The Influence of Sprinkler and Drip Irrigation on the Incidence and Severity of Bacterial Disease in Onions Grown in Northeast California

Overhead sprinkler irrigation and its effect on leaf wetness is often referenced as a key factor for bacterial disease spread in onions grown in arid climates. A study was conducted in 2021 and 2022 in Tulelake, CA to compare the incidence and severity of bacterial disease under sprinkler and drip irrigation. The experiment was a split-plot randomized complete block design with drip and solid-set sprinkler as the main plot treatments and inoculated and non-inoculated with plant pathogenic bacteria as the split-plot treatments. The entire study area was irrigated with solid-set sprinklers from planting until the five-leaf stage to facilitate uniform onion germination and chemigation of herbicides for weed control. After the five-leaf stage, irrigation treatments were imposed for the remainder of the growing season. Total irrigation water amount and irrigation frequency were similar for both irrigation methods. Foliar disease severity and the incidence of bulb rot were 170% and 186% higher, respectively, for solid-set sprinkler compared to drip. Leaf wetness within the plant canopy during onion bulbing was on average 36% higher in solid-set sprinkler compared to drip while relative humidity and temperature were similar between irrigation methods. Drip maintained lower soil water tension and increased average bulb size by 9% compared to solid-set sprinkler. This study confirms historic observations that overhead irrigation can extend leaf wetness and increase bacterial disease in onions in arid production areas, and highlights the potential benefits associated with the adoption of drip irrigation.