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 (http://blogs.ext.vt.edu/tree-fruit-horticulture/2017/04/26/apple-carbohydrate-thinning-model-updates-for-april-23-30-2017/) in your bookmarks and go directly to the “Recent Posts” icon on the right-hand side of your screen to remain updated.

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.

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.

The outputs of the carbohydrate model for Central Virginia area is based on the Batesville (Crown Orchard) weather station. The solar radiation and temperature values suggest carbohydrate deficiency through this week (until Friday, April 21). Since it’s rainy today in most parts, thinning applications (at the standard rate) tomorrow and till Friday should be efficient. Although it will be sunny and warm tomorrow and probably Saturday, leading to some carbohydrate accumulation (the blue line), interruptions with hot and/or cloudy days will reduce the 4-day average (red line) balance and promote fruit abscission.

For Winchester area, I ran the model based on the data retrieved from our weather station at the AHS Jr. AREC. As it appears in the figure below, the outputs look similar to that of Batesville and hence the timing for thinning applications will be similar as well. However, the predicated temperature and solar radiation values for Winchester don’t support any carbohydrate accumulation this week and hence it’s recommended to decrease the chemical thinner rate by 15%.

In modern apple production systems, chemical thinning of young fruitlets within the first four weeks after petal fall has become a key management element. Inadequate thinning of fruit during this period increases the need for hand thinning, which is generally more laborious and expensive. On the other side, if trees are left without thinning at all, the chances of having low quality fruits and less return bloom in the following year are very high. To achieve adequate thinning using chemical thinners, many factors should be taken into consideration. These include, but are not limited to, fruit size, carbohydrate reserve status of the tree, weather conditions and -to a lesser extent- variety sensitivity. As a general rule, sunny days and cold nights promote the accumulation of photoassimilates and consequently trees become less prone to shed any of their fruits and are less sensitive to thinning applications. Whereas, cloudy days and warm nights lead to carbohydrate deficit and therefore trees tend to retain only a limited number of fruits and become more sensitive to chemical thinning. This associating between weather conditions and the total carbohydrate reserve has encouraged scientists (Alan Lakso and Terance Robinson) at Cornell University to establish a model known as “The Cornell Apple Carbohydrate-Thinning Model” that provides instructions on the dose and timing of thinning applications based on the solar radiation, temperature and day length records acquired from a weather station in a grower’s orchard blocks. This model can be accessed online through the Network for Environmental and Weather Applications (NEWA)(http://newa.cornell.edu/index.php?page=apple-thin).
It is a good practice to visit the NEWA website and make yourself familiar with the inputs and outputs of this model, especially if you have a weather station in your orchard that is connected with NEWA server, or you know the one that is close to your operation and that you can depend on.
As we are moving toward a heavy crop load and, luckily, less frost damage for apples this year, we have to consider looking at the outputs of the carbohydrate model in order to decide the timing and the dose of the chemical thinner. Typically, the efficiency of thinning applications applied from petal fall until a fruit size of 20 mm correlates well with the carbohydrate reserve of the tree and these are the times where the model outputs will serve as useful guidelines.
This year, we, at AHS Jr. AREC, Winchester, will run the model weekly and provide the outputs of this model as graphs that show the carbohydrate reserve of apple trees, and the recommended applications. The following table (Table 1) describes the recommended dose of chemical thinners based on the carbohydrate balance of the tree.
Table (1): Decision rules for using the output of the carbohydrate model to adjust chemical thinning rate.

Fruit size is another major factor that determines the efficiency of chemical thinning. When fruits are young (~ 6 mm), their demand of carbohydrate is not much and hence thinning becomes relatively challenging. However, chemical thinners such as naphthaleneacetic acid (NAA) and Sevin (carbaryl) and, to some extent, naphthaleneacetamide (NAD; Amid-Thin) can be successfully used at this stage. As fruits grow to the size of 7-14 mm, their cells divide rapidly and their demand of carbohydrate becomes more than what vegetative tissues can supply, especially if weather conditions do not largely support photoassimilation. At this stage, fruits become more sensitive to chemical thinners such as 6-benzyladenine (6-BA), NAA and NAD. However, if thinning is not adequate at this stage due to unsuitable weather conditions, then another round of thinning application will be essential. As fruit develops to the size above 20 mm, the chemical thinning becomes a tangible challenge. At such an advanced stage of development, fruits become more tolerant to abscission as they have more carbohydrate reserve and the seeds produce auxin which interferes with ethylene action preventing abscission. Chemical thinning at this stage (known as delayed/ rescue thinning), relies mostly on the combined application of 2-chloroethylphosphonic acid (ethephon) and 1-naphthyl methylcarbamate (carbaryl). The following table (Table 2) shows the recommended thinning applications in each stage of flower/fruit development and the dose of each application.
Table (2): Materials, rates and timing of chemical thinning application (Source:2017 Spray Bulletin for commercial tree fruit growers).

Having some snow on trees and ground, even if it is not that heavy like the one we had last night, is not bad at all. Having a layer of dry snow on the ground should provide good insulation for soil and roots against the expected low temperatures we will face tonight and tomorrow morning. Also, large limbs that are still loaded with snow should get some protection. Using sprinklers will worsen the situation as wet snow works as a good thermal conductor and will transfer these freezing temperatures to roots much faster than dry snow.

The simple and direct answer is YES, BUT you have to consider the right time/conditions.
When you use sprinklers to reduce frost damage, you are simply using the internal (latent) energy of water molecules as a source of heat to your trees. Simply, water exists in three forms (liquid, solid (ice) and gas (vapor), and the transition among these forms can produce energy (exothermic) or consume energy (endothermic) as appears in the diagram below. So, when the air temperature is below freezing points (< 32 oF), and you use sprinklers, you indeed encourage the transition of water from its liquid phase into ice in order to release the latent energy into sensible energy (heat) that plant tissues can use to warm up. Great! Where is the problem?

There are three things you have to understand and consider in order to get this positive effect of sprinklers and not to harm your plants. First, you have to watch the wind. The Wind encourages the transition of the water from the liquid phase into the gaseous phase (vapor) and while this happens, heat in the air and around your plants will be consumed in an endothermic reaction. So, it’s always recommended that you don’t use sprinklers if the wind is above (10 mph). The table below (Table 1) should help you decide the amount of water needed (inches/h) under different wind conditions (mph) and under different temperatures.
Table (1): Amount of water (inches/h) that should be provided for frost protection under different wind speeds (mph). (U of Florida Ext. Circ. 287)

Second is the dew point: without putting complication to the topic, low dew points implies low humidity and if the dew point is too low, the water you add through sprinklers will be soon evaporated to compensate for the low air humidity and consequently cools down air around your plants. So, under moderate dew points, a part of water you add through sprinklers will turn into vapor and consumes heat and another part will turn into ice and produces heat, but AFTER SOME TIME, the net energy will be positive and you will get the benefit of using sprinklers. However, if the dew point is too low, it means that the air is too dry and it will take MUCH TIME to be saturated with vapor. During this time, the air temperature might reach to the critical temperature at which your buds/flowers will be damaged. In this case, sprinklers will cause more damage to your plants than without sprinklers. To help you decide whether sprinklers will be useful for your case or not, use the following table (Table 2) to determine the temperature at which you should turn on/off your sprinklers. If the dew point and the critical damage temperatures are not in the range shown in the table, don’t use sprinklers.
Table (2) shows the minimum temperature at which sprinklers should be turned on/off (UC-Davis, FP005 Quick Answers).
Simply, select the critical temperature for frost damage (the columns) and the dew-point temperature (the rows). The temperature where the row value crosses with the column value is the minimum air temperature at which you can turn on/off your sprinklers.

– you can use this tool to determine the dew point (https://www.weather.gov/epz/wxcalc_rh).
Or, you can get the dew points from Intellicase (use your zip code) http://www.intellicast.com/Local/Weather.aspx?location=USVA0837

The third thing to consider is the amount of water that your sprinklers can provide. If you are not sure that your sprinklers can provide sufficient amount of water, don’t use them at all. Table (1) should help you determine the amount of water required under different conditions.

Orchard management and profitable production of tree fruits has become driven by robust science and thorough research. In light of this fact, this multidisciplinary questionnaire has been put together by a group of AHS Jr.-AREC scientists to help them identify and set research questions that reflect the interests of Virginia tree fruit growers and stakeholders. A similar questionnaire was distributed 6 years ago and has served to guide AHS Jr.-AREC scientists designing their field-blocks and shaping their research directions. Based on this survey, many experiments in pomology, plant pathology and entomology have been initiated. This includes, but is not limited to, experiments on crop load management practices, orchard tree planting density, variety and rootstock evaluation, physiological disorders, fungicides and insecticide efficacy trials, mating disruption trials and organic practices. This year’s questionnaire is meant to investigate the new trends and interests of Virginia fruit producers that may ignite new ideas or further promote researching the existing ones.

We are expecting responses from tree fruit growers, private agricultural consultants, agribusiness field representatives, VCE specialists and VCE extension educators in Virginia. An online version of the 2017 questionnaire can be accessed here (https://virginiatech.qualtrics.com/SE/?SID=SV_ezHdupTmKNcdO05). Please complete before March 15^th.

A paper version of this questionnaire will be distributed during the upcoming fruit school meetings (Feb 13-17).