Tubb Canyon Desert Conservancy

 

Winter 2025

TCDC Announces Completion of Phase II of Quest to Check the Spread of Sahara Mustard

Borrego Springs, CA – The Tubb Canyon Desert Conservancy (TCDC) announced today that Phase II of its three-phase project begun in 2014 to discover a biocontrol agent for Sahara mustard has been completed and the results have been published in the journal Molecular Ecology. The study led by Daniel E. Winkler, Ph.D. reveals that the invasive desert weed Sahara mustard (Brassica tournefortii) has spread around the world not by chance, but by hitching rides along ancient and modern human trade routes. Using genome-wide DNA analysis of 312 plants from 31 sites across Africa, Europe, Asia, Australia, and the United States, Dr. Winkler’s team reconstructed the species’ global invasion history—and found human fingerprints at every turn.

A Weed with a Passport

Native to North Africa, southern Europe, and western Asia, Sahara mustard was long cultivated and foraged as a leafy vegetable before becoming an agricultural and environmental pest. Its genetic trail, the study shows, reflects centuries of human contact. Within its native range, populations from Morocco, France, Italy, and Egypt display extensive genetic mixing—so much so that Egyptian genotypes appear to form a bridge connecting the western Mediterranean and the Middle East. The authors suggest that Egypt, a hub of trade and agriculture for millennia, served as a “genetic crossroads,” mixing lineages that were later exported abroad.

Crossing Oceans in the Age of Agriculture

The study’s genetic analyses pinpoint Egypt—and perhaps adjacent regions in Morocco and France—as the likely source of Sahara mustard populations now thriving in Borrego Springs and the rest of the American Southwest. The weed first appeared near Palm Springs, CA, in 1927, coinciding with the U.S. import boom of date palms from North Africa and the Middle East. The researchers argue that mustard seeds probably traveled as contaminants in the damp soil and fiber wrappings used to ship the palms. Once established in the Coachella and Imperial Valleys, the plant spread explosively through the Sonoran and Mojave Deserts.

Signatures of Selection and Human Mixing

Perhaps the most striking finding is that nearly all populations—native, agricultural, and invasive—show unusually high genetic diversity and “excess heterozygosity.” In plain terms, individuals are more genetically varied than expected for a mostly self-fertilizing plant. This pattern, the authors note, likely reflects centuries of human selection, cultivation, and hybridization with related crops such as canola and Chinese mustard. In effect, human farming practices may have inadvertently bred a weed with the genetic flexibility to thrive anywhere.

Implications for the Borrego Valley

Sahara mustard’s ability to thrive seemingly anywhere is known well in the Borrego Valley where there have been numerous examples of former wildflower fields being type-converted into nothing but a sea of Sahara mustard. Nowhere have there been more dramatic examples than in the flower fields north of Henderson Valley Rd (See photos below) and the flower fields north and south of S22 four miles east of the Borrego Springs Airport.

 

Because of Sahara mustard’s vast expansion in the southwestern US and northern Mexico, the only means of diminishing its impact on native flora is a biocontrol agent that can be dispersed on an ecosystem-wide level. Phase III of the TCDC project is designed to produce that result. Armed with the knowledge that the Sahara mustard in the US originated in North Africa (Egypt and Morocco), researchers from the USDA’s European Biologic Control Lab have, and will continue, to search in Egypt and Morocco for those biological agents that keep Sahara mustard in check in its native range. Once found, these agents will be subjected to exhaustive testing for safety and efficacy in the European biocontrol laboratories and then again in secure, bio-containment labs in the US. If found to be safe and effective, such an agent can be introduced to areas such as the American Southwest, including Borrego Springs, to reduce the unchecked spread of this destructive invasive plant. Though significant progress has been made in this three-phase project, the use of such a biocontrol agent is still 5-10 years away.

(Major funding for this project has been provided by Audry Steele Burnand and Jim Dax.)

• Winkler Phase II Report (PDF)

  J. David Garmon, MD
  President, Tubb Canyon Desert Conservancy

   

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