Who would guess that changing pest management practices in carrots could reduce ozone in the San Joaquin Valley? That’s because fumigation is used to control nematodes and diseases that commonly plague carrots.
Many pesticides, including fumigants, contain volatile organic compounds (VOCs), which evaporate from fields and combine with nitrogen oxides to produce ozone. Ozone formation is not unique to pesticides. Other sources include trucks, cars, burning, oil and gas production, consumer products, even wine production, each of which contribute less than 10 percent of the total. Pesticides, including fumigants, are estimated to be only about 6 percent of the total.
However, to further reduce ozone, the Department of Pesticide Regulation (DPR) plans to impose stricter rules on the use of soil fumigants. Soil fumigants are polluting gases that account for about one-half of all pesticides applied on crops in the San Joaquin Valley. DPR has also asked manufacturers to begin reformulating more than 700 insecticides, herbicides and other pest-killing chemicals to reduce their emissions.
Because of fumigant use, carrots are one of the leading application sites for pesticides that give off VOCs. This is because fumigants are used to protect carrots from highly damaging diseases and nematodes.
Root-knot nematodes are major pathogens of vegetables throughout the United States and world, impacting both the quantity and quality of marketable yields. In addition, root-knot nematodes interact with other plant pathogens, resulting in increased damage caused by other diseases.
Nematode resistant carrots
Phil Roberts, in the Nematology Department at University of California, Riverside, and Joe Nunez, farm advisor for the UC Cooperative Extension in Bakersfield, are trying to reduce VOCs from fumigant use, and provide cheaper and more reliable pest management, by using root-knot nematode resistant carrots as an alternative to fumigation.
Root-knot nematodes attack a wide range of plants, including many common vegetables, fruit trees, and ornamentals. They are difficult to control and can be spread easily from field to field in soil and plant parts. That’s why fumigation is used to control them.
During ongoing work under a long-term breeding and genetics project funded by the California Fresh Carrot Advisory Board, Roberts and carrot breeder, Phil Simon, USDA at the University of Wisconsin, identified excellent root-knot nematode resistance in carrot germplasm, together with gene markers that are useful to breeders for advanced selection.
The resistance is effective against the common root-knot species found in San Joaquin Valley and southern desert valley carrot fields, and it has been bred into high quality fresh carrot breeder lines. These lines have been made available to commercial carrot seed companies in recent years. The seed companies are in the latter stages of incorporating the resistance into elite carrot varieties for commercial release.
In a complementary project funded by the UC Statewide IPM Program, Roberts and Nunez tested these nematode-resistant carrots on two trial sites in commercial carrot fields in Kern County last summer. Both sites had high levels of root-knot nematode at planting. Fumigation reduced the amount of taproot forking and galling caused by nematode infection in each field. Similar trials in additional carrot fields are being conducted in 2006.
“The resistant carrots had significantly lower levels of nematode-induced carrot damage compared with the susceptible lines,” says Roberts. “We are now evaluating how nontarget soil pests and diseases might impact nematode-resistant carrots grown without soil fumigation and are assessing yield performance.”
The identity of the nematode species and its damage potential under controlled conditions is being assessed in greenhouse tests. All plots were monitored for occurrence of other pathogens and growth constraints including weeds that may be present under non-fumigation conditions. No secondary pathogens or weeds occurred in fumigated or non-fumigated plots in both fields.
“In the long run, the hope is that through the use of resistant varieties of carrots, along with cultural practices and applications of less toxic materials, the need for soil fumigation to control nematodes may be decreased,” says Roberts.
Another research project funded by the UC Statewide IPM Program is tackling one of the key pathogens for which fumigation is used, cavity spot in carrots. Mike Davis, plant pathologist for UC Davis, is also working with Joe Nunez to develop a model to assess the risk of cavity spot.
Cavity spot is characterized by elliptical to irregularly shaped, depressed lesions oriented across the mature carrot taproots. Infections occur anywhere along the taproot, but lesions tend to be more abundant on the upper third of the root and are often found where lateral roots emerge from the taproot. Lesions begin as pinpoint, sunken spots and generally enlarge as roots mature.
“Almost all of the carrot acreage in California receives multiple applications of the fungicide Ridomil-Gold, but this material has lost efficacy in some fields,” says Davis. “Isolates of the fungus that are not responsive to the fungicide have been identified, and breakdown of Ridomil is enhanced to ineffective levels in fields with a long history of its use. If we can find a way to predict cavity spot based on indirect measurement of cavity spot buds or spores in the soil, we could limit pesticide usage in those fields with the highest risk.
“We’re also trying to develop a method for extracting fungal DNA directly from soil and combine this with detection of fungus-specific DNA sequences by the polymerase chain reaction. PCR is a common method of creating copies of specific fragments of DNA. It rapidly amplifies a single DNA molecule into many billions of molecules. We will then need to correlate DNA levels in the soil with the severity of cavity spot actually experienced in the field.”
A model for doing this is from Queensland, Australia, where soil samples collected prior to planting were used to determine the levels of pathogens in the fields and correlate this with yield losses, addressing the routine use of fumigation for tomatoes.
By a combination of these strategies, it should be possible to make carrot production less expensive and more reliable, all while making the air a bit easier to breathe.