Gibberellin to increase blueberry fruit set
Cold, wet or windy weather inhibits bee activity and pollination. Spray of gibberellin can increase berry numbers and size when pollination is limited.
Gibberellin (GA) is naturally produced by blueberry seeds. In well-pollinated berries, GA is abundant and promotes normal berry growth. Without pollination and seed production, berries abort or do not size adequately. When bee activity and pollination are limited by extended periods of cold, rainy, windy weather, GA applications can sometimes increase the average size or number of berries.
Several GA products are labeled for highbush blueberries, including ProGibb 40% and ProGibb 4% (Valent BioSciences), and several formulations of GibGro (NuFarm Americas), and possibly others. GA can be applied in a single spray during bloom (80 gram active ingredient per acre) or two 40 g sprays, one during bloom and the second 10 to 14 days later. Higher spray volumes (40 to 100 gallons per acre) may improve coverage and effects. Slow-drying conditions also increase absorption. Make sure your spray water pH is not above 7.5.
The cost of GA treatments is significant ($75 to $100 per acre), so it is important to know when benefits are likely. 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 to provide adequate pollination. Consistently cold, rainy or windy weather through bloom causes pollination problems. A clue that pollination was inadequate is that the corollas stay on the bushes longer than usual and turn reddish-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 GA. Jersey, for example, is relatively unattractive to honeybees, and berry numbers and size are often limited by inadequate pollination. GA does not always provide a benefit and effects can be subtle, so always leave non-treated check rows. This is the only way to tell if your money was well spent.
Work in the southwestern states has shown that GA may help overcome slight freeze damage to Rabiteye blueberry flowers. Here, GA appears to trick partly injured flowers into setting berries. If flowers are severely injured, GA has no effect. The rabbiteye recommendation is to apply 24 to 32 g GA just after a damaging frost event and again in 10 to 18 days. This technique has not been studied on northern highbush blueberries. If you use GA after a frost event, please leave untreated areas as checks.
Contact me by phone (517-355-5191 ext. 1386) or email as I would like to assess the effect.