What causes biennial bearing?
Resting spurs do not appear to be essential for return bloom.
Recent research is providing new insights into biennial bearing and indicates that good spur quality plays a major role in consistent cropping.
Biennial bearing in apples is a greater problem than it has ever been before, notes horticulturist Dr. Peter Hirst of Purdue University in Indiana, and has negative impacts all through the system, from the orchard, to the warehouse, and also in the marketing channels. With the high cost of establishing an orchard today, the impact of losing a crop or even a partial crop is much more significant than it was when apples were cultivated less intensively, he said during the Washington State Horticultural Association’s annual convention.
Biennial bearing varies among cultivars, and many of the varieties being grown today have a greater tendency for biennial bearing than the traditional varieties.
Development of an apple is a two-year process, Hirst explained. There’s a signal that tells a bourse bud whether to flower or not, though scientists still aren’t sure what the signal is or when it arrives. The first visual sign of a flower is a doming of the apex of the bud in July to August. Differentiation of king and side blooms occurs in August to September. The bud overwinters, and the following spring, a flower emerges. If it is pollinated and the fruit sets, it develops into an apple.
It used to be thought that the fruit on a tree inhibited flowering the following year, perhaps because it drained the energy from the spurs, said Hirst, who has been studying floral initiation in apples for the past 15 years. It was also thought that resting spurs were essential for return bloom, but Hirst said this appears not to be true.
It appears now that it is not the fruit, but the gibberellins produced in apple seeds that inhibiting flowering. When fruit are thinned, there are fewer seeds on the trees, less gibberellic acid, and therefore better return bloom.
One of the earliest studies to show this was completed by Cornell University, New York, in the 1960s. The research was done with Spencer Seedless, a variety that will set fruit without pollination, although apples from unpollinated flowers have no seeds.
In the experiments, when there were seeded fruit on a spur (regardless of number), only about 7 percent of the bourse buds formed flowers the following year. Where there were no fruit on the spur, all of the buds formed flowers the following year. However, where there were unseeded fruit, all the flowers came back the following year also. Hirst said scientists concluded if there are any seeds at all on the spur, they will inhibit flower initiation for the following year.
In Hirst’s research with Fuji, he found that even fruitful spurs could produce flower buds, which also challenges the long-held idea that fruit on a spur inhibits flower initiation in the bourse bud of the spur.
Hirst has been exploring whether biennial cultivars produce more gibberellins than annual bearers. When he looked at the levels of GA in Gala and Fuji, he found that Gala, a regular bearer, produces more GA than Fuji, an alternate bearer. But there are many different gibberellins, some that inhibit flower formation and some that don’t. For example GA7 inhibits flowering, while GA4 does not inhibit flowering and may even promote it. Gala, it turns out, has a better ratio of GA4 to GA7. Hirst wonders if GA4 protects buds from the inhibitory effects of GA7 in Gala. “GAs seem to be involved, but it’s more complicated than more GA, more biennial bearing.”
Hirst said his research has shown that buds on large, high-quality spurs all flower the following year, whether the spur is thinned or not, and smaller, low-quality buds do not come back, even if they are thinned.
“Spur quality is important for buds coming back the following year,” he said. “If you had trees full of high-quality spurs, there was very little bienniality in those spurs. If you had trees full of small or poor quality spurs, they were very biennial. The quality of the buds is critical in reducing biennial bearing. We need to focus on things that influence spur quality, such as light, pruning, and nutrition.”
He also found that whether a spur was floral or fruiting had no effect on whether they would form flower clusters or on the quality of the flowers formed for the following year.