Irrigation Management
When to Water How Do Irrigation Frequency and Final Irrigation Timing Influence Onion Bacterial Diseases? By Tessa Belo, Gabriel LaHue, Lindsey du Toit and Tim Waters, Washington State University
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nion bacterial diseases can pose a significant economic threat to growers, as they can lead to crop losses of more than 50 percent under the worst conditions. Previous research has found that overhead irrigation, especially after bulb initiation, and other sources of moisture in the canopy during field curing are risk factors for onion bacterial diseases. Less is known about managing onion bacterial diseases based on the frequency of irrigation during the growing season or through optimizing the timing of the final irrigation. Given that moisture and splashing water are significant drivers of bacterial diseases, strategic irrigation management is a critical and often overlooked part of managing crop losses caused by pathogenic bacteria while maintaining the desired bulb yields.
2020 Irrigation Trials
Trials evaluating the effects of irrigation frequency and final irrigation timing were conducted under sprinkler irrigation in Pasco, Washington, using the cultivar Calibra. The irrigation frequency trial was designed to test the hypothesis that more frequent irrigation would exacerbate onion bacterial diseases by comparing the effects of irrigating twice as often (2X) but for shorter time periods to longer, less frequent irrigations (control). The control plots were irrigated when 40 percent of the plant available water in the root zone was depleted, while the 2X treatment plots were irrigated at 20 percent depletion, based on a water balance calculated from publicly available weather data. Irrigation frequency treatments began at the third to fourth leaf stage, after which the 2X plots were watered 59 times and the control plots were watered 35 times, but all plots received just over 19 inches of irrigation total. The irrigation cut-off timing trial tested the hypothesis that irrigating later into the growing season would lead to a larger number of bulbs with bacterial rot. This trial compared three irrigation cut-off
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Onion World • December 2021
Figure 1. This bulb shows bacterial rot symptoms after five months in storage. All bulbs were cut from the neck to the basal plate for rating disease incidence, as shown.