Good Fruit Grower

March 15

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42 MARCH 15, 2015 GOOD FRUIT GROWER www.goodfruit.com Take the guesswork OUT OF THINNING Pollination Tube nucleus Sperm Pollen grain Pollen tube Germinating pollen tube H orticulturists in Washington State say a new predictive model helps to take the guesswork out of chemical thinning of apples and could help reduce the need for postbloom thinning sprays. "We're using less petalfall and post-bloom sprays because we're get- ting enough off during bloom," Kevin Larson, production manager at Roche Fruit Company in Yakima, said during a discussion on the Pollen Tube Growth Model at Washington's winter hort meeting. Harold Schell, director of fi eld services for Chelan Fruit Cooperative, said he began testing the model three years ago. He wanted to know how to better apply multiple bloom sprays. He used it on Gala, Golden Delicious, and Fuji. His objectives were to ensure consistent return bloom, reduce the amount of hand thinning needed (though not necessarily eliminate it), and produce a consistent volume of fruit with consistent quality and size. During bloom, pollen is deposited on the sticky fl ower stigmas on the tips of the styles (see fi gure). Pollen grains germinate on the stigmas, sending out pollen tubes that grow downward through the styles to reach the ovary of the fl ower. The pollen tubes carry the male gametes (sperm) to the ovary where they fertilize the eggs, which then have the potential to develop into seeds. Seed initiation stimulates hormone production, which causes the ovary to develop into an apple. How long it takes for a pollen tube to grow down through the style and for the eggs to be fertilized depends on the weather. Scientists at Virginia Tech, who have spent over a decade studying pollen tube growth, developed a temperature-based, variety-specifi c model that calculates the time required to fertilize a blossom after pollination. Much of the work was funded by the Washington Tree Fruit Research Commission. The model was developed, using fi eld data from Washington collected by Research Commission staff, specifi cally for Washington growers. For the past fi ve years, a number of growers and horticulturists have been beta testing the model, which is available at the Washington State University's AgWeatherNet website (weather.wsu.edu). Using the model Users need to set up a free AgWeatherNet account. On the Pollen Tube Model page of the website, the model can be set up for each orchard block by identifying the variety and selecting which weather station the model should pull data from. As soon as the trees begin to bloom, the grower should mea- sure the longest style on 25 to 50 blossoms throughout the block to calculate the average style length, which is entered into the model. The AgWeatherNet Web site gives instructions on how to measure the style. The grower should know in advance what the target yield is on a per-tree basis. For example, if the target yield is 60 bins per acre of 88 size fruit, that is 126,720 apples per acre. With a spacing of 3 feet by 12 feet (1,210 trees per acre), that works out at 105 fruit per tree. The grower should then count open fl owers on fi ve to ten representative trees, and when there are enough blooms open on the trees to achieve the target yield (for example, 105), the model should be started. It will predict pollen tube growth (as a percentage of the average style length), based on the weather data. The grower should check the model routinely and apply a chemical thinner about the time when the pollen tube is predicted to have grown the full length of the style. At this point, the desired number of blooms should be fertilized and the thinning product should prevent further fruit set. Filament Nectary Ovule Sepal Petal Anther Anther Apple blossom $VHFWLRQRIDQDSSOHEORVVRPHQODUJHGDERXWWLPHV$IWHU½RZHUVDUHIXOO\ opened, the anthers pop open and release pollen. Pollen grains stick to the stigmas on the tops of the styles and germinate, sending out pollen tubes that JURZGRZQWKURXJKWKHVW\OHVWRUHDFKWKHRYXOHV Ovary Style Stigma A predictive model enables growers to be in a planning mode rather than a crisis mode at chemical thinning time. by Geraldine Warner Style Stigmatic surface Pollen SOURCE: Pollination and Fruit Set of Fruit Crops, Cornell Cooperative Extension Information Bulletin 237 by Roger Way. Stigma

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