Pruning to give spur leaves light
I was recently reading an article in Good Fruit Grower magazine about the research of Alan Lakso who, during a long career at Cornell U., studied apple tree physiology to determine best pruning and other management strategies to get highest productivity from apple trees.
A couple of his observations and discoveries may be of use to those of us called upon to prune apple trees, especially vigorous, free-standing ones (M7 and up).
First of all, spur leaves, the small rosette of leaves surrounding fruit clusters, devote all their available energy to the fruit in their center. Nearby shoots do not, and only send their energy forward to new leaves and wood until they stop growing- sometime around mid-summer. It is only at this point that they send it back to help support fruit, wood and root behind.
I think his most important discovery was that after a certain period of being shaded from the sun, spur leaves lose their ability to photosynthesize. If you summer prune to expose them to light after they've been shut out they will be unable to supply the fruit with any energy. He recommends that you avoid this atrophy by pruning the trees open enough so the spur leaves are never excessively shaded by annual shoot growth.
The most productive orchards have a higher percentage of light reaching the spur leaves- least productive have more light harvested by growing shoots.
It seems to me, that, for the home grower, it might be well worth it to do some spring pruning to maintain a high level of light on the spur leaves. Commercial growers tend to only prune once during the growing season, usually in mid-summer to help apples get color. Earlier pruning would increase labor costs because of regrowth.
It seems possible, however, that maximum size, quality and productivity could be encouraged by making sure spur leaves get optimum light throughout spring into mid-summer by removing a percentage of the shoots that are creating shade- perhaps leaving the less vigorous shoots to nourish the fruit after mid-summer and for spur renovation.
I just finished opening up the apple trees in my own orchard- just a few days after completing thinning of fruit (it's been about 3 weeks since petal fall of last apples). I know now that all the spur leaves are getting plenty of light. I'm curious if this will help me defeat biennial bearing on a few of my less cooperative trees. I expect it will.
I also suspect that other species tend to operate in a similar manner, with growing shoots sending most energy outward once they've reached a certain distance from the fruit. I'd love to see more research on this general topic as it applies to plums, pears, peaches and other common fruits.