Research Project: Strawberry Crop Improvement through Genomics, Genetics, and Breeding

Location: Genetic Improvement for Fruits & Vegetables Laboratory

2022 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, breeding once-fruiting strawberry cultivars, an early-season selection with high yield of large, showy, sweet fruit with noticeably creamy texture and good postharvest shelf life, was identified for testing by commercial nurseries and probable patenting. Data were collected for possible patent application in 2023. In support of Objective 2, breeding repeat-fruiting strawberry cultivars, three selections performed comparably to the reference cultivar in the 2021 replicated evaluations and were used heavily in cross pollinations. They tested virus free and will be evaluated again in 2023. In support of Objective 3, dissection of molecular, genetic, and environmental factors affecting strawberry production-efficiency traits, a segregating F2 population was planted and scored to determine that the previously characterized mutant phenotype, male sterility, may not result from a single gene mutation. To determine which genes are affecting pollen development in this mutant, tissue was collected and stored for transcriptome analysis and for bulk segregant sequencing analysis. In support of Objective 3, dissection of molecular, genetic, and environmental factors affecting strawberry production-efficiency traits, three genes previously identified as potential candidates for harboring mutations leading to loss of ability to produce runners were cloned and sequenced. Plasmids were produced for testing the function(s) of these genes. Crosses were made to determine the interactions among genes known to affect runner production and an F2 population is being scored. In support of Objective 3, dissection of molecular, genetic, and environmental factors affecting strawberry production-efficiency traits, measurements of cell size and cell number showed that a mutation resulting in severely shortened inflorescences, short runners, and puckered leaves likely affects cell size. Exogenous gibberellin was found to alleviate the inflorescence and runner phenotype, indicating that the mutant gene is likely involved in gibberellin biosynthesis or signaling. In support of Objective 4, determining genetic control of resistance to anthracnose crown rot, segregation ratios of individual strawberry breeding families planted Fall 2021 in a naturally infested field revealed that resistance is controlled by more than just the one gene identified in Florida. The first seedlings resulting from an agreement between ARS and The University of Florida were evaluated Fall 2021 and Spring 2022. Several selections were made of seedlings resistant to anthracnose crown rot and having many large, sweet fruits, satisfying breeding goals of both parties.

Accomplishments
1. ‘Cordial’ strawberry cultivar was released (USPP33636P2). Strawberries fruit for only a few weeks in spring, and one strategy growers use to extend their marketing season is to grow different cultivars with early, mid, and late season production. Current cultivars available to Mid-Atlantic growers for the late season are very susceptible to anthracnose fruit rot and either too soft and tender for consumer acceptance or produce so late that the fruit are lost to summer heat. ARS researchers at Beltsville, Maryland, released and patented ‘Cordial’, a late-season, “once-fruiting” strawberry resistant to anthracnose fruit rot, and with very high yield of large firm berries with superior skin integrity and outstanding shelf life. The berries can have cotton-candy aromatic overtones to the flavor, and are not acidic, even in cool overcast and rainy growing conditions. ‘Cordial’ is expected to have greatest value to growers in the Mid-Atlantic and other hot growing conditions where other cultivars fail to perform. Plants of ‘Cordial’ were sent to two U.S. nurseries and have been requested by a nursery in Canada. A Canadian patent application is in process.

Review Publications
Lewers, K.S., Enns, J.M. 2022. ‘Cordial’ strawberry. HortScience. 57(2):231–235. https://doi.org/10.21273/HORTSCI15808-21.
Richardson, M.L., Arlotta, C.G., Lewers, K.S. 2021. Yield and nutrients of six cultivars of strawberries grown in five urban cropping systems. Scientia Horticulturae. https://doi.org/10.1016/j.scienta.2021.110775 .