Greetings

The following is an updated version of a factsheet my former M.Sc student (Mr. Chester Allen) and I published last year on the VCE website (https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/spes/spes-110/SPES-110.pdf). I updated it to reflect what we have learned from our research in 2018 and 2019, particularly as it relates to the concentrations of lime sulfur and oil that could achieve good thinning activity and fruit quality.

What is Chemical Apple Blossom Thinning?

• Chemical blossom thinning in apple orchards is the practice of spraying chemicals during the blossom time to thin the crop.
• Chemical blossom thinners are sprayed multiple times during the bloom period in order to reduce or prevent the fertilization of a portion of the blossoms that would cause undesired crop loads if left to set fruits.
• Chemical blossom thinning can be used as a supplemental and/or alternative practice to standard fruit thinning sprays.

What are the Benefits of Apple Blossom Thinning?

• Increased fruit size, optimized crop load, improved annual bearing, increased return bloom, and more predictable thinning results are all benefits of chemical blossom thinning in apples.
• Additionally, some blossom thinners can provide partial early season disease suppression and are registered for organic use.

What Chemicals & Rates can be used for Apple Blossom Thinning?

• Currently, the only products registered for use as apple blossom thinners in Virginia are Rex Lime Sulfur Solution (28% Calcium Polysulfide) and NovaSource Lime-Sulfur Solution (29% Calcium Polysulfide). Label rates for these products range from 4% to 12% of formulated product per volume of spray solution when spray oil is NOT used. When spray oil is used, label rates of these products range from 1% to 3% of formulated product per volume of spray solution.
• Mineral oils (dormant & summer) and fish oils may be combined with lime sulfur solutions and used as spray adjuvants to increase thinning effectiveness. Label rates for oil range from 0.5% to 2% depending on the type of oil used.
• Based on our research in 2018 and 2019, application rates of 3% lime sulfur and 2% JMS stylet oil causes severe russeting and reduces fruit packout significantly, whereas low rates (e.g. 1% lime sulfur and 1% JMS stylet oil) have no significant thinning effect. Blossom thinning using 1.5-2% lime sulfur and 2% oil showed good thinning results and less fruit russeting.
• If blossom thinning is to be followed by chemical fruit thinning (e.g. by 6-BA and NAA), we would suggest using 1.5% lime sulfur and 2% oil for blossom thinning. Our research also indicated that both forms of lime sulfur (Rex Lime Sulfur Solution and NovaSource Lime-Sulfur Solution) were equally effective.
• Blossom thinning using high rates of lime sulfur (3%) and oil (2%) can also be used for defruiting young apple trees.

When & How Frequently Should Blossom Thinners be Applied?

• Determining when to apply the first blossom thinning spray and how frequently to reapply blossom thinning sprays are the most important decisions to make when using blossom thinning sprays to thin and manage the crop load.
• The first blossom thinning spray should be applied once a sufficient number of blossoms have been fertilized to set the desired crop load.
• After the first blossom thinning spray, additional blossom thinning sprays will need to be re-applied approximately every 2-4 days, depending on temperature, to prevent the remaining blossoms from becoming fertilized and setting fruit. No more than three applications of lime sulfur solution can be applied for blossom thinning per year.
• Our research on Gala apples has indicated that applying blossom thinners at 20% open bloom, followed by another application 48h or 72h later, results in smaller crop loads and better fruit size compared to untreated (unthinned) trees.

What is the Pollen Tube Growth Model (PTGM)?

• The Pollen Tube Growth Model (PTGM) model, developed by researchers at Virginia Tech, is a useful tool for assisting in deciding when to first apply and re-apply blossom thinner sprays.
• The model collects weather data and uses style length (mm) to predict the time a pollen tube would take to reach an ovule and achieve fertilization. 
• Based on inputs provided by weather stations and users (e.g. cultivar, style length and the model start time), the model provides the day/time for the first thinning application and subsequent thinning sprays if required.  
• The cultivar-specific model has been generated and tested for seven apple varieties, including Red Delicious, Golden Delicious, Cripps Pink, Honeycrisp, Fuji, Gala and Granny Smith.
• The model is available on the NEWA webpage (https://ptgm.newa.cornell.edu/) which is sponsored and managed by Cornell University.
• To get more information about using the model for blossom thinning, you can follow the instructions listed in our previous blog post (https://blogs.ext.vt.edu/tree-fruit-horticulture/2019/04/10/).
• Our research on Gala apples indicated that applying thinning treatments at 20% bloom + 48h or 72h later, produces similar results to thinning applications guided by the PTGM.

Sherif M. Sherif, PhD, Assistant ProfessorVirginia Tech, School of Plant and Environmental Sciences,AHS Jr. Agricultural Research and Extension Center
595 Laurel Grove Road, Winchester, VA 22602
Te. 540-232-6035, Email. ssherif@vt.edu