Acta Hortic. 1228. ISHS 2018. DOI 10.17660/ActaHortic.2018.1228.11 Proc. XI Int. Symp. on Integrating Canopy, Rootstock and Environmental Physiology in Orchard Systems Ed.: L. Corelli Grappadelli

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Shade netting reduces sunburn damage and soil moisture depletion in ‘Granny Smith’ apples L. Kalcsits1,2, L. Asteggiano1,2, T. Schmidt3, S. Musacchi1,2, S. Serra1,2, D.R. Layne2 andG.Mupambi11TreeFruitResearchandExtensionCenter,WashingtonStateUniversity,Wenatchee,WA,USA; 2DepartmentofHorticulture, Washington State University, Pullman, WA, USA; 3Washington Tree Fruit Research Commission,Wenatchee,WA,USA.Abstract

The benefits associatedwith the use of protective netting for apple orchardshavebeenassessed in severalgrowing regions,butno studiesexist forWashingtonState,USA.Thisregionexperiencessemi-aridconditionscharacterizedby longdays,highlightintensityandhightemperatureswhichincreasestheriskofplantstressandfruitsunburndamage.Theaimofthisstudywastoassesstheeffectofanti-hailnetson orchardmicroclimate, soilmoisture and fruit sunburn. Seven-year-old ‘GrannySmith’/‘M9-T337’ apple trees were covered in 2014 with 20% white shade netshelteringarectangulararea.Uncovered treesservedascontrol.Shadenetreducedthe maximum daily photosynthetically active radiation (PAR) by 32%, with amaximumPARvalueonasunnydayof1649and1222μmolm-2s-1inthecontrolandnettedareas,respectively.Canopyairtemperaturewasslightly lowerduringthedayandhigheratnightunder thenetcompared to thecontrol.Shadenetsreducedsoilmoisture depletion at 0-30 cm depth, resulting 20% higher than the control. Nosignificantdifferenceswereobserveddeeper inthesoilprofile.Nettingsignificantlydecreased fruit sunburn,with 92% of harvested fruit having no sunburn damagecompared to control in which 51% had no sunburn damage in 2014. Althoughpreliminary, these results show that shade netting has the potential to reducesunburn damage in ‘Granny Simith’ apples and water requirements in semi-aridenvironments andmight represent an important tool for increasing yields of highqualityfruit.

INTRODUCTIONInWashingtonState,eastoftheCascadeMountainrangewherealargeportionofthefresh apples are grown in the United States, environmental conditions in the summerincludelonghotdays,minimalprecipitationandhighlightintensities.Asaresult,significanttreestressandahighincidenceoffruitphysiologicaldisorderssuchassunburndamagecanoccur.OtherapplegrowingregionssuchasAustralia,SouthAfricaandChilealsoexperiencesimilar, high-light conditions that can cause sunburn. In Australia,McCaskill et al. (2016)reportedthatupto10%oftotal fruitcanbelostduetosunburndamageforcertainapplecultivars. Under South African growing conditions, producers estimate losses due tosunburndamageat10-20%,butinsomeyearsashighas30-50%ofthetotalyield(Wandetal., 2006).Racsko andSchrader (2012) estimated that sunburndamage costsWashingtongrowersover$100millionperyear.Growers primarily use overhead cooling or protective sprays to reduce the fruitsurfacetemperatureduringthehottesthoursofthedaytolimittheoccurrenceofsunburn(Racsko and Schrader, 2012). Overhead cooling usually consists of the application ofirrigationwaterabove theorchard for short, intermittentperiodsand toallow theenergyabsorption of water evaporation to cool the fruit (Gindaba and Wand, 2005). Whileprotective sprays such as reflectance particles or protective waxes have shownimprovements in sunburn prevention, they are often inadequate in completely limitinglossesfromsunburn(GlennandPuterka,2004).Nettinghasbeenshowntoreducesunburninapples(Shahaketal.,2004).Unlikeredandbi-colorapples, thedevelopmentof anyover coloror “blush” ingreenapple cultivars

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like‘GrannySmith’:redcolordevelopmentcanactuallyreducethegradeofthefruit.Nettinghasbeenshownasaviableoptionforreducingsunburnin‘GrannySmith’orchardsgrowinginother climates (DayiogluandHepaksoy,2016).Here,wesought to identifyhownettingmodifies the orchard environment including soil conditions, as well as of its effects onsunburnincidenceandblushdevelopmentof‘GrannySmith’appleinadesertenvironment.MATERIALSANDMETHODS

OrchardsiteNetting was installed in 2014 at Washington State University’s Sunrise researchorchard(RockIsland,WA) inablockof7-year-old ‘GrannySmith’appletreesonM9T337rootstock planted at 0.9m in-row spacing and 3.6m between rows and trained to a tallspindlesystem.Whitenetting(ExtendayUSA,UnionGap,WA)was installed tocompletelycoverthetopandsides(totheground)foreachpod;4podsinreplicatedfashionthroughouttheblock.Podscoveredarectangularareaof7treerowswith12treesperrow(18×11×5.2m high). Nettingwas deployed after petal fall.Within the same block, uncovered trees ofequal plot size served as control. The nettingwas a non-transparent polyethylene plasticwovennettinginahexagonalweavedesignthatreducedlightby~30%.EnvironmentalmonitoringWater was supplied to trees using a combination of daily micro-sprinkler and dripirrigationapplications.Monitoringlocationswithineachreplicatewereselectedbasedonapriori evaluation of irrigation patterns within the orchard. Water collection trays wereplaced at several locations within the orchard and environmental sampling spots werechosen based on uniformity of water application within each replicate. Soil moisturemonitoring tubes(DeltaT-DevicesLtd.,Cambridge,UK)were installed to60-cmdepthsatsamplingspotswithineachreplicate.Soilmoisturewasmeasuredperiodicallyat10,20,30,40 and 60 cmdepth throughout July andAugust, 2014 using a handheld soil capacitanceprobe(PR2,Delta-TDevicesLtd.).The netting effects on canopy conditions were determined by recording airtemperature1.5mabovethegroundat10-minintervalsfromAugust1to31,2014(HoboProv2,OnsetComputerCorp.)withoutradiationshields.Photosyntheticallyactiveradiation(PAR)wasrecordedhourly(6amto4pm)ontwosunnydays(August21and27,2014)byplacingahandheldPARsensoranddatalogger(QSO-S,ApogeeInstrumentsInc.)3mabovetheground,butstillwithinthetreecanopy.PARlightintensitywasrecorded(μmolm-2s-1)andrepeatedforeachofthefournettedandcontrolreplicates.SunburnevaluationImmediately prior to harvest, sunburn damage was assessed visually on 200 fruitsthatweretakenfromsun-exposedregionsofthetreeatrandomfrom40non-bordertreesper replicate. Fruit was scored for sunburn severity using a six ranking sunburn scaleadaptedfromSchraderetal.(2003)andalsoevaluatedforthedevelopmentofredblushthatleadstocommercialdowngradingof‘GrannySmith’apples(Figure1).

Figure1. Sunburn scale for visual assessment of damage (Schrader et al., 2003).Originalscalewas for ‘Fuji’ apple. Scalewasmodified for ‘GrannySmith’ by I.Hanrahanand M. Mendoza (WA Tree Fruit Research Commission, pers. commun.). Cleanfruit indicates no sunburn while Y1 to Y3 indicate increasingly severediscoloration. Tan represents epidermal damage and Black indicates cell deathandnecrosis.

Clean Y1 Y2 Y3 Tan Black Blush

 

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RESULTSANDDISCUSSION

Nettingreducedlightintensity,in-canopyairtemperatureandwater-usein‘GrannySmith’appleThe shade netting installed after bloom in 2014 reduced light intensity byapproximately 32%,with themaximum value on a sunny day being 1649 μmolm-2 s-1 innon-covered control compared to 1222 μmol m-2 s-1 under the netting (Figure 2). Thematerial used in this netting was non-transparent, resulting in lower scattering of lightcomparedtoothertypesofnettingusedinotherstudies(Shahaketal.,2004;Bastıasetal.,2012).Therefore,atlowsunangles,theshadingeffectwouldbestrongercomparedtohighsunangles intheorchard.Forexample, theshadingeffectonAugust21and27,2014waspronouncedat09:00wheretheaveragelightintensityofthecontrolexceeded800μmolm-2s-1whilethenettedtreatmentaveraged100μmolm-2s-1.Earlierintheseason,inMay,June,andJuly,sunrisewouldoccurearlierandsunangleswouldbegreater.Therefore,theimpactofthesunangleontheshadingeffectinmid-morningwouldbereduced.

Figure2. Photosynthetically active radiation (PAR, μmolm-2 s-1) measured on August 21and27,2014,from06:00to16:00undernettingandanuncoveredcontrol.Air temperature was significantly lower under the net compared to the uncoveredportion of the orchard (30.9±3.2°C vs. 31.9±3.3°C, respectively) (Figure 3). Reporteddifferences in air temperature between netted and non-netted orchards have beeninconsistent. Middleton and McWaters (2002) and McCaskill et al. (2016) reported nodifferences incanopytemperaturebetweennettedandnon-nettedtrees.However, Iglesiasand Alegre (2006) reported decreased temperatures under netting compared to anuncoveredcontrol.Studiesshowingnochangesinairtemperaturewereassociatedwiththeuse of a radiation shield covering the temperature sensor. The sensors used here did nothaveradiationshieldsandweresimilartothoseusedinIglesiasandAlegre(2006).Soil moisture was measured at 10, 20, 30, 40, and 60 cm depth. The orchard wasirrigated daily and therefore, significant daily variation in soil moisture content was notobserved (Figure 4). Shade net significantly reduced soilmoisture depletion at 10-30 cmdepth,resultinginsoilwatercontent20%greaterthannon-coveredcontrol(Figure5).Nosignificantdifferenceswereobservedat40and60cmdepth.Theimpactofairenvironmentand overall water-use was less at greater soil depths. Soil moisture was less variable atgreaterdepthscomparedtomoreshallowlocationsinthesoilprofile.

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Figure3. Maximum(circles)andminimum(squares)canopyairtemperature(°C)intreescovered by netting (open symbols) compared to an uncovered control (closedsymbols)fromAugust1-12,2014.

Figure4. Soilvolumetricwatercontent(%vol/vol±SE,n=4)at20cmdepthundernetandanuncoveredcontrol.JulyandAugust,2014.

Figure5. Meansoilvolumetricwatercontent(%vol/vol±SE,n=4)undernettingcomparedtoanuncoveredcontrolat10,20,30,40and60cmdepthmeasuredweeklyatmid-day.July17-September24,2014.

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NettingsignificantlyreducedfruitsunburnincidenceandseverityTheincidenceofsunburndecreasedfrom49%forfruitharvestedfromtheuncoveredcontrolto8%forfruitfromthenettedtreatments(Figure6).Theseverityofsunburnwasalsomarkedlyhigherinfruitfromthecontrolcomparedtothenettedtrees.Morethan30%ofappleswereclassifiedasYellow2 (Figure7)orworse in thecontrol,which indicatesaprobable decrease in commercial fruit quality, in contrast to only 2% under netting.Interestingly, the west side of the tree consistently had more severe sunburn. Similarly,Racsko et al. (2005) reported higher fruit surface temperature and sunburn on theWestsides of apple trees. This is in contrast to Fouche et al. (2010) where there were nodifferences in sunburn incidencebetween thewestandeast sideofnorth-south rows.Airtemperatures generally peaked between 2:30 and 4:00 pm. This is also the period of daywith the lowest relative humidity and leafwater potential,which bothmay contribute toincreasedfruitsurfacetemperatureandsunburn.Withlightexposurebeinggreateronthewestsideofnorth-southrowsduringthisperiodofmaximumplantstress,theincidenceofsunburnshouldbegreateronthisexposureaswell.

Figure6. Thepercentageof fruitbelonging to thesixsunburnclassesandwithredblushfor‘GrannySmith’appleharvestedfromsun-exposedregionsundernetcomparedtouncoveredcontrol.

Figure7. Percentageof fruit belonging to seven sunburn classes for ‘GrannySmith’ appleharvested from either the east orwest sides of sun-exposed regions under netcomparedtoanuncoveredcontrol.

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CONCLUSIONShade netting can alter the orchard microclimate and reduce stressful conditionscaused by excessive light and heat load. Shade nets reduced sunburn damage and soilmoisture depletion for ‘Granny Smith’ apple in the semi-arid environment of CentralWashington. Overhead netting represents an important tool for decreasing stress-relatedlossesofmarketablefruitfor‘GrannySmith’apple.ACKNOWLEDGEMENTSAppreciationisextendedtoExtendayUSA(UnionGap,WA)forprovidingthenettingand infrastructure for this experiment. We thank the Washington Tree Fruit ResearchCommission staff for help in field data collection and the WSU farm staff for orchardmaintenance.LiteraturecitedBastıas,R.M.,Manfrini,L.,andCorelliGrappadelli,L.(2012).Exploringthepotentialuseofphoto-selectivenetsforfruitgrowthregulationinapple.Chil.J.Agric.Res.72(2),224–231https://doi.org/10.4067/S0718-58392012000200010.Dayioglu,A., andHepaksoy, S. (2016). Effects of shadingnets on sunburn andqualityof ‘Granny Smith’ applefruits.ActaHortic.1139,523–528https://doi.org/10.17660/ActaHortic.2016.1139.90.Fouche, J.R., Roberts, S.C., Midgley, S.J., and Steyn, W.J. (2010). Peel color and blemishes in ‘Granny Smith’applesapplesinrelationtocanopylightenvironment.HortScience45(6),899–905.Gindaba,J.,andWand,S.J.(2005).Comparativeeffectsofevaporativecooling,kaolinparticlefilm,andshadenetonsunburnandfruitqualityinapples.HortScience40(3),592–596.Glenn, D.M., and Puterka, G. (2004). Particle film technology: an overview of history, concepts and impact inhorticulture.ActaHortic.636,509–511https://doi.org/10.17660/ActaHortic.2004.636.63.Iglesias, I.,andAlegre,S. (2006).Theeffectofanti-hailnetson fruitprotection,radiation,temperature,qualityandprofitabilityof‘MondialGala’apples.J.Appl.Hortic.8,91–100.McCaskill,M.R.,McClymont,L.,Goodwin,I.,Green,S.,andPartington,D.L.(2016).Howhailnettingreducesapplefruit surface temperature: a microclimate and modelling study. Agric. For. Meteorol. 226-227, 148–160https://doi.org/10.1016/j.agrformet.2016.05.017.Middleton,S.,andMcWaters,A.(2002).HailnettingofappleorchardsAustralianexperience.CompactFruitTree35(2),51–55.Racsko, J.,andSchrader,L.E.(2012).Sunburnofapple fruit:historicalbackground,recentadvancesandfutureperspectives.Crit.Rev.PlantSci.31(6),455–504https://doi.org/10.1080/07352689.2012.696453.Racsko,J.,Nagy,J.,Szabo,Z.,Major,M.,andNyeki,J.(2005).Theimpactoflocation,rowdirection,plantdensityandrootstockonthesunburndamageofapplecultivars.Int.J.Hortic.Sci.11(1),19–30.Schrader,L.,Zhang,J.,andSun,J.(2003).Environmentalstressesthatcausesunburnofapple.ActaHortic.618,397–405https://doi.org/10.17660/ActaHortic.2003.618.47.Shahak,Y.,Gussakovsky,E.E.,Cohen,Y.,Lurie,S.,Stern,R.,Kfir,S.,Naor,A.,Atzmon,I.,Doron,I.,andGreenblat-Avron, Y. (2004). ColorNets: a new approach for light manipulation in fruit trees. Acta Hortic. 636, 609–616https://doi.org/10.17660/ActaHortic.2004.636.76.Wand, S.J.E., Theron, K.I., Ackerman, J., andMarais, S.J.S. (2006). Harvest and post-harvest apple fruit qualityfollowingapplicationsofkaolinparticle filminSouthAfricanorchards.Sci.Hortic. (Amsterdam)107 (3),271–276https://doi.org/10.1016/j.scienta.2005.11.002.