Several genes will be needed for future carrot varieties having improved resistance to root knot nematodes, the chief economic pests of the California carrot industry, says award-winning USDA plant geneticist Philipp W. Simon.
Simon’s breeding program at the University of Wisconsin at Madison has been evaluating breeding materials from various sources to develop resistance to a group of root knot species. Carrot growers have depended on nematicide treatments to manage the underground pests.
Meloidogyne javanica, as well as M. incognita and related species, damage roots and cause stunting which, if not controlled, can claim half a crop. M. javanica, in particular, is responsible for serious forking, galling, and stubbing of carrots.
Simon found resistance to M. javanica in Brasilia, a Brazilian carrot, in 1990. He has since been using it and hundreds of diverse, wild cultivars in a search for resistance to M. incognita.
He has an international reputation for breeding improved carrot, onion, garlic, and cucumber cultivars and in 2001 received USDA’s "Area Senior Scientist of the Year" award for breeding of carrots for use in developing countries.
A group of his "inbred" selections have good resistance to M. javanica, plus some improvement in the traits sought by the California industry, including high color, smoothness, flavor, and resistance to Alternaria blight. Strong resistance to M. incognita, however, remains elusive.
Part of his work has been a breeding project for the California Fresh Carrot Advisory Board, which is funded by California carrot handlers.
Reporting on his 2003 breeding trials during the board’s recent research symposium at Bakersfield, Simon said, "Hybrids combined with inbreds having nematode resistance make quite acceptable carrots. There’s still room for improvement in color and flavor, but much to our surprise, combinations of inbreds with hybrids performed better than expected in comparison to the inbreds themselves."
Seed companies, he added, are currently testing some of the combinations in hopes of releasing improved commercial varieties.
"So the current status of the project," he said, "is that resistance to M. javanica imparts some resistance to M. incognita, but we’ve found the M. incognita resistance is still incomplete in the hybrids. That makes the selection process more difficult.
"Several genes are likely involved, so in the next few years we will need to unravel the resistance to M. incognita and look at some of the intercrosses we’ve made."
Simon’s greenhouse seed production was done at the University of Wisconsin, disease resistance was evaluated in Florida, and resistance to black crown, cavity spot, and Alternaria were screened by cooperators in California.
Resistance to Alternaria blight found in wild carrots, Simon noted, takes several forms: failure to develop symptoms, very slow symptom development, and ability to retain leaves when attacked by the disease. He continues to test his selections for it.
The board’s research projects also extend to the Pacific Northwest, where 75 percent of the U.S. carrot seed is produced, to find ways to conquer bacterial blight, which reduces seed yield significantly. Seed companies bear costs of treating infected seed, and foreign markets reject infected seed.
Lindsey du Toit, plant pathologist at Washington State University, said her multi-year trials in Washington and Oregon seed-carrot fields monitored Xanthomonas campestris pv.carotae (Xcc), the source of bacterial blight.
Although seed lots having Xcc can be detected and treated, the disease persists and is exchanged between regions. While seedborne inoculum is sent out on seed grown in central Washington and central Oregon, in turn, it can come back through steckling carrots produced in California for seed production.
Among the conclusions from the research is the "green bridge" effect where dust from seed harvesting, particularly in Oregon fields, was found to disperse the disease from infected fields to newly-planted fields up to one mile away.
The disease is usually carried by splashing water, so sprinkler irrigation and rains pose a hazard for infection. Direct-seeded crops tended to have more disease than steckling crops, she said.
Honeybees, essential for pollination of the crop, are also suspected of transmitting Xcc, although tests were not conclusive.
"We do know that carrot seed free of Xcc can be grown if it is managed carefully enough. Infested seed shipments need to be treated, and the disease problem is greater later in the season," du Toit said. However, she added, environmental conditions, such as temperatures needed to set off the disease, are also important.
Copper applications have been used for the disease, but, she said, stopping the spread of inoculum is a more effective control.
Collaborating with du Toit on Xcc research for the carrot board was Fred Crowe, plant pathologist at Oregon State University, whose studies gave support to the long-standing suspicion that the disease is carried inside carrot tissue, as well as seed. That points to the role of steckling plants.
His trials showed that analyses of "dry tissue found in roots in both 2002 and 2003 strongly suggest reproduction by Xcc in root tissue without symptom development." He said data for those two seasons suggest that the disease can lurk in and on carrot tissue without damage during generally cool conditions in carrot-seed production areas.
"While eradication of Xcc from true seed seems routine, movement of Xcc with stecklings could be an even more substantial and untreatable problem than true seed – except that increase in Xcc foliar populations seems delayed in steckling fields in the Pacific Northwest in comparison to seed-to-seed fields."
In a separate but related project for the board, Robert Gilbertson, plant pathologist at the University of California, Davis, reported that his data show contaminated seed to be the most important source of Xcc.
"The moderate to high incidence of the disease in Oregon and, to a lesser extent in Washington, together with the common contamination of seed from these areas with Xcc, warrants routine treatment and assaying of all seed lots prior to planting in California."
Seed treatments with water at 55 degrees C. for 25 minutes eradicate Xcc.
Gilbertson, who also monitored Alternaria dauci, or Alternaria blight, said local sources are its most important sources of inoculum. "These sources are likely to be carrot debris and volunteers. Thus, sanitation efforts, on an area-wide basis, are important for disease management."
Gilbertson added that reliance on fungicide treatments alone is unwise because of the potential for A. dauci to develop fungicide-tolerant strains. New fungicide products need to be rotated.
Generally lower levels of foliar diseases in California carrots during 2003, he reported, may indicate improved practices are successful in reducing incidence and severity of the diseases, although other factors, including weather, encourage infections.
Gilbertson said a new, carrot-seed assay is now available to do testing in 24 hours rather than the month required by older methods. But the new test has a shortcoming: it also reveals dead Xcc on seed. Seed-wash assays are still required to confirm that hot water treatment has eradicated Xcc.