Crop load management of apples by chemical thinning

In modern apple production systems, chemical thinning of young fruitlets within the first four weeks after petal fall has become a key management element. Inadequate thinning of fruit during this period increases the need for hand thinning, which is generally more laborious and expensive. On the other side, if trees are left without thinning at all, the chances of having low quality fruits and less return bloom in the following year are very high. To achieve adequate thinning using chemical thinners, many factors should be taken into consideration. These include, but are not limited to, fruit size, carbohydrate reserve status of the tree, weather conditions and -to a lesser extent- variety sensitivity. As a general rule, sunny days and cold nights promote the accumulation of photoassimilates and consequently trees become less prone to shed any of their fruits and are less sensitive to thinning applications. Whereas, cloudy days and warm nights lead to carbohydrate deficit and therefore trees tend to retain only a limited number of fruits and become more sensitive to chemical thinning. This associating between weather conditions and the total carbohydrate reserve has encouraged scientists (Alan Lakso and Terance Robinson) at Cornell University to establish a model known as “The Cornell Apple Carbohydrate-Thinning Model” that provides instructions on the dose and timing of thinning applications based on the solar radiation, temperature and day length records acquired from a weather station in a grower’s orchard blocks. This model can be accessed online through the Network for Environmental and Weather Applications (NEWA)(
It is a good practice to visit the NEWA website and make yourself familiar with the inputs and outputs of this model, especially if you have a weather station in your orchard that is connected with NEWA server, or you know the one that is close to your operation and that you can depend on.
As we are moving toward a heavy crop load and, luckily, less frost damage for apples this year, we have to consider looking at the outputs of the carbohydrate model in order to decide the timing and the dose of the chemical thinner. Typically, the efficiency of thinning applications applied from petal fall until a fruit size of 20 mm correlates well with the carbohydrate reserve of the tree and these are the times where the model outputs will serve as useful guidelines.
This year, we, at AHS Jr. AREC, Winchester, will run the model weekly and provide the outputs of this model as graphs that show the carbohydrate reserve of apple trees, and the recommended applications. The following table (Table 1) describes the recommended dose of chemical thinners based on the carbohydrate balance of the tree.
Table (1): Decision rules for using the output of the carbohydrate model to adjust chemical thinning rate.

Fruit size is another major factor that determines the efficiency of chemical thinning. When fruits are young (~ 6 mm), their demand of carbohydrate is not much and hence thinning becomes relatively challenging. However, chemical thinners such as naphthaleneacetic acid (NAA) and Sevin (carbaryl) and, to some extent, naphthaleneacetamide (NAD; Amid-Thin) can be successfully used at this stage. As fruits grow to the size of 7-14 mm, their cells divide rapidly and their demand of carbohydrate becomes more than what vegetative tissues can supply, especially if weather conditions do not largely support photoassimilation. At this stage, fruits become more sensitive to chemical thinners such as 6-benzyladenine (6-BA), NAA and NAD. However, if thinning is not adequate at this stage due to unsuitable weather conditions, then another round of thinning application will be essential. As fruit develops to the size above 20 mm, the chemical thinning becomes a tangible challenge. At such an advanced stage of development, fruits become more tolerant to abscission as they have more carbohydrate reserve and the seeds produce auxin which interferes with ethylene action preventing abscission. Chemical thinning at this stage (known as delayed/ rescue thinning), relies mostly on the combined application of 2-chloroethylphosphonic acid (ethephon) and 1-naphthyl methylcarbamate (carbaryl). The following table (Table 2) shows the recommended thinning applications in each stage of flower/fruit development and the dose of each application.
Table (2): Materials, rates and timing of chemical thinning application (Source:2017 Spray Bulletin for commercial tree fruit growers).

2017 Early Season Commercial Tree Fruit Meetings – Winchester Area

Below are the dates for the upcoming commercial tree fruit meetings.  Drs. Chris Bergh, Sherif Sherif, and Keith Yoder will be providing updated information and will be available for discussions and concerns regarding the upcoming season. 

Thursday, March 30.  In-Depth Meeting 7:00 p.m. – 8:30 p.m.  Seasonal Updates

Program: Dr. Keith Yoder (Pathologist – Virginia Tech AHS Jr. AREC)


Thursday, April 13.  Breakfast Meeting 7:00 a.m. – 8:00 a.m.

Seasonal Updates and Breakfast Provided


Thursday, April 27.  In-Depth Meeting 7:00 p.m. – 8:30 p.m.  Seasonal Updates

Program: Dr. Sherif Sherif (Horticulturist – Virginia Tech AHS Jr. AREC)


Thursday, May 11.  Breakfast Meeting 7:00 a.m. – 8:00 a.m.

Seasonal Updates and Breakfast Provided


Thursday, May 25.  In-Depth Meeting 7:00 p.m. – 8:30 p.m.  Seasonal Updates

Program: Dr. Chris Bergh (Entomologist – Virginia Tech AHS Jr. AREC)


All meetings will be held at the Alson H. Smith Jr. AREC (Winchester Fruit Lab) at 595 Laurel Grove Road, Winchester, Virginia.  Directions from I-81: take Stephens City exit (Exit 307).  Go west into Stephens City on Fairfax Street and proceed straight through the traffic light onto Rt. 631 (Fairfax Street becomes Marlboro Rd.) and continue west approximately 3.5 miles.  Turn right (north) onto Middle Road (Rt. 628) at the “T”.  Go 1.5 miles north on Middle Road and turn left (west) onto Laurel Grove Road (Rt.629).  Go 0.8 miles to the AREC on the left.


Mark Sutphin

Associate Extension Agent | Agriculture and Natural Resources, Horticulture | Unit Coordinator (Frederick)

Serving the counties of Frederick, Clarke, Page, Shenandoah, & Warren

Virginia Cooperative Extension – Frederick County Office | 107 North Kent Street | Winchester, VA 22601

Phone – 540.665.5699 | Fax – 540.722.8380 | Cell – 540.398.8148 | Email –


2017 Rappahannock-Fauquier-Madison Orchard Meetings

The Rappahannock Extension Office invites each of you to attend a series of in-orchard meetings scheduled from April through July. These meetings will be held in orchards located in Rappahannock, Fauquier, and Madison Counties.

We will meet at the host orchard at 11:00 a.m. for a tour of the orchard, followed by a discussion of current orchard management recommendations. Virginia Tech fruit specialists from the Alson H. Smith Jr. Agricultural Research and Extension Center will be present to address specific topics. We encourage you to bring samples of insects, diseased foliage or scaffolds for treatment recommendations. We will adjourn around 2:00 p.m. A schedule of meeting locations and directions to the orchards are listed below.

Please call the Extension Office at 540-675-3619 for additional information about these programs.

April 12th In Orchard Meeting
Williams Orchard, Tommy and Eddie Williams, Flint Hill Rt. 211 east to Ben Venue. At the crossroads, turn north on to Rt. 729 and go approximately 2½ miles. Orchard is on the left.

May 10th In Orchard Meeting
Jenkins Orchard, James Jenkins, Woodville, From Rt. 231, turn right on to Rt. 621, go about 2 miles. The orchard is on the left by the packing shed.

June 14th In Orchard Meeting
Stribling Orchard, Robert Stribling and Alex Jeffries
From Flint Hill: Rt. 522 N, turn right on Rt. 635, turn N/left on Rt. 688, the orchard is located near the intersection of Rt. 688 and Rt. 55 in Markham; 11587 Poverty Hollow Ln, Markham, VA 22643

July 19th In Orchard Meeting
Graves’ Mountain Farm, Jimmy Graves, Syria
Rt. 231 to Rt. 670, meet at the picnic shelter on left just past Syria.

If you are a person with a disability and desire any assistive devices, services or other accommodations to participate in this activity, please contact Kenner Love, VCE at (540-675-3616/TDD*) during business hours of 8 a.m. and 4:30 p.m. to discuss accommodations 5 days prior to the event. *TDD number is (800) 828-1120.

Snow is not bad after all…keep it dry.

Having some snow on trees and ground, even if it is not that heavy like the one we had last night, is not bad at all. Having a layer of dry snow on the ground should provide good insulation for soil and roots against the expected low temperatures we will face tonight and tomorrow morning. Also, large limbs that are still loaded with snow should get some protection. Using sprinklers will worsen the situation as wet snow works as a good thermal conductor and will transfer these freezing temperatures to roots much faster than dry snow.

Sprinklers for frost protection: are they worth using?

The simple and direct answer is YES, BUT you have to consider the right time/conditions.
When you use sprinklers to reduce frost damage, you are simply using the internal (latent) energy of water molecules as a source of heat to your trees. Simply, water exists in three forms (liquid, solid (ice) and gas (vapor), and the transition among these forms can produce energy (exothermic) or consume energy (endothermic) as appears in the diagram below. So, when the air temperature is below freezing points (< 32 oF), and you use sprinklers, you indeed encourage the transition of water from its liquid phase into ice in order to release the latent energy into sensible energy (heat) that plant tissues can use to warm up. Great! Where is the problem?

There are three things you have to understand and consider in order to get this positive effect of sprinklers and not to harm your plants. First, you have to watch the wind. The Wind encourages the transition of the water from the liquid phase into the gaseous phase (vapor) and while this happens, heat in the air and around your plants will be consumed in an endothermic reaction. So, it’s always recommended that you don’t use sprinklers if the wind is above (10 mph). The table below (Table 1) should help you decide the amount of water needed (inches/h) under different wind conditions (mph) and under different temperatures.
Table (1): Amount of water (inches/h) that should be provided for frost protection under different wind speeds (mph). (U of Florida Ext. Circ. 287)

Second is the dew point: without putting complication to the topic, low dew points implies low humidity and if the dew point is too low, the water you add through sprinklers will be soon evaporated to compensate for the low air humidity and consequently cools down air around your plants. So, under moderate dew points, a part of water you add through sprinklers will turn into vapor and consumes heat and another part will turn into ice and produces heat, but AFTER SOME TIME, the net energy will be positive and you will get the benefit of using sprinklers. However, if the dew point is too low, it means that the air is too dry and it will take MUCH TIME to be saturated with vapor. During this time, the air temperature might reach to the critical temperature at which your buds/flowers will be damaged. In this case, sprinklers will cause more damage to your plants than without sprinklers. To help you decide whether sprinklers will be useful for your case or not, use the following table (Table 2) to determine the temperature at which you should turn on/off your sprinklers. If the dew point and the critical damage temperatures are not in the range shown in the table, don’t use sprinklers.
Table (2) shows the minimum temperature at which sprinklers should be turned on/off (UC-Davis, FP005 Quick Answers).
Simply, select the critical temperature for frost damage (the columns) and the dew-point temperature (the rows). The temperature where the row value crosses with the column value is the minimum air temperature at which you can turn on/off your sprinklers.

– you can use this tool to determine the dew point (
Or, you can get the dew points from Intellicase (use your zip code)

The third thing to consider is the amount of water that your sprinklers can provide. If you are not sure that your sprinklers can provide sufficient amount of water, don’t use them at all. Table (1) should help you determine the amount of water required under different conditions.

What’s important to look at if concerned about potential frost?

Two things-the developmental stage of your buds and the critical temperature for frost damage
– For deciduous like pome and stone fruits, dormant buds are protected enough and not too much affected by the frost and freeze conditions. However, once the buds swell and become ready to burst, low temperatures are a major threat for the developing floral organs and the fresh vegetative tissues. For the northern parts of Virginia, and from what I see in our apple orchard at AHS Jr. AREC, I would not raise any red flags regarding the frost that might occur tonight (March 10) or over the weekend. Why? because most apple varieties are still between the silver tip and the green tip stage and we would need some degrees between 5-10 F or below to worry about bud damage, which is not the case based on the weather forecasting for this period.
Here is the table that has been developed by WSU for the critical temperatures that might cause damage to apple and other fruit trees.

To download the whole table, use the link

– Another source that may be useful for some of you is the apple freeze risk tool developed by Cornell University. Link:

CSF Apple Stage / Freeze Damage Probability

In this tool, you can select the location (using ZIP code), the variety (only three apple varieties), and whether you would like to see the full season trend (choose full season) or the current (this is the default setting). The tool takes into consideration the minimum temp., the developmental stage, and the temp. where 50% buds could be killed due to frost. If the minimum temp line (the blue line), crosses with the critical temp (the orange line), then the expectation is 50% bud damage.
If you look at the chart above, you can easily realize why I am stating that it might be useful for SOME (not all), because we at Winchester area are not even close to the tight cluster in Red Delicious (or any other variety) as this tool suggests; and hence the prediction of the 50% damage during the weekend (March 11, 12) is unrealistic. However, if this tool hits the right stage for you, I would say use it as a reliable source for frost prediction. Otherwise, use the above-mentioned table to assess the damage, if any.

2017 Tree Fruit Research Priorities Questionnaire

Orchard management and profitable production of tree fruits has become driven by robust science and thorough research. In light of this fact, this multidisciplinary questionnaire has been put together by a group of AHS Jr.-AREC scientists to help them identify and set research questions that reflect the interests of Virginia tree fruit growers and stakeholders. A similar questionnaire was distributed 6 years ago and has served to guide AHS Jr.-AREC scientists designing their field-blocks and shaping their research directions. Based on this survey, many experiments in pomology, plant pathology and entomology have been initiated. This includes, but is not limited to, experiments on crop load management practices, orchard tree planting density, variety and rootstock evaluation, physiological disorders, fungicides and insecticide efficacy trials, mating disruption trials and organic practices. This year’s questionnaire is meant to investigate the new trends and interests of Virginia fruit producers that may ignite new ideas or further promote researching the existing ones.

We are expecting responses from tree fruit growers, private agricultural consultants, agribusiness field representatives, VCE specialists and VCE extension educators in Virginia. An online version of the 2017 questionnaire can be accessed here ( Please complete before March 15th.

A paper version of this questionnaire will be distributed during the upcoming fruit school meetings (Feb 13-17).

2017 Tree Fruit Schools

Carroll Patrick Fruit School in Cana, Virginia: Monday, February 13, 2017 – agenda and registration details: ( carroll-patrick-fruit-school )

Botetourt-Roanoke Fruit School : Tuesday, February 14, 2017 – agenda and details ( roanoke-area-fruit-school )

Southside Fruit School in Blackstone, Virginia: Wednesday, February 15, 2017 – agenda and registration details: ( southside-fruit-school )

Nelson-Albemarle (Central Virginia ) Fruit School : Wednesday, February 15, 2017 – agenda and details ( 2017-fruit-school-registration-lovingston )

Rappahannock-Madison Fruit School in Syria, Virginia: Thursday, February 16, 2017 – agenda and details: ( rappahannock-madison-fruit-school )

Winchester Fruit School in Winchester, Virginia: Friday, February 17, 2017 – agenda and registration details: ( winchester-fruit-school )

Mid-Atlantic Fruit and Vegetable Convention in Hershey, PA

Save the dates, January 31 – February 2, 2017, for the Mid-Atlantic Fruit and Vegetable Convention in Hershey, PA. The conference schedule is now available at the website:

Registration through the Virginia State Horticultural Society should be available soon. Contact Liz White (540.667.9101 or for VSHS registration information.

FDA Food Safety Modernization Act (FSMA) Produce Safety Rule Training – December 9, 2016

Virginia Cooperative Extension will be holding an FDA approved FSMA Produce Safety Rule Training on Friday, December 9, 2016, at MidAtlantic Farm Credit – Winchester Office.  This building is located at 125 Prosperity Drive, Winchester, Virginia 22602.  The program will run from 8:30am to 4:30pm with registration starting at 8:00am.

Space is limited for this training and is geared towards operations that are covered by the FDA FSMA Produce Safety Rule. See the attached flowchart to help determine if your operation is covered by the ruling and thus requires compliance. “Covered” farm operations are required by law to have at least one “supervisor or responsible party” complete a training utilizing FDA approved curriculum. Full details regarding FSMA can be found on the U.S. Food & Drug Administration website:

There will be a $150.00* registration fee for the primary attendee from each operation. This will include program materials, certificate, and lunch. Registration for additional attendees from an operation will be $50.00* each and include a completion certificate and lunch.  Pre-registration is required. Please complete and return the enclosed registration form with payment (checks made payable to: Treasurer, Virginia Tech) by December 2, 2016.  Walk-in attendees will not be permitted.  If you have any questions, I can be reached at the Frederick Office or by email:

Registration form and details: fsma-produce-safety-rule-training-12-9-16

*Cost reduction may apply for Virginia operations through VDACS grant funding. Please contact Mark Sutphin ( or 540.665.5699) for specifics.