Climate change effects on the processing tomato growing season in California using growing degree day model

California has a unique Mediterranean climate, well suited for irrigated agriculture. The climate is an important factor in making California a global leader in production of many high value crops, including procesing tomatoes (Lycopersicon esculentum Mill), with production of approximately 95% nation's and 30% of world's processing tomatoes. However, climate change poses many immediate and long-term challenges for state's highly productive agricultural industry. In order to help growers manage risks, it is important to study locally relevant agronomic indicators that are viable and aligned with growers' interests. Growing degree day models translate raw climate data into meaningful agricultural indicators which growers can utilize for immediate and long-term strategic decisions. Objective of this study was to analyze growing season trend in top five processing tomato-producing counties in California through the use of growing degree-days model and historical and future climate scenarios generated from the general circulation model (GCM). Based on the findings, the models indicated a significant decrease in the number of days between transplanting and maturity, with an expected harvest 2-3 weeks earlier than normal under current conditions and cultivars. Results from this study could be utilized to make strategic decisions such as variety selection, planting and harvest dates, agricultural water management, and studying trends in pests and diseases due to shifts and lengthening of tomato growing season in the tomato production areas of California.