Carrot Country

Nematode-Resistant Carrot Varieties Are on the Way

Fall 2006
Carrot Country

The day is coming soon when commercial growers will be able to plant carrots in fi elds infested with root-knot nematodes and still produce a marketable crop, while limiting the need to treat the soil with nematicides.

So predicts Phil Roberts, a nematologist at the University of California-Riverside. Working with Phil Simon, a USDA geneticist and plant breeder at the University of Wisconsin, and Joe Nunez, a vegetable/plant pathology advisor with the UC Cooperative Extension in Bakersfield, Calif., Roberts is optimistic about the new nematode-resistance breeding materials now being released to commercial seed companies to incorporate into their own elite carrot varieties.

Simon is doing the actual breeding, with carrot lines screened for resistance by Roberts in California. Roberts and Nunez are testing and monitoring the materials in California commercial carrot fi elds. Two key carrot processors are also cooperating in the fi eld studies, Grimmway Farms and Bolthouse Farms. Joe Voth is the representative for Grimmway; John Guerard is the go-through for Bolthouse.

Funding for the ongoing project is provided by the California Fresh Carrot Advisory Board. The objective is to come up with carrot breeding materials with resistance to two species of root-knot nematode of primary concern to the California carrot industry, Meloidogyne javanica and Meloidogyne incognita.

Identified a Nematode-Resistance Gene
“We’ve identifi ed a major resistance gene called Mj-1 for Meloidognye javanica,” says Roberts. “Used in the primary screens, the gene has a Brazilian background and has been incorporated to achieve resistance in several advanced breeder lines. We’ve been working for a number of years doing rounds of screening, selection and breeding to get to this point. Several commercial seed companies involved in carrot breeding are now working with these materials to add nematode resistance to some of their commercial offerings.”

Simon, Roberts and Nunez also continue to look for additional sources of resistance to broaden the nematode resistance of their breeder lines. The reason is single gene resistance is more vulnerable to future breakdown by various pathogens while multiple gene resistance is more difficult to overcome.

In their studies, the researchers have used a combination of laboratory, greenhouse and fi eld screening to test their lines. Much of the fi eld screening has been done at the UC Kearney Ag Center in Parlier, in the San Joaquin Valley, near Fresno.

“We continue to screen materials with and without the Mj-1 gene,” Roberts points out. “The search continues for alternative genes that will ensure even longer lasting nematode resistance.”

Two trials are under way at the Kearney Ag Center this summer, one in a field infested with Meloidogyne javanica and the other in a fi eld infested with Meloidogyne incognita.

“In those screenings, during the latter part of the summer, we will dig carrots and score them for nematode symptoms on the tap and feeder roots,” Roberts explains. “If infected and susceptible, the carrots will have galling and disfi guration of the tap root.”

Typically, when nematodes are present, they cause the tap root to divide or ramify into two or three smaller tap roots. Only one or two nematodes need be present to cause this type of disfi guration. The nematode-damage symptoms in carrots are often referred to as cosmetic injury, rendering the carrots unmarketable, Roberts explains. The plants may actually grow fairly well but the tap root distortions ruin eye appeal and turn consumers off.

“Carrots are highly sensitive to root-knot nematodes,” Roberts stresses.

Hope to Reduce Fumigation Needs
The second part of the study is focused on seeing how well Simon’s breeder release materials do in the absence, or with reduced rates, of soil fumigation relative to other pathogens that may be present. This is connected to interest in the industry in reducing or limiting the need for soil fumigation, either with products such as Vapam (metam-sodium) or Telone II (1,3-Dichloropropene).

“Obviously, we also want to insure that the resistance of these materials to nematodes holds up in a commercial setting,” stresses Roberts, “and then to look at the potential for reducing or eliminating the need for fumigating the soil prior to planting carrots. One of the concerns beyond nematodes is whether there are other root pathogens that could be problematic should a grower not fumigate.” Another reason to cut back on regular fumigation is public concern over reducing VOC (volatile organic compounds) emissions, the researcher says. VOC emission affects the atmosphere, and carrots are one of the crops that contribute significantly to VOC emissions because they almost always are planted in fumigated soil.

All said and done, Roberts says he is excited about the future for nematode-resistant carrot varieties.

“The challenge now is for the private seed company sector to incorporate these nematode-resistant breeder release materials into acceptable fresh market
varieties,” he says. “The good news is they are already working on getting the job done. We expect to see some resistant lines in the commercial marketplace in two to three years.”

Simon, Roberts and Nunez began their search for nematoderesistant breeder release materials in the early 1990s. It has been a long-term commitment, but the payoff is soon at hand, Roberts
feels. While some breeder release materials are unique in appearance, others are more diffi cult to distinguish from common commercial varieties.

“What’s important is for the seed companies to incorporate the nematode-resistance gene(s) in their lines without sacrifi cing other marketable traits such as taste, shape and color,” Roberts summarizes.