Good Fruit Grower

February 1

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GOOD TO KNOW A research report from Julie Tarara, Bernardo Chaves, Luis Sanchez, and Nick Dokoozlian Taking the guesswork out of yield estimating Trellis tension technology could improve accuracy of crop estimating in grapes. T rellis-tension technology developed by U.S. Department of Agriculture scientists to improve crop estimating in grapes is ready to be commercialized by private companies. Estimating the yield in a vineyard is an important, though often difficult task. Accurate yield projections help ensure that enough tank space and barrels will be available to process the fruit, as it must be processed immediately at harvest and cannot be stored for later handling. Estimating yields involves sampling cluster counts and bunch weights in vineyards, earch our archives online a labor-intensive task that must be completed during at a short window of time. The lag phase of berry growth for the previous articles, in grapes often is used as the trigger to start sampling "Real-time crop information" in the because that is the period of slowest berry growth. September 2006 issue and "Crop Lag phase occurs between the first period of rapid Monitoring" in the May 15, 2008, growth after flowering and the second period of rapid issue. growth during ripening. Yield estimates are likely to be more accurate when sampling is completed during the lag phase because it is when berry weight changes the least. The lag phase can be very short and the acreage to sample large, which makes yield estimation on a per-block basis a challenge. It can also be difficult to properly identify the lag phase in the field. Berries in the lag phase are on the cusp of softening, which is ambiguous. Soft berries with slight color change already have reached veraison and are in the second phase of rapid growth, so counting such berries could skew crop estimates. S Julie Tarara Trellis tension The trellis-tension technology, developed by USDA in collaboration with wine grape industry representatives, involves using an automated device (Figure 1) that continuously monitors and records tension changes in the cordon wire. Research to measure the viability of the technology to take some of the guesswork out of identifying the lag phase was conducted in two wine grape vineyards in California in 2007 to 2009. The device's electronic output provided an indirect indication of vine and fruit growth and in most cases showed the distinctive double-sigmoid growth curve (Figure 2). The solid lines in both panels of Figure 2 are the results of our modeling the processed tension output to make the shape of the curve clearer for a grower to identify lag phase from the data. We defined the onset of lag phase as the first shoulder that precedes the flatter part of the curve. The second shoulder occurs well into ripening and was not part of the study. Human vs. math calculation Two industry personnel examined the cordon wire tension curves visually to look for the slowing of growth in the curve and identify the shoulder. Our rationale was that if growers were to use changes in cordon wire tension to track crop growth and development, then the simplest and most straightforward way to use the information would be to look at the curves on a daily basis to find the shoulder in the growth rate. We compared the industry representatives' selected dates for the onset of lag phase with those identified through a calculus-based mathematical analysis of the wire tension curves. Data for 2008 were not used in the study because the research vineyards were hit by frost about a month after budbreak, which defoliated the vines, resulting in a secondary bud break and a compressed growth cycle. This made it almost impossible for the industry representatives to find a shoulder in the curve, as can be seen in Figure 2. (Continued on page 34) 32 FEBRUARY 1, 2014 GOOD FRUIT GROWER

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