Issue link: http://read.dmtmag.com/i/467731
w w w. v w m m e d i a . c o m M a r - A p r 2 015 | V I N E YA R D & W I N E RY M A N A G E M E N T 6 5 from any one individual farm. Nev- ertheless, recent indications from California's Air Resources Board (ARB) are that the issue is not going away for California agriculture. Ludwig said, "In the latest ARB scoping plan (issued in 2014) which lays out not only how ARB plans to meet the 2020 deadline within AB32, but also measures to meet the 80% GHG reduction by 2050, ARB does state it expects to direct- ly regulate GHG emissions from agriculture. When asked how, they have no clear answer, as they know it is extremely difficult to do so." SOIL MOISTURE AT BUD BREAK Keller of Washington State pre- sented new findings that demon- strate a strong link between soil moisture near bud break and spring growth, including flower and fruit formation. "Based on our studies to date, we advise growers to measure soil moisture before bud break and to irrigate if the moisture level is 3% below field capacity or lower," Keller said. "Alternatively, irriga- tion could be initiated if sap bleed- ing does not start despite warming moisture is so low that if you don't irrigate, your vines may not recov- er) for their soil. In studies from 2011 to pres- ent, WSU scientists found that sap flow started almost immediately (within one to three days) in vines that were irrigated to FC. This was followed by rapid and uniform bud break and shoot growth. Shoot growth and sap bleeding rate were strongly correlated with soil moisture. Shoot vigor (elonga- tion and leaf expansion) was great- est when soil moisture levels were at FC, to 2% to 3% less than FC. When soil moisture declined fur- ther, vine vigor declined and fruit set also declined. Overall, vines in soil that was near PWP had very poor growth, or neared death; however, vines at 1% above the PWP survived, although inflorescence abortion occurred. Preliminary tests showed that if vines initially maintained at PWP were watered back to FC before the buds died, they would resume normal bud break and shoot development. In 2014, Keller and Gohil con- d u c t e d s t u d i e s t o a n s w e r t h e follow-up question: How soon to irrigate drought-stressed vines once soil and air temperature are conducive to bud break, to prevent long-term detrimental effects? If irrigation was delayed by two weeks, vines were able to catch up in terms of shoot growth compared to those with no delay. Fruit set was slightly affected, though, with higher variability. Delaying further would only worsen crop yield. The best way to ensure normal growth and yield was to irrigate within a week of bud break, if not sooner. Janet White is a science writer and editor specializing in agricultural, environmental and nutritional sci- ences. From 1991-2013 she was the executive editor of University of California's California Agricul- ture journal. She left UC in 2013 to pursue science writing and further training in information science. Comments? Please e-mail us at feedback@vwmmedia.com. soil temperatures (when they rise above 45°F) or if bud break seems erratic despite favorable air temper- atures (above 50°F)." Hemant Gohil, technology trans- fer specialist working with Keller, explained further, "Now we know that dry soil before and during bud break can lead to uneven bud break, poor canopy development and fruit set, and reduced yield. If the grower can irrigate the vineyard just prior to or during bud break, especially after a winter drought, vines can recover and catch up in terms of shoot growth and produce normal yield." After a historic drought that had entered its third year this winter, California received a bump in pre- cipitation in December. Whether this increase changes root dynam- ics in the season to come will depend on how much rain collects in the root zone before bud break, Keller said. G o h i l a d v i s e d t h a t g r o w e r s assess their soil profile, including the field capacity (FC, when soil moisture is so great that addition- al water has to be drained or lost through surface runoff) and perma- nent wilting point (PWP, when soil According to research by Markus Keller of Washington State University, vines can't initiate sap flow after a dry winter, because the soil is too dry for root pressure to work properly. This may lead to delayed bud break, stunted shoot growth, aborted clusters and poor fruit set.