Author Archives: S. Sherif

Apple carbohydrate thinning model this week (May 23) for Winchester

The weather network predicts a sunny and warm weather this week, and you might feel it’s the time to give thinning your best shot. So, let me remind you that thinning treatments take at least 7 days to show an effect if the weather was warm; and almost two weeks (if not more) if the weather was cold. With all the cold weather we had in the past two weeks, if you applied a thinning treatment two weeks ago, you should start seeing an effect on the tree now. However, if your last thinning treatment was a week ago, you should give it 4 more days (from today) to check your trees and see if more thinning is required.

If you will apply any thinning treatment tomorrow or Monday (May 24, 25), the carbohydrate model suggests increasing the rate of thinning materials by 30%. I, however, would recommend applying the standard rate. Temperatures are expected to be above 85 OF for several days by the end of this week, which may put the trees under stress, turning them oversensitive to thinning materials. This year, it’s normal to see on the same tree fruit that are 18-22 mm in diameter and others that range between 11-14 mm. Thinning treatments by 6-BA and NAA will only be effective on fruit that are less than 18 mm in diameter.

The carbohydrate thinning model for Gala apples in Winchester, VA. Green tip date= 3/16; Bloom date=4/20; Percent flowering spurs (51-75%).

Sherif M. Sherif, PhD
Assistant Professor
Virginia 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 Carbohydrate Thinning Model-Winchester-May 20, 2020

If you plan to put any thinning treatments tomorrow or Friday, the model suggests increasing the chemical thinning rate by 30%. I would recommend postponing thinning treatments until Saturday or any day next week for three reasons: a) 6-BA and NAA treatments will be more effective as average daily temps. are predicted to be above 70 oF. b) only apple clusters from late bloom will be responsive to your thinning treatments whereas those resulted from early bloom and that represents the primary crop would be less affected by thinning next week; and c) Fruits affected by past frost events would be more likely fallen by that time, allowing you to make a better assessment of your crop load and the need for any further thinning.

Sherif M. Sherif, Assistant Professor of Horticulture, Virginia 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:

The freeze warning tonight (5/8/2020)

Subfreezing temperatures as low as 30 oF is expected midnight tonight (Friday) to 10:00 am Saturday for northwest and western Virginia. Our cherries, peaches, and most, if not all, apple varieties are at the post-bloom stage now. The critical temperatures that can cause 10% and 90% of damage at this stage are 28 oF and 25 oF, respectively. At this stage of apple crop development, frost can cause damage to the fruits and seeds. Fruits with damaged and aborted seeds stop growing and eventually drop off. Therefore, it’s important to assess your crop first for any frost injury before starting thinning applications this coming week. You can do so by picking a few fruitless from different locations in the orchard and cut them in half by a pocket knife and observe the flesh and seed color. Damaged fruitlets will show brownish flesh and seed color. In most cases, the damage can also be observed on the fruit surface as a frost ring (See images below). 

CSF Apple Stage / Freeze Damage Probability on the Climate Smart Farming, a program of Cornell University.
The model predicts apple freeze damage potential for the Red Delicious apples in Winchester, VA, at petal fall. However, the date presented in the chart are also suitable for other apple varieties. The yellow horizontal line indicates that temperature at which 50% of fruits can be killed. The blue color refers to the mini temp forecasted, which is 31 oF for Saturday, May 9.
Brown flesh color as indication of frost/freeze damage in Gala. Photo courtesy of Bill Mackintosh
Frost ring on a Gala apple fruitlets. Photo courtesy of  Chester Allen

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

The Carbohydrate Model & Thinning Recommendations for Winchester-May 1, 2020

The average fruit size for our Gala, Pink Lady, Red Delicious, Fuji, and Honeycrisp in the Winchester area is 6.00, 6.11, 6.04, 5.72, and 6.37 mm, respectively. So, you may be wondering whether you should put any thinning application this week and if yes, when to spray and at what rate?!

The short answer is yes. You may consider applying chemical thinners, e.g. 6-BA or NAA, for your apple trees this week, preferably tomorrow, May 2nd, and Sunday (if it’s not raining). Based on the 10-d weather forecast for Winchester, it will be only Saturday and Sunday this week with temp. above 70 oF. Both 6-BA and NAA work better when the temp is greater than 65 oF. As for the rate, the carbohydrate model recommends increasing chemical thinning rate by 30%. Two things to consider here: a) Apple cultivars respond differently to thining materials. Golden Delicious, Fuji, and Cameo are usually hard-to-thin, whereas Pink Lady, Gala, Honeycrisp and Granny Smith are easier to thin. For the rates and materials recommended for Gala, Fuji, Honeycrisp, Red Delicious, Pink Lady, and Golden Delicious, use this link ( b) The rates mentioned on 6-BA and NAA product labels are for dilute applications (100 gal/acre). To convert from dilute to concentrate, please refer to my other post at ( 

Apple Carbohydrate Thinning Model for Gala. Winchester (VT AHS AREC) weather station
Gala’s green tip and bloom dates are 3/16 and 4/20, respectively.
Winchester, VA 10-Day Weather Forecast

Sherif M. Sherif, Assistant Professor at 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

The Carbohydrate model and thinning recommendations for Central Virginia

Fruit size for Gala apples in the Central Virginia area is ranging between 10-12 mm now, which is a perfect size window for post-bloom thinning applications (e.g. by 6-BA and NAA products). Therefore, I ran the carbohydrate thinning model on NEWA website today, April 28, to predict: a) the tree carbohydrate level and b) thinning rates. Since 2017, we have been using the Batesville weather station for the central Virginia area and we will keep using the same location to help us compare among the year. We did not detect any considerable differences in the model outputs when we used the Afton weather station. However, if you have access to the model from your own computer, I would recommend that you run the model yourself based on the nearest weather station to your location. As you may be aware, any differences in the elevation levels would change the average daily temperature which is a critical input for calculating the tree carbohydrate level, and this will in turn change model outputs and recommendations.

As you will see on the NEWA website, there are two carbohydrate models, the old version which we have been using in the past three years and the 2019 new model (Apple CHO Thinning 2019) which I will use this year. One of the differences in the new model is the percent flowering spurs as one of the inputs you have to add to the model. Despite the fact that Gala is considered as an annual bearing variety and has no major issues with return bloom, I would recommend setting the model at 50-75% flowering spurs instead of 75-100%. The reason for that is to account for the extended period of cold weather we had during bloom and the frost damage the has been reported in some locations. These two factors might have affected flower pollination and total fruit set to some degree. The other two modifications in the 2019 model are: a) the tree carbohydrate balance column in the new model is using 7-day average values instead of 4-day average which makes it more accurate; b) the accumulated 4 oC degree days (DD) column which predicts the number of DD since bloom at which thinning would be highly effective. As a rule of thumb, 200-250 DD from bloom is considered the sweetest spot for thinning applications. Interestingly, this period often matches with the 6-12 mm fruit size window that we have long been recommending for thinning. 

With all of that in mind, I ran the new model today for Central Virginia based on Gala’s green tip and full bloom dates (March 10 and April 4, respectively) that were kindly provided by our respected growers in the Central Virginia area. As shown in the image below, the model predicts that we have between 214-251 DD between April 28 and May 1st, which is our target window. The model also suggests carbohydrate surplus during that period which on one hand is good for supporting cell division and consequently good fruit size; but on the other hand, it will make thinning a bit harder. Therefore, the recommendation is to increase thinning rates by 30% during that period. For instance, if you would normally use 6-BA at 48 oz/100 gal, you should increase it to 62 oz/100 gal. This is still among the rate range of 48-64 oz/100 gal we were recommending. To convert from dilute to concentrate applications, please refer to my blog post at ( Also, for thinning materials to be used for other varieties, e.g. Fuji, Honeycrisp, Pink Lady, Golden Delicious, Red Delicious, and Ginger Gold, please refer to my other blog post at (

One final point to highlight here is the effect of temperature on the activity of thinning materials. It has always been suggested that the best thinning activity by 6-BA and NAA is achieved when temperatures at and after the day of application are greater than 65 oF. By taking this into consideration and based on the 10-day weather forecasting for Batesville, this coming Saturday and Sunday, May 2 and 3, would be good days for thinning applications.

Should you have any questions/comments, please feel free to use the comments box below, call me at 5402326035, or shoot me an email at: Wish you all good luck with your thinning applications.

Apple Carbohydrate Thinning Model for Batesville, Virginia. April 28, 2020
Green tip and Bloom dates for Gala are March 10 and April 4, respectively.
Batesville, VA 10-Day Weather Forecast

Chemical options for defruting young apple trees

Defruiting newly planted and young apple trees (e.g., 2nd and 3rd leaf) allows trees to fill their allotted bearing space, grow to the top wire and become ready to bear a decent crop by the fourth and fifth year. Although defruiting can be performed manually by removing flower clusters or by hand-thinning small fruitlets, many chemical options can make defruiting much more comfortable and less labor-dependent. These include:

a) Using blossom thinners: Two products of lime sulfur are now labeled for blossom thinning in Virginia. These are Rex lime sulfur and NovaSource lime sulfur. Two to three applications of lime sulfur at 3% (3 gallons/100 gallon per acre) mixed with oil (e.g., JMS Stylet-oil) at 2% will be enough to prevent the fertilization and fruit set of the majority of king and side blossoms. However, blossoms thinning sprays should be avoided when daytime temperatures are favorable for fire blight infections, e.g., between 75 to 85 oF, or when fire blight models (e.g., Maryblyt) predict blossom infections.

b) Using post-bloom thinners (recommended): chemical products containing 6-BA (e.g., Maxcel and Exilis plus) or NAA (e.g., Fruitone L, PoMaxa and Refine 3.5WSG) can be applied in combination with carbaryl (e.g., Sevin XL plus) to remove apple fruitlets. One application at petal fall and another application at fruit size 6-12 mm, will be sufficient. 6-BA at 100 ppm (64 fl oz) or NAA at 15 ppm (6 fl oz) combined with carbaryl at 1 qt/100 gal/acre should achieve satisfactory results. Please note that some forms of 6-BA and NAA may contain a higher percentage of the active ingredient. So, it’s always advisable to read the label and use the fl oz amounts that correspond to 100 ppm and 15 ppm of 6-BA and NAA, respectively. It should also be noted that NAA is not recommended for Fuji and Delicious. Defruiting with 6-BA or NAA will be more efficient if trees are under carbohydrate deficit conditions. The carbohydrate thinning model on NEWA ( predicts the carbohydrate status of the tree based on daytime temperatures and solar radiation values. The best defruting will be obtained when the 4-day average balance is between -40 to -80 g/day. If the model is not accessible, 6-BA and NAA sprays should be applied when daytime temperatures are ≥ 85 o.  Cloudy days at and after spray applications should also be targeted. Adding a non-ionic surfactant (e.g., Regulaid) to the spray tank will also enhance the efficacy of defruting applications.

The following figure shows the effect of two defruting treatments on the fruit set (%) and crop load (fruit/branch cross-sectional area) of Honeycrisp and GoldRush trees, respectively. Defruiting sprays were applied twice at fruit size 6-18 mm.

c) Ethephon and carbaryl application (less recommended, especially for trees on dwarfing rootstocks): This combination will remove most fruit, but may cause tree growth reduction and increase flowering in the following year. The recommended rate for this application is 1 – 1.5 pt of ethephon tank mixed with 1 qt of carbaryl and 1 qt of Superior oil/100 gal/acre. Apply when fruit size is between 10 and 15 mm and daytime temperatures are between 75 – 90°F.

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
Tel. 540-232-6035; Email:

Mixing Kudos and Ca is not recommended


                Please find below an email I received from Mr. Dudley Dabbs (Fine Americas, Inc.) regarding mixing Kudos in the spray tank with Ca products.

“For years, Fine has cautioned against using Calcium containing formulations in the spray tank with Kudos PGR. The attached data (below) indicates that when using a (more expensive) highly complexed or Chelated Calcium product that has NO Free Calcium in it’s formulation may be mixed with Kudos PGR effectively. In addition, the attached data indicates that using a Calcium product with Free calcium in the formulation Mixed in the spray tank with Kudos will render Kudos ineffective as a PGR.  IT WILL NOT WORK.

Please NOTE: This data is specific to KUDOS and does not include other Prohexadione Calcium formulations such as APOGEE. APOGEE and Kudos formulations are not the same. They are different from each other and you should NOT make the assumption that certain Complex Calcium Formulations that work OK with Kudos may NOT WORK OK with APOGEE”.

Here are the powerpoint slides showing results of Kudos and Ca tank mixes on pear

You may contact Mr. Dudley Dabbs at Email: for more information on Kudos

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:

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

Tree row 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