Biostimulants in Raisin Grapes
Joy Hollingsworth, UCCE Table Grape Advisor, Tulare, and Kings Counties
Biostimulants are a broad category of products that use naturally occurring ingredients intended to stimulate plants into better performance. Officially recognized in the 2018 Farm Bill, they were defined as “a substance or micro-organism that, when applied to seeds, plants, or the rhizosphere, stimulates natural processes to enhance or benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, or crop quality and yield.” Biostimulant products all have unique components and modes of action which might not be clearly identified. Product categories include but are not limited to: humic, fulvic, amino, and organic acids, beneficial microbes, extracts, biopolymers, and inorganic compounds. Private research conducted in the San Joaquin Valley suggested that some biostimulant products enhanced the quality of table grapes, improving the uniformity of berry size and color (Vasquez, personal communication). With the success of biostimulant applications experienced in table grapes, a similar trial was organized in an underperforming raisin grape vineyard to see if yield or quality could be improved.
In the spring of 2021, a trial was established in a six-year-old raisin vineyard (Vineyard A) in Fresno County with a history of inadequate yield and quality. Several years of poor winter precipitation, record-breaking heatwaves, and irrigation pump failures made it challenging to sufficiently water the vines, which had poor growth and low capacity, with yields of less than one ton of raisins per acre. The industry average for a mature overhead raisin vineyard is four to five tons per acre. The vineyard is on Tujunga sandy soil (Table 1) and was planted with Selma Pete on Freedom rootstock. The vines were head-trained and cane-pruned on an overhead trellis. Treatments included three different commercial biostimulant products (microbial, carbon-based, and seaweed-based) in addition to the grower’s standard fertilizer program representing the control. All three products were injected through the vineyard’s drip irrigation system (Image 1) similar to the grower’s fertilizer. The products were applied three to four times per year, depending on the companies’ standard protocols, while the control was applied at the grower’s discretion.
Image 1. Biostimulant product being injected through the drip system
Treated vines were monitored throughout the season. Leaves and petioles were sampled at bloom, veraison, and pre-harvest and sent to a commercial lab for nutrient analysis. Prior to cutting the canes in mid-August, 30 berries per vine were collected from each treatment replicate. Average berry length, width, and weight were determined, and then the berries were crushed and total soluble solids (TSS) were measured from the juice. After the canes were cut and the grape clusters sufficiently dried into raisins, each replicate was hand harvested, and yield, moisture, and quality were determined.
The experiment was repeated in 2022 after improvements were made by the addition of compost and more organic amendments applied through the drip system by the grower. Additional water was also applied to the trial site by the grower throughout the season, which improved canopy growth.
A second vineyard site (Vineyard B) was added to the trial in 2022. The second vineyard was in close proximity to the first but planted on a Hanford fine sandy loam. Vineyard B mirrored the first vineyard, having the same variety, trellis, and was approximately the same age. However, it was better performing with yields matching the industry average of 4 tons per acre. The addition of the second vineyard was to determine if biostimulant effectiveness was consistent when tested in a vineyard with stronger vines and in which cultural practices may be somewhat different.
The same protocols were used in 2022 as in 2021, with the exception that one product had changed from four yearly applications to three.
Results and Conclusions
Statistical analyses indicated that neither year nor location interacted with the treatments to affect yield or quality, so data from both sites were combined. Combining the data from both sites increased the strength of the study but, even so, it was determined that none of the products significantly affected raisin yield or quality more than the grower control (Table 2). However, there were significant effects of vineyard and season (Table 3). The grower from Vineyard A was able to dramatically increase his yields and quality in the second year, but improvements were not the result of the biostimulant treatments. However, the yield increase did not meet the industry average for Selma Pete grown on an overhead system. In contrast, Vineyard B was able to produce three more tons than the industry average in 2022 in this trial.
Table 3. Year and location effect on yield and quality
This trial did not find any significant benefits to any of the three products that were tested. However, it should be acknowledged that this was a relatively small trial, that was only conducted over two years in two locations. It should be considered preliminary, and more, larger studies may be needed to come to a robust conclusion.
If a biostimulant product interests you, and you want to see if it can help you solve a problem in your vineyard, it is advisable to do an on-farm trial, possibly in partnership with the manufacturer, a Certified Crop Advisor, or a UC farm advisor. They can help you design a trial that can be easily managed, determine what kind of data should be collected, and show you how to keep good records and analyze the results.