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Thursday, July 18, 2024

How the strawberry breeding program at UC Davis is cultivating ‘the next best strawberry’

UC Davis’s selectively bred, high-quality strawberry varieties are used in approximately 60% of the world’s strawberry production


By LYNN CHEN — features@theaggie.org

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A customer visiting the supermarket most likely has no second thoughts while picking up their usual carton of strawberries. They may glance here and there for any under ripe ones, but then they move on to the next item on their list. However, getting those strawberries into a shopping cart is a surprisingly long, complex and scientific process, designed to perfectly cater to consumer taste buds. Everything begins with the development of a quality strawberry variety, an area of expertise at the Strawberry Breeding Program at UC Davis.

Researchers at the Strawberry Breeding Program have been breeding commercially useful varieties of strawberries since 1952. The program is currently housed in the Department of Plant Sciences in the College of Agricultural and Environmental Sciences and includes a teaching component for UC Davis students to learn hands-on. Varieties developed here at UC Davis constitute about 60% of the strawberries consumed worldwide. 

In order to develop high-quality strawberries for growers, researchers at the program enhance different key traits of the berries, such as disease resistance, aroma, flavor and post-harvest traits like firmness, which are important for packaging and shipping.

“Disease resistance is a big one,” Dominique Pincot, Ph.D., a postdoctoral researcher with the program said. “There are so many different diseases that are of concern for growers.” 

In recent years, a disease on growers’ radar was the Fusarium wilt, a soil-borne disease that, as the name implies, causes wilt in strawberry plants, according to Glenn Cole, a staff research associate. 

Fusarium was a disease that [took] off the last 10 years and even more rapidly in the last few years,” Cole said. “The disease started to pick up because a lot of acres are grown with susceptible varieties.” 

According to Mitchell Feldmann, Ph.D., an assistant professor of strawberry genetics at UC Davis and direct-elect of the breeding program, the spread of the Fusarium pathogen was originally combated by the fumigant methyl bromide. However, in 2005, the product was banned by California legislation. 

“Methyl bromide was a huge tool that protected strawberries against a ton of pathogens,” Feldmann said. “Unfortunately, it’s also a really bad greenhouse gas and carcinogen.”

A year after the ban on methyl bromide, Fusarium spread from the soil, causing increasing cases of wilt in strawberries. It even led to concerns that a Fusarium epidemic could destroy the strawberry crop in California. Fortunately though, in mid-April this year, UC Davis announced the release of five new strawberry varieties, or cultivars: UC Eclipse, UC Golden Gate, UC Keystone, UC Monarch and UC Surfline — the first to be resistant to the deadly fungal disease.

“It was critical for us to release this current set of varieties to address the [Fusarium],” Cole said. “There have been other products on the market with Fusarium resistance, but they’re older genetics, so they don’t yield as well. Growers have started to move away from those varieties, and so we need to have a new set of higher-yielding varieties that are more relevant for today’s market.”

Other than having high yield rates and resistance to Fusarium wilt, the new varieties also have improved flavor and characteristics that enable year-round growing in California. According to Feldmann, the new varieties are tailored to grow in various regions that belong to three different market segments distinguished by temperature and daylight hours, including fall-planted, short-day varieties; fall-planted, day-neutral varieties; and summer-planted, day-neutral varieties.

Developing these strawberry cultivars was no easy task. To find plants that had the Fusarium wilt, the team at the Strawberry Breeding Program had to obtain and analyze the DNA of thousands of plants in a five-acre strawberry selection nursery. In September, new breeding crosses are planted in the field, and these seedlings grow throughout the winter until they begin to flower and fruit in spring. It is during this time that Cole collects data on the new experimental varieties.

“When fruiting begins, I walk the fields and make phenotypic or visual observations for different trait qualities for the strawberry varieties that we’re trying to develop,” Cole said. 

This process has been made faster through genetic tools such as markers. 

“Instead of picking fruit from 10,000 plants, we can pick fruit from 2,000 plants because we know which ones have the traits we want,” Cole said.

Breeding strawberries does not end with the development of immunity to one disease, though. 

“There’s a very common analogy that you see pop up in plant pathology classes: There’s a constant arms race between a pathogen and the crop that you’re breeding resistance to,” Pincot said. “That’s the whole Fusarium story. We had race one in California. We found five different resistance genes that protect against that race. And now a race two has popped up, and we have to catch up finding a deliverable gene and get that into our material.”

Researchers have to be constantly wary of new pathogens, such as the destructive Macrophomina. According to Feldmann, the research program has placed a new plot of seedling nursery in Wolfskill Experimental Orchards to develop genetic resistance to future cultivars of the soil-borne disease.

Along the way, strawberry growers from different regions are heavily involved in the process of selecting strawberry varieties. Growers eventually help whittle down the advanced selections of cultivars UC Davis plans to release to commercialized farmers.

With enough time, strawberries that make the cut end up beside a person’s kitchen sink as a juicy study snack they might casually munch on. The berries they just washed represent hours of experimentation and selection by a group of people in a research facility.

While it is an endless, arduous task to develop the “perfect” strawberry, it is nevertheless rewarding to continuously improve and pursue the next variety. 

“My favorite strawberry is the ‘next strawberry,’” Cole said. “The strawberries we’re releasing right now — I’ve looked at them for three to five years. I know their values, and I know what’s good about them, but I also know what’s coming. There are [strawberries] that are going to have better disease resistance [and] better taste. And so, I think it’s the next best strawberry. That’s my favorite strawberry.” 

Written by: Lynn Chen — features@theaggie.org