Author Archives: S. Sherif

How to calculate the tree raw volume (TRV) and chemical rates for concentrate spraying?

Tree raw volume TRV is the water volume needed to spray the tree canopy to the point of runoff. This is also defined as the dilute volume. Some might believe that 100 gals/acre is the dilute volume, but this is not true in all cases. TRV can considerably vary from one orchard block to another depending on planting density, tree height, and treewidth. Please see the equation below:

TRV = (Tree height X Tree width X 43,560 X 0.7) / (Between row spacing X 1000).

Example 1: You have a 7-year old Gala block planted at spacing 4’X12’ and the trees reached 1’ above the top wire (10’). How much water volume is needed to adequately cover all trees/acre to the point of runoff?

TRV = (11 X 8 X 43,560 X 0.7)/ (12 X 1000) = 223 gallons (~ 200 gal/acre).

Please note that younger trees and trees on very dwarfing rootstocks (e.g. B.9) might require less than 200 gals (probably between 100-150 gal), despite being planted at the same spacing (4’X12′). For instance, if you have a 5-year-old Honeycrisp/B.9 orchard planted at 4’X12′, and trees haven’t filled their allotted space yet. In this case, tree width would be around 6 feet (3′ in each side) and therefore TRV for this block would be 167 gals (You may round it to 160 gals) instead of 200 gals.

It is crucial to determine the TRV for each block in order to make further calculations for the chemical materials that you should apply at concentrate spraying. For instance, if your TRV is 200 gal/acre, and you set-up your sprayer at 100 gal/acre. In this case, you should add 2X the rate of each chemical (except surfactant and oil) in the spray tank, to get the same effect you would get if you applied the regular rate at the point of runoff (200 gal/acre).

Example 2: The recommended rate of thinning materials at petal fall for Gala is (48 oz of 6-BA) and (1 pint of sevin)/100 gal dilute. How much chemical materials you should add to a 500-gal spray tank if the TRV for your Gala block is 200 gals/acre?

  • In this case, you need to add 2X rate of each chemical (at dilute/acre).
    • 6-BA = 48 X 2 X 5 = 480 oz/500 gal +
    • Sevin = 1 X 2 X 5 = 10 pints/500 gal to cover 5 acres at concentrate.

Example 3: You decided to turn-off the bottom half of the nozzles and focus your thinning sprays on the top part of the tree (which I generally recommend for thinning sprays). In other words, each acre will receive only 50 gals of the spray mixture. How much 6-BA and Sevin you should add to your 500 gal spray tank if the TRV is 200 gals/acre?

  • In this case, you need to add 4X rate of each chemical (at dilute/acre).
    • 6-BA = 48 X 4 X 5 = 960 oz/500 gal +
    • Sevin = 1 X 4 X 5 = 20 pints/500 gal to cover 10 acres at concentrate.

Should you have any questions or comments, please don’t hesitate to call me at 540-232-6035, or email me at:  You can also use the comments section below to send questions and comments.

Sherif M. Sherif, Assistant ProfessorVirginia Tech, School of Plant and Environmental Sciences, AHS Jr. Agricultural Research and Extension Center
595 Laurel Grove Road, Winchester, VA 22602

Fruit Thinning Scenarios-2020

Please note the following:

  • Thinning rates used at bloom (liquid lime sulfur 1.5% and 2% JMS stylet-oil) will not cause enough thinning if used as a stand-alone thinning treatment, but it will likely remove some blooms that will be tough to thin at later stages. One blossom thinning treatment at 20% open bloom is enough for this purpose.
  • 6-BA applications at petal fall will serve a dual purpose: a) to improve fruit size, especially in Gala, and b) to open up fruit clusters.
  • Using NAA at petal fall will also sever a dual purpose in Honeycrisp: a) to improve return bloom, and b) to open up tight clusters.
  • Thinning at 6-12mm fruit size is the main fruit thinning window for most apple varieties. Thinning rates and timing at this stage should be determined based on the outputs of the apple carbohydrate thinning model available at NEWA website ( I will run the model as usual and share with you the model outputs and recommendations through this blog.  
  •  Using 6-BA and NAA with carbaryl at 13-18 mm can also provide satisfactory results, but the efficacy of these chemicals will also be dependent on the tree carbohydrate level. So, it’s recommended to follow the model outputs at this stage as well.
  • Rescue thinning treatments (at 19-25 mm fruit size), might be necessary if earlier thinning attempts did not achieve satisfactory crop loads.
  • To improve return bloom in Honeycrisp it is highly recommended to apply four applications of NAA (2 oz/100 gal) at 10 days intervals, starting from 26-35 mm fruit size. Because NAA causes pygmy fruits in Fuji, NAA should be replaced by ethephon applications at 0.5 pt/acre to improve return bloom in Fuji.
  • Some varieties, e.g. Pink Lady and Ginger Gold, are easier to thin than others, and it can be easily thinned at the 6-18 mm fruit size window.
  • Important: all the rates suggested in these two charts are based on 100 gal/acre.

I hope you find this information useful. Should you have any questions or comments, please don’t hesitate to call me at 540-232-6035, or email me at:  You can also use the comments section below to send questions and comments.

Sherif M. Sherif, Assistant ProfessorVirginia Tech, School of Plant and Environmental Sciences,AHS Jr. Agricultural Research and Extension Center
595 Laurel Grove Road, Winchester, VA 22602

Chemical Fruit Thinning in Apple


I just wanted to share with you some of the slides and information I presented this past Tuesday via zoom in case you haven’t had a chance to attend the meeting. You may also consider reading my previous blog post on blossom thinning ( and the factsheet on the Virginia Cooperative Extension webpage ( if you want to get a better idea about the science and application of chemical thinners, e.g. 6-BA, NAA and ethephon.

Table 1: Chemicals and plant growth regulators labeled for fruit thinning in Virginia

Notes: Exilis 9.5 SC contains 9.51% of 6-BA, compared to 1.9% in Maxcel and 2% in Exilis Plus.
There are two forms of Refine, Refine 3.5 and Refine 6.25. The later contains a higher concentration of NAA.

Table 2: Thinning chemicals and rates recommended for petal fall-5 mm fruit diameter

Notes: NAD and NAA cause pygmy fruits when applied to Red Delicious and Fuji.
Carbaryl can be applied as a carbaryl only thinning spray between petal fall and 5 mm fruit diameter, but it will be less effective as fruits are getting bigger.

Table 3: Thinning chemicals and rates recommended for 6 mm-15 mm fruit diameter

Table 4: Chemicals and rates recommended for rescue thinning applications (at 16-25 mm fruit diameter)

Sherif M. Sherif, Assistant ProfessorVirginia Tech, School of Plant and Environmental Sciences,AHS Jr. Agricultural Research and Extension Center
595 Laurel Grove Road, Winchester, VA 22602
Tel. 540-232-6035, Email:

Apple Blossom Thinning: General Overview and Updates


The following is an updated version of a factsheet my former M.Sc student (Mr. Chester Allen) and I published last year on the VCE website ( I updated it to reflect what we have learned from our research in 2018 and 2019, particularly as it relates to the concentrations of lime sulfur and oil that could achieve good thinning activity and fruit quality.

What is Chemical Apple Blossom Thinning?

  • Chemical blossom thinning in apple orchards is the practice of spraying chemicals during the blossom time to thin the crop.
  • Chemical blossom thinners are sprayed multiple times during the bloom period in order to reduce or prevent the fertilization of a portion of the blossoms that would cause undesired crop loads if left to set fruits.
  • Chemical blossom thinning can be used as a supplemental and/or alternative practice to standard fruit thinning sprays.

What are the Benefits of Apple Blossom Thinning?

  • Increased fruit size, optimized crop load, improved annual bearing, increased return bloom, and more predictable thinning results are all benefits of chemical blossom thinning in apples.
  • Additionally, some blossom thinners can provide partial early season disease suppression and are registered for organic use.

What Chemicals & Rates can be used for Apple Blossom Thinning?

  • Currently, the only products registered for use as apple blossom thinners in Virginia are Rex Lime Sulfur Solution (28% Calcium Polysulfide) and NovaSource Lime-Sulfur Solution (29% Calcium Polysulfide). Label rates for these products range from 4% to 12% of formulated product per volume of spray solution when spray oil is NOT used. When spray oil is used, label rates of these products range from 1% to 3% of formulated product per volume of spray solution.
  • Mineral oils (dormant & summer) and fish oils may be combined with lime sulfur solutions and used as spray adjuvants to increase thinning effectiveness. Label rates for oil range from 0.5% to 2% depending on the type of oil used.
  • Based on our research in 2018 and 2019, application rates of 3% lime sulfur and 2% JMS stylet oil causes severe russeting and reduces fruit packout significantly, whereas low rates (e.g. 1% lime sulfur and 1% JMS stylet oil) have no significant thinning effect. Blossom thinning using 1.5-2% lime sulfur and 2% oil showed good thinning results and less fruit russeting.
  • If blossom thinning is to be followed by chemical fruit thinning (e.g. by 6-BA and NAA), we would suggest using 1.5% lime sulfur and 2% oil for blossom thinning. Our research also indicated that both forms of lime sulfur (Rex Lime Sulfur Solution and NovaSource Lime-Sulfur Solution) were equally effective.
  • Blossom thinning using high rates of lime sulfur (3%) and oil (2%) can also be used for defruiting young apple trees.

When & How Frequently Should Blossom Thinners be Applied?

  • Determining when to apply the first blossom thinning spray and how frequently to reapply blossom thinning sprays are the most important decisions to make when using blossom thinning sprays to thin and manage the crop load.
  • The first blossom thinning spray should be applied once a sufficient number of blossoms have been fertilized to set the desired crop load.
  • After the first blossom thinning spray, additional blossom thinning sprays will need to be re-applied approximately every 2-4 days, depending on temperature, to prevent the remaining blossoms from becoming fertilized and setting fruit. No more than three applications of lime sulfur solution can be applied for blossom thinning per year.
  • Our research on Gala apples has indicated that applying blossom thinners at 20% open bloom, followed by another application 48h or 72h later, results in smaller crop loads and better fruit size compared to untreated (unthinned) trees.

What is the Pollen Tube Growth Model (PTGM)?

  • The Pollen Tube Growth Model (PTGM) model, developed by researchers at Virginia Tech, is a useful tool for assisting in deciding when to first apply and re-apply blossom thinner sprays.
  • The model collects weather data and uses style length (mm) to predict the time a pollen tube would take to reach an ovule and achieve fertilization. 
  • Based on inputs provided by weather stations and users (e.g. cultivar, style length and the model start time), the model provides the day/time for the first thinning application and subsequent thinning sprays if required.  
  • The cultivar-specific model has been generated and tested for seven apple varieties, including Red Delicious, Golden Delicious, Cripps Pink, Honeycrisp, Fuji, Gala and Granny Smith.
  • The model is available on the NEWA webpage ( which is sponsored and managed by Cornell University.
  • To get more information about using the model for blossom thinning, you can follow the instructions listed in our previous blog post (
  • Our research on Gala apples indicated that applying thinning treatments at 20% bloom + 48h or 72h later, produces similar results to thinning applications guided by the PTGM.

Sherif M. Sherif, PhD, Assistant ProfessorVirginia Tech, School of Plant and Environmental Sciences,AHS Jr. Agricultural Research and Extension Center
595 Laurel Grove Road, Winchester, VA 22602
Te. 540-232-6035, Email.

Notching and PGRs to induce branching in newly planted, non-bearing and bearing apple trees

Q: Can I do notching for trees right after planting to increase the number of feathers?

Yes, if you get trees from nursery with a little number of branches (feathers), you can use notching to induce branching. This is mainly important for trees intended for high-density training systems (e.g. tall spindle) and to avoid or reduce the chances of blind wood. Younger trees are more responsive to notching than older trees. You can use a hacksaw or a small knife to cause a notch above the bud as shown in the image attached. In notching, we remove a very small piece of bark without hurting/damaging the wood. This promotes branching by disturbing the hormone (auxin) that prevents the formation of lateral shoots. You should mainly target the area between 30-60 inches above the soil surface for notching. Although you can start notching directly after planting, waiting until new shoots are between 1-3 inches long would be better as you can easily distinguish the buds that failed to break.

Q. Does applying PGRs to the notched area increase branching in the newly planted trees?

That’s right. You may consider adding a 6-BA compound (e.g. Promalin) to the notched area by using a high concentration (12 oz in 5 gallons of water) and a backpack sprayer. Other researchers even suggested mixing Promalin and Maxcel and spraying them together to induce branching and enhance the growth of the new shoots. To do so, you need to mix 5 tablespoons of Maxcel, 1.5 tablespoons of Promalin and 2.4 ml of Regulaid (a surfactant) in a 1-quart spray bottle and spray it as a mist to the notched area. This will enhance branching significantly.

Q. Is notching and PGR applications effective with older apple trees?

For 1 and 2-year old apple trees, notching alone can be effective but again, a mist spray of Promalin alone or Promalin and Maxcel as indicated above will enhance branching and help the tree fill the space much quicker. This time of the year (between the tight cluster and open cluster) is just perfect for both notching and PGR applications. You may still do that until full bloom and after bloom, but the risk of shoot blight increases after bloom and notched areas will serve as entry sites for the fire blight bacterium. This risk can also be partially with seasonal streptomycin sprays. The same rules apply for enhancing branching on the blind wood of older apple trees (> 3 years).

Q. Can I just use PGRs applications without notching to induce branching in non-bearing and bearing apple trees?

Yes, you can use 6-BA products alone but you have to follow the instructions on the label for the concentration and application timing for bearing and non-bearing apple trees. You can use Maxcel at 250-500 ppm (128 oz/40 gal/acre) to induce branching in non-bearing apple trees. Applications should be made when terminal shoots are 28-30 inches long. For bearing apple trees, you can use spray Promalin solution to the blind wood at a concentration of 0.25-1 pint in a 5 gal of water, applied at 1-3 inches of new terminal growth.

Q- I have a lot of blind wood in the top part of the tree; is there anything other than notching and PGRs I can do to fix that?

At the time of dormant pruning, I would suggest you remove 2-3 large limbs by making a bevel cut and follow this with notching and PGR sprays two weeks before bud break. Leaving many large limbs in the tree sucks up the carbohydrate resources from the main trunk and makes it hard for any new branches to develop in the leader, causing blind wood.

Dr. Sherif M. Sherif, Assistant Professor, Virginia Tech, School of Plant and Environmental Sciences, AHS Jr. Agricultural Research and Extension Center, 595 Laurel Grove Road, Winchester, VA 22602. Email:; Tel: 540-232-6035 

Flower bud development in apples and stone fruits-3/19/2020


               I just wanted to update you on where we are right now in terms of flower bud development in apple and stone fruits and whether we will potentially be affected by the cold weather this weekend (Sunday). As for apples, almost all our apple cultivars in the ASH Jr. AREC’s research farm are in the silver tip, green-tip or ½ inch green now (see the table below). Pink Lady and Zestar are the only cultivars we have that show buds in tight-cluster and first-pink stages. None of the buds I investigated in Pink Lady and other commercial cultivars showed signs of frost damage so far. Based on the weather forecast for the Winchester/Frederick county area, temp-Max will range between 41-78 oF and temp-Min will range between 31-42 oF in the next 10 days. If this turns out accurate, then we should not really be worried about frost damage to our apple blossoms for the next 10 days.

Bud development stages of commercial apple cultivars (3/19/2020)
Schlect Red Delicious/M.26 ½ inch green
Honeycrisp/B.9 Green tip
Ramey York/M.9 Green tip
Pink Lady/M.9 Tight cluster – first pink
Gala/M.9 ½ inch green
Daybreak Fuji/M.9 Silver tip
Elstar/G.16 Swollen bud
Golden Delicious/B.9 Green tip
Zestar/M.9 Tight cluster – first pink
Pixie Crunch/G.935 Green tip
Gold Rush/G.11 ½ inch green
Liberty/B.9 ½ inch green
Granny Smith/M.26 ½ inch green – tight cluster

What about stone fruits? Well, based on the few apricot and plum trees we have in the lab, our plum and apricot trees are in the full-bloom and post-bloom stages now and although we had temp. of 31 oF this past Monday (Mar 16) for a couple of hours, the damage to apricot and plum blossoms that could be due to frost is less than 5%. It’s also unlikely that temps this Sunday would cause any significant damage to apricot and plum flowers unless they will drop below 25 oF. Our sweet cherries, on the other hand, are still between the green-tip and tight-cluster stages and therefore, the risk of frost damage is also minimal. Most of our peach cultivars are moving slowly toward 1-2% bloom and with temps of 78 oF tomorrow (Friday), this percentage might jump to 5-10%. At this stage, the temperature that can kill 90% of buds is 24 oF, which is not the case this Sunday, at least for most parts of the state. For more information about critical temperatures for bud developmental stages, please use this link/PDF ( You may also need to print a color version of this figure and keep it in your house/office as a reference.

If you are not done with apple pruning yet, for very obvious reasons, and wondering whether you should proceed with the blocks that are not pruned, I would not generally recommend doing any pruning at this stage, for two reasons: 1) trees have already lost their cold hardiness and they would be more sensitive to any low temperatures during and after pruning; 2) there is a high risk of fire blight/shoot blight if the pruned wood did not heal well enough before the full-bloom window. As for peaches and nectarines, I usually recommend pruning the trees 4-6 weeks after full bloom.

If you have any questions or comments, please send them in the “comments” window below, send me an email at:, or call me at my office number: 540-232-6035 and I will make sure to get back to you as soon as I can.

Stay Safe, everyone.

ReTain inhibits ethylene production in fruit. Hence it is used to delay ripening, reduce pre-harvest drop, and extend the harvest season. Since it’s about the time for the pre-harvest drop control sprays, I wanted to share with you a field trial we conducted last year in one of Galize Apples’s orchards, in Winchester. The purpose of this trial was to examine the effect of different rates and application timing of ReTain on fruit drop of Gala apples. The tables below show the effects of two application timings (1 and 3 weeks before harvest (WBH) and two rates (1/2 and full-rate) on fruit drop and fruit quality. To accurately evaluate fruit drop (%), we assigned 6 trees/treatment, and we flagged 100 fruit/tree. We counted these fruits every week starting 1 WBH and through four weeks after the normal harvest date. Last year, the expected harvest date of Gala was Aug 28, so we applied our sprays on Aug 9 and 23.
– As you can see from Table 1, Retain treatments decreased fruit drop percentage compared to the untreated controls, but the differences between untreated trees and treated trees were not STATISTICALLY significant at 1 WBH, at harvest and 1 WAH. However, for fruits that were left on the tree for 2, 3 and 4 weeks after the normal harvest date, ReTain treatment applied 3 WBH at full-rate (333 g/acre) has shown SIGNIFICANT reductions in the percentage of fruit drop compared to untreated trees (28% vs. 58%). These reductions in fruit drop can be translated to a 40-50% increase in the yield and two weeks extension to the harvest season. We also found that two applications of ReTain (1 and 3 WBH) at half-rate can give a similar effect to a single application (3 WBH) at a full-rate.
– As far as fruit quality is concerned, fruit samples collected from treated trees and untreated trees at harvest indicated that ReTain applied at the full rate 3 WBH has significantly delayed fruit ripening. Fruits treated with ReTain were firmer than untreated controls and had lower sugar, starch, and color values (Table 2). Similar results were obtained when fruits were collected 2 weeks after the anticipated harvest date (Table 3), but the differences between treated and untreated fruits were not statically significant.











Late Thinning Application-May 13, 2019

The average fruit size for Pink Lady, Gala, Fuji and Honeycrisp in Winchester is 16.67, 17.02, 16.3 and 16 mm, respectively. At such advanced stage of apple fruit development, 6-BA and NAA will not be effective as chemical thinners and it is the time to start the “rescue thinning” sprays if more thinning work is still needed. Ethephon is the only effective thinner at this stage. Ethephon thins more effectively when the temperature is in the 70s to low 80s. Although the weather forecast predicts relatively cool temperatures this week, increasing the rate of chemical thinners by 30% to compensate for the high carbohydrate levels, as shown below, might mitigate the negative impact of low temperatures.

The recommended materials and rates for late (rescue) thinning are:

  • Ethephon (1- 1.5 pt) +Sevin (1pt)/100 gallon for Fuji and Spur-type Red Delicious.
  • Ethephon (0.75 pt) +Sevin (1pt)/100 gallon for Gala, Cameo, Goldrush and Jonagold
  • Ethephon (0.4-0.5pt) +Sevin (1pt)/100 gallon for Golden Delicious, Rome Beauty and Enterprise.

Fruit Thinning Treatments-May 9, 2019

The average fruit size for Gala, Honeycrisp and Pink Lady in Winchester today is 13.19, 13.08 and 14.06 mm, respectively. Although 6-BA-carbaryl or NAA-carbaryl mixes can still be effective at this stage, thinning applications will not be possible during the rest of this week & weekend due to the rain. On the other hand, the thinning model predicts a carbohydrate surplus next week, making thinning a bit challenging. The recommended options for next week are: a) Increase the rate of thinning materials by 15-30% according to the model outputs (note: another update will be posted on my blog on Monday, May 13), b) Wait until your apples approach an appropriate size (19-25 mm) for rescue thinning applications.

Tuesday, May 7: Another Chance For Thinning Applications

If you are not done yet with your thinning sprays and feel that another application is needed, tomorrow, Tuesday-May 7, is another good chance for thinning. Fruit sizes for Gala, Pink Lady and Honeycrisp in Winchester are 12, 12.5 and 10.5 mm which are still within the recommended range for thinning with 6-BA/carbaryl or NAA/carbaryl mixes. Temperatures will drop to 65F this Wednesday making thinning with 6-BA and NAA a bit challenging. The addition of 1 pt/100 gal of oil (e.g. 70-sec superior, JMS Stylet oil) can help increase the response to thinning sprays. However, the addition of oil may cause russeting on some cultivars. Also, Captan should be avoided within 7 days of oil applications, as it may lead to sever russeting. As per the carbohydrate thinning model, the rates of chemical thinners should be increased by 15% for Wednesday applications to compensate for the potential increment in the tree carbohydrate level.