Mechanical blossom thinning effects on Gala and Honeycrisp apples

Mechanical blossom thinning effects on Gala and Honeycrisp apples

Summary

This year we successfully conducted and evaluated two string thinner studies on mature ‘Gala’ and ‘Honeycrisp’ apple trees with a cooperator in NY. All thinning treatments were applied on May 10 and 11 2015. Preliminary results of this technique have been promising. First evaluations began in 2014. More detailed studies were conducted in 2015 with SARE funding. We have had the concern that mechanical string thinning can damage apple trees and spread fire blight in a high density apple orchard. In this study (and at another conducted in 2014) the incidence and severity of fire blight was not increased for either cultivar by using the string thinner. Damage to the spur leaves, reproductive, and vegetative structures was more significant when the string thinner was applied at 240rpm in 2014. Increasing the rotational velocity of the string thinner (240 rpm vs. 220 rpm vs. 200 rpm) caused more string contact with the narrow canopy and increased leaf damage but this was not formally quantified in 2014. The string thinner at 240 rpm removed more Honeycrisp blossom clusters, spur leaves, and broke fruiting shoots compared with Gala. The most severe treatment removed 100 more Honeycrisp spur leaves compared to the control. In 2015 we quantified the damage to the spur leaves, reproductive, and vegetative structures. It was reduced when the string thinner was applied at 200 rpm and 220 rpm instead of 240 rpm.

Maxcel applied at 8-10mm did improve fruit size and had a better effect than in 2014 when the string thinning at 240 rpm caused as much as 40 gr decrease in fruit weight compared to the control trees. At the 2015 harvest the string thinning at 220 rpm and the string thinning at 220 rpm followed by Maxcel caused as much as a 15 gr and 22 gr decrease in fruit weight compared to the control trees, respectively.

Objectives/Performance Targets

1. To evaluate the use of a single spindle string thinner on two high-value apple cultivars in the Western NY fruit region by (a) determining the proper thinning severity, (2) the optimal application timing for Honeycrisp and Gala, (3) quantifying the potential spreading of fire blight, (4) supplementing the mechanical thinning with other chemical treatment, and (5) measuring return bloom and potential yields.

Accomplishments/Milestones

In May 2015, we conducted two string thinner studies on mature ‘Gala’ and ‘Honeycrisp’ apple trees on B.9 rootstock at 2X11 ft. spacing. The ‘Gala’ and ‘Honeycrisp’ trees were 7 and 9 years old, respectively. Both were trained to a Super Spindle apple system at Lamont Fruit Farm Inc., Waterport, New York. The shape of the canopy was a narrow tree wall. The string thinner (Darwin machine) consisted of a tractor –mounted frame with a 10 ft. tall vertical spindle in the center of the frame. Attached to the spindle were 54 steel plates securing a total of 216 plastic cords each measuring 2 ft. in length. Speed of the clockwise rotating spindle was adjusted with a hydraulic motor. The string thinner spindle was operated at 200 or 220 rpm, and the mechanical thinner was driven through the orchard rows at 5 miles/hour. All treatments were applied at 70-80% bloom stage for the king flower and balloon stage for the laterals for Gala on May 10 and for Honeycrisp on May 11, 2015.

      The specific treatments for ‘Gala’ were: (1) Control apple trees that received the grower standard chemical thinning program; 2pts/Acre Sevin XLR 5/27, 1 Galon/Acre Maxcel + 2pts/Acre Sevin XLR 6/1, 1 Galon/Acre Maxcel + 2pts/Acre Sevin XLR 6/9 (without the use of the string thinner), (2) Apple trees thinned with the string thinner at 200 rpm and 5 mph, (3) Apple trees thinned with the string thinner at 200 rpm and 5 mph, followed by Maxcel @85ppm sprayed at the 8-10mm fruitlet stage on June 1, 2015 (4) Apple trees thinned with the string thinner at 220 rpm and 5 mph, and (5) Apple trees thinned with the string thinner at 220 rpm and 5 mph, followed by Maxcel @85ppm sprayed at the 8-10mm fruitlet stage on June 1, 2015.

      The specific treatments for “Honeycrisp/B.9” were: (1) Control apple trees that received the grower standard chemical thinning program; 2pts/Acre Sevin XLR 5/27, Fruitone L 4.25oz/Acre + 2pts/Acre Sevin XLR 6/2 (without the use of the string thinner), (2) Apple trees thinned with the string thinner at 200 rpm and 5 mph, (3) Apple trees thinned with the string thinner at 220 rpm and 5 mph, (4) Apple trees thinned with the string thinner at 200 rpm and 5 mph, followed by Maxcel @50ppm sprayed at the 8-10mm fruitlet stage on June 1, 2015, (5) Apple trees thinned with the string thinner at 220 rpm and 5 mph, followed by Maxcel @50ppm sprayed at the 8-10mm fruitlet stage on June 1, 2015.

The two field experiments were designed as randomized complete blocks with five replications consisting of 50 trees per plot. Before string thinning treatments were applied, the initial number of blossom clusters per flagged data tree was recorded. Immediately after treatment, blossom clusters were counted on the data trees. One extra data tree per plot was used to quantify the potential damage to the bark, shoots, leaves, and buds, and for assessing fire blight. In July 2015, ten trees were selected within the center of each plot to ensure consistent treatment, uniformity, and were used to quantify the final number and diameter of fruitlets per tree. In September 2015, whole tree fruit counts and fruit weight of three data trees per plot were collected at harvest one time for Gala and two times for Honeycrisp.

Impacts and Contributions/Outcomes

Effects of mechanical blossom thinning treatments applied on May 2014 on return bloom of Honeycrisp and Gala:

String thinner treatments had a significant negative effect on return bloom of Honeycrisp. There were fewer blossom clusters per tree when the string thinner spindle was operated at 240rpm in 2014. Based on the negative effect on return bloom of Honeycrisp we decided to decrease the rotational speed from 240rpm to 220rpm in 2015.

String thinner treatments had a significant positive effect on return bloom of Gala. There was no significant reduction in blossom clusters per tree in the low, medium, or upper section of the trees. The 220rpm treated trees had the higher number of blossom cluster per tree.

Effects of mechanical blossom thinning treatments on number of fruitlets and clusters removed per tree in 2015:

We have had the concern that mechanical string thinning can damage apple trees and spread fire blight in a high density apple orchard. In this study (and at another conducted in 2014) the incidence and severity of fire blight was not increased for either cultivar by using the string thinner. Damage to the spur leaves, reproductive, and vegetative structures was more significant when the string thinner was applied at 240rpm in 2014. Increasing the rotational velocity of the string thinner (240 rpm vs. 220 rpm vs. 200 rpm) caused more string contact with the narrow canopy and increased leaf damage but this was not formally quantified in 2014. The string thinner at 240 rpm removed more Honeycrisp blossom clusters, spur leaves, and broke fruiting shoots compared with Gala. The most severe treatment removed 100 more Honeycrisp spur leaves compared to the control. In 2015 we quantified the damage to the spur leaves, reproductive, and vegetative structures. It was reduced when the string thinner was applied at 200 rpm and 220 rpm instead of 240 rpm.

Effects of mechanical blossom thinning treatments on fruit set, fruit number, fruit size, fruit weight, and yield in 2015:

Gala: All string thinning treatments reduced fruit set and did not significantly reduced fruit size and fruit weight. The string thinning treatment applied at 200 and 220 rpm followed by Maxcel caused greater thinning. Maxcel applied at 8-10mm did improve fruit size and had a better effect than in 2014 when the string thinning at 240 rpm caused as much as 40 gr decrease in fruit weight compared to the control trees. At the 2015 harvest the string thinning at 220 rpm and the string thinning at 220 rpm followed by Maxcel caused as much as a 15 gr and 22 gr decrease in fruit weight compared to the control trees, respectively. The grower standard chemical thinning treatment used multiple sprays of Maxcel which likely stimulated cell division and gave larger fruit size while the string thinner treatments reduced spur leaf area which likely inhibited initial fruit growth and cell division. There are incentives in the market for growers to produce large “Gala”; however, the smaller fruit size we obtained with the string thinner was a more serious problem for this strategy with Gala in 2014 than in 2015.

Honeycrisp: All string thinning treatments reduced fruit set and did not significantly reduced fruit size and fruit weight. The string thinning treatment applied at 220 rpm followed by Maxcel caused greater thinning. Both string thinning treatments followed by Maxcel increased fruit weight by August 19 and at harvest. There were as much as 48 gr and a 22 gr increase in fruit weight from the string thinning treatments at 200 rpm and 220 rpm followed by Maxcel at harvest, respectively. The highest yield reduction was achieved with the string thinning treatment at 200 rpm without Maxcel and there was a 56 gr decrease in fruit weight compared to the control trees.

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