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Carrot tests seek nematode control

Carrot varieties resistant to root knot nematodes are on the developmental horizon, but once they are available, California growers will still need help in management of other nematode species.

With this in mind, Becky Westerdahl, Extension nematologist at the University of California, Davis, is evaluating combinations of controls, some promising so far, for the root knot group, Meloidogyne sp., as well as Paratrichdorus sp., or stubby root nematodes, and Longidorus africanus, or needle nematode. These species form galling on carrots making them unmarketable.

Meloidogyne and Paratrichdorus occur statewide, while Longidorous africanus is important to carrots in the Imperial Valley.

The California Fresh Carrot Advisory Board is funding her studies and she gave a progress report at the board's recent Research Symposium at Bakersfield.

Westerdahl said the uncertain future for chemical nematicides is great enough that alternatives using two or more techniques, each contributing partial control, are being considered.

She has collaborated in trials, some continuing this season, with Joe Nunez, Kern County farm advisor, and John D. Radewald, Extension nematologist emeritus, UC, Riverside.

An element of one approach is trap cropping, a nematode management technique evaluated from time to time since the 1800s. The other component is Valent's biological nematicide DiTerra, only recently registered in California.

Trap cropping

With trap cropping, she said, 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.

The plants are then destroyed before the life cycle of the nematode can be completed, trapping nematodes within the root.

“By itself,” Westerdahl explained, “trap cropping is not likely to provide the same level of control as a chemical nematicide such as Telone II, because not all nematodes are induced to enter the roots.

“However, the potential for loss of registration of this and other chemical nematicides for various environmental reasons is great enough that an IPM approach is warranted, using two or more techniques in combination, that will each provide partial control of the nematode population.”

The 2002 trials at Shafter and Irvine indicated that trap cropping with DiTerra was promising, and harvest results of 2003 trials will determine which treatments are repeated in 2004.

In evaluating the 14 different combinations of treatments, she said, “Telone II at 9 gallons per acre is my typical standard to compare against. In testing with DiTerra at 25 and 50 pounds per acre, sometimes we get better results than others, and we are trying to figure out how to use it in the best manner.”

Sampling timing

She said timing of soil sampling is critical for accurate nematode population counts in carrot fields. It should be done near harvesttime rather than just before planting of the next crop when populations have declined with the absence of a host.

And if growers sample soils several weeks after planting, the results may also be skewed, since the nematodes have an opportunity to move into roots and not show up in the samples.

In reporting on another study supported by the carrot board for the past three years, Tim Hartz, vegetable crops specialist at UC, Davis, laid much of the reason for carrot cracking at the doorstep of nutrition, particularly excessive nitrogen.

After surveying nitrogen management in fields in Kern County, Cuyama, and the Imperial Valley, Hartz said, “there is no justification for putting on more than about 160 pounds of nitrogen per acre per season.” Some growers, he added, were applying as much as 300 pounds.

“Increasing nitrogen application distinctly increased cracking sensitivity. Nitrogen manipulation can increase cracking by about 25 percent.”

His results are counter to long-standing recommendations of the Western Fertilizer Handbook and other references, which have called for midseason petiole nitrate values of 5,000 to 7,000 parts per million.

Instead, he found that 2,000 ppm at two months before harvest and 1,000 ppm at one month before harvest were sufficient.

Variety effects

Variety also has much to do with cracking. Prime Cut was found to have 53 percent cracked roots, while Trinity had 14 percent. The midrange percentages included Top Cut, 44; Columbia, 43; and Caropak, 42.

He evaluated the tendency to crack with a standardized test with the roots cooled to 40 degrees. In the test, a steel blade is forced down into a root and a computer records the force required to break it.

The cracking phenomenon is governed by the strength of a root's periderm, or skin, which acts like a rubber band to resist cracking. The stronger the skin of the variety, the less sensitive it is to cracking.

Irrigation management and root water status play only a minor role in cracking.

Hartz said the studies suggested three recommendations to minimize cracking:

  • Minimize nitrogen use. Account for residual soil nitrogen in fertilizer recommendations, since with some rotational crops, such as other vegetables, large amounts of residual nitrogen may be present.

  • Variety selection can be important, since significant differences in cracking sensitivity exist among currently available varieties. New varieties can and should be screened objectively for cracking sensitivity.

  • Problem fields are likely to be of moderate to heavy soil texture, and such fields may be especially problematic for late spring through early fall harvest. In these situations, variety selection and nitrogen management may be particularly important.

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