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.

ReTain and Harvista For Pre-Harvest Drop Control of Apples

The pre-harvest drop refers to the abscission of fruits from the tree before horticultural maturity. Depending on the cultivar and growing season, yield losses due to pre-harvest drop can reach 30%. Factors such as heat and drought stress, heavy insect infestation, and late summer pruning can increase the severity of fruit drop. Early-maturing cultivars (e.g., Gala and Honeycrisp) are usually more prone to fruit drop than late-maturing cultivars (e.g., Fuji and Pink Lady). Ethylene, the ripening hormone, is considered the primary driver of pre-harvest drop, and therefore ethylene inhibitors are used to control the pre-harvest drop in apple orchards. ReTain (from Valent Bioscience) and Harvista (from AgroFresh) are the two ethylene inhibitors labeled for pre-harvest drop control in Virginia. ReTain inhibits the biosynthesis/production of ethylene, whereas Harvista prevents ethylene reception and action. The purpose of this blog post is to share with you the results of two experimental trials we conducted in the 2018 and 2019 seasons to explore the effects of different rates and application timings on the efficacy of these materials.

In 2018: we examined two rates of ReTain (166g/acre and 333g/acre) and three application timings (1 week, 3 weeks and (1 & 3) weeks before the anticipated harvest date) on the percentage of fruit drop (%) of Gala apples. We used 6 trees per treatment. We flagged and counted 100 fruit/tree at 4 weeks before harvest (WBH), and we counted the fruits on these trees every week starting at 1 WBH through 4 WAH. The percentage of fruit drop (%) at each stage was calculated relative to the initial fruit count. In addition to fruit drop, we also investigated the effects of different treatments on fruit quality parameters (fruit weight, size, firmness, color, sugar content, starch index, and acidity). The findings of this trial can be concluded in four points: a) The effect of ReTain treatments on fruit drop was more evident at 1 and 2 weeks after harvest. b) Applying ReTain at the full rate (333g/acre) 3 weeks before the anticipated harvest date reduced fruit drop by 40-50% and extended the harvest season by 1-2 weeks. The same results were obtained when ReTain was used at half rate (166g/acre) at 1 and 3 WBH. C) fruits treated with ReTain (full rate) were firmer, had a lower starch index and lower brix than the untreated checks. However, ReTain-treated fruits were generally less colored than untreated checks. D) no differences in fruit size were found between ReTain- treated and untreated fruits.  (Please refer to Tables 1 & 2).

Table 1: The effects of ReTain rate and application timing on the pre-harvest fruit drop (%) of Gala apples-Season 2018.

Silwet L-77 (0.1 %; 388 ml/100 gal) was added to all spray applications.
Values sharing the same letters are not statistically significant. 
WBH: weeks before harvest; WAH: weeks after harvest.

Table 2: Gala fruit quality as affected by ReTain applications- 2018.

Color was measured by a DA meter; lower IAD values indicate advanced fruit maturity.

In 2019: we examined 2 rates of ReTain (333 g/acre and 666 g/acre), 1 rate of Harvista (121 fl oz/acre), 3 application timings for ReTain (at 1, 3 and (1&3) weeks before harvest) and 2 application timings for Harvista (at starch index 2 and at starch index (1.5 and 3). Again, six trees were used per treatment. The total number of fruits on each tree was counted four weeks before harvest. The percentage of fruit drop was calculated starting at 1 WBH and through 3 WAH. We also examined the effects of different treatments on the fruit quality of Gala apples at harvest. Our results indicated that: a) a full rate (333 g/acre) of ReTain applied at 3 WBH was not statistically different than a double rate (666 g/acre); b) Harvista applied twice (at starch index 1.5 and 3) showed better results on fruit drop than the single application (at starch index 2); c) There were no significant effects on fruit size and weight when either ReTain or Harvista were used; d) Fruits treated with ReTain were generally firmer than control, but had poor coloration; e) Harvista did not improve fruit coloration, but also did not reduce it compared to control; f) ReTain applications at a full rate significantly reduced the fruit’s ethylene content, but it required two applications of Harvista to acquire the same effect. (Please refer to Tables 3 & 4)

Table 3: ReTain and Harvista effects on the pre-harvest fruit drop (%) of Gala apples-2019

DBH: days before harvest; WBH: weeks before harvest

Table 4: ReTain and Harvista effects on fruit quality of ‘Gala’ apples-2019

Color was measured by a DA meter; lower IAD values indicate advanced fruit maturity.  Ethylene (ET) content was quantified per each fruit using gas chromatography. Higher ET levels indicate advanced maturity.

Other general notes:

  • 6h drying time after ReTain and Harvista applications is required for achieving better results.
  • For optimal results, ReTain and Harvista should be applied with 100 gal/acre as a complete spray.
  • ReTain label allows the use of up to two pouches per acre either as a single spray or as a split application for apple.
  • Harvista can be applied close to anticipated harvest date (within 3 days prior to harvest).

Apple Fruit Thinning-Winchester-May 14

Our fruit size averages for ‘Gala’, ‘Red Delicious’ and ‘Honeycrisp apples in Winchester are 15.66, 14.9 and 13.23 mm, respectively. If you applied thinning treatments on May 2 or 4, you should be able to see thinning responses by now. Fruitlets to be abscised will be loose and easily pulled off. In order to assess the response of thinning applications applied on May 4, we marked the fruitlets of 10 clusters of Gala, Honeycrisp and Reds and tracked their sizes in the last three days using a digital caliper. The idea behind this was to monitor the fruit growth rate and determine how many fruitlets will eventually abscise. Fruitlets to be abscised are believed to have a growth rate that is less than 50% of the fastest growing fruit on the tree. For instance, the fastest growth rate for Gala in the past three days was 0.6 mm per day. A fruit in the cluster that shows less than 0.3 mm/day will likely abscise soon. Based on this, we have found that our thinning applications on May 4, will result on 50%, 67%, 82% abscission rate for Reds, Gala and Honeycrisp, respectively. On other words, we should expect more triples, doubles and singles/cluster for these three cultivars, respectively. Our per acre thinning rates for Gala and Reds were (64-96 fl oz of Exilis + 1 qt of Sevin + 1 pt of Regulaid). For Honeycrisp, our rate was (4.5 oz of Refine + 1 qt of Sevin + 1 pt of Regulaid). If you applied similar rates on the same day for the same cultivars, your responses will be likely the same.

The Apple Carbohydrate Thinning Model at NEWA.Cornell. For this location (Winchester, VA) I used March 20 and April 16 for the green tip and bloom dates, respectively. Precent flowering spurs (51-75%). The model was run on Saturday May 15 at 1:10 am. Winchester VT AHS AREC weather station was used for this location.

If, after assessing thinning responses in your orchard, you still feel that another thinning application is required, it’s still possible to apply 6-BA and NAA applications until your average fruit sizes are ~ 18 mm. NAA would likely result in better thinning responses at this stage. As shown in the figure below, the carbohydrate model predicts carbohydrate surplus in the next three days and therefore the recommendations are to increase the rate of thinning materials by 30%.  

 For cultivars exceeding an average fruit size of 20 mm , NAA and 6-BA applications won’t be effective. If more thinning is required, the only chemical option available would be ethephon. You will find more information about rescue thinning by ethephon in the following blog post (

Potential Fire blight Infections

A message from Dr. Srdjan Acimovic:

In any apple trees blocks still at bloom (young planted trees, and any orchards with rat tail bloom): fire blight infections have occurred on 5/3 in or around Charlottesville, Lynchburg, Wakefield and on 5/2 and 5/3 in Red Hill, and in Red Hill and Wakefield the infection will occur again tomorrow 5/4 and 5/5. Please examine attached NEWA EIP print screens. Protect by applying 1.5 lbs/A Harbour or Agri-Mycin plus Regulaid 1 pt/100 gal, before rain/dew triggering infection or up to 24 h after that rain, on any apple tees in bloom. You can still apply 5/3 for infection that occurred on 5/2. If you did not protect against the infection/s on 5/2 apply Apogee at 12 oz/100 Gal as soon as you can, best 1-3 days after infection event that you did not apply streptomycin for. Use of Apogee for and unprotected infections beyond the 24 h after infection has occurred will help you prevent fire blight cankers to develop from the currently incubating infections. 

Fruit Thinning Recommendations-Winchester-May 2, 2021


Looking at the carbohydrate thinning model and weather forecast for this week (May 2- May 9), you can realize that we, at Winchester/Fredrick county area, have only a couple of days that could be suitable for our primary fruit thinning applications. As shown in the carbohydrate model below, we are approaching the recommended degree days of 200-250 DD by this Wednesday (May 5), which would be perfect for applying our regular thinning treatments if it’s not raining that day. However, it will be apparently cold and/or rainy for the rest of the week, leaving us with only two days with temperatures above 80 oF (Sunday, May 2 and Tuesday, May 4), which is good for the activity of our main thinning materials, 6-BA and NAA. These thinners don’t work well when temperatures are below 70F. Therefore, I would recommend targeting these two days (May 2 and 4) for thinning applications. Although our current DD is slightly under the recommended range, our fruit sizes lie within the perfect range (7-13 mm) for thinning applications. In our last zoom meeting this past Wednesday, we discussed the materials and rates you can use for thinning Gala, Fuji, Honeycrisp and several other cultivars. You can access this information by following this link:

As you can see in the model page below, the last column refers to the recommended application rate based on the 7-day carbon balance. The model suggests a standard rate of thinning material for Sunday applications and 30% increase in thinning rates for Tuesday. In other words, if your standard rate for MaxCel is (64 fl oz + 2 pt of carbaryl/acre); you need to add (83 fl oz of MaxCel + 2.6 pt of carbaryl/acre) on Tuesday. This 30% increase doesn’t apply to the surfactants you apply with the thinning mix. Please also note that some forms of 6-BA (such as Exilis 9.5) have 5X active ingredient and therefore the amounts used in the example above will be different. Similarly, for NAA there are two forms of Refine (Refine 3.5 and Refine 6.25) that contain different concentrations of ai. Please read the product label before applying any of these chemicals. Finally, feel free to approach me via the email and telephone number mentioned below.

The Apple Carbohydrate Thinning Model at NEWA.Cornell. For this location (Winchester, VA) I used March 20 and April 16 for the green tip and bloom dates, respectively. Precent flowering spurs (51-75%). The model was run on Sunday May 2nd at 2:35 am. Winchester VT AHS AREC weather station was used for this location.

Disease Update: Fire blight

Dear All: Please find below a message from Dr. Srdjan Acimovic.

In any apple trees blocks still at bloom, fire blight infection will be possible if weather forecast from NWS changes from what is now used to run EIP NEWA fire blight model: i.e. if dew or rain occurs. The currently orange labeled EIP cells with values above 100 could turn to Red color and word Infection if dew or rain occurs (attached), keep looking at model every day. Some locations (airports) I attached show that red cell change to Infection due to wetting. If this change occurs, apply 1.5 lbs/A Harbour or Agri-Mycin plus Regulaid 1 pt/100 gal, before rain/dew triggering infection or up to 24 h after that rain, on any apple or pear trees in bloom.

Updates on Fire Blight Infections


Please see below the updates from Dr. Srdjan Acimovic on the fire blight situation in Virginia.

If the data from Roanoke International Airport station are correct there will be fire blight infection today 4/14 triggered by the rain, in and near Roanoke. Growers should apply strep 1.5 lb + Regulaid 1 pt/100 before rain or apply this mix up to 24 h after the rain event that triggered the infection. What puzzles me is that Floyd (CLH), not so far away from Roanoke, has no infection predicted for 4/14, leading me to believe that it either one of the two stations is not calibrated properly OR it will be hit or miss in terms of where rain/dew lands. Next, the infection occurred in Esmont (Rees), Lynchburg, and Red Hill yesterday 3/13 and if the growers did not apply streptomycin before the rain/dew on 3/13 triggering the infection, they must and can still apply strep 1.5 lb + Regulaid 1 pt/100 today i.e, up to 24 h after the rain event that triggered the infection for it to be effective. In Lynchburg infection is also predicted today 4/14 and the spray today 4/14 will cover for both infections 4/13 and 4/14. In Richmond, if the station is plugging in the correct weather data into the model, there was contiguous infection 4/9-12 which is not good, and proper response as per my previous blog was to spray on 4/10, and 4/12 to cover for all 4 infections and both times growers should have applied strep 1.5 lb + Regulaid 1 pt/100 to get 24 h kick-back and thus cover for all 4 infections. This is based on my experiences from eastern NY and is different from the VA spray guidelines that currently state that a streptomycin application remains effective 3-5 days. Based on this experience with the outbreaks I saw after similar block of infection in NY, I disagree with that 3-5 days as per my blog published here: 1. Frightful Blight Model Cases How Long A Streptomycin Spray Lasts; 2. Major Scab Infection/s Possible 25-30 or 28-29 May All East NY posted on May 25, 2020. In Rustburg (Crown), I would apply strep 1.5 lb + Regulaid 1 pt/100 tomorrow 4/15 if the “High” risk turns into “Infection” for 4/14 in NEWA’s EIP model. Danville has had infections continuously from 4/8-12, assuming correct weather data. Most other weather stations do not indicate a risk from infection, assuming they are functioning properly in collecting the weather data. Please check your closest weather station in NEWA daily, and apply protection according to the EIP model, and make sure it is functioning properly before you can trust the model outputs from it.

You can reach Dr. Acimovic at 517-449-0905

Fire blight in Roanoke and many other locations in VA predicted or already occurred


     Please find below an email I just received from our new plant pathologist, Dr. Srdjan Acimovic, regarding the fire blight situation in different Virginia locations. Dr. Srdjan will communicate his recommendations directly to you through his blog when he officially starts his job with Virginia Tech by the end of May 2021.

I am not sure are the weather stations accurate i.e. functioning properly in the airports or orchards, but there is a fire blight infection predicted for today and tomorrow in Charlottesville, and there were three fire blight infections in Roanoke on 4/8, 4/9 and today 4/10, and the same is in Lynchburg. I hope the growers are on top of looking the EIP model in NEWA daily during bloom and caught this and already applied on 4/9 streptomycin + Regulaid in Roanoke and Lynchburg. It they did not, I would still recommend to apply this mix today on anything in bloom. In Charlottesville they need to apply streptomycin 17% @ 1.5 lb/A + Regulaid 1 pt/100 gal today or tomorrow (24 h after the infection on 4/10) on anything that is in bloom. As I recommend on my talks earlier in Feb & March, growers should look at the EIP in NEWA daily. In Manassas, fire blight infection is predicted for 4/12, in Rustburg there is an infection today 4/10, and there was an infection block from 4/6-8. I hope they protected against that block. In Staunton there is an infection today 4/10, in Washington (G cellars) there was an infection 4/7. In Newport News there is an infection tomorrow 4/11, In Norfolk there is an infection today 4/10 and 4/11. In Richmond infection occurred yesterday 4/9, and it still warrants an application of strep+ Regulaid today when another infection is predicted also. In Staunton infection is predicted today, they can spray up to 24 h after infection today. Same spray mix recommended above will work. 

Now, for all the locations that did not apply streptomycin on time in Roanoke and Lynchburg (for infection 4/8) they should immediately apply 12 oz/100 gal of Apogee or equivalent rare of Kudos, at 2-3 days from the infection on 4/8, and they should still apply streptomycin + Regulaid mix above today or tomorrow for infection today. They can mix all three, Apogee, strep, Regulaid. If no Regulaud accessible use LI700, but Regulaid is preferred. 

More frost damage recorded in Apples and Sweet Cherries in Winchester

Our digital temperature data loggers placed in four different blocks in the AHS Jr. AREC’s research farm indicated that we were hit by killing freezing temperatures in the mornings of Friday (April 2) and Saturday (April 3), with the latter being more aggressive and long-lasting. In fact, the lowest we received on Friday was 24oF, but on Saturday, our trees were hammered by temps as low as 21oF for more than 30 min. 21 oF for 30 min or more are enough to kill 90% of apple flower buds at ‘tight cluster’, ‘first pink’ and ‘full pink’.  Today (April 5), we collected flower clusters from Gala, Fuji, Pink Lady, Red Delicious, Zester, and Honeycrisp. Except for Zester, all the cultivars are only moving from tight cluster to first pink. Zestar with flowers at ‘full pink’ showed 65% mortality. Other cultivars showed 45% (Fuji), 45% (Gala), 42% (Pink Lady), 15% (Red Delicious) and 5% (Honeycrisp). Unsurprisingly, king blossoms showed far more damage than side flowers. As for sweet cherries, we have recorded 93% flower mortality for flower buds at different stages (mostly first white and first bloom). Luckily and -to some extent- unexpectedly, our ‘Sunhigh’ peach flowers showed less than 25% flower despite being at the pink to the first-bloom stage. I have attached some images below to show some examples of frost damages we detected in apples and sweet cherries.

Frost damage in apple and sweet cherry

The weather forecasting predicted below freezing temperatures this past Friday (April 2nd) and this morning (Saturday, April 3rd) in the Winchester area (22602). The temperature data as it appears now on the weather network show 2h of 29 oF on Friday morning, 3h at 26 oF and 1h at 25 oF this morning. All these temperatures are above the threshold that causes 90%-kill for flower buds, but can certainly result in 10% damage for apples, sweet cherries, and peaches. To get a better idea of how much damage has occurred after the Friday frost, my research team and I collected flowers from two apple cultivars, a peach cultivar, and a cherry cultivar and evaluated flower viability after 9h of frost (around 3:00 pm, yesterday/Friday). Unfortunately, our data showed 46% and 56% mortality rate in Pink Lady apple and ‘Ebony Pearl’ sweet cherry, respectively. We observed 0% damage in the flowers of ‘Honeycrisp’ apple and ‘Sunhigh’ peach. It’s worth noting that the 46% damage observed in ‘Pink Lady’ flowers is from the King flowers only. We haven’t examined any of the side flowers because they were relatively smaller and hard to observe. It’s also important to know that ‘Pink Lady’ is at the ‘first pink’ stage now, whereas ‘Honeycrisp’, like most of our apple cultivars, are still in the ‘tight cluster’ stage. The majority of our sweet cherry and peach cultivars are at the ‘white bud’ and ‘Pink’ stage, respectively. We will examine these cultivars again tomorrow, Sunday, April 4th, to determine the final percentage of damage after the two frost events. Knowing this information is crucial before starting blossom and fruit thinning. Below are some images to show the extent of frost damage in different species. 

We used a sharp razor blade to prepare longitudinal sections of sweet cherry flowers at the ‘white bud’ stage. Notice the green style and ovary tissues in the middle, as indications of flower viability. We assessed the damage for 30 flowers collected from different branches and trees.
Longitudinal sections of ‘Pink Lady’ apple flowers at the ‘first pink’ stage. More than 30 king blooms were assessed for damage after 9h of a frost event that occurred on Friday, April 2, 2021 in Winchester, VA.
Longitudinal sections of ‘Ebony Pearl’ sweet cherry flowers at the ‘white bud’ stage. Blossoms were assessed for damage after 9h of a frost event that occurred on Friday, April 2, 2021 in Winchester, VA.
Longitudinal sections of ‘Sunhigh’ peach flowers at the ‘Pink’ stage showing alive flower styles and ovaries after a frost event that occurred on April 2nd, 2021.