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Carrot research probes foreign cultivars

BAKERSFIELD, Calif. — Carrot germplasm from Uzbekistan, Poland, and Canada may someday offer traits for improved commercial cultivars for California, according to Phil Simon, USDA/ARS carrot breeder based at the University of Wisconsin, Madison.

Last summer in Uzbekistan the internationally recognized vegetable breeder collected 16 wild-carrot cultivars, some of which are have been grown for generations in home gardens there.

These and other "novelty" types from Canada and Poland have various shapes and colors, along with diverse genetic advantages, Simon said.

Purple-colored carrots from Canada, for instance, may have resistance to the Pythium-induced cavity spot disease important to commercial carrot production in California.

While the more familiar orange pigment is associated with nutritional values, purple, yellow, and red pigments have other health benefits. "So we are now breeding for carrots that have color as well as good flavors," he said.

The research with the foreign germplasm is presently not directly connected with Simon’s continuing breeding project funded by the California Fresh Carrot Advisory Board.

In reporting recently in Bakersfield to CFCAB on his primary research during 2004, Simon said field and laboratory tests continue with derivatives of the Brasilia cultivar.

The Brasilia group has good resistance to Meloidogyne javanica, a root knot nematode of concern to the California carrot industry because it causes losses from deformities such as forking, galling, and stubbing. In 1990, Simon made a breakthrough by isolating a single gene from Brasilia that provides that resistance.

His most recent trials seek to incorporate resistance to the related M. incognita nematode, which has greater economic importance in California, as well as other pests and diseases, with non-Brasilia material.

Selections progress

A group of selections evaluated for three years and showing strong resistance to M. incognita will be the basis of expanded seed production. The work has also generated molecular markers that Simon’s project and seed companies can use to achieve resistance.

The process to come up with improved germplasm for seed companies to use in new varieties will take several years and has the goal of developing carrots that can be grown without nematicides.

For California carrot growers, emphasis is also placed on bright orange color, smoothness, flavor, resistance to Alternaria blight, and long root length.

Meanwhile, Phil Roberts, nematologist at the University of California, Riverside, is collaborating with Simon in another CFCAB-funded project screening the genetics for resistance to Meloidogyne species, particularly M. incognita.

Using a system to rate cultivars’ susceptibility to the nematodes as expressed by galling, Roberts scored entries and shipped the best performers back to Simon’s lab at the University of Wisconsin for further breeding and selection.

A by-product of Roberts’ work is screening to remove an undesirable trait inherent in the Brasilia materials. "We have been making taproot roughness evaluations," he said, "because we observed a trait in the Brasilia derivatives of swollen tissue around the base of the lateral roots that is independent of nematode infection but can be confused with nematode-induced galling of the harvested taproot."

Fungi importance

Mike Davis, plant pathologist at UC, Davis, told the board that the growth of carrots is highly dependent on mycorrhizal fungi to mine essential nutrients from the soil.

He surveyed Glomas species fungi collected from three conventional farms, three organic farms, and three non-cultivated areas in Kern County. Two species make up 90 percent of the mycorrhizal fungi populations.

The fungal diversity, he said, "will ultimately allow us to determine the possible effects that fungicides and fertilizers may have on the mycorrhizal fungal populations."

He found that structures of the fungi function as root extensions of the crop. "The carrot plant is much, much more efficient in picking up nutrients, especially phosphorous," with these fungi. This is important because phosphorous does not move in the soil.

If sufficient phosphorous is present, mycorrhizal fungi are not needed. But, on the other hand, Davis added, the expense of applying phosphorous could be saved if adequate amounts of the fungi are present.

While grain crops enhance mycorrhizal fungi, mustard cover crops, as well as applications of Vapam fumigant, discourage them.

Cavity spot

Davis also investigated fungicides on cavity spot in Kern County. The two new-chemistry fungicides, Ranman and Reason, applied by overhead irrigation, showed promise as controls.

Cavity spot is typically linked to P. sulcatum, although P. ultimum can also cause the disorder. Control of the fungal disease is currently solely dependent on Ridomil Gold.

"We have seen resistance to Ridomil, and in fields with a history of Ridomil use, we have seen a faster breakdown of it. So there is a clear need for a replacement," he said.

In another cavity spot project for the board, James J. Farrar, assistant professor of plant pathology at California State University, Fresno, said he has established a cavity spot nursery at the campus.

Farrar said three successive crops of susceptible carrots were inoculated with five Pythium species, including P. sulcatum, P. ultimum, and P. violae, and after the fourth crop, incidence of the disease in the untreated portion of the nursery was 12 to 16 percent.

He said treatments with Raman and Reason compared favorably with Ridomil Gold, although he said more tests should be made to confirm the efficacy under different field conditions.

The nursery soil, Farrar explained, does not have a history of Ridomil Gold applications so the fungicide did not rapidly degrade. And since the inoculum used was not resistant to Ridomil Gold, the product was useful as a comparison.

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