Good Fruit Grower

March 15

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18 MARCH 15, 2014 GOOD FRUIT GROWER Model helps to time thinning sprays W ashington apple growers have a new tool to help them add a little science to the art of chemical thinning. A computer model is available to help them apply chemical blossom thinners at the optimum time to achieve their target crop load. Historically, growers have applied lime sulfur and fi sh oil at 20 per- cent and 80 percent full bloom, with generally good results, said Tory Schmidt, research scientist with the Washington Tree Fruit Research Commission. "There's never been anything magical about those timings," he said. "We just felt it was important to get a chemical thinning spray on early in bloom and one late during bloom, and that's generally worked out okay for a lot of folks." The apple fertilization process begins after pollen is deposited on a fl ower stigma. Male gametes from the pollen are transported through a pollen tube that grows from the pollen grain down through the style to the ovules where the egg is fertilized. How long it takes for the pollen tube to grow down through the style—in other words, for the fruit to set—depends on the temperature. Scientists at Virginia Polytechnic Institute and State University have spent the past decade studying pollen tube growth and have developed a temperature-based, variety-specifi c model that calculates the time required to fertilize a blossom after pollination (see sidebar). The model was devel- oped specifi cally for Washington growers through controlled experi- ments in growth chambers at Virginia Tech and validation through beta testing in Washington State orchards. The Research Commission pro- vided funding. The model can be found at the Washington State Univer- sity AgWeatherNet Web site at Schmidt said the model should help reduce the risk of over- or under- thinning. It can give growers a better idea of how many fl owers have been fertilized at a certain time, so they don't have to go by gut feeling as they have in the past. The model has a forecasting component and can predict the amount of pollen tube growth within the next 72 hours, allowing growers to have their spray crews ready to go at the appropriate time. What it can't do is make up for any previous management mistakes relating to pruning or nutrition, Schmidt said. "It can only work with the situation that's out there now." If the weather happens to be wet or windy at the recom- mended spray timing, grow- ers will have to make their own adjustments. Models are available for four varieties: Gala, Fuji, Golden Deli- cious, and Cripps Pink. The proj- ect team hopes to add models for Honeycrisp, Red Delicious, and Granny Smith by next year. Schmidt said that for Gala, which is a naturally small apple, a grower's strategy might be to optimize fruit size by setting as many early king blooms as possible and thinning off later fl owers. Say the target yield is 60 bins per acre of size 88 apples. Assuming a packout of 24 boxes per bin, that's a target crop of 126,000 apples per acre. In a planting with 1,200 trees per acre, that's an average of 105 apples per tree. The grower would want to set the fi rst 105 fl owers per tree that open and no more. To track when 105 fl owers have opened, the grower will need to count the open fl owers on fi ve to ten representative trees during bloom and keep checking at intervals until the 105th fl ower opens. "As soon as the 105th fl ower opens, that's when you start the clock on the pollen tube growth model," Schmidt said. The grower also needs to calculate the average fl ower style length by sampling 25 to 50 fl owers from different trees throughout the block. The average style length is then used as a benchmark for fl ower fertilization in the model. Instructions on how to measure the style can be found, along with the model, on the AgWeatherNet Web site. Once the model indicates that the pollen tube has theoretically reached the base of the style, chemical thinners can be applied to pre- vent any further fruit from setting. The spray should be applied when the model shows that the style has reached 100 to 110 percent of the aver- age style length on the tree. Waiting until 110 percent allows a few more fl owers to be fertilized, if growers don't want to cut it too fi ne, Schmidt said. Once the spray is applied, the model resets to track the pollen tube growth of the next fl owers that open in order to prevent any further fer- tilization. Subsequent sprays should be applied well before the model reaches 100 percent of the average style length. Schmidt recommends applying thinners at 75 percent of the projected style length to account for fl owers that have shorter than average styles. After each spray, the model resets. Model limitations The model assumes that bee activity is optimal and the pollen is viable. Most of the data that drive the model are based on pollen from Snowdrift, Schmidt said, and there's some indication that other pollen sources, such as Manchurian crab or Golden Delicious might not behave the same way. It also assumes that the chemical thinners don't thin off anything that's been fertilized, which Schmidt said is not necessarily the case in real life. The model was designed to help with the timing of lime-sulfur and oil applications, and might not be relevant for bloom thinners with different modes of action. It will not recommend timings for postbloom thinning. Schmidt described the model during the North Central Washington Apple Day and encouraged growers to consider giving it a try this season. "I would not do it fence-to-fence across your whole operation but maybe try it out on a small block," he suggested. If growers are not comfortable making decisions with the model this year, they can do a dry run, but still apply chemicals at the usual timings. He encouraged growers to go to the AgWeatherNet site ahead of bloom to familiarize themselves with the model. "It's just a tool," he said. "It's one small step forward in this great black box of understanding crop management. It's not a silver bullet. It's not going to guarantee you a specifi c result, but, hopefully, it will make it a little more precise and a little more predictable for you." • A pollen-tube growth model takes some of the guesswork out of chemical thinning. by Geraldine Warner Ovary Sepal Ovule Style Stigma Anther Filament Pistil (female organ): Stigma; Style; Ovary Stamen (male organ): Anther; Filament Flower parts Crop Management

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