Estimation of Airborne Sporangia Numbers of Lettuce and Spinach Downy Mildew Pathogens in Three California Valleys

California is the leading producer of lettuce and spinach in the United States. Production of these crops is beset by a continual challenge from downy mildews caused by the obligate oomycete pathogens Bremia lactucae and Peronospora effusa, respectively. Managing these diseases requires repeated fungicide applications that are costly and increase the risk of fungicide resistance developing in the pathogen populations. Increased understanding of the dispersal patterns of sporangia in California is necessary to support more judicious application of fungicides. In this study, qPCR was used to estimate the numbers of airborne sporangia of P. effusa and B. lactucae from three valleys in California, one on the central coast and two in the southern desert, from September through March of 2021/2022 and 2022/2023. The analyses of these data uncovered a relationship between estimated airborne sporangia concentration and maximum and minimum temperatures. In consecutive seasons, airborne sporangia of both pathogens were not detected during 3- to 4-week periods of high temperatures in the Imperial Valley early in the growing season, suggesting that the primary inoculum was not from distant airborne sporangia that were introduced into the system. This work provides insight on lettuce and spinach downy mildew sporangia numbers in different California agricultural production valleys that may be useful to improve downy mildew management and the strategic timing of fungicide applications. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 “No Rights Reserved” license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2026.