COMBATING ONION BACTERIAL DISEASES THROUGH NITROGEN AND IRRIGATION MANAGEMENT

Sahil Thapa

doi:10.7273/000007943

The onion industry in the United States is worth about US $1.6 billion, and onion bacterial diseases cause annual losses of approximately US $60 million. Chemical controls have limited effectiveness in most regions, which calls for a holistic approach that integrates cultural practices including nitrogen and irrigation management. High nitrogen application rates or late-season nitrogen application may delay maturation of onion plants, providing a window for pathogenic bacteria to establish in the necks of plants when the necks are beginning to senesce and are most susceptible to infection. However, this has not been investigated under the high-yielding, irrigated production conditions of the western United States. Furthermore, late-season sprinkler irrigation has proven detrimental for onions as free moisture on the plants favors bacterial growth, yet whether the frequency of sprinkler irrigation affects bacterial diseases is less clear. Trials were conducted in the Columbia Basin of Washington State to assess the impact of N application rates, in-season N application timing, and irrigation frequency on the incidence of onion bacterial diseases, bulb size distribution, and yield. The N application rate trial was conducted in 2021, 2023, and 2024 with total available N ranging from residual N only (0 kg N ha-1 applied) to 403 kg N ha-1. Results demonstrated that N application rates can affect total yield and the incidence of onion bacterial bulb rot, such that N application rates needed to maximize marketable yield (total yield minus bulbs culled for rot or other causes) are likely lower than those required to maximize total yield, though carefully managing N application rates to minimize bacterial bulb rot and maximize marketable yield can be challenging in practice. In contrast, late-season N application (up to two weeks after bulb initiation) did not affect yield or bacterial bulb rot incidence compared to stopping N application at bulb initiation or two weeks before bulb initiation. Irrigating twice as frequently but for half the duration each time had mixed effects, with both total yield and bacterial bulb rot incidence increasing in plots with more frequent irrigation in one of the two years; it was posited that the observed response to irrigation frequency was a result of late termination of irrigation and that irrigation frequency is unlikely to affect the incidence of onion bacterial diseases if irrigation is stopped before 50% of the tops of the onion plants fall over (a natural part of the senescence of onion leaves). In conclusion, careful management of irrigation and nitrogen is recommended for onion production for many reasons, though adjusting N application timing or irrigation frequency throughout the season may be less impactful than other cultural management practices like optimizing N application rates, terminating sprinkler irrigation sufficiently early, or switching to drip irrigation.

COMBATING ONION BACTERIAL DISEASES THROUGH NITROGEN AND IRRIGATION MANAGEMENT

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