Good Fruit Grower

March 15

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The apple industry uses Manchurian crab as a pollinizer because of its compatibility with the major apple varieties. This Manchurian is pictured in a Gala orchard. It also needs to bloom annually and have good pollen viability and germination. Research at the Virginia Polytechnic Institute and State University, suggests that the pollen of Manchurian has a relatively slow pollen tube growth rate and so might not be the most efficient pollinizer (see "Optimizing pollination"). Now, it appears that disease resistance is important, too. "I think it's time to get rid of Manchurian," said Auvil, who has planted Manchurian in every block in his own orchard since 1990. The ornamental wholesale nursery J. Frank Schmidt & Son, of Boring, Oregon, lists 39 crab apple varieties on its Web site, but little is known about their suitability as pollinizers in commercial orchards. Dolgo, Jackii, and Pink Spires are the earliest bloomers. Dolgo is thought to be a good candidate as a pollinizer as it has an upright growth habit, white blossoms, and good disease resistance. It is also one of the hardiest crabs. Jackii produces profuse and long-lasting white blossoms and is hardy. It is resistant to scab, cedar-apple rust, mildew, and has some resistance to fireblight. The tree, however, is rounder (growing 20 feet tall and 20 feet wide) and not as tall as Dolgo. Pink Spires would not fit the apple industry's needs because of its bright pink flowers and reddish purple foliage. Sugar Tyme blooms a little later but has potential because of its white flowers and upright growth habit. photos courtesy of willow drive nursery Trial Mike Willett, vice president for scientific affairs at the Northwest Horticultural Council, said good crab apple candidates need to be identified and then evaluated alongside the standard Manchurian crab. The most efficient way to do this would be in a research trial, rather than by growers individually. The question is, who would be able to organize such a trial? Auvil said promising cultivars need to be screened for compatibility and performance with major fresh varieties that the industry grows. Characteristics that need to be evaluated include attractiveness to bees, tree structure, and disease tolerance. Washington State University recently appointed horticulturist Dr. Desmond Layne to a new technology transfer position with the tree fruit industry, and Dr. Stefano Musacchi, a horticulturist from Italy, will join WSU in August. Auvil said it's possible that one of the new faculty might propose to lead such a project. • Optimizing pollination Scientists hope to find out how the pollen source affects pollination and final fruit size and quality. by Geraldine Warner R esearchers at Virginia Tech are studying pollen tube growth in apple pollinizers in an effort to help growers optimize pollination. The fertilization process begins after pollen is deposited on a flower 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, resulting in fruit set ("An apple flower"). Dr. Keith Yoder, horticulturist at Virginia Tech, said understanding the progression of pollen tube growth is critical in order to apply bloom thinners at the right time. Style Stigma Yoder and his colleagues developed a temperature-based model that calculates the Anther time required to fertilize the king bloom after pollination. The model has been beta tested in Washington for the past two years via the AgWeatherNet Web site, where it is integrated with current and forecasted weather data. The idea behind the model is to help growers predict when a large enough percentage of the flowers has been fertilized Filament Nectarto set a good crop of large fruit. Then, they secretory Sepal can apply bloom thinners, such as liquid glands lime sulfur and fish oil, before more pollen Ovary tubes reach the flower ovules and set more fruit. The goal is to help them save money Ovule by applying sprays at the optimal timing Embryo and avoiding unnecessary thinning sac sprays. "It's a way to add precision to the way we're using bloom thinners, instead of saying apply them at 20 or 80 percent bloom," Dr. Gregory Peck, horticulturist at Virginia Tech, reported to the Washington Tree Fruit Research Commission, which has helped fund the work. While the pollen tube growth rate is built into the model, the grower must measure the average style length to determine the distance the pollen tube has to grow, and enter this into an Excel spreadsheet. Feedback from testers has been positive and is helping the scientists improve the model. Further research has shown that the growth rate of the pollen tube varies depending on the fruit variety and weather conditions. For example, data from Washington orchards shows that the time between pollination and fertilization can exceed 100 hours for Cripps Pink but that Fuji can be fertilized in just over 60 hours. an apple flower An apple flower Pollen source "It's a way to add precision to the way we're using bloom thinners." Questions have arisen about how different pollen sources might affect the accuracy of the model. Manchurian is the most common pollinizer in Washington, followed by Snowdrift. —Gregory Peck The data that the model was based on came mainly from the maternal side of the pollination question—the fruit variety, Peck said. Now, the scientists want to explore the impact of the paternal side (the pollen). Pollen used in developing the model came from Snowdrift. He and his colleagues plan to study other pollinizers to better understand the impact that the pollen source has on pollen tube growth rate. In preliminary studies, when they looked at the growth rate of pollen tubes from eight different crab apples used to pollinate Gala flowers, they found a fourfold difference in growth rate. A day after pollination, pollen tubes of the Thunderchild crab apple were the longest; Snowdrift was intermediate; and Manchurian had among the shortest pollen tubes. In future research, the scientists plan to screen an array of crab apples and fruit varieties at different temperatures to assess the pollinizers' performance in the varied weather conditions typically seen during bloom. They also hope to find out how important the pollen source is in determining final fruit size and quality. • GOOD FRUIT GROWER March 15, 2013 23

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