Regarding the freeze warning tonight (4/18/2018)

As you may be aware, temperature are going to drop to a freezing point (32 oF) tonight and tomorrow night in most parts of Virginia (see the freeze warning: Most peach and cherry varieties are in full bloom now and this weather might cause a slight damage to the open blossoms, but we are not close to the critical temperatures that would cause serious damage to the crop. Here is the chart of critical temperatures for frost damage.

Source (

If you would consider turning-on your sprinklers to mitigate the effect of these freezing temperatures, here is the link to my previous post on using sprinklers for frost mitigation (
As for apple, this cold weather should not be a concern at this this point of bud development. Most apple varieties in our research farm in Winchester are still in the pink or loose cluster stage. Only Pink Lady and Arlet have some clusters with king bloom. Temperatures that would cause serious damage at this stage should be below 25 oF, which is luckily not the case for tonight or for the rest of this week.

Pink Lady in King bloom since Sunday April 15, 2018

Promoting return bloom in apple

The initiation of the next year flower buds occurs 2-6 weeks after bloom. Therefore, if the tree is exhausted in producing more fruits and/or vegetative tissues during this period, the initiation of flower buds will be negatively affected, and less return bloom will be obtained. Fruit thinning during this period, either chemically or by hand, helps the tree to direct some of their photoassimilates to the production of the next season’s flower buds. It has also been proven that treatments with some PGRs, particularly NAA and ethephon, after the thinning period ends (~ 6 weeks after bloom), can also promote bud initiation and subsequently return bloom.
Based on the results of many field trials, four bi-weekly sprays of NAA (5 ppm) and ethephon (150 ppm) beginning six weeks after full bloom promote return bloom in apple. These sprays can be added to the cover spray. However, if the tank is mixed with oil or a surfactant, the rates of NAA and ethephon should be reduced by 1/3. These rates should also be adjusted to the tree raw volume. Increasing water amount by 3-4X will enhance the positive effect of these treatments.

Updates on the carbohydrate thinning model- (May 16-21)

    The following is a note of caution from Alan Lakso, the developer of the apple carbohydrate-thinning model (May 11, 2017):

“• The apple carbohydrate model assumes that the trees are healthy, normal vigor, have no significant stress (frost, drought, nutrient) and no significant carry-over effects from previous years that might change the tree response to the weather. With the severe drought in 2016, there may be orchards that will have a weakened state coming into 2017 which would likely make the tree abnormally sensitive to thinners. So in those cases the model may suggest a stronger thinner concentration than is appropriate. Conversely, if irrigated last year, with the warm season and lots of sun, those trees may be in better than normal condition and harder to thin.
• With the current cold period, flowering, pollination, initial set and early fruit growth may not be very well modeled as we did not have much data under these very cold (or also excessively hot) conditions when the model was developed.
For all these reasons, you should have less confidence in the model this year. Use your experience with your unique situation.”

    Here is my take on this:

– I have not observed any unusual or extreme conditions in Virginia in the last few weeks that would seriously affect the accuracy of the model in predicting the timing and rate of chemical thinners.
– We have applied chemical thinners (e.g. 6-BA, NAA) to our Fuji and Gala trees in Winchester lab based on the model recommendations and we got fruit thinning enough to call it good.
– The fruit size in Winchester and Central Virginia is above 20 mm and rescue thinning using ethephon would be the only chemical thinning option at this time. For ethephon, the variety sensitivity and temperatures are more important factors to consider than the carbohydrate level.

    Therefore, I would recommend the following for this week’s thinning treatments:

– The weather forecast predicts temperatures close to 90 oF this week, which will increase the responsiveness to ethephon applications and might lead to over-thinning if the proper rates are not used.
– The carbohydrate model for Winchester and Central Virginia predicts carbohydrate deficit for the following six days (until May 22), which should also increase the responsiveness to ethephon. The model has already stopped providing any recommendations for the rates. However, if you will apply ethephon this week, I would recommend decreasing the rate by 30% at least and not to add oil or a surfactant to the ethephon-carbaryl mix.
– Rome Beauty and Golden Delicious are highly sensitive to ethephon and under the above-mentioned conditions; it might be risky to use ethephon with these varieties.

The following is the carbohydrate model for Winchester and Central Virginia.

Apple Carbohydrate Thinning Model-Updates-(May 08 – 12)

At such advanced stage of apple fruit development, 6-BA and NAA are no longer effective as chemical thinners and it is the time to start the “rescue thinning” applications. The fruit size for most varieties in Winchester is close to 20 mm and it would likely be above 23 mm in Central Virginia. Ethephon is the most effective thinner at this stage. Ethephon thins more effectively when the temperature is in the 70s to low 80s. Although the weather forecast predicts cool temperatures this week, increasing the rate of chemical thinners by 30% might mitigate this negative impact of low temperatures. Mixing carbaryl (1 pint/100 gal) and oil (1-2 qt/100 gal) with ethephon (0.5-1.5 pints/100 gal) enhances its effectiveness and it might show desirable effects under the weather conditions of this week. The addition of NAA (5 ppm) to ethephon/carbaryl mix might be an option if more aggressive thinning is still needed. However, this application should be avoided with Delicious and Fuji as it will lead to the development of pigmies. Adjusting the pH to 3-5 is recommended with ethephon applications. Non-ionic surfactants (e.g. Regulaid) can also increase the response to ethephon applications.
As shown in the graphs below, the apple carbohydrate-thinning model predicts carbohydrate surplus for the whole week and likely during the weekend in Winchester and Central Virginia. Under such conditions, the recommendations are to increase the rate of chemical thinners by 30%.

Worker Protection Standard Meeting – Wednesday, May 10, 2017 1:00pm to 4:00pm at the Alson H. Smith, Jr. AREC in Winchester, VA

Worker Protection Standard Meeting – May 10, 2017 1:00pm to 4:00pm at the Alson H. Smith, Jr. AREC in Winchester, VA

This is an opportunity to remind you of the revised Worker Protection Standards (WPS) and to provide resources from VDACS to address questions that you/we have on implementation of the revised standards. Personnel from VDACS’s Office of Pesticide Services, who are responsible for on-site compliance checks related to the WPS, will be on hand. These will be great opportunities to ask questions that you might have on the specific requirements of the revised WPS.

10 May 2017 (Wednesday)

1:00 – 4:00 pm

AHS Jr. Agricultural Research and Extension Center

595 Laurel Grove Rd., Winchester VA 22602


    • Seasonal viticultural updates, canopy management, nutrient management
    • Seasonal pest management updates
    • Discussion of revised Worker Protection Standards, time for questions and answers
    • Vineyard walk-through for those interested

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

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

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 – |


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



Extension is a joint program of Virginia Tech, Virginia State University, the U.S. Department of Agriculture, and state and local governments.


Apple Carbohydrate Thinning Model-Updates-(May 02 – 05)

Unlike last week, weather forecast predicts conditions that will likely lead to carbohydrate surplus and subsequently, less thinning efficiency this week (May 01-05). As shown in the images below, the carbohydrate model predicts accumulation of up to 35g/day carbohydrate this Friday. This, in turn, will necessitate a 30% increase in the rate of chemical thinners that will be applied tomorrow, Thursday or Friday. I believe we are still within the good thinning window in terms of fruit size, but this week might be the last chance for (6-BA + Carbaryl), or (NAA + Carbaryl) sprays to show an effect.
The following is the carbohydrate model for Winchester and Central Virginia.

Apple Carbohydrate Thinning Model-“Updates” for April 28 – May 1

The weather forecast predicts a hot and cloudy day for tomorrow (Saturday, April 29), which certainly will lead to more carbohydrate deficit. As you see from the graphs below, the carbohydrate balance will increase again by Tuesday and Wednesday, but the carbohydrate reserve will be below 20 g/day, which is not too high to influence thinning treatments (if needed) at this time.
For defruting you young apple trees (to fill the space and get a good tree structure), tomorrow might be a good day to do so. The temperature will be high (>85 oF) in Winchester and Central Virginia and the carbohydrate balance is very low (< -60 g/day) which together encourage for more fruit abscission. To defruit your trees, you can apply:
A) NAA (5-8 ppm) + Carbaryl (1-2 pt) + Superior oil (1qt)/100 gal; or
B) Ethephon (1-1.5 pt) + Carbaryl (1-2 pt) + Superior oil (1qt)/100 gal.
Although Ethephon should result in more aggressive thinning at such high temperatures; it might also cause tree growth reduction and might increase flowering the following year.
The following are the carbohydrate model outputs for both Winchester and Central Virginia.

Is it “Risky” to thin tomorrow?

Although the carbohydrate model still suggests thinning applications until Friday, I would consider any thinning application tomorrow and for the rest of this week as RISKY.
Today (April 26) is a perfect timing for thinning as you still have at least three days before the carbohydrate balance reaches to critical levels and it’s highly likely that fruitlets that will respond to today’s spray will be found on the ground in the weekend or early next week.
However, the weather forecast predicts conditions that will lead to more carbohydrate deficit this week and probably next week; and the carbohydrate model shows critical levels of carbohydrate balance (-60 to -80 g/day) this Saturday. At such low levels of carbohydrate reserve, the natural abscission of small fruitlets is highly likely. So, even if you do not apply any thinning treatments tomorrow (Thursday, April 27) or after tomorrow, some natural thinning will likely occur within and after the weekend.
The following are the recent updates of the model outputs, for Winchester and Central Virginia.

Note: I have noticed some delay between the time I post on the Horticulture blog and the time the updates can be seen by the public. I would advise that you keep this link ( in your bookmarks and go directly to the “Recent Posts” icon on the right-hand side of your screen to remain updated.

Apple Carbohydrate Thinning Model-“Updates” for April 23-30, 2017

As I mentioned in my last post, this week is a perfect timing for chemical thinning. However, I thought of sending daily updates as the weather forecast predicts more cloudy/ and or hot days this week that may lead to more carbohydrate deficit. Therefore, we should keep our eyes open for any change in the rates of chemical thinners that we intend to apply today (Wednesday, April 26) or tomorrow.

The following are the carbohydrate model outputs for Winchester and Central Virginia.

Apple Carbohydrate Thinning Model for April 23-30, 2017

This week of April 24-30 is the perfect timing for chemical thinning applications for both Winchester and Central Virginia. It’s as the books says about the optimal thinning conditions. Apples are in the size of 6-9 mm, compete aggressively with each other and consume a lot of the tree carbohydrate reserve; temperatures are above 80’s in most days and subsequently the transpiration rate will be high; some days are partially cloudy and will lead to fewer photoassimilates; and more importantly, the carbohydrate model predicts carbohydrate deficiency up to -60 g/day. The only thing that works against us is the high chance (90%) of rain tomorrow (Monday) and after tomorrow (Tuesday). However, we still have this Wednesday and Thursday as two ideal days to chemically thin our fruitlets.
If you were wondering about the best thinning applications to use at this stage, I would recommend using 6-BA for most varieties. Best results will be obtained when you mix 6-BA with carbaryl. NAA is another thinner you can use at this stage and also will give better results when mixed with carbaryl (i.e. Sevin). The mix of 6-BA and NAA might work well with most varieties and there is a good logic of having these two thinners mixed together at this time. However, this mix (6-BA + NAA) SHOULD NOT BE USED with Delicious and Fuji as it will likely result in the development of pygmies. With the coming hot weather this week, adding oil to the tank may result in overthinning. There are many variations among apple varieties and some are more sensitive to chemical thinners than others. The following table (Table 1) gives some indications about the sensitivity of different apple varieties to chemical thinners.

One final note, I run the model for Winchester and Central Virginia based on the information I retrieved today (April 23) from the weather stations in AHS Jr. AREC, Winchester and Batesville, respectively, and there is no guarantee that weather forecasting will remain unchanged the whole week. However, if I have detected any major change in the temperature and solar radiation values over the week, I will run the carbohydrate model again and keep you posted.