We’ve all heard of apple June drop, but what is it, what are the causes and why should we care? June drop is more correctly called physiological fruit drop and is caused by a shortfall of carbohydrates in the tree. This shortfall is because the trees have limited leaf area for photosynthesis (supply of carbohydrates) but a lot of developing fruit all demanding carbohydrates to grow. So there’s a competition for resources and the strong survive while the weaker competitors drop off. The common post-bloom thinners increase this shortfall and therefore increase fruit drop, but they do this in different ways. NAA reduces photosynthesis and therefore decreases the supply of carbohydrates. Maxcell increases respiration so carbohydrates are consumed more quickly and decreases the pool of carbohydrates available for fruit development. Sevin decreases translocation of the carbohydrates from the leaves to the fruit, and so also increases the shortfall of carbohydrates. These different mechanisms of action help explain why the combination of thinning materials can be so effective.
So what are we seeing this year? With a lot of cool, cloudy weather soon after bloom, we would expect increased fruit drop from lower photosynthetic rates. Surprisingly, this is not what we’re seeing in many places this year, and this has caught out some growers. Refer to the accompanying article on thinning and which materials can be most effective as fruit continue to grow.