Issue link: https://read.dmtmag.com/i/473131
30 MARCH 15, 2015 GOOD FRUIT GROWER www.goodfruit.com A carbohydrate model for thinning apples is catching on rapidly in eastern apple growing areas. "This model is fantastic. It is driving people to the NEWA network as never before," said Dr. Juliet Carroll, who is in charge of NEWA, the Network for Environment and Weather Applications, at Cornell University, New York. For years, NEWA has offered apple growers access to forecast models for three diseases, six insects, and irri- gation scheduling. But the newest one—the one Carroll says is driving growers to the network as never before—is the carbohydrate model for apple thinning. This carbohydrate model, which was developed from research by Cornell's Dr. Alan Lakso and field ver- ification by Dr. Terence Robinson, predicts how sen- sitive the tree may be to thinners, based on recent weather and the forecast. The researchers found that apple thinning responses were stronger during periods of carbo- hydrate deficit, for example on hot, cloudy days. During those times, apples are easier to thin because the plant is having difficulty providing an adequate supply of carbohydrate to support all the rapidly growing young fruits. Carbohydrate deficit can come from periods of high carbohydrate demand (due to warm tempera- tures and a lot of fruit) or low carbohydrate supply, and these in turn are caused primarily by weather. Both Lakso and Carroll spoke to growers at the Mid-Atlantic Fruit and Vegetable Convention in Hershey, Pennsylvania, in January. NEWA started at Cornell in 1995 and has since spread to 291 stations in seven states. Growers in Minnesota want to connect 12 stations there to the network. Growers in Michigan use the carbohydrate model as well, using data from the state's Enviro-weather network of some 70 stations across Michigan to work the model. But Enviro-weather is not formally part of the NEWA network. Michigan State University exten- sion fruit educator Phil Schwallier works the model for Michigan growers. Access to NEWA Growers anywhere can access the NEWA website (newa.cornell.edu). They can find a nearby weather station where weather would be about the same as on their farm, and then use the models. But more and more, growers are finding it to their benefit to put in their own stations. And since this data then is shared, the more stations there are, the better the data becomes. Carroll tells growers to buy only RainWise AgroMET and IP100 Weather Stations, which cost $1,890. The stations come with software, a solar panel for power, an Ethernet interface that transmits data by line-of-sight transmission, and eight integrated sensors. The sensors monitor temperature, relative humidity, rainfall, leaf wetness, solar radiation, wind speed and direction, and barometric pressure. Data are sent every 15 minutes to NEWA's server. Carroll said weather stations need to be cali- brated every two years, and that it is not something that can be done in the orchard. "You have to send them back to RainWise," she said. "You can't do it in the field." How models work Models can be developed after scientists figure out what makes things tick. The development of many insects is driven by temperature, Carroll said, so models for insects like codling moth, oriental fruit moth, obliquebanded leafroller, plum curculio, and apple maggot can be built from knowledge of when eggs are laid relative to accumulated growing-degree days, for example. Calculations are based on a biofix date, which may be determined by first trap catches or by plant growth stage, such as petal fall, or whatever is a good base point from which to monitor the activity of an insect or disease. Diseases like apple scab, sooty blotch, and fly- speck develop according to temperature and leaf wetness, and fire blight organisms also multiply according to temperature and moisture conditions. The biofix date for fire blight, for example, is the date of first flower opening. Carbohydrate model In the case of the carbohydrate model, Lakso said, the basic biofix date is green tip, the date of bud- break, which is the starting of canopy development. It takes leaves and photosynthesis to provide carbo- hydrate to the plant for root, shoot, and fruit growth. "Once that date is established, the only required inputs are daily maximum and minimum tempera- tures and daily radiation," Lakso said. "Basically, sunlight drives carbohydrate supply, and temperature drives carbohydrate demand," he said. Ideal conditions for thinning are when weather is warm, driving demand for carbohydrate, but radia- tion is low, providing little new carbohydrate supply. Warm nights create demand but no supply at all. Other factors that make thinning easier include heavy bloom and heavy crop, humid conditions, weak spurs, and vigorous shoot growth, Lakso said. It is hard to thin when weather is cool (lower demand) but sunny (high supply), he said. Light bloom, strong spurs, and slow shoot growth make thinning harder. The model works pretty well, but a key tree response complicates the situation. "Weather con- ditions in the three days after the thinner is applied are more important than conditions before the thin- ner is applied," he said. "So we need to use weather forecast data to estimate what the post-treatment period will be like for carbohydrates. That can be a real limitation no matter how accurate the model is. Forecasting radiation is the hardest." The basic decision the model offers is not whether to thin but how much to adjust the thinner concen- tration, compared to what a grower would normally use in a thinning program. There are seven possible results of the model on any given day, depending on the carbohydrate A model for THINNING the 12-mm to 15-mm stage. If the predicted fruit set is still too high, then follow up thinning can be done again. This process allows for precision and will usually fine- tune the thinning process to a near perfect thinning job. However, there will always the need to hand thin small, defective, and damaged fruitlets. Hopefully this becomes a minor activity. Growers using PCM A few years ago, PCM was introduced to Michigan apple growers with the express purpose of increas- ing annual thinning success. At the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan, last December, three apple growers reported their 2014 experiences with the "Michigan Precision Cropload Management Project." The growers are Kim Kropf, who farms with her hus- band, Chris, at Kropf Orchards near Lowell; Jill Blattner, who farms with her husband, Tony, at Twin Bee Orchards near Lowell; and Bridget Engelsma, who farms with her father, Jim, at Engelsma Apple Barn near Walker. "I do the hiring for hand thinning," said Jill Blattner. "So I knew that cutting that down would be wonderful. We have never been able to thin our Gala block down to a manageable cropload—we were always too heavy—but this year, using this PCM approach, we, for the first time, successfully thinned the Galas. We had great Gala size." They dormant pruned their Gala block only to discover they were only down to a 2x budload level and had to dormant prune their block for a second time to get down closer to 1.5x budload. Jill and her daughter Bethany worked as a team—Jill measuring fruitlets and Bethany recording the data. "We used a laptop, and that saved a lot of time. If the same person does all the measuring, it works better," Jill said. Kim Kropf and her daughter Emily teamed up, too, with Kim marking and measuring fruitlets while Emily recorded the data for later entry. The Kropfs used PCM on three blocks of Gala, Jonagold, and Fuji. "It took time to set up and make the first measurements—about three hours per block to set up the clusters and two hours each time to measure," Kim said. "But the confidence we developed in predicting the cropload was more than worth it." "With so much data to enter, I soon retrieved my lap- top and in the orchard, directly entered measurements into the spreadsheet model," she said. Growers need to hire two people to measure fruitlets to make the job work smoothly. "Growers do not have time to stay with the protocol, they have too many things on their mind, too many jobs to do, and get too many interruptions to get the measurements all done on time," she said. The Engelsma PCM objectives are to obtain consistent high quality packouts and reduce the human stress of thinning by having more thinning options. They've been using PCM for three years and have fine-tuned some of the recommendations for their situation, Engelsma said. "We reduced the number of flower clusters marked and measured in each block," said Bridget Engelsma, who teamed up with her sister Nicole in the work. "This project has trained me to see fruitset even before I measure the fruitlets. It has given me confidence Carbohydrate model and good weather information support grower decisions for apple thinning. by Richard Lehnert COURTESY PHIL SCHWALLIER Jill Blattner and her daughter Bethany work as a team, measuring apples and recording the data.