Good Fruit Grower

April 1

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W ine grape growers in eastern Washington hold special power in their hands. By turning or not turning their irrigation valve, they can impose vine water stress to control vine vigor and berry growth. The challenge is to apply the right amount without negatively impacting fruit quality. In many wine regions, like Bordeaux, that receive more than 35 inches of rain annually, grapes are farmed without irrigation. But in eastern Washington, with only seven to nine inches of precipitation annually, irrigation is needed. Yun Zhang, postdoctoral research associate at Wash- ington State University, said irrigation involves knowing how much to apply and when, how to measure vine water and soil moisture status, and how to manage water stress without negatively affecting vine growth and fruit quality. "It's a challenge for eastern Washington wine grape grow- ers to understand all about irrigation." Monitor When to irrigate depends on the vines' water status and soil moisture. Growers should look for signs of stress in the vine to know when to irrigate, Zhang said during the annual meeting of the Washington Association of Wine Grape Growers. Check ten- drils, leaf orientation, and shoot tips to see if a vine is actively growing. Monitoring soil moisture is also important and can be done using neutron probes, capacitance sensors, tensiometers, and other tools. Weather conditions also infl uence irrigation. Evapotranspiration is the combined loss of water through soil evaporation and vine transpiration. Low ET values are associated with cloudy, cool, or humid days when the vine and soil are not losing lots of water; high ET is associated with warm, sunny, dry, and windy conditions and long days. Zhang noted that WSU's AgWeatherNet weather stations collect and publish ET values, but warns that using such numbers solely can be problematic because they are not site specifi c or specifi c to wine grapes. Vines receiving suffi cient water will produce large can- opies, but that's not always a good thing in wine grapes. Large canopies can result in too much shade in the fruit zone and reduce fruit set and cluster ripening. In areas with abundant soil moisture, growers often use hedging, shoot thinning, and leaf removal, to open up the canopy and improve light penetration. Because soils in eastern Washington are quite dry, growers are able to use defi cit irrigation techniques to their advantage to manipulate vine growth, Zhang said. How much and when "The fastest-growing tissues—fl owers, shoot tips, and pre-veraison berries—are the most sensitive to water stress," she said. "Because of that sensitivity, when you sub- ject the vine to mild to moderate water stress, you can control shoot growth and berry enlargement. But you have to be careful to avoid severe water stress." Most wine grape growers in Washington follow regulated defi cit irrigation, known as RDI, maintaining vine water status within a prescribed range of water stress for a defi ned part of the growing season. For red wine grape varieties, this means stressing the vines from fruit set to veraison. But she warns that the water stress needs to be removed after the defi ned period, and vines should then be irrigated to 100 percent ET. "If the soil is not fully refi lled after this period and the stress removed, the vines may be vulnerable to heat stress later in the season." Partial root zone drying is another defi cit irrigation method. PRD involves alternately irrigating different sides of the vine, spatially separating the root zone. Vine roots are exposed to dry and wet soil, with each side alternately irrigated usu- ally every 5 to 14 days, depending on the site and weather. The vine reacts to water stress on the dry side by slowing vegetative growth, but 18 APRIL 1, 2014 GOOD FRUIT GROWER www.goodfruit.com Grapes A season of wine grape stress 8MQMRKSJHI½GMXMVVMKEXMSRHITIRHWSRKVS[XLWXEKI (I½GMX Phenology Good Bad applications Tips &YHFVIEO 2SVQEPFPIIHMRKFYHFVIEO 2SSVPMXXPIFPIIHMRK 2S 'LIGOWSMPQSMWXYVIMJHV][MRXIV WXYRXIHWLSSXKVS[XL &PSSQXSJVYMXWIX +SSHJVYMXWIX 6IHYGIHJVYMXWIXMRGVIEWIHWLSX 2SXSQMPH 7IRWMXMZIWXEKI FIVVMIWRIGVSXMGMR¾SVIWGIRGI 4VIZIVEMWSR 'SRXVSPPIHWLSSXKVS[XL :MKSVSYWWLSSXERHPEXIVEP =IW[MXLGEYXMSR (IKVIISJWXVIWWHITIRHWSR KVS[XL ZMRI]EVHKSEP&IWXERHQSWXGVMXMGEP XMQIXSGSRXVSPFIVV]WM^I :IVEMWSRXS 2SPEXIVEPKVS[XL 0EXIVEPKVS[XL]IPPS[PIEZIW 1MPH 2SJIEVSJMVVMKEXMSR TVILEVZIWX LIEPXL]GERST] HIL]HVEXIHFIVVMIW EWPSRKEWRSRI[WLSSXKVS[XL 4SWXLEVZIWX 2SPEXIVEPKVS[XL 'SPPETWIHGERST] 2SXSQMPH 6I½PPWSMPQSMWXYVIJSV LIEPXL]GERST] SZIV[MRXIVMRK * WSU research has shown there is no dilution effect of berry quality from preharvest irrigation. SOURCE: Yun Zhang, WSU THE GOOD & BAD of defi cit irrigation Partial root zone drying defi cit irrigation has potential for white varieties. by Melissa Hansen "It's a challenge for eastern Washington wine grape growers to understand all about irrigation." —Yun Zhang

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