A line of cultivars developed from a Brazilian carrot by USDA plant geneticist Philipp W. Simon in Wisconsin shows promise in helping the California carrot industry deal with root knot nematodes.

Simon was on hand to report on his latest research funded by the California Fresh Carrot Advisory Board recently at the board's annual research symposium at Bakersfield.

His ongoing trials, based at the University of Wisconsin at Madison, in 1990 isolated a single, dominant gene in “Brasilia,” a Brazilian cultivar that confers resistance to the nematode, Meloidogyne javanica.

Since M. javanica is only one of the root knot and other nematode species which cause forking, galling, and stubbing on carrots in California, he said the question became how Brasilia would stand up to M. incognita, a related nematode having greater economic importance to the industry.

The resistance from Brasilia in his experimental plants is not as strong for M. incognita as for M. javanica, he explained. But his survey of hundreds of diverse carrot cultivars from around the world continues in a search for M. incognita resistance.

Simon, recognized internationally for his breakthrough work in carrots having improved beta-carotene and other healthful compounds, has made some progress in combining M. javanica resistance with resistance to carrot diseases. He has a global reputation for his breeding of improved onion, garlic, and cucumber cultivars.

‘Inbred’ selections

He developed seven “inbred” selections from Brasilia having traits sought by the California industry, including color, smoothness, flavor, and resistance to Alternaria blight. He continues to search for the longer length preferred by California growers.

From this material, 23 new sources having some resistance to M. incognita were identified in trials at Coachella, Irvine, and Parlier during the 2001-2002 season.

Winter greenhouse seed production for the project was done at the University of Wisconsin, disease resistance was evaluated in Florida, and horticultural traits are being evaluated by California cooperators.

“We are intercrossing them with each other and with carrots that have the proper root shape and good flavor for California production,” Simon said.

The fact that the research has not revealed the degree of resistance to M. incognita as to M. javanica, he added, suggests several genes may be involved in the resistance to M. incognita.

More seed production

He has expanded seed production of the M. javanica resistant entries to establish larger-scale field trials that will hopefully lead to stronger plants that can be combined with lower inputs of nematicides for control of M. incognita for marketable yields.

Related research at the University of California, Riverside is aimed at finding the relationship between nematode resistance in carrots and high temperatures during the growing season. In the case of tomatoes, for example, extreme summer heat can break down resistance to M. incognita.

The carrot advisory board also supports research projects at Washington State University and Oregon State University on bacterial blight, or Xanthomonas campestris pv. carote, of carrot-seed crops. About 75 percent of the U.S. carrot seed crop is produced in those states, and most carrot seed for California comes from them.

Infections of bacterial blight can reduce germination significantly. Seed companies incur costs of treating infected seed lots, and foreign markets reject infected seed.

Lindsey du Toit, a plant pathologist at Washington State, said one finding in the 2001-2002 trials in the two states was a “green-bridge effect” in spread of the disease.

This meant that new, susceptible plants emerging near mature seed crops picked up the disease from windblown, infested debris during threshing.

The researchers quantified the amount of disease inoculum with special equipment to collect air samples during harvesting. “We were able to detect airborne Xanthomonas up to a mile downwind of seed crops being harvested,” she said.

Splashing water transmits the disease, and contamination was particularly evident when the new crops were sprinkler irrigated, providing moisture to bacteria settling on foliage.

The seedborne disease can infect various parts of the plant and leaf symptoms go from small chlorotic areas to water-soaked lesions and on to infection of umbels and new seed.

Residue, rotation

She said the above-ground bacterium is restricted to carrots. It can survive underground, however, and prompt incorporation of carrot crop residue after harvest and rotation every two or three years are important in controlling it.

Stecklings, mature carrots with ideal shape and other traits selected for seed production or breeding, can also carry Xanthomonas inoculum.

Du Toit also reported on an Oregon State project evaluating the potential for Xanthomonas to spread systemically within the carrot plant. Results thus far are inconclusive.

On greenhouse grown plants free of the disease, researchers treated leaves, petioles and roots with the bacterium, which was later recovered in some, but not a majority, of the samples.