Estimating the impact of seasonal weather patterns and irrigation methods on carrot seed yield, economic outcomes, and potential for establishment of Xanthomonas in commercial carrot seed crops in the Pacific Northwest region of the US

Successful carrot seed production requires a delicately balanced climate. Yield and quality attributes are highly sensitive to temperature as well as moisture-related weather conditions that can serve as a conduit for establishment of diseases such as bacterial blight of carrot Daucus carota L. subsp. sativus. This study sought to improve understanding of the relationship of seasonal weather patterns and irrigation methods as they relate to carrot seed yield, economic outcomes, and potential for establishment of Xanthomonas hortorum pv. Carotae. Data was collected from 46 commercial scale carrot seed fields in Oregon and Washington from 2020 to 2023. Ordinary Least Squares regression was used for the estimation. Explanatory variables included the number of days from May to July where average relative humidity was 75% or greater, the number of days from May to July where maximum daily temperature was 35 °C or greater, irrigation methods, and the number of growing degree days from May to July. Despite the relatively small sample size and limited number of explanatory variables, basic relationships between weather and yield in commercial scale carrot seed production in semi-arid desert regions of the PNW were established. Results indicated every day of average relative humidity that was 75% or greater resulted in a yield loss of about 14.5 kg/ha. The estimated marginal cost of every day of high relative humidity was about $414/ha. Every day of heat that was 35 °C or greater resulted in a yield loss of about 11.13 kg/ha. The estimated marginal cost of every day of high temperature was about $318/ha. Every additional growing degree day resulted in a yield increase of about 0.40 kg/ha. The estimated marginal value of benefits from every additional growing degree day was about $11.34/ha. Irrigation methods were not found to have a significant impact on carrot seed yield.