Predicting phenological events of California processing tomatoes

A method for predicting tomato plant phenology using heat units and incorporating hi temperatures which appear to delay maturity is described. Degree-day >10 degrees C accumulation to 21 plant growth stages was determined using 536 data sets collected from commercial processing tomato fields in California's Sacramento Valley over 4 years. Accumulations from seedling emergence to harvest was greater for later planting dates, and in warm years. Regression analysis indicated that tomato plant development was delayed when temperatures exceeded 30 degrees C. A model which utilizes degree-days and incorporates a developmental delay whenever the daily maximum temperature exceeds 30 degrees C is proposed to predict time from emergence to 9 of the plant growth stages. Model validation indicated significant accuracy and robustness of predictions compared to observed dates of when stage 9 growth (75-90% ripe fruit) is reached.