Comparison of Growing Degree-Unit Models for Primocane-Fruiting Blackberry

Four growing degree unit (GDU) models have been developed to predict bloom dates of floricane-fruiting blackberries. These models, one multi-linear and three asymmetrical curvilinear, were compared to number of days to bloom and node formation for primocane-fruiting blackberries in combination with harvest season manipulation treatments. Two experiments were conducted to compare these models. One compared predictions in the context of three cane-mowing treatments. Treatments were mowing canes to the ground 1x, 2x, and 3x during the growing season. The other experiment compared genotypes Prime-Jan^®, Prime-Jim^®, and APF-46 under one cane-mowing treatment. Both experiments were replicated under hightunnels and ambient conditions. Nodes were counted weekly and bloom dates were recorded in both experiments. GDU were calculated for each model based on half-hourly temperature data under hightunnels and ambient conditions. Results indicate that node formation is consistent with the response models proposed. However, the linear model had the lowest coefficient of variation (proportion of variance to mean) for units required to flower. Total GDU at bloom and total node formation were affected by cane treatment and by genotype. For any given cane treatment, using simply the number of days was accurate to within 1 week.