For those who responded humbly to the recommendations of my previous blog post and sprayed their Galas, Fujis and other hand-to-thin cultivars with chemical thinners on May 4 and 5…Lucky You…I believe that was a wise decision. We had several cold and/or rainy days in the past few days, along with carbohydrate surplus conditions that were unsuitable for any thinning treatments. This week, there is another potential window for thinning for those who chose to wait for a larger fruit size or more perfect thinning conditions. According to the carbohydrate model outputs (see below), the accumulated degree days will be within the perfect range (200-250 DD) on May 13 & 14. There is also a potential decline in tree’s carbohydrate level which should increase the response of apple trees to thinning materials applied in these two days. As of today, May 9, the average fruit size for our Fuji, Gala, Goldens and Honeycrisp in Winchester’s research farm is evolving around 10 mm which is also the perfect size for thinning. Having said that, I think it won’t be possible to spray on Friday and Saturday due to the rain; so, you better apply your thinning treatment on Wed and Thu this week (May 11 and 12); or Sunday (May 15) if weather allows. For blocks treated for thinning on May 4&5, no additional thinning treatments are needed at this time. You need to wait at least 2 weeks after the 1st application to decide if additional thinning is required.
It’s worth noting that the outputs and recommendations of the model above are based on our Gala’s green tip date of Mar 15 and full bloom date of April 17. If you have different dates for your Gala or other apple cultivars, you will see different model outputs. To access the model, use this link: https://newa.cornell.edu/apple-carbohydrate-thinning.
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 (https://newa.cornell.edu/apple-carbohydrate-thinning) 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.
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.
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
Table 3. Thinning materials and rates for 6-18 mm fruit diameter
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 (https://newa.cornell.edu/apple-carbohydrate-thinning) 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: https://blogs.ext.vt.edu/tree-fruit-horticulture/?s=Accede
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 https://www.pubs.ext.vt.edu/SPES/SPES-134P/SPES-134P.html.
For both Winchester and Central Virginia, the Malusim model is predicting a mild to no carbohydrate stress over the next week. Although temperatures at the time of application, are less important then the carbohydrate running average (2+4 Running Average), expect chemical thinners applied Sunday (10 May) or Monday (11 May) to cause more thinning than applications made late last week or for the rest of this week. Expect a “typical” response from chemical thinners applied Tuesday (12 May) through the rest of this coming week.
The National Weather Service is predicting sunny weather later this week, as well as daytime temperatures in the 70s. This should result in ideal conditions for plant growth, and thus less carbohydrate stress in the apple trees.
One of the weaknesses of weather-based models, is the need to use forecast data to make decisions about future events. When I ran the MaluSim models for both Winchester and Central Virginia last Thursday, the forecast was for hot, cloudy weather. Although it was in the mid-80s in most of Virginia over the weekend, there was also a lot more sunlight than predicted. Temperatures are generally a lot more reliable than cloud cover predictions, but both seemed to be less severe than originally forecasted.
Cloudy weather predicted for later in the week is causing wild swings in the MaluSim model output. My expectation is that actual carbohydrate levels will be more moderate and that running average values will remain in the 20 to -20 g CHO/day range.
Due to warm days (>80F) and intermittent cloud cover, the MaluSim carbohydrate model is showing a strong carbohydrate deficit for both Winchester and Central Virginia over the next four to five days. Expect an aggressive to very aggressive response to chemical thinners applied today through early next week. Reduce rates, and/or do not include oil or other surfactants if overthinning is a concern.
Pay close attention to the weather forecasts. If actual temperatures are closer to 90F and/or there are prolonged periods of cloud cover, then overthinning will likely occur.
The greatest amount of thinning occurs when fruitlets are on average between 8-12 mm in diameter. Thinning when fruitlets are slightly smaller or larger will result in less thinning, which may be desirable if there is a severe carbohydrate deficit.
Cooler temperatures forecasted for the middle to end of next week should result in less carbohydrate stress and reduced chances of overthinning. However, fruitlets that are greater than 15 mm in diameter are more difficult to thin with NAA or 6-BA.
During these warm days, expect the average fruitlet size to increase by at least 0.5 mm per day.
Warmer temperatures and cloud cover from some rainstorms later in the week will cause a moderately stronger carbohydrate deficit for Central Virginia.
Thinning applications made Tuesday through Thursday will result in the strongest response this week. Long term weather forecasts predict days in the 80’s and nights in the 60’s for the next 10 days. Carbohydrate levels will likely remain in the 0 to -40 range during that period. To get the strongest response possible, make your applications when average fruit size is 8-12 mm in diameter.
Due to a technical glitch, I was unable to run the MaluSim model for Winchester. I hope to have the problem resolved Tuesday morning. I will post an update for Winchester as soon as possible.
Moderate temperatures and adequate sunlight are resulting in slight carbohydrate deficits according to the MaluSim model.
This means that chemical thinners applied today through the first part of next week should cause an “average” response. Warmer temperatures are forecast for the middle of next week, which should result in greater carbohydrate deficits. I will run another simulation for Central Virginia, as well as the first Winchester run on Monday.
Today, I ran the first Central Virginia MaluSim model for 2015. I will start the Winchester models later this week.
For Central Virginia, I use data from a weather station set up at Silver Creek Orchards and managed by my colleague, Dr. Mizuho Nita. A big thank you to Dr. Nita for allowing me to install the necessary instrumentation and for maintaining the station and associated software.
This year, I will be using forecast data from the National Weather Service. In past years, I used Intellicast.com data because they provided 10-day forecasts with cloud cover predictions. Intellicast no longer provides the cloud cover data in an easy to use format, so I am switching to the National Weather Service’s forecasts. Note that these forecasts only project five days into the future.
We will discuss the details of the model at Tuesday’s meeting at Saunders Brother.
However, this first model run suggests that the relatively cool weather, and mostly sunny weather that has occurred since bud break has resulted in a moderate carbohydrate deficit in apple trees. Growers can expect average results from chemical thinners applied the last couple-few days through to the weekend. In blocks where significant thinning is needed, growers should apply carbaryl at petal fall and then look to the warmer weather that is predicted for the weekend and early next week for their 10 mm application.
Central Virginia weather data, MaluSim data, and an interpretation chart are in the pdf linked to below:
I ran a MaluSim model using data from the weather station located at the Alson H. Smith, Jr. AREC in Winchester. This will be the last model run for the year.
No forecast data was used in this run. Based on the output, I would expect chemical thinners that were applied May 14-16 to have an average response, while thinners that were applied on and after May 17 to have minimal effect. The effectiveness of thinners is greatly influenced by fruit size, so if the model showed a carbohydrate status that was greater than 20 g per day, and fruitlets were greater than 15 mm at the time of application then chemical thinners may not have been particularly effective.
Growers should compare the results of their chemical thinning program with the modeled carbohydrate status to determine if the model accurately predicted the level of thinning you expected from your applications.
I would appreciate hearing from growers about how they used the model to make chemical thinning decisions. Both positive and negative feedback is appreciated. You can use the comment feature in the blog site, or just send me an email.
On Thursday, I ran the MaluSim model for Winchester.
As I suggested in my last post, the strongly negative carbohydrate balance forecasted for May 20 and 21 were a product of poor cloud cover forecasting. In fact, since May 16 the carbohydrate status has been in the positive range. Chemical thinning applications made from May 17 through the next several days will likely have an moderate to poor response. Now that fruitlets are 15 to 20 mm in diameter, NAA and 6-BA will have minimal thinning efficacy. Growers who need to thin additional fruit from their trees should consider ethephon applications.
Table 1. Fruitlet sizes at the Alson H. Smith, Jr. AREC in Winchester, VA.
As I did for the Central Virginia models, I will run one more season recap model early next week.