When Terence Robinson speaks of precision pruning, he does so with passion. In the end, the Applied Fruit Crop Physiologist from Cornell University leaves nothing unsaid, even if it takes some reiteration.
“I want to introduce two physiological concepts that I don’t think that I’ve explained that well in the past,” Robinson told attendees of the annual Cornell NYS Tree Fruit Conference. “And I hope that this will help you understand why we push precision pruning so hard.”
CONCEPT NO. 1
In the spring, at green tip, the tree mobilizes reserves of carbohydrates and hormones, particularly cytokinins from the roots, and moves them up the tree. Meanwhile, the tree, with a spring flush of growth, takes in primarily nitrogen along with other nutrients from the soil and transports them up the tree.
When the tree has an excessive number of buds, that amount of cytokinin, carbohydrates, and nitrogen transported to the top is divided among numerous buds, leaving each bud with less than the optimal level of nitrogen, carbohydrates, and cytokinins.
“That results in weak buds,” Robinson said, and weak buds have low fruit set, produce small fruit, and are more biennial. But “if we can reduce the bud number through pruning …” he pondered.
Such pruning, again, would have to be done before green tip. Done later, the tree has already pushed many of its reserves into the soon-to-be eliminated buds. While waiting to prune until after growth starts would be good horticultural strategy to weaken the vigor in overly vigorous trees, with trees that are mature and low in vigor — which is often the case with ‘Gala’, Robinson noted — it results in buds that already are weak in getting reserves and then cutting them off.
Therefore, the pruning for ‘Gala’ has to be done before green tip, Robinson said. “If that’s done, each of the remaining buds — those 170 buds that we’re going to leave — will get more nitrogen, more carbohydrates, and more cytokinins, resulting in more vigorous buds, larger fruit size, and less bienniality.”
Robinson’s mention of 170 buds is a benchmark that he frequently emphasizes. Specifically, he recommends that precision pruning — the process of reducing the number of flower buds to a predetermined number through pruning while using the rules of tall spindle pruning and then spur extinction pruning — results in maximums of 170 flowering spurs for Tall Spindle ‘Gala’ and 131 in Tall Spindle ‘Honeycrisp’.
CONCEPT NO. 2
If an excessive number of flower buds are left on a tree, the result is a lot of little fruitlets from bloom until they can be thinned off at either 10 millimeters (mm) or by hand thinning at 30 mm. But that excessive number of fruits has a lot of seeds, and every seed produces the gibberellin hormone and exports it out of the seed and into the bud on that same spur. “Gibberellin is the anti-flowering hormone. In fact, if you sprayed gibberellins at this time, you could reduce flowering the next year,” Robinson said. “But these seeds are potent factories of gibberellins, and having excessive seed numbers on the tree by leaving too many buds causes biennial bearing.”
In the first of two scenarios noted by Robinson, ‘Honeycrisp’, when pruned to the recommended bud load of 131 floral buds, with each flowering spur having five flowers per cluster, and each fruitlet having 10 seeds, results in approximately 6,550 seeds on the tree at bloom. “Some of them won’t set, and you’ll thin off some of them, but you start with a lot of seeds,” Robinson said.
In the second ‘Honeycrisp’ scenario, when trees are not pruned to the recommended floral bud load number of 131, as is often the case in New York orchards, where Robinson has measured an average of 219 floral buds after pruning, the result, using the aforementioned calculation, is 10,950 seeds. “Almost double,” Robinson said. “And you just think about every one of those seeds producing gibberellin. That’s why, when you have too many seeds, you don’t get any flower initiation.”
CARBOHYDRATE MODEL UPDATE
Turning to post-bloom thinning, Robinson called 2020 a “very different year” in New York state, particularly in the Hudson Valley. Following a May 4 full bloom, a heat wave struck on May 18, at the timing of petal fall thinning sprays being applied. This heat wave caused an intense carbohydrate deficit, which continued almost unabated until the 10 mm thinning spray. Not until the 18 mm spray did it moderate to a point of almost no deficit.
“That natural heat wave and naturally induced carbohydrate deficit caused natural thinning. It was almost as if you had a thinning spray you put on,” Robinson said.
Despite numerous Zoom meetings on the topic, crop load wound up being lighter than optimum in the Hudson Valley. “It was just a weird environmental hand of cards that Mother Nature dealt us,” Robinson said.
The heat wave was evident but less severe in the Champlain Valley, where a neutral carbohydrate balance did arrive by the critical 10 mm spray.
In retrospect, Robinson praised the Cornell MaluSim carbohydrate model, which had predicted excessive carbohydrate deficits in 2020. The forecast wound up dictating thinning rates and timings to avoid overthinning.
“When we did thinning studies and compared sprays at bloom, petal fall, 10 mm, and 18 mm, bloom thinning worked quite well for us in 2020,” Robinson said. “But petal fall thinning, with either NAA (naphthaleneacetic acid) plus Sevin (Carbaryl) or NAD (naphthalene acetamide) plus Sevin, worked extremely well. In fact, 90% of the thinning we got in our thinning trials came out of that petal fall spray. The 10 mm spray gave only modest thinning. The 18 mm spray gave almost no thinning.
“This shows that the carbohydrate balance model was essential to understand what to do. Even having it, we could not prevent overthinning in the Hudson Valley, but, by using the model, we did a pretty good job of not overthinning in western New York and Champlain Valley.”
Nonetheless, Robinson has made improvements to the carbohydrate model, which he had developed with Alan Lakso. After spraying at any point — petal fall or 10 mm — growers should assess the result of that spray using the fruit growth rate model, which utilizes fruitlet diameter measurements at two times after the spray.
“The problem we’ve had is that, in hot years, that interval between measurements is about four days. But in cool years it can be up to six to seven days,” Robinson said. “So sometimes in cool years people are measuring a second time only four days after the first measurement, but we still don’t get the right answer because we’re measuring too soon. By making the first and second measurements based on degree days, we’ll be better tuned to make the correct decision with the fruit growth rate model.”
For 2021 the carbohydrate model will advise to make the first fruit diameter measurements after 50 degree days (DDbase 39°F) have accumulated since that spray and to make the second fruit diameter measurement after 120 DD have accumulated after that spray.