Alternaria leaf blight, the California carrot industry's most damaging foliar disease, already exists in local fields, while its look-alike, bacterial leaf blight, is likely spread by seed, according to studies commissioned by the California Fresh Carrot Advisory Board.
Robert Gilbertson, plant pathologist at the University of California, Davis and a member of the investigating team, detailed the findings at the board's recent research symposium at Bakersfield.
Noting that incidence of Alternaria leaf blight in California was low to moderate in 2002, he said the similarity of types detected in California fields, plus the absence of it in seed from Oregon, suggests local sources.
Alternaria, a common disease during warm and rainy fall periods, causes heavy losses due to defoliation as well as weakened tops that cause many carrots to be left in the field after mechanical harvesting.
Gilbertson said sophisticated molecular tests distinguished the Xanthomonas bacterium, the cause of bacterial leaf blight, from the Alternaria dauci fungus responsible for Alternaria leaf blight. Symptoms of the two are similar and often require a trained eye to differentiate.
DNA-extraction test kits are being used to identify samples of Xanthomonas from leaves and seed lots more rapidly than with previous tissue methods. Improvements will enable even coated seed to be assayed quickly for the bacterium.
“Bacterial leaf blight is very common in the Oregon seed-producing areas, with all three Xanthomonas types there in similar amounts,” he said.
While some progress has been made in removing the bacterium with hot water treatments of Oregon seed destined for California fields, he said they must be continued and verified with testing afterward.
Even though seed may come from fields that may show few symptoms of the disease, seed from them can still have heavy concentrations of disease inoculum, he added. Inoculum already in California fields provides additional disease pressure. Seed samples from cooperating growers will be analyzed and evaluated with disease pressure in commercial fields this season.
In addition to use of pathogen-free seed, other management steps are timely fungicide or bactericide application, and field sanitation.
Beyond initial inoculum in fields and contaminated seed, other possible sources are windborne spores and debris from neighboring fields, carrot residue on field equipment, volunteers, and wild carrot.
The continuing project is being done with the goal of defining how the disease spreads and then developing an improved control strategy.
Collaborating with Gilbertson are plant pathologist Mike Davis, postdoctoral researcher Xiang Meng, both of UC, Davis, and Joe Nunez, Kern County farm advisor.
In reporting on a separate board-funded project to find controls for cavity spot, Davis said he is evaluating blends of white and brown mustard cover crops incorporated in the soil as a biofumigant.
Cavity spot is root damage caused by at least two Pythium species, although some research indicates nutritional deficiencies and insect damage can also bring it on. Various fungicides have with specific action can reduce Pythium.
The disease is associated with continuous carrot production in the same fields, and once it becomes established it persists for years. The amount of damage in an infested field varies from year to year and is also linked to time of planting.
The brassica cover crops, Davis said, contain compounds known as glucosynalates, which when turned under break down into thyocyanates that have a toxic effect equal to Vapam on microorganisms.
“In fact,” he said, “some studies show thyocyanates have greater activity than Vapam, although they are active for only a few hours.”
The practice is to let the mustards grow in the fall until they start to flower and then disk them under. The organic matter, some 4 tons of dry weight to the acre, from the mustards also promotes microbial activity antagonistic to the Pythium in the following crop of carrots.
Davis said he was not able to demonstrate an immediate reduction in Pythium in soil analyses after the incorporation of brassicas, although he added that some Pythium species, such as P. sulcatum, are more susceptible to thyocyanates than others.
Several vegetable crops host P. sulcatum and according to improved assays in recent years, it is the species most commonly associated with cavity spot of carrots in California.
“We will have to plant carrots this season and see what we get,” he said.
Meanwhile, he continues to search for alternatives to Ridomil Gold for cavity spot control. “The carrot industry is at risk since cavity spot management in California is currently dependent on Ridomil alone,” he said, adding that multiple isolates of Pythium species have been found to be strongly resistant to the fungicide. He is testing several materials, including Thiram, long used on cotton and vegetable seeds, this season.