By: Prof. Ted DeJong and Sarah Bradley Castro
Sarah Bradley Castro is a Staff Research Associate III at University California, Davis, and manages the Prunus domestica Cultivar Development Program. Sarah’s research interests include increasing cost efficiency for California Prune growers using new varieties as well as other innovations in fruit and nut orchard systems.
Prof. Ted DeJong is a Distinguished Professor, Emeritus, who worked as a fruit tree crop physiologist in the Department of Plant Sciences at the University of California, Davis, from 1981 to 2016. In addition to teaching, research and extension responsibilities he has overseen the UC Davis Prune Breeding Program since 1985.
For the past several decades the California prune industry has relied on one single variety for the entire state’s crop. This monoclonal situation lends itself to vulnerability of widespread disease, pest outbreaks and year-to-year variations in statewide yield. In addition to the risks of a monoculture system, the entire industry harvests and dehydrates the crop all within a few weeks. The development of new, acceptable or superior, prune cultivars will increase the efficiency and sustainability of California prune production and give some protection against the risks involved with a monoculture.
In 2000, the program released ‘Sutter’ prune as well as the ‘Tulare Giant’ fresh plum. These items, though very different in shape and use, were known for their high sugars and good flavors. The ‘Sutter’ prune gained popularity throughout the years, but was then dismissed by processors for various reasons. ‘Tulare Giant’ was released as a fresh market plum, ripening in California’s San Joaquin Valley in early July. It is still grown in some parts of the valley today. Since the releases of these items the breeding program has adjusted its attention to only breeding trees that would be approved by prune processors as well as save the growers money in operational costs. While the breeding program does not focus on breeding for fresh plums, if promising fresh cultivars arise, the breeding program is willing to share these trees with interested growers on a trial basis.
In recent years the UC Davis breeding program has focused on tree and fruit characteristics that will be particularly helpful in reducing grower costs while improving the dried fruit products. To that end, the program has put a greater emphasis on tree structure and fresh fruit characteristics that may influence fresh-to-dry ratios and ease of dried fruit handling. To ensure dried fruit quality, California does not sun dry their prunes but rather tunnel-dries the crop. Fruit that partially dries on the tree before harvest would use less energy to dry, thus fruit that dries on the tree could substantially help California growers save money in drying costs while sustaining quality. Such innovations can ensure the viability of California farmers in the competitive international market.
A new cultivar, ‘Yolo Gold’, is set to be released soon by the UC Davis breeding program. It is an upright growing tree that produces oval, yellow fruit. ‘Yolo Gold’ has excellent dried fruit attributes, and its mahogany color complements its smooth, fruity taste. The tree produces fruit on previous seasons shoots as well as spurs and should not be long pruned; crop loads are consistently high and, in most years, thinning is needed. ‘Yolo Gold’ blooms 4 to 6 days before ‘Improved French’ and harvests typically 1-2 weeks after ‘Improved French’ which is typically in early September in California. The fruit is typically larger than California standard ‘Improved French’ and has superior fresh and dried flavor in comparison to ‘Improved French’. The flavor is so superior that it would not mix well with dried ‘Improved French’ fruit and would likely need to be marketed separately. The 2019 fruit was pitted via an Ashlock pitter and the fruit was given out as samples during the February, 2020 California Prune Summit. These samples created excitement amongst industry members. The taste and appeal of the ‘Yolo Gold’ has processors and farmers seeking this fruit not only to try in their operations, but they seek the fruit also for their personal consumption. Larger processors such as Mariani and Sunsweet have test processed the ‘Yolo Gold’ at their plants to further test its suitability in California’s commercial industry.
‘Yolo Gold’ is not only a candidate for the California commercial industry but would also be an excellent option for a backyard tree. The yellow fresh fruit is attractive and delicious and the extra fresh fruit could be easily dried. Once dried, the fruit is a pleasant mahogany color and then gets darker the longer the dried fruit sits after drying. The tree is adapted to California’s hot summer climate and has not been known to pit burn, drop fruit prematurely or split pit. It has yet to be tested in colder, non-Mediterranean climates such as the Pacific Northwest.
The goal of the breeding program is to produce trees that have dense, non-juicy fruit that are easy to dry. The breeding program looks forward to releasing two or three more promising purple fruited cultivars within the next 5 years. The purple, Agen-type fruit of current new selections can partially dry on the tree before harvest and are very precocious. This precocity will make them good candidates for high density orchards and conventional orchards alike. The ability to partially dry on the tree allows the fruit to slightly shrivel on the tree and will not only save growers costs for drying, but also the dense fruit is less likely to be damaged by mechanical harvesting and processing as well as maintain more attractive appearance during drying and processing. These purple cultivars typically have a fresh to dry ratio of 2.5 or less and brix of over 30º.
The California industry could benefit from the development of new prune cultivars that have cost saving characteristics such as improved tree structure that would require less pruning, improved fruit dry matter content that would decrease drying costs, and increased tolerance to pests and diseases. It has taken a longer time than initially expected to develop and identify new prune cultivars that have the quality characteristics required to provide California growers better variety options for establishing new orchards, but we are excited about recent progress in the prune breeding program. After many years of improving our germplasm and refining our selection processes we are now confident that we have several potential new cultivars that will enhance the sustainability of the California prune industry for the next several decades.