HEAT UNITS TO PREDICT TOMATO HARVEST IN THE SOUTHEAST USA

Planting and first harvest dates of tomato (Lycopersicon esculentum Mi ll.) from 2 seasons in 3 years at eight locations in Georgia, North Ca rolina and South Carolina formed 38 environments which were used to de termine the most reliable method to predict first harvest date of toma to based on daily maximum and minimum air temperature. Eleven methods of calculating heat units were chosen for comparison based on their pe rformance as described in the literature. The most reliable method was defined as the one with the smallest coefficient of variation (CV). C Vs were calculated for each method over both seasons and locations, fo r each season over all locations, each location over all seasons, and for each season at each location. All heat unit summation methods had smaller coefficients of variation (CV) than the standard method of cou nting days from planting to first harvest. Heat unit summation methods improved harvest date prediction accuracy compared with the counting day method for tomatoes in the South Atlantic Coast (SAG) region. Pred iction using location/season specific models were less variable than t he models over all seasons and locations. Incorporating daylength impr oved model prediction accuracy when applied over all locations and sea sons, all locations by season, and all seasons by location, Based on t he results of this study, the heat unit summation technique recommende d for this region (where the location and season specific models are n ot available) is the reduced ceiling method multiplied by daylength. ( C) 1997 Elsevier Science B.V.