There’s more to carrots than meets the eye
The cultivated carrot contains an amazing amount of genetic diversity and is cultivated around the world by a diversity of producers. This diversity was highlighted at the recent 2013 International Carrot Symposium.
The evolution of the carrot
The 2013 International Carrot Symposium held Aug. 15-16 in Madison, Wis., hosted professors, breeders and carrot producers from around the world, highlighting both the genetic diversity of carrots and the varied types of growers who produce them. Something that became quickly apparent is that carrot varieties contain an amazing diversity in size, shape and color. Who would have thought that the humble carrot might have such colorful appeal and amazing diversity?
To understand this, consider that both cultivated and wild carrots (Queen Anne’s Lace) belong to the same species, Daucus carota, and can freely interbreed. Queen Anne’s Lace, native to Eurasia, but introduced and considered a weed in the United States, is a flower often seen along roadsides whose taproot is a far cry from the carrots usually served at the dining table. Centuries of breeding have drawn on genetic diversity hidden in the lowly wild carrot to produce cultivated carrot varieties of many shapes and sizes.
Carrot varieties come in a multitude of colors, demonstrating the genetic diversity inherent in the species. Photo credit: Stephen Ausmus, USDA-ARS
More specifically, carrots belong to the subspecies Daucus carota ssp. sativus, while wild carrots are a different subspecies, Daucus carota ssp. carota (Queen Anne’s lace). Cultivated carrots were likely first domesticated from wild carrots in Central Asia. Traits that evolved during its domestication include a large taproot with reduced lateral branching and increased accumulation of carotenoid pigments, which give cultivated carrots their color. In contrast, wild carrots have a small, tough, white root with many branches.
Tapping into carrot diversity
The cultivated carrot contains genetic variation for many traits which has allowed breeders to produce varieties differing in size, shape and color. For example, over time varieties have been developed that have different levels and types of carotenoids, giving different varieties either a red, yellow, purple or orange color. The orange varieties of carrots most Europeans and Americans know are actually a more recent development than most other colored varieties that some U.S. growers now produce for fresh market sales at farmer’s markets. Interestingly, carrot varieties with different pigments may have different health benefits.
Recent work will allow breeders to develop carrots with desired traits much more rapidly. Specifically, geneticists at the University of Wisconsin and collaborators have sequenced the majority of the carrot genome, and these and other researchers have developed genetic markers. This is the first species within this family of plants to have its genome sequenced. With this information, breeders will have a genetic roadmap to help them more quickly and accurately select for genes and groups of genes with desired traits, and then track them in the offspring of crosses as they select for them. Examples of traits that researchers are examining include color and disease and nematode resistance.
Diverse production systems
Attendees hailed from a variety of countries, including Australia, Canada, China, France, the Netherlands, New Zealand and the United States. Carrot production systems were varied. For example, Wisconsin and Michigan primarily grow processing carrots, Ontario produces a mix of fresh and processed carrots, California grows “Imperator” type carrots which are long, slender, varieties of carrots that are used to produce “cut-and-peel” baby carrots, and Australia focuses mostly on fresh production.
Production challenges were also varied. For example, in Michigan and Wisconsin there are three main pests that are consistently the target of control: aster leafhoppers, Alternaria leaf blight and Cercospora leaf blight. In Ontario, these are also important. However, there are other important diseases and insects that are the focus of pest management research, including cavity spot, white mold, and carrot rust fly. In California and Australia, one of the major challenges reported by participants is a lack of alternative soil fumigants for control of soilborne disease and nematodes. This highlights the diversity of challenges that carrot producers face in different parts of the world, and the importance of international conferences that allow producers to share and draw from each other’s experiences.