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WSU Tree Fruit

Reducing cold damage in apple and sweet cherry

Written by Matthew Whiting, WSU, February 2019

Every year, tree fruit growers lose money from cold damage to reproductive buds or flowers.  The U.S. Food and Agriculture Organization reported that cold damage has caused more economic losses to crops in the U.S. than any other weather hazard. The potential losses from cold damage are devastating, and predicted to become more commonplace with increasingly variable spring weather.  Despite the significant perennial threat of cold damage, growers have no new reliable means for protecting developing buds, depending largely on wind machines that are ineffective against advective freeze events.

Cellulose nanocrystals (CNC) represent a new generation of renewable nano-biomaterials with unique physical and chemical properties, and numerous potential applications. A particular benefit of CNC films is their low thermal conductivity (i.e., they provide excellent insulation). Our team with WSU’s Center for Precision and Automated Agricultural Systems (Changki Mo, Matt Whiting, Qin Zhang, and Xiao Zhang) hypothesized that a CNC dispersion could reduce cold damage to fruit buds when applied to trees.  We have recently synthesized a CNC dispersion that can be sprayed onto trees, forming a thin, uniform and insulating film on the surface of the buds. Here we report briefly on the preliminary but promising results in apple and sweet cherry.

Figure 1.  Sweet cherry flower buds exhibiting various degrees of cold damage – dark flower are dead while green tissue is alive.

Figure 2.  Cellulose nanocrystals at various concentrations.  Pure powder (left), a 5% CNC dispersion (middle) and a 12% dispersion (right).



In 2018 we conducted several trials evaluating the ability of CNC treatments to improve bud hardiness.  All treatments were made with a single-nozzle electrostatic sprayer, courtesy of On Target Spray Systems (Figure 3).  Pressure was provided from a portable air compressor.  For each experiment the CNC dispersion was prepared in Dr. X. Zhang’s lab in WSU Tricities campus.  The necessary volume (typically ca. 1 L) was loaded into a plastic reservoir secured above the sprayer.  Application volume was ca. 50 gal/acre and calibrated by collecting sprayer output for 30 sec intervals and determining the volume sprayed at a constant pressure (ca. 14 PSI).  Applications were made by holding the sprayer about 2 – 3 feet from the target trees and applying the treatment by moving the sprayer in a Z-pattern from the top to the bottom of each tree.

Figure 3. WSU graduate student Jassim Alhamid uses a single-nozzle electrostatic sprayer for CNC applications to apple trees. The black arrow indicates the ‘tank’ containing the CNC dispersion.  The white arrow indicates the nozzle.


In a trial in a mature ‘Scifresh’ apple block northeast of Prosser we compared two concentrations of CNC (1% and 2%) with untreated control. This trial was conducted on 18 April when trees were at about 20% full bloom (i.e., all king flowers in lower half of trees were open).  CNC treatments improved hardiness of ‘Scifresh’ apple flower buds (Figure 3).  CNC at 1% was marginally effective at improving hardiness, and it did not reduce the lethal temperature required to kill 10% of buds (LT10).  In fact, clusters treated with 1% CNC exhibited greater pistil death than untreated at 26.6 and 21.2 F.  In contrast, treatment with 2% CNC improved hardiness, reducing pistil death at every temperature.  LT10 was 30.2 for untreated flowers and 26.6 for flowers treated with 2% CNC (an improvement of 3.6 F).   The greatest improvement in flower hardiness was observed at 21.2 F, a temperature at which 80% pistil death was recorded for control, 90% for 1% CNC, and only 30% for 2% CNC (Figure 4).  The protective effect of 1% CNC was variable, and lost by ca. 21 F.  In contrast, the protective effect of 2% CNC was significant, and not lost until ca. 16 F.  Based on these results, untreated trees would have complete crop loss at about 19 – 20 F, and trees treated with 2% CNC would have ca. 50% crop remaining, not losing the entire crop until ca.16 F.



Figure 3.  Effect of field-applied dispersions of CNC at 1% and 2% on the incidence of pistil mortality in ‘Scifresh’ apples.  Treatments were applied 18 April and hardiness was assessed on randomly selected clusters 24 hr after treatment.





Figure 4.  Pistil death (% mortality) in ‘Skeena’ (left) and ‘Selah’ (right) sweet cherry trees treated with 2% CNC on 26 March, at ‘side green’ stage of bud development.  Assessments made 24 hr after field treatment.


In a sweet cherry trial on ‘Skeena’ and ‘Selah’, we similarly found significant improvements in flower hardiness with applications of 2% CNC (Figure 4).  Treatments were made on 26 March in a block at the WSU-Roza experimental orchard north of Prosser.  Average bud development was similar for ‘Skeena’ and ‘Selah’ at side green.  In ‘Skeena’, LT10 was 24.8 F for untreated and ca. 22 F for trees treated with 2% CNC, an improvement of about 3 F.  In ‘Selah’, LT 10 for untreated flowers was 26.6 F and ca. 22.3 F for trees treated with 2% CNC, again, an improvement of about 4.3 F.   In ’Skeena’ the greatest protective effect occurred near 23 F where untreated flowers exhibited 60% death and treated flowers were 100% viable.  Complete crop loss would have occurred in untreated trees at ca. 19.4 F whereas treated trees exhibited only 40% pistil mortality at this same temperature.  The degree of mortality for ‘Selah’ was similar, with 100% pistil mortality at 19.4 F – in contrast, buds from trees treated with 2% CNC exhibited only 50% pistil mortality at this temperature.


Matthew Whiting

WSU, Horticulture

24106 N Bunn Rd.
Prosser, WA 99350

Phone: 509.786.9260