Cavity spot, a significant fungal disease of carrots grown in the San Joaquin Valley, was not stopped by isothiocyanates produced by mustard cover crops in field trials in 2003.
Isothiocyanates, the very same compounds that impart the pungent taste and odor to horseradish, onions, radishes, and mustards, were tested in a search for alternative controls for cavity spot.
Earlier research suggested that mustard covers, once chopped and incorporated into the soil as a biocide, have the same effect as Vapam fumigant.
According to Mike Davis, Cooperative Extension plant pathologist at the University of California, Davis and leader in the research, studies in Australia showed these compounds from Brassica cover crops did reduce some species of Pythium and other pathogens.
However, trials in carrots at Bakersfield, Lancaster, and Cuyama last year did not do the same. "The bottom line is we have not controlled cavity spot with mustards," he said in reporting the results at a recent symposium held by the California Fresh Carrot Advisory Board (CFCAB), which funded the studies.
Pythium sulcatum, Davis noted during the Bakersfield event, is the primary cause of cavity spot in California. It is able to survive without symptoms on all field crops and vegetables typically rotated with carrots. Infected, mature carrots show irregular, depressed lesions across taproots but no above ground symptoms.
Although Pythium species have been managed for more than a dozen years with the carrot industry’s sole, registered control against cavity spot, Ridomil, at least two species showed resistance to the fungicide last year.
"We know that in fields with a long history of Ridomil use, its efficacy lasts for shorter and shorter amounts of time," said Davis.
P. sulcatum, he added, was found in his trials to be sensitive to two experimental fungicides now being developed for registration in California: fenamidone or Reason made by Aventis, and cyazofamid or Ranman 400SC by ISK.
A major obstacle in dealing with P. sulcatum is the lack of a soil assay to detect it so growers might avoid infested fields. Davis explained that existing methods reveal the aggregate amount of various Phythium species but not the culprit most responsible for cavity spot.
Another source of frustration was the high volatility of the isothiocyanates, which disappear quite rapidly from soil samples before they can be measured. Although cavity spot control was not achieved, Davis said the mustard species, including the plots of the varieties Pacific Gold, Absolute, ICHI 20, and Blend 119, lent other benefits.
"The total organic matter, microbial activity, and water infiltration of the soil, along with erosion and dust control, increase after incorporation of the mustards. In general, in terms of plant health, those are positive."
In related laboratory tests, Davis found that soil incorporated with mustard crop residue failed to control Rhizoctonia in cotton, a crop highly susceptible to that pathogen.
Davis is also investigating the relationship between beneficial mycorrhizal fungi, which mustard crops antagonize, and cavity spot in carrots. CFCAB funds for cavity spot are also taking shape in a nursery devoted to study of the disease at California State University, Fresno.
James J. Farrar, assistant professor of plant pathology there, is collaborating with Joe Nunez, Kern County farm advisor, in inoculating the quarter-acre plot with cavity spot-infected carrots. The plot will be monitored to segregate which Pythium species are present and for fungicide resistance studies.
Farrar said once problems with weeds, ground squirrels and the irrigation system are ironed out, he expects to have high levels of the disease. That will enhance research projects by guaranteeing cavity spot is present rather than assuming it will occur in commercial field plots.
Other projects by the board, continuing to investigation of root knot nematodes, the chief nematode pest of carrots, and Phil Roberts, nematologist at UC, Riverside, leads research in screening carrot germplasm resistant to the Meloidogyne species.
Selections have been released to seed companies, and Roberts said he expects resistant commercial varieties will be available "fairly soon."
Roberts has been working with lines of carrots developed by Philipp Simon, a USDA plant breeder at the University of Wisconsin, from Brazilian sources. These carry resistance to M. incognita, M. javanica, M. arenaria, and M. hapla.
"More effort," Roberts said, "has been placed on M. incognita resistance in the last three years, because the most advanced breeder-release material with excellent M. javanica resistance showed less resistance to M. incognita in some previous tests."
Another front against root knot, as well as other nematode species, continues in the research of Becky Westerdahl, nematologist at UC, Davis, to find methods to head off the potential loss of chemical nematicides.
Even with the advent of nematode resistant varieties, Westerdahl said they would mainly work against root knot species, so other means are needed to deal with stubby root and needle nematodes.
She has been teaming biological nematicides, such as DiTera, a Valent product that was recently registered for use in California, with trap cropping, since neither method is sufficient on its own.
Trap cropping, she explained, has been evaluated periodically since the 1800s. "A susceptible host is planted, and larvae of a sedentary parasitic nematode such as root knot are induced to enter and establish a feeding site.
"Once this has occurred, and the female begins to mature, she is unable to leave the root. The plants are then destroyed before the life cycle of the nematode can be completed, trapping nematodes within the root."
Westerdahl said not all nematodes can be made to enter the roots, so the method is only partially effective. However, partial control of DiTera and partial control of trap cropping, when combined with a resistant variety, could provide greater control than one method alone.