Gibberellic acid doesn’t need to be applied at high volume to be effective | WSU Tree Fruit

Written by Matthew Whiting, Professor, Scientist, Extension Specialist, Washington State University, May 2021

Most cherry growers worldwide agree that an application of gibberellic acid (typically in the form of GA₃) is beneficial for fruit quality on several levels – improved firmness, fewer defects, and, generally, a delay in fruit maturity. Yet, despite decades of investigation, both formal and informal, there remains much debate about key factors determining the effectiveness of GA₃ treatments. These include the rate of GA₃, the spray volume per acre, timing of application, and the effects of crop load and genotype, among others.

In 2020 the Stone Fruit Physiology program at WSU-IAREC setup several trials comparing the effects of GA concentration and sprayer technology on fruit quality. In every trial, applications were made by the collaborating grower with standard, commercially-available application systems. These were not replicated trials, rather, treatments were applied to large, contiguous, blocks in uniform areas of the orchard so that treatment effects could be compared, within reason. At commercial harvest (the same day for all treatments in an orchard), fruit samples were collected randomly from bins throughout the treatment blocks. Fruit samples were transported to WSU-IAREC where quality was analyzed on an individual fruit basis, with 400-500 fruit samples. In this short summary, results from a ‘Chelan’ and ‘Santina’ orchard are presented. We conducted three other, similar trials in other cultivars – look for results from these trials in a later report.

‘Chelan’. Here we compared 24 oz of GA₃ per acre applied by Rears airblast sprayer at ca. 350 gallons per acre (ca. 20 ppm) with 12 oz of GA₃ per acre applied by On Target Spray Systems electrostatic sprayer at ca. 50 gallons per acre (ca. 75 ppm). Falgro® 4L (4% GA₃) was used in each trial.

The yield in this block was about 6 tons/acre. Fruit color was unaffected by GA₃ treatment (Fig. 1). In both treated blocks the bulk of the fruit (ca. 35%) were color 6 on the CTIFL scale (a mahogany), ideal for ‘Chelan’.

Figure 1. Effect of GA₃ treatment on ‘Chelan’ fruit exocarp color.

Fruit firmness was also unaffected by rate and application system (Fig. 2). In general, firmness was reasonably good with 74% and 79% of fruit being 250 g/mm or greater from airblast and electrostatic treatments, respectively. Both treatments had about 30% fruit of 300 g/mm or greater.

Figure 2. Effect of GA₃ treatment on ‘Chelan’ fruit firmness (g/mm).

Fruit size was improved slightly by the higher concentration and lower volume application using the On Target Spray System sprayer (12 oz of product applied at 50 gallons per acre, ca. 75 ppm; Fig. 3). This improvement came from an increase in the percent of 10.5-row fruit and a decrease in smaller size categories. Overall, fruit from the airblast treatment had 57% 10.5-row and larger whereas the On Target treatment had 64% in the same size categories (from a 7% increase in 10.5-row size fruit).

Figure 3. Effect of GA₃ treatment on ‘Chelan’ fruit size.

In summary, there were no apparent differences in fruit color (i.e., harvest timing) nor firmness between ‘Chelan’ fruit treated with 24 oz GA₃ at 350 gallons per acre or 12 oz GA₃ at 50 gallons per acre. Fruit size however was slightly improved from treatment of 12 oz at 50 gpa via electrostatic sprayer, compared to the grower standard of 24 oz GA₃ at 350 gpa.

‘Santina’. Here again we compared 24 oz of GA₃ per acre applied by Rears airblast sprayer at ca. 350 gallons per acre (ca. 20 ppm)with 12 oz of GA₃ per acre applied by On Target Spray Systems electrostatic sprayer at ca. 50 gallons per acre (ca. 75 ppm). In this block we again saw no effect of treatment on fruit color (data not shown). We assessed about 400 fruit individually per treatment and found that firmness was reasonable overall, with 67% and 65% of fruit being 250 g/mm or firmer from airblast and On Target, respectively (Fig. 4). Average firmness, across all fruit was similar for both treatments, at about 240 g/mm.

Figure 4. Effect of GA₃ treatment on ‘Santina’ fruit firmness.

Fruit size was good overall, for Santina, with average row size being 10.7 and 10.4 for airblast and On Target treatments, respectively. There was again a slight improvement in fruit size from the treatment of 12 oz GA₃ in 50 gpa via On Target Spray System’s electrostatic sprayer (Fig. 5). The percent of fruit that were 10.5-row or larger was 55% and 67% for airblast and On Target, respectively.

Figure 5. Effect of GA₃ treatment on ‘Santina’ fruit diameter.

In summary, we again see no treatment effect on ‘Santina’ fruit color nor firmness, but a slight improvement in size with the 75 ppm (12 oz) treatment applied using On Target Spray System’s electrostatic sprayer compared to the standard high volume via airblast. Like most research projects, this work has raised further questions regarding GA₃ rate vs. application technology, cultivar differences, crop load effects, etc. This work was not setup in a replicated experimental design so care needs to be taken in interpreting these results. There is more work ongoing in 2021 as we continue to investigate means for more efficient orchard operations and the production of superlative fruit.

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