops is increasing among nursery producers due to the rise of water scarcity and water regulation in many parts of the U.S. However, concerns about potential crop damage from contaminants in recycled water, including residual pesticides, may be a barrier for growers to adopt irrigation recycling technologies. Past research suggests that pesticide concentrations in recycled water are generally safe for woody ornamental plants, but visual injury and reductions in physiological responses and growth are sometimes reported. It is widely assumed that sensitivity of plants to residual pesticides is greater at early growth stages but few systematic studies have been conducted. In this trial, we evaluated the effect of simulated nursery runoff containing two pesticides, either oxyfluorfen (0.02 ppm) or oryzalin (8 ppm) on various growth stages of Hydrangea paniculata ‘limelight’ in terms of physiological responses, growth and visible injury. Dormant plants in #3 (12L) containers that had been over-wintered in an unheated hoop-house were placed in a heated (22°C) greenhouse. Once all plants began a new flush of growth, four sub-sets of 5 plants received runoff exposure via an overhead irrigation system for 10 days. Runoff irrigation began either 5, 15, 25, or 35 days after the initiation of growth. One sub-set of 5 plants (control) was irrigated with well-water throughout the experiment. Plants in each exposure group were watered with well-water before and after the 10-day exposure. After simulated pesticide runoff treatment, we measured net photosynthesis (A_max) and light adapted chlorophyll fluorescence (fv/fm), along with visual injury (leaf browning, misshapen growth) and growth index 1, 10, and 20 days after treatment and at the end of the study. Plants that were exposed to pesticide at earlier stage of growth had greater visible injury compared to later exposure dates. Oxyfluorfen produced maximum damage immediately after exposure but for oryzalin, damage was progressive and maximum damage was observed at 10 days after the exposure. Physiological responses (A_max and fv/fm) were also affected by date of exposure. These results suggest that nursery managers may be able to reduce potential risks associated with residual pesticides in runoff irrigation by minimizing exposure to plants immediately after new flush of growth.