Location: Genetic Improvement for Fruits & Vegetables Laboratory
2020 Annual Report
Objectives
Objective 1: Generate once-fruiting strawberry selections and varieties for the Mid-Atlantic and surrounding region, for use in traditional matted-row and/or annual plasticulture production systems, with emphasis on high yield; excellent fruit quality; long shelf life; and resistance to Colletotrichum, Botrytis, and foliar and fruit-rot diseases. [NP301, C1, PS1A, PS1B]
Objective 2: Generate repeat-fruiting strawberry breeding selections with an open plant architecture; adequate runner production; high continuous yield; large fruit with excellent quality; and resistance to Colletotrichum, Botrytis, and foliar and fruit-rot diseases for use in developing varieties for extended-season production systems. [NP301, C1, PS1A, PS1B]
Objective 3: Dissect the molecular, genetic, and environmental factors affecting strawberry production-efficiency traits, especially disease resistance and control of plant architecture, through initiation and development of plant organs such as stolons, branch crowns, and inflorescence structures. [NP301, C3, PS3A]
Objective 4: Identify or generate new strawberry mutant genetic stocks for determining the functions or regulation of genes affecting disease resistance. [NP301, C3, PS3A]
Approach
Standard plant breeding methods will be used to generate superior strawberry cultivars for traditional production practices and fruiting for the traditional short spring season. Novel evaluation practices for fruit quality and flavor will be developed and incorporated into the annual breeding cycle. A seedling screen for resistance to anthracnose crown rot, an emerging disease of worldwide importance, will be developed to identify resistant strawberry plants and increase the breeding population’s average resistance to the disease. New cultivars resulting from selection based on increased disease resistance, fruit quality, yield, and shelf life will be released. To help satisfy demand for year-round availability, similar methods will be used to generate improved strawberry plants that fruit for an extended season from April through December. Because the longer-fruiting plants will face weather and pest challenges that are not problems during the traditional fruiting season, new comparison methods will be developed to facilitate identification of plants that produce fruit within the traditional season, and produce equally well outside the traditional strawberry season. Additional research will be done to optimize the season-extending “low-tunnel” production system developed in the previous Project Plan to better evaluate advanced breeding selections. Inheritance of the strawberry’s capacity for continuous fruiting will be studied with both classical and molecular genetics. Characterization of novel mutant diploid lines with microscopy, hormone physiology and analysis, genetics, and genomics will illuminate genetic control and regulation of stolon production, a trait of vital importance to strawberry nurseries and growers.
Progress Report
In support of Objective 1, sixty-eight crosses were planned and sixty-two were executed. Over 7,000 once-fruiting seedlings were planted in Fall 2019. Twelve cultivars and seventeen breeding selections were planted in replicated evaluation plots; an observation plot was established for destructive testing of fruit for each cultivar and selection in the replicated plots, plus 40 new selections. Data were collected for a patent application for a new late-season strawberry, ‘Cordial’, tested as B2360. When our successful cultivar, ‘Flavorfest’ was reported to succumb to an unknown crown rot, we coordinated efforts between university pathologists and a private crop consultant to identify the pathogen as a North Carolina-originated hybrid between two Phytophthora species, sensitive to available pesticides that can be used by nurseries to produce “clean” plants for growers.
In support of Objectives 1 and 2A, a CRADA between the strawberry breeding programs at Beltsville and the University of Florida was established to improve our population’s resistance to Colletotrichum gloeosporioides, and their population’s flavor and shelf life; four reciprocal crosses at each location were planned and executed to test project logistics. To support a future automated high-throughput seedling evaluation system using deep learning, an application web site, created in cooperation with our partner Farmwave, was tested and established, with 32 recruited members. In further support of this vision, a non-disclosure agreement was established with TRIC Robotics to develop image analysis techniques. A hand-held gloss meter was obtained after determining that glossiness was the primary indicator consumers responded to for apparent freshness in strawberry. Improved moisture sensors were established to help develop a timed pulse-regulated irrigation program to irrigate our fields with reduced water waste.
In support of Objective 2A, fifty-three crosses were planned and executed, and over 10,000 repeat-fruiting seedlings were planted in Spring 2020. A low-tunnel planting was established for replicated- and observation-plots in Spring 2020. Three new selections had sufficient plant numbers to establish them in replicated plots with three reference cultivars in preparation for testing selections of sufficient quality to be possible cultivars; this is a small test of logistics. The three selections and three cultivars in this test, plus 24 other new selections, also were planted in observation plots. One of these selections has the desired plant architecture and firm fruit of similar size to the reference cultivars, and has far better flavor.
In support of Objective 2B, a manuscript describing project research on the performance of repeat-fruiting cultivars under low tunnels covered with four films with differing light transmission properties was published. The study identified the best film to use in future work for this objective.
In support of Objective 2C, unsuccessful inquiries were made to identify a collaborator willing to help map one of the two new repeat-fruiting loci identified by this project, with phenotypic data, parental and progeny leaf tissue collected, and parental marker data available.
In support of Objective 3, four diploid strawberry mutants with no, or little, fruit production due to abnormal anther formation were identified and characterized. A manuscript describing these mutants was submitted and returned as accepted with revisions. A diploid strawberry mutant with severely shortened flowering structures, named “shortened inflorescence” was identified and characterized. Inflorescences and crowns of both mutant and wild-type plants elongate in response to gibberellin application. A diploid strawberry mutant with highly elongated fruit shape was identified and characterized genetically, physiologically, and anatomically. Using this mutant, it was shown that strawberry fruit shape is determined by the shape of the receptacle early in flower bud development, and that auxin applied after fertilization did not change the predetermined shape.
In support of Objective 4, an isolate of Colletotrichum in the gloeosporioides species complex was obtained on the North Farm of the Beltsville Agriculture Research Center (BARC). Because it was obtained on site, this isolate can be used to screen seedlings without concern for spreading an exotic pathogen. However, because there is no confirmed resistance to this isolate, we still intend to use appropriate measures to prevent spread in the greenhouses and fields at BARC. The isolate was obtained and frozen shortly before the COVID19 pandemic caused implementation of maximum telework restrictions, so no further work has been done to culture it.
Accomplishments
1. Genetic control of continuous-fruiting strawberries. Most strawberries fruit once a year for only a few weeks. Strawberries that fruit continuously through the year are highly desirable because consumers demand strawberries year-round. ARS researchers at Beltsville, Maryland, discovered that continuous fruiting strawberries are controlled by three genes, one dominant, one recessive, which-functions to mask the activity of a third gene that confers continuous fruiting. Knowledge of the existence of these two genes, and their control over the third, helps strawberry breeders better understand which parents to choose, how many seeds to plant, and when to evaluate the resulting plants to develop continuous-fruiting strawberries for farmers and consumers more quickly.
Review Publications
Whitaker, V.M., Knapp, S.J., Hardigan, M.A., Edger, P.P., Slovin, J.P., Bassil, N.V., Hytonen, T., Mackenzie, K.K. 2020. A roadmap for research in octoploid strawberry. Horticulture Research. https://doi.org/10.1038/s41438-020-0252-1.
Dong, W., Lu, Y., Yang, T., Trouth, F.J., Lewers, K.S., Daughtry, C.S., Cheng, Z. 2019. Effect of genotype and plastic film type on strawberry fruit quality and post-harvest shelf life. International Journal of Fruit Science. https://doi.org/10.1080/15538362.2019.1673873.
Lewers, K.S., Castro, P.R., Hancock, J.F., Weebadde, C.K., Die, J.V., Rowland, L.J. 2019. Evidence of epistatic suppression of repeat fruiting in cultivated strawberry. Biomed Central (BMC) Plant Biology. https://doi.org/10.1186/s12870-019-1984-7.
Lewers, K.S., Fleisher, D.H., Daughtry, C.S., Vinyard, B.T. 2020. Low-tunnel strawberry production: Comparison of cultivars and films. International Journal of Fruit Science. https://doi.org/10.1080/15538362.2020.1768616.
Lewers, K.S., Newell, M., Luo, Y., Park, E. 2020. Consumer preference and physiochemical analyses of fresh strawberries from ten cultivars grown in Maryland. International Journal of Fruit Science. https://doi.org/10.1080/15538362.2020.1768617.
