Watermelon Production in Utah: Exploring Biostimulants to Reduce Water and Fertilizer Use

The impacts of climate change and drought threaten water supplies that are important for watermelon production. Additionally, rising fertilizer costs and off-site effects from improper fertilizer applications cause concerns for the sustainability of watermelon production. Biostimulant products have shown promise in helping plants maintain production under reduced irrigation conditions and may offset fertilizer addition. Biostimulants are any microorganism or substance that is applied to a plant with the goal to increase nutrition efficiency, crop quality traits, or tolerance to environmental stresses. Most of the production in the U.S. uses seedless watermelon which are typically grown in greenhouses and transplanted due to high seed cost and poor establishment in the field. Seven commercially available biostimulant products were tested on greenhouse- grown watermelon seedlings (‘Crimson Sweet’) to measure effects on seedling emergence and growth. Three bacterial, two mycorrhizal, one humic, and one seaweed extract products were used. All products were applied to the soilless potting media. Leaf area and root/shoot fresh and dry weight were measured over a 30-day period after stand establishment. No significant difference in plant performance (size, weight) was observed between plants treated with biostimulant products and the untreated control. In 2023 and 2024, two watermelon cultivars (one seeded and one seedless) were treated with three biostimulant products (two bacterial and one mycorrhizal) and then subjected to reduced fertility (67% of recommended rate) or reduced irrigation (75% evapotranspiration (ET) in the late season). Watermelon seedlings were grown using local production practices. Biostimulant products were reapplied after transplanting. We found that biostimulant applications had no effect on plant growth, fruit yield, or fruit quality in the field. Reducing irrigation from 100% ET to 75% ET in the late season did not reduce yield in either year but did have positive effects on soluble solids content. A 33% reduction in fertilizer applications had no effect on yield or fruit quality in 2023, although a reduction in vine growth was observed. In 2024, the recommended fertility treatment had a higher yield and fruit number per meter squared than the reduced fertility treatment. ‘Crimson Sweet’ tended to have a greater vine length and higher yield than ‘Fascination’ in both years and experiments. Biostimulants show promise in improving plant productivity, and field applications may be viable. This warrants further investigation before recommending their use for commercial melon production.