Crunchy & sweet

Fresh carrots have always been a staple in grocery stores, where consumers seek uniform, blemish-free, bright orange carrots. However, recent breeding work has resulted in more interesting carrots.

Those marketing fresh organic vegetables directly from the farm or at markets have taken advantage of breeders’ work to develop colorful varieties loaded with flavor and nutritional value.

In working with the Carrot Improvement for Organic Agriculture (CIOA) project, Dr. Philipp Simon, USDA-ARS geneticist and professor of horticulture at the University of Wisconsin-Madison, focused research on carrot pests and diseases, crop diversity and origin and nutritional quality and flavor. He noted that while carrots aren’t a major crop, they are a money maker for those who grow them.

“There’s a long process that underlies agriculture, and that’s an important part of the story,” said Simon. “Like other crops, carrots are very genetically diverse in appearance. Diversity is a result of the domestication process and is important in crop improvement.”

Simon noted that the organic sector continues to grow: “As of 2019, about one carrot in six was grown under organic management. That has been going up since organic statistics have been taken, and it’s because carrots are a relatively easy crop to grow under organic management.”

Research goals included genetic diversity to disease resistance, pest and disease management and flavor improvement in organic carrot production. A grower survey among organic and conventional producers, as well as seed companies and processors, revealed the top three issues as resistance to rootknot nematodes, Alternaria leaf blight and cavity spot. Simon’s team looked at a wide range of recent cultivars as well as local and worldwide heirloom cultivars.

“When growing carrots from diverse sources, most of the time there are susceptible carrots,” said Simon. “In some cases, we found some carrots that were resistant. Carrots were grown in fields infested with nematodes and in greenhouses.”

Genetic sources of resistance were found in carrots from Brazil, Syria, Europe, China, Australia, South Africa and Tunisia. Researchers grew carrots then sent roots to Simon for seed production and breeding for resistant stock.

Alternaria leaf blight (A. dauci) is an important foliar disease and regarded as the most impactful disease of carrots. When disease is in an area, yield losses typically range from 28% to 75% due to reduced plant growth or weak tops, resulting in carrots lost during mechanical harvest. Alternaria is characterized by leaf lesions, foliar loss and low-quality carrots.

Simon’s research hasn’t discovered any carrot cultivars with strong Alternaria resistance, but there are sources of partial resistance. The screening process for Alternaria is similar to isolating for nematode resistance. Some trial fields had a reliable natural occurrence of A. dauci; other fields were inoculated with A. dauci to increase disease intensity.

Another notable issue for carrots is cavity spot (Pythium species), which usually isn’t a significant problem until post-harvest storage. Post-harvest damage has led to cavity spot being considered the most important disease of carrots in post-harvest cold storage. Some crops with severe cavity spot are abandoned at harvest.

“The selection process involves growing carrots in infested fields and evaluating resistance in the plots,” said Simon. “Among the approximately 75 carrot breeding lines being advanced in the CIOA project, nine have consistent resistance ratings [for cavity spot] among the top 20% of carrots being evaluated.”

Another important aspect of growing carrots both conventionally and organically is weed competition. Simon noted that carrots are started from seed in high density systems and develop slowly, giving weeds an opportunity to thrive.

“We’ve been looking for a genetic variation for several traits we thought would contribute to better competition with weeds,” said Simon. “Early germination and growth, early season vigor, plant height, leaf area, allelopathy and canopy architecture have been looked at in other crops, and we looked at those traits in carrots.”

Tall-top carrots are desirable because they require less hand weeding than short-top carrots. From among the 75 carrot breeding lines in the project, nine have relatively tall tops. Yield loss is significantly higher among short-top varieties compared to tall tops.

Soil types and nutrients are among the major differences between organic and conventional production. Work with other crops has focused on nitrogen availability and mycorrhizal colonization; CIOA research focuses on mycorrhizal and endophyte activity. Carrots for these research trials were grown in organic fields, conventional fields and in a greenhouse.

In organically managed carrots, mycorrhizal fungi had a greater influence on the growth of heirloom open-pollinated carrots than on modern hybrids. Soils under organic management have more abundant organic matter and microbial biomass. Carrot taproot endophyte composition diversity varies among carrot genotypes in these soils.

Simon predicts that plant/microbe interactions may become a trait evaluated in future carrot breeding programs.

“Consumer preferences are important for both conventional and organic growers, but organic growers have much more interaction with consumers,” said Simon. “There’s a lot of variation in carrot quality, flavor, texture, nutritional value and novelty. This has a significant impact on what consumers buy. The CIOA project has looked at breeding carrots with variable color, flavor and texture to address consumer interests.”

Breeding projects have tapped into consumers’ interest in color and shape. Primary carrot colors include yellow, red, purple, orange and white, along with shape differences.

“The same variation in color is borne out with texture and flavor,” said Simon. “The project has gone beyond color to look at those variables because consumers pay attention to texture and flavor when selecting carrots.”

Marketers and consumers of organically grown vegetables often place higher value on flavor, texture, nutritional value and novelty. CIOA is working on incorporating such traits into cultivars for organic production as well as evaluating flavor, texture and post-harvest storage.

Simon emphasized the broad genetic diversity in carrots that provides growers with disease and pest resistance and weed competitiveness.

“We evaluated 36 breeding lines for four years in two locations comparing productivity traits under organic and conventionally managed trials,” he said. “The management system had some influence on performance, but there were no clear trends that applied across genotypes and environments. Breeding in either conventional or organic has the potential to address organic production challenges related to slow early crop development and weed competition. Genetic improvements in conventional systems have delivered and can continue to deliver improved cultivars for organic systems. However, selection in organic systems may speed up the process of weed competitiveness. Testing cultivars in organic conditions is recommended to ensure cultivar assortment is optimum for organic systems.”

Learn more about the CIOA at eorganic.info/carrotimprovement/about.

by Sally Colby