Consider Gibberellin use for fruit set on blueberries

Gibberellin acid (GA) is naturally produced by blueberry seeds. In well-pollinated berries, Gibberellin is abundant and promotes normal berry growth. Without pollination and seed production, berries abort or do not size fully. When bee activity and pollination are limited by extended periods of cold, rainy, windy weather, Gibberellin applications can sometimes increase the average size and number of berries. When Gibberellin is beneficial, yield increases are usually in the modest 10-20% range. These yield effects are hard to recognize by looking at bushes.

Several Gibberellin products are available for highbush blueberries. Apply Gibberellin in a single spray during bloom (80 grams active ingredient per acre) or as two sprays of 40 grams; one during bloom and the second 10-14 days later. Higher spray volumes (40 to 100 gallons per acre) may improve coverage and effects. Slow-drying conditions such as at night also increase absorption. Spray water pH needs to be between 4.0 and 8.0.

Judging when Gibberellin treatments will be beneficial is difficult. If weather has been reasonably good for bee activity and the white corollas (petals) fall easily from the bushes, pollination is probably adequate. Keep in mind that blueberries can bloom over a long time, and often only a couple days of good conditions is enough for adequate pollination. Cold, rainy and windy weather through bloom limits pollination. A clue that pollination was inadequate is that some corollas (petals) hang on the bush longer and turn red/purple before falling. The corollas of pollinated flowers drop readily while still white. Varieties with fruit set problems (Jersey, Coville, Earliblue, Berkeley, Blueray) are most likely to benefit from Gibberellin. Some older varieties, such as ‘Jersey’, respond somewhat consistently to Gibberellin because fruit set is often poor. Michigan State University undergraduate student Lexie Kelsey conducted trials in East Lansing, Michigan, in 2017 and 2018 to determine if several popular cultivars will respond to Gibberellin.

‘Duke’, ‘Bluecrop’ and ‘Draper’ were tested in 2017 and the varieties Duke, Bluecrop, Draper and Nelson were tested in 2018. Treatments were an untreated control, 80 grams active ingredient per acre Gibberellin applied at full bloom, and a split treatment of 40 grams active ingredient per acre applied at full bloom and petal fall. Treatments were applied to single bushes (four replicates) with a hand sprayer to the point of drip. Flowers and resulting fruit on four branches per bush were counted to calculated percent fruit set. Mature fruit were harvested, counted and weighed to determine yield and average berry weight.

Varieties varied in their response to Gibberellin treatments, with some types responding and others not (Tables 1 and 2). Often, Gibberellin increased fruit set without a corresponding increase in yield. In 2017, the split treatment (bloom plus petal fall) was superior whereas the bloom only application was more consistent in 2018. These inconsistent results are similar to initial studies years ago that lead to the labelling of Gibberellin on blueberries. Pollinators and weather conditions influence the response to Gibberellin from year to year. These results suggest that the newer varieties Duke, Draper and Nelson are responsive to Gibberellin when conditions are right. Consider Gibberellin treatments on these varieties, knowing that responses are possible. Since Gibberellin does not always provide a benefit and effects can be subtle, always leave non-treated check rows to see if your money was well spent.

Table 1. Effect of Gibberellin treatments on fruiting of three blueberry varieties in East Lansing in 2017.
Gibberellin rate per acre, timing y	Fruit set (%)		lb/bush		g/berry
Duke
Control	43	az	1.7	a	1.27	b
80 grams at bloom	84	b	2.2	ab	1.09	a
40 grams at bloom and petal fall	78	b	3.4	b	1.06	a
Bluecrop
Control	72		2.0		1.36
80 grams at bloom	87		1.8		1.23
40 grams at bloom and petal fall	89		1.9		1.39
Draper
Control	50	a	1.9	a	1.59
80 grams at bloom	72	ab	1.7	a	1.37
40 grams at bloom and petal fall	82	b	3.4	b	1.27
y Bloom sprays applied on May 5 (Duke) or May 15 (Bluecrop, Draper). Petal fall sprays applied on May 16 (Duke) or June 1 (Bluecrop, Draper). z Means followed by common letters not significantly different.

 

Table 2. Effect of Gibberellin treatments on fruiting of blueberry varieties in East Lansing in 2018.
Gibberellin rate per acre and time y	Fruit set (%)		lb/bush		g/berry
Duke
Control	49	az	1.6		1.23
80 grams at bloom	84	b	1.9		1.00
40 grams at bloom and petal fall	62	ab	1.8		1.17
Bluecrop
Control	54	a	2.8		1.27
80 grams at bloom	81	b	2.5		1.13
40 grams at bloom and petal fall	78	ab	2.6		1.14
Draper
Control	53	a	1.1	a	1.27
80 grams at bloom	84	ab	2.1	b	1.30
40 grams at bloom and petal fall	86	ab	1.8	ab	1.37
Nelson
Control	34	a	1.4	a	1.46
80 grams at bloom	65	ab	2.3	b	1.54
40 grams at bloom and petal fall	80	b	1.7	ab	1.30
y Bloom sprays applied on May 23 (Duke, Bluecrop), May 25 (Draper) or May 27 (Nelson). Petal fall sprays applied on May 31 (Duke, Bluecrop, Nelson) or June 1 (Nelson). z Means followed by common letters not significantly different.

– Eric Hanson and Lexie Kelsey, Michigan State University, Department of Horticulture