Ecklonia maxima and Glycine–Betaine-Based Biostimulants Improve Blueberry Yield and Quality

The consumption of blueberries has increased in recent years due to their excellent flavor and high antioxidant activity, which promote human well-being and health. Several sustainable cultural practices, such as biostimulants, have been applied to increase crop performance. The pre-harvest foliar application of two doses of a biostimulant based on the macroalgae Ecklonia maxima (EM), two doses of a glycine–betaine (GB) biostimulant, and two doses of the combination of the biostimulants in ‘Duke’ and ‘Draper’ blueberry cultivars in 2022 and 2023 were studied. The application of 4 L ha−1 EM significantly increased the yield of ‘Draper’ blueberries, while the same occurred in ‘Duke’ fruits treated with 4 L ha−1 EM + 4 kg ha−1 GB. The blueberries sprayed with both doses of EM + GB were heavier and larger. ‘Duke’ fruits treated with EM were firmer and, following 4 L ha−1 EM + 4 kg ha−1 GB treatment, presented a lower concentration of organic acids, determined by HPLC, greater sensorial sweetness, and lower acidity. Therefore, these biostimulants are recommended for a more environmentally friendly production perspective, as they may increase blueberry yield and improve fruit quality.