Tianyou (Hope) Xu and Yun Yin, Department of Food Science and Technology, Virginia Tech, Blacksburg; Jayesh Samtani and Patricia Richardson, Hampton Roads Agricultural Research and Extension Center, Virginia Beach; and Amanda McWhirt, Department of Horticulture, University of Arkansas

Blackberry (Rubus spp.) is a popular fruit due to its delightful taste and notable health benefits.  The objective of this study was to determine the effectiveness of pre-harvest foliar treatments and shade application on yield, white drupelet disorder, post-harvest attributes and aroma profile of two blackberry varieties Prime-Ark® Traveler and Prime-Ark® Freedom.

Virginia study

In Virginia Beach, a field study was repeated in 2021 and 2022 growing seasons at the Hampton Roads Agricultural Research and Extension Center in a completely randomized design. Grower standard control (GSC), shade cloth with 30% light reduction (SHA), calcium (CAL) and salicylic acid (SAL) foliar applications were randomly assigned to each variety (Photo 1). The grower standard control plants and plants in all other treatments were given ~80 to 100 lb N/acre over spring, summer, and fall each year by alternating fertigation with water-soluble Plantex 20-20-20 (Master Plant-Prod Inc., Brampton, ON), Multi-K® potassium nitrate (Haifa Chemicals, Haifa Bay, Israel) and Calcinit® calcium nitrate (Yara North America, Inc., Tampa, FL) using the Dosatron® drip fertilizer injector (Gempler’s, Janesville, WI) every two weeks. The foliar calcium spray (Nutri-Cal®; CSI Chemical Corporation, Bondurant, IA) was applied as a fine mist to ensure good overall coverage of the blackberry plants but without causing runoff. The SAL treatment is not registered for blackberry crop production and was only for research purposes. A 30% light reduction shade cloth was purchased from Greenhouse Megastore (West Sacramento, CA) and installed in late May to provide a reduction of direct solar radiation and heat stress for the plants.

Photo 1. Plants upfront in the photo include grower standard control plots to the left and shade-treated plants to the right.

Table 1. Application rates and dates for treatments applied in 2021 and 2022 growing seasons.

Treatment1Application Rate2021 Growing Season2022 Growing Season
CALFour applications at concentration of 2 fl. oz/gal6/15; 6/24; 7/1; 7/136/15; 6/25; 7/5; 7/14
SALTwo applications at concentration of 276 mg/L6/15; 7/136/15; 7/14
SHA30% light reductionInstalled on 6/2Installed on 6/15

1 CAL: Calcium; SHA: Shade cloth; SAL: Salicylic acid

Fruit yield and physicochemical attributes of the blackberry were collected and analyzed and aroma-active compounds in blackberries were identified by use of headspace-solid-phase microextraction-gas chromatography-mass spectrometry-olfaction (HS-SPME-GC-MS-O).

In Virginia, shade cloth significantly reduced the white drupelet disorder (WDD) in Prime-Ark® Freedom but it also reduced the Total soluble solid content (°Bx) and °Bx/%Titratable acidity (a ratio indicates sweet & sour balance for fruit) for both varieties. No significant improvement was found in the yield, °Bx, TA, and firmness of blackberries treated with CAL and SAL. Sixteen consistent aroma-active compounds were found across treatments for both varieties and growing seasons. Foliar and shade application did not alter the aroma profile of either blackberry variety. However, higher volatile contents were found in 2021 than in 2022, possibly due to climate variation. Clear distinction on aroma profiles of the above two varieties were also observed: PrimeArk® Freedom was higher in compounds possessing “fruity” and “floral” notes, while PrimeArk® Traveler featuring more “green” and “fresh” characteristics.

Arkansas study

During the 2021 growing season in Arkansas, CAL applications were made to floricane fruit on Prime Ark® Traveler and Osage varieties at a grower’s location in White County, AR  and compared to water applications (zone 7b). These applications were made using a pump sprayer and to ensure good coverage but not to drip. The application of foliar CAL was not found to impact any measured characteristic of fruit quality or post-harvest quality in Arkansas.


Regional berry growers should be more conservative when adopting foliar and shade applications due to potential seasonal variations surpassing the significance of agronomic treatments. There were some distinctions in aroma profiles for the two varieties evaluated in Virginia.

Funding acknowledgment. Virginia Department of Agriculture and Consumer Services, Specialty Crop Block Grant and Southern Region Small Fruit Consortium.