Author Archives: S. Sherif

About S. Sherif

Dr. Sherif Sherif is an Assistant Professor of Horticulture at the School of Plant and Environmental Sciences at Virginia Tech, USA. He received both his bachelor's and master's degrees in horticulture from Alexandria University, Egypt, and his doctorate in plant agriculture from the University of Guelph, Canada. Dr. Sherif's broad research experience in molecular biology, plant development, and tree physiology has led him to lead several research projects focusing on the biotic and abiotic factors affecting fruit trees' production, sustainability, and competitiveness. Sherif's current research program at Virginia Tech focuses on frost mitigation strategies, precision crop load management, high-density production systems, molecular regulation of critical horticultural traits, and germplasm development.

Fruit Thinning Decisions for Apple Orchards in Winchester/Frederick County- May 3, 2022.

Tomorrow and after (May 4-5) could be a good (but not ideal) time for fruit thinning applications for apple orchards located in Winchester and the surrounding area. In our Winchester research orchard, the average fruit diameter for Gala, Honeycrisp, Pink Lady, Red Delicious, and Fuji is about 6.5 to 8.5mm, which is inside the size range (6-15mm) we usually target for thinning treatments. Also, the carbohydrate thinning model shows low carbohydrate levels in the past two days, today, and the following four days, which should also ensure a good response to thinning treatments applied tomorrow and Thursday (May 5). However, it should be noted that we are not within the “perfect” thinning window yet. We typically target accumulated degree days between 200-250 DD. We got only 143 DD, which is not bad…but also not ideal. Also, based on the weather forecast, it will be rainy tomorrow until 8:00 am and windy (>10 mph) by noon, which gives you only a few hours to apply thinning treatments. The daytime temperature tomorrow and Thursday is fine for 6-BA and NAA treatments, but I would generally wait for extended periods of 75 to 85 oF for a better thinning response.

The second potential thinning window will likely be on May 10-12. As we approach there, fruit size will be around 11-12 mm, degree days will be within the perfect range (200-250), the temperature will be just perfect (based on the current forecast), but we can’t predict if the tree carbohydrate balance would remain on the deficit side by that time. We can, however, increase the rate of thinning materials to compensate for high carbohydrate levels.

So, after considering all these factors, I would focus tomorrow on thinning the hard-to-thin cultivars (e.g. Gala, Fuji, Golden Delicious, and Gold Rush) that generally require more than one thinning treatment. But, for other cultivars, I would wait for the May 10-12 window. It should be worth noting that blocks you decide to spray tomorrow can’t/shouldn’t be resprayed with thinning treatments on May 10-12. For these blocks, I would suggest that you give it 2-3 weeks at least to see the effect of the first thinning treatment and decide if you will need additional applications. For tomorrow’s thinning applications, the model recommends that you apply the standard chemical thinning rate.

The Cornell Apple Carbohydrate Thinning Model (May 3, 2022).

Finally, if you are nearby, you may need to consider attending our in-depth meeting tomorrow, May 4 at 7:00 pm at the Alson H. Smith, Jr., Agricultural Research and Extension Center (AHS AREC): 595 Laurel Grove Road, Winchester, VA 22602. I will cover critical topics regarding crop load management and return bloom, and other specialists will provide seasonal updates on disease and insect management.

The Carbohydrate Model & Thinning Recommendations-Central Virginia-May 1, 2022

Our collaborating apple growers in Central Virginia informed me that fruit sizes for most apple cultivars are now between 6-12 mm; the prime window for fruit thinning applications. I, therefore, ran the apple carbohydrate thinning model this morning to determine the optimal application timing based on the carbohydrate status, growing degree days, and thinning efficacy. As you will see below, we have already approached accumulated degree days of 235 (base temp= 4C), which is within the optimal range (200-250) for thinning applications. This range often coincides with fruit size 6-15 mm, which has always been considered the sweetest spot for fruit thinning. As for the carbohydrate status, the daily carbohydrate level is predicted to be in the deficit status for the next three days (May 2-4), and the 7-days weighted average will be at -2.49 g/day for tomorrow.

As you know, trees respond better to thinning materials when the 7-day weighted average values are in the range of -10 to -40 g/day, which is unlikely to be achieved in the following three days and therefore, the model predicts a mild response to thinning materials, and suggests increasing thinning rates by 30%. So, if you decide to apply thinning sprays tomorrow and if you usually use per acre rates of (64 fl oz of Maxcel and 1 qt of Carbaryl, and 1 pt of Regulaid/100 gal) for thinning your Gala trees, you should use (83 fl oz of Maxcel and 1.3 qt of Carbaryl and 1 pt of Regulaid/100 gal/acre) to compensate for the mild thinning action. As you may have noticed, the rate of the non-ionic surfactant remains the same per 100 gal.

My recommendations: after considering the fruit size, the carbohydrate level, and the predicted thinning action, I would seriously consider applying thinning sprays in the following three days (May 2-4); and with the forested thunderstorms on Tuesday and Wednesday, I would surely consider finishing a major portion of thinning applications tomorrow (Monday, 5/2). The temperatures in the following three days are around 80 F, perfect for NAA and 6-BA uptake and effectiveness. Also, the cloudy days on May 5-7 might reduce the carbohydrate level, at least partially, allowing for better thinning. If the current forecast is accurate, the second potential thinning window will be on Monday and Tuesday next week (May 9-10). I would use this window for cultivars currently at 6-8mm fruit diameter.

The Cornell Apple Carbohydrate Thinning Model (May 1, 2022).

Important remarks regarding the Cornell apple carbohydrate thinning model:

  • The model interface on NEWA ( is slightly different this year, but the model inputs and outputs are the same.
  • The model collects data for temp and solar radiation from the weather station to calculate the tree’s carbohydrate daily balance and 7-day average (2 days before, the day of thinning and the 4 days after ).
  • For orchards in Central Virginia, I usually use the weather station in Tyro (Silver Creek), but I could not see this station listed on NEWA this morning, probably for a technical reason, and therefore I used the Crozet (Chiles peach orchard) instead. It would help if you used the nearest station to your location.
  • The model requires inputs for the green tip and full bloom dates. For the model I ran today, I used a green tip date of March 15 and a full bloom date of April 10 for Gala apples. If your dates are different, the outputs and recommendations will be different.

Apple Fruit Thinning: An Overview & Recommendations

After all the frost events and subfreezing temperatures in the past four weeks, we (the state of Virginia) still have a medium-heavy apple crop, and we should start planning for the fruit thinning treatments. However, given the geographical distribution of apple orchards throughout the state, which results in different developmental stages, growing degree days, elevation, etc., thinning recommendations would vary from one location to another. So, this post aims to give a general overview of thinning materials, rates, and application timing as well as a few suggestions to consider when thinning certain cultivars or using specific materials. But I will follow this with other blog posts focusing on particular locations as required.

Table 1: Common chemical thinners, tradenames, and manufacturers.

  • 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: Effective thinning sprays at petal fall to 5 mm fruit diameter

The application rates are based on PoMaxa for NAA, Amid-Thin for NAD and Carbaryl 4 L for Carbaryl.

Table 3. Thinning materials and rates for 6-18 mm fruit diameter

The application rates are based on PoMaxa for NAA, Maxcel for 6-BA, Amid-Thin for NAD and Carbaryl 4 L for Carbaryl.
  • Do not use NAD and NAA with Red Delicious or Fuji, as this will result in pygmy fruits. 
  • To achieve better results with NAD, use it in 100 gallons or more per acre.
  • Carbaryl can be used alone for fruit thinning between petal fall and 15 mm fruit diameter. However, it is better to combine it with either NAA or 6-BA for thinning fruits at 7-15 mm. 
  • Carbaryl is not rate-responsive when used alone, so increasing the rates of carbaryl will not necessarily improve thinning efficiency.
  • 6-BA enhances cell division and fruit size compared to other fruit thinners. Therefore, it is recommended to use it when thinning small-fruited cultivars such as Gala and Ginger Gold. 
  • Temperatures between 75 – 85 oF are ideal for 6-BA uptake and effectiveness. 6-BA is not effective when temperatures are below 68°F.
  • NAA can reduce the overall fruit size of the harvested crop compared to other chemicals used in fruit thinning.
  • NAA applications for fruit thinning can also enhance return bloom in biennial bearing cultivars, such as Honeycrisp and Golden Delicious. Research from my lab at Virginia Tech showed that NAA applications within the first 30 days of bloom are critical for flower bud formation and return bloom. NAA applications at 40 and 50 days of bloom have minimal effect on return bloom in Honeycrisp. 
  • Adding a non-ionic surfactant (e.g. Regulaid @ 1pt/100 gal) to 6-BA and NAA spray mixtures improves thinning efficiency. 
  • We use the Cornell Apple carbohydrate thinning model on the NEWA website ( to determine the optimal timing for thinning treatments. The model collects temperature and solar radiation data from the nearest weather station to your location to predict the carbohydrate status of the tree. Under the carbohydrate deficit status, trees become more responsive to thinning treatments; and the opposite is true under the carbohydrate surplus conditions. 
  • Sunny, cool days and cold nights promote carbohydrate accumulation, resulting in low thinning efficiency; whereas cloudy, hot days, and warm nights lead to carbohydrate deficiency, which is good for fruit thinning.
  • The upper parts of the canopy are harder to thin than the lower parts.
  • If you have a heavy crop, thinning will be easier than having a light crop.
  • Vigorous trees are harder to thin compared to compact and dwarf trees. 

Table 4: Effective thinning treatments at 16 mm-25 mm fruit diameter.

  • Ethephon should be applied as a “rescue thinning” treatment if first thinning sprays were insufficient. It is most commonly used when fruit size is between 18 and 26 mm.
  • When aggressive thinning is needed, mix carbaryl or NAA with ethephon.
  • Ethephon can result in severe over-thinning, particularly at high temperatures (> 90 oF).
  • Accede, a new thinning product from Valent USA, can also be used at this stage. For more information about Accede, read our blog post:

Table 5: Easy, moderate, and hard to thin apple cultivars.


Application rates mentioned in this article are based upon a concentrate spray volume of 100 gallons per acre and product labels at the time of publication. When applying chemical thinners use the rates indicated on the labels of the products that you are using. The degree of thinning action are listed according to the author’s personal field experiences. The degree of thinning action may vary from orchard to orchard and block to block.

For more information regarding apple fruit thinning, you can read our extension pub “Crop Load Management in Commercial Apple Orchards: Chemical Fruit Thinning” at

Another, probably less devastating, frost event is expected tomorrow, April 18

Another frost/freeze event is expected from midnight today to 9:00 am tomorrow (Monday, April 18), with temp as low as 30 oF in some portions of central, northern, and northwest Virginia. On their webpage, the National Weather Service stated that “this frost and freeze conditions will kill crops, other sensitive vegetation and possibly damage unprotected outdoor plumbing”. I can’t comment about plumbing, but I can probably provide insights into how this frost event would impact apples, peaches, and apples in the state. In our location in Winchester, VA, most apple cultivars, including Honeycrisp, Gala, Fuji, Reds, and Goldens, are between 20%- 90% bloom. Some early cultivars, e.g., Pink Lady and Zestar, are already post-bloom. For cultivars at early bloom and full bloom, 10% kill happens at temperatures below 28 oF for > 30 min; and 90% kill happens if the temp drops below 25 oF for >30 min. Based on the current forecast, the lowest temp in most of the locations I looked at in Frederick, Rappahannock/Maddison, Roanoke, Nelson, and Carroll counties, is 30 oF, which should not, theoretically, cause any significant damage to apple blossoms. For cultivars at post-bloom and petal fall, subfreezing temp (below 32 oF) can cause damage to the skin of developing fruitlets, leading to “frost rings”. Fruits with a frost ring will likely grow normally until harvest, but blossoms damaged by the frost will likely abscise within the next 2-4 weeks. Temperatures above 28 oF should not cause significant damage to flowers and developing fruits of peaches and sweet cherries.

Early season applications to improve fruit size and control bitter pit in apple. 

Prohexadione calcium to reduce bitter pit incidence: 

– Shoot growth reduction by prohexadione calcium (PC) early in the season (at Pink) can significantly reduce bitter pit incidence in ‘Honeycrisp’ apples.

– PC applications at pink also reduce the incidence of shoot/fire blight.

  • Use at the rate of 6 oz/acre.
    • PC should not be mixed with calcium or boron.
    • Adding a non-ionic surfactant (e.g., Regulaid or LI700) to the tank increases the efficiency of PC applications. 
    • If the water source contains high levels of calcium carbonate (hard water), add spray-grad ammonium sulfate (AMS) to the spray tank at a rate of (1 lb AMS to 1 lb PC) or according to the product label.
    • Don’t spray if rain is expected within 8h of application.
    • Both Kudos and Apogee are PC products labeled for use at the pink stage.

Calcium applications:

– Between 15-50 lb of calcium chloride (CaCl2) should be added per acre per year to reduce bitter pit incidence. In other words, 2-8 pound/cover spray.

  • Early season applications of calcium (starting @Pink) are more critical than late-season applications.
    • Foliar applications of Ca nitrate are not recommended for Delicious and York. They might cause cork-spot-like symptoms. 
    • Avoid CaCl2 applications at temperatures above 75 oF or under slow drying conditions (e.g., early morning) as this might cause damage to the foliage, especially in sensitive apple cultivars (e.g., Idared and Golden Delicious).  
    • Soil applications of Ca are not efficient.
    • Boron deficiency may reduce Ca movement in the tree.

Boron applications:

– Boron is an essential element for flower development and fruit set, and boron deficiency can negatively affect yield and fruit size.

– Boron is also crucial for Ca movement, and Boron deficiency may lead to Ca-deficiency disorders, including bitter pit.  

  • Maintenance rate: 0.5-1 lb of Boron/acre (2.5 – 5 lb of Solubor/acre)
    • Apply at Pink stage, tank mixed with calcium chloride. 
      • OR, 7-10 after petal fall; or with the first cover spray.
    • Research at WA state showed a significant positive relationship between splitting in Gala and fruit boron content. So, it is recommended to use the lowest rate of boron with Gala.

Urea applications:

Foliar applications of urea at bloom (3 lb/100 gal) and at petal fall and early cover sprays (5-6 lb/100 gal) can promote cell division, especially for Gala. These applications are particularly important when king blooms, which usually give the largest fruit, are lost to frost.


Freeze Injury Updates (Winchester-March 29): Sweet cherry blooms lost to freezing temperatures; apples and peaches made it through.

Temperature profile of orchard blocks in the AHS Jr. AREC’s research farm (Winchester, VA) on March 28-29, 2022.

Our digital temperature data loggers recorded temperatures as low as 17 oF that continued for > 2h this morning (Tuesday, March 29). Some of our apple cultivars, including Gala and Pink Lady, were at the tight cluster and first pink; whereas, others like Fuji and Honeycrisp were between half-inch green and tight cluster. Temperatures below 21F were enough to wipe out the flowers of these cultivars completely, but we were pleasantly surprised to find out that all apple cultivars we assessed this afternoon survived this frost event with minimal damage. We recorded between 15 – 35% bud mortality in the king blooms and less than 15% in the side-blooms of Honeycrisp, Gala, and Pink Lady, with the latest showing the highest percentage of damage. Similarly, the four peach cultivars (Rich May, Sunhigh, Red Haven, and John Boy) we evaluated showed less than 15% flower mortality.

I generally believe that 10-30% flower mortality after frost is not a bad thing and could even be considered desirable as a natural thinning process and to ensure a good return bloom next year. It is generally evident that thinning during bloom has a very positive impact on return bloom; and whether this thinning is done chemically (e.g. liquid lime sulfur), mechanically (e.g. hand thinning), or by mother nature, the impact on return bloom will probably be the same.

On the other hand, our sweet cherry cultivars failed to sustain these low temperatures, and despite being still in the tight-cluster stage, they were severely damaged, with an average bud mortality rate of around 80% in Ebony Pearl and Selah. Regina, a relatively late-blooming cultivar, was still at the swollen bud stage, showing 0% damage.

It is also worth noting that flower development can vary among cultivars, species, and locations in the state. Also, temperatures recorded this morning in Winchester were seriously low, but they might not be the lowest in other areas. So, you can depend on this report only if your trees are in the same developmental stage and your temperature didn’t go below 17 oF this morning. If otherwise, I suggest that you evaluate the damage in your block as per the method and images I shared in previous posts.

Flower buds were collected from three apple cultivars (Pink Lady, Gala and Honeycrisp) and assessed for damage after the frost event that occurred on the morning of March 29, 2022 in Winchester/Frederick county area.
Flowers were collected from four peach cultivars (Sun High, Rich May, John Boy and Red Haven) and assessed for damage after the frost event that occurred on the morning of March 29, 2022 in Winchester/Frederick county area.
Flowers were collected from three sweet cherry cultivars (Ebony Pearl, Regina and Selah) and assessed for damage after the frost event that occurred on the morning of March 29, 2022 in Winchester/Frederick county area.

Hard Freeze is Expected This Tuesday (March 29)

Based on the weather forecast for Tuesday, March 29, temperatures will drop to 19 oF for a couple of hours (6:00 – 8:00 am) in the Frederick County and Rappahannock-Madison area, which could be devastating for the majority of apple cultivars and some early-bloom peach cultivars grown in these locations. Our apple cultivars in Winchester are currently in the tight-cluster stage, and some (e.g. Pink Lady and Zestar) are in the full pink-stage now. The majority of peach cultivars are in the pink stage now, with some early-blooming cultivars (e.g. John Boy) are already in bloom. Sweet cherries, on the other hand, are still in tight-cluster stage. Based on the critical temperature tables below, 90% of apple buds at tight-cluster can be killed if exposed to temperatures at 21oF and below for more than 30 min. Similarly, peach flowers at full bloom can be killed at the temperatures forecasted for Tuesday. Sweet cherry cultivars that are still in tight cluster will be likely OK, and only 10% can be damaged by this freeze event.

This is a typical advective freeze, accompanied by wind > 5 mph and dry air. Unfortunately, none of our frost mitigation measures, e.g., wind machines, overhead and under-tree sprinklers, can be of utility with this type of frost. Using any sprinklers, or spraying the tree with water using an air-blast or any other sprayer, will cause more damage than benefit. Heaters, on the other hand, can provide some protection if used for several hours, probably starting from midnight.

Tree fruit growers in the Nelson County may also face a brief, less damaging frost event, with the lowest temperature predicted to reach 24 oF before sunrise. Based on the current forecast, no frost/freeze is predicted for Roanoke and Carroll-Patrick counties.

Source:New Mexico State University
Source:New Mexico State University

Accede: a new chemical thinning material for apples and peaches

Accede is a new thinning product from Valent BioSciences that you may need to consider for apple fruit thinning and peach blossom thinning. The active ingredient of this product is ACC (1-aminocyclopropanecarboxylic acid (ACC); a chemical that is produced naturally in plant as a precursor for ethylene (the ripening hormone). Unlike other ethylene-based products (e.g. ethephon), Accede doesn’t cause gummosis in peach and nectarine; and is not as aggressive when thinning apples. 

For apple fruit thinning: Accede can be used when the king fruit’s average fruit size ranges from 8-25 mm. However, this product is mainly intended for later thinning treatments (fruit size 15-18mm) when the primary thinning materials (e.g. 6-BA and NAA) are no longer effective. This product should be applied at a 23-46 fl oz/acre rate. A reduced rate should be considered if the temp on the day of application is >90 oF, and the best results are achieved under slow drying conditions (e.g., early morning, at night). 

For peaches and nectarines: Accede is the only labeled chemical thinning product. Unlike apple, Accede is mainly used as a bloom thinning treatment for peaches and nectarines. It should be applied between bloom and petal fall at 34-69 fl oz/acre rate. However, it was found that two applications (at 10% bloom and full-bloom), each at 34 fl oz/100 gal/acre, can achieve the best thinning results. The product works by causing early senescence to the flowers, thus preventing fertilization and fruit set of the unwanted crop. According to research conducted in MSU in 2021, two applications of Accede, reduced fruit set significantly and cut the time needed for hand-thinning in half, compared to untreated trees. 

Source: Dr. Anna Wallis, Apple Production Specialist at MSU

Precautions when using Accede for peaches and nectarines:

This product should not be applied to stressed, unhealthy trees as it may cause over-thinning. If frost occurs during bloom, you should wait until assessing the percentage of frost damage before applying Accede. The product should not be applied if rain is expected within 8h of application. Finally, this product should not be used for peaches and nectarines after petal fall.  

Here is a link for the product label for more information about Accede.

2021 Tree Fruit Extension Survey


            The following is a link for a very brief survey that should take less than 2 min to complete. The results of this survey will help us assess our tree-fruit research and extension program at the AHS Jr. AREC. This survey will also help us identify priority areas and extension activities most preferred by Virginia tree fruit growers and stakeholders.

Here is the survey link:

We appreciate your help!

Apple Maturity Report-Winchester Sep 30-2021

DA readings refer to the level of chlorophyll (green color) in the fruit, just below the skin. DA values decrease proportionally with fruit ripening. Starch index, fruit firmness, and, to some degree, Brix values are commonly used to determine the harvest date for most commercial apple varieties. Generally speaking, fruits intended for CA should be picked at starch index (3-5), firmness (> 16 lb), and DA reading (around 0.60). Fruits intended for fresh consumption should be harvested at starch index (5.5-7), firmness (13-16 lb), DA reading (0.35), and Brix (12-14%).