Issue link: http://read.dmtmag.com/i/481870
www.goodfruit.com GOOD FRUIT GROWER APRIL 1, 2015 25 More extreme irrigation treatments were studied in the second trial: —100 percent ET from fruit set to harvest —60 to 70 percent ET from fruit set to harvest (industry standard) —25 percent ET from fruit set to harvest —25 percent ET from fruit set to verai- son, 100 percent ET from veraison to harvest Timing In both trial sets, irrigation effects were consistent over the three years, said Keller, who shared results during the annual meeting of the Washington Association of Wine Grape Growers. Vine growth and productivity were measured by collecting data on leaf layers at verai- son, pruning weight, cane weight, yield, cluster numbers per vine, and berry weight. Some of the research findings were expected, others more interesting. "Full-season deficit of 25 or 35 percent ET led to lower vigor and yield," he said. In both trials, yields of the full season 25 and 35 percent ET were half to a third (2.4 and 2.5 tons per acre, respectively) lower than the industry standard of 4.8 and 3.9 tons per acre, respectively. "Pre-veraison deficit of 25 or 35 per- cent and then watering 70 or 100 percent after veraison resulted in smaller ber- ries—what winemakers want—without significantly reducing yield," he said. Interestingly, researchers found no difference between applying 70 or 35 percent ET from veraison to harvest. "There was no effect from cutting the post-veraison water supply in half," said Keller, noting that reducing irrigation after veraison to reduce berry size was ineffective. "So if you think you can correct any overwatering done early in the season by applying deficit after veraison, it won't work. "Conversely, increasing irrigation after veraison following a pre-veraison deficit did not increase berry size. So the fear of many winemakers that late-season irrigation somehow dilutes quality is unjustified." Impacts on weed growth and cluster sun exposure are additional benefits of RDI. Keller observed significant reduced weed growth in the 25 percent ET treat- ment compared with 100 percent ET. Also, there were fewer shadows under the canopy in the deficit treatment, which indicates the canopy is more open and clusters more exposed to light. Light exposure is needed to bring out flavor qualities of fruit and eliminate vegetative or "green" flavors. But too much heat and solar radiation can sunburn berries and impair color accumulation. Keller used small devices called iButtons to monitor temperatures of sun-exposed and shaded clusters in the 25 and 100 percent ET treatments throughout the season. Temperatures of up to 120°F were recorded in sun- exposed clusters in the 25 percent ET before veraison. "There is such a thing as too much fruit exposure caused by having such a small canopy from the season-long deficit irrigation." Some stress is good and can achieve desired results, but more is not necessarily better. "The extreme deficit of 25 percent ET all season long is too much water stress and simply not enough water for Cabernet Sauvignon," Keller said. "The 25 percent ET decreased almost everything we measured. There was a loss of vine capacity, loss of productivity, and exces- sive fruit exposure. It's not sustainable for Cabernet." Keller was not surprised that vines did well in the 35/70 percent ET regime (35 percent ET fruit set to veraison/70 percent ET veraison to harvest). He was a little more surprised by how well they did with the 25/100 percent ET. "It seems that 35 percent ET is as low as you can go over the berry development period," he said. "Though 25 percent ET is too much water deficit if you apply it for the entire period, if you start out with low water amounts from fruit set to veraison and then increase water, you get all the benefits," he said. "You get small berries, an open can- opy, which helps with disease control and weed control, and good sun exposure, all while maintaining vine capacity with no decline in vine productivity." •