Researchers examine new ways to control nematodes in carrots

As most carrot growers know, root knot nematodes, which afflict almost every carrot field in California, are one of the biggest challenges they face. Nematodes are parasitic, microscopic, wormlike organisms that feed on bacteria and fungi. They can cause serious problems in carrots because they attack carrots right at the root, causing forking, galling and stubbing.

"Carrots can't tolerate any nematode damage because essentially what we're selling to the consumer is the root, so it has to be perfect," said Joe Nunez, University of California Cooperative Extension vegetable plant pathology farm advisor in Kern County.

Root knot nematodes generally grow best in sandy soils, as do carrots.

"That's one of the reasons that makes growing carrots so hard," Nunez said. "Nematodes and carrots both like the same soils."

To make the challenge tougher, new regulations being imposed by the Department of Pesticide Regulation have caused researchers to scramble to find viable alternatives to traditional fumingants used to control root knot nematodes in carrot fields.

"Over the past five years these fumigants have been highly restricted, and there may be even more restrictions coming in the future," Nunez said.

Nunez has done several experiments with various alternative treatments over the past two years, including three new products—Nema-Q, Ecozin, and MeloCon, which have recently been registered and available for commercial use by growers.

"We knew about organic farmers using some of these products, but there was very little information about the efficacy of them, so we wanted to do our own experiments and see what we came up with," Nunez said.

Nunez did experiments with these materials in 2008 and 2009 at the UC Shafter Research and Extension Center on land that had prior history of root knot nematode infestations. During the trials, the plots were shaped into 30-inch beds, 30 feet long and six beds wide. All pre-plant applications were applied by hand onto the top of the beds and then the beds were re-shaped with a rotovator/shaper implement. A single drip line with 18-inch spaced emitters was placed on the top of each treated bed.

As was expected, the metam sodium plot did the best with 24 percent nematode damage and the untreated control plot, with 42.5 percent damage, did the worst. However, the Ecozin and Nema-Q in combination with MeleCon had 27.4 and 30.8 percent nematode damage, respectively.

In addition to this, a second trial was conducted at a grower's carrot field who had already pre-fumigated his fields with metam sodium, but began noticing severe nematode damage taking place as the season progressed, Nunez said.

"So, we decided to investigate to see if any other post-plant applications could help reduce the amount of damage that was occurring," Nunez said.

The control field, which was the one that had already been treated by metam sodium, had 72 percent nematode damage. The fields that had the least nematode damage after the application of alternative treatments were the Dragonfire-CPP, which had 44.2 percent nematode damage, and the MeloCon, which had 40.4 percent damage.

There were some inconsistencies as to how these products performed at the two trials. At Shafter, Ecozin, Nema-Q and MeloCon looked promising. However at the grower's field site, only MeloCon and Dragonfire-CPP showed significant control.

For growers who are reluctant to try using these alternative treatments, some of them may want to consider trying them provisionally on the buffer zones around their fields where the DPR prohibits them from treating anyway, Nunez said.

"The buffer zones can be anywhere from 300 to 500 feet around a field," Nunez said. "And that means in conventional fields, hundreds of acres end up being left untreated and overrun by nematodes. So instead of letting that happen, some of these growers might try some of these alternative treatments."

Nunez said he plans to take a more in-depth look at other sesame oil based products as well as Nema-Q, Ecozin and MeloCon.

"Those products are definitely worth looking at more closely," Nunez said.

The bottom line, though, with a lot of these products is that there isn't going to be a single one that will control nematodes 100 percent effectively, Nunez said.

"You also have to apply these products correctly, because otherwise they don't do any good," Nunez said.

This is an extra challenge for growers, especially since metam sodium works best when applied by sprinkler.

"But because of issues with sprinkler applications, most everyone is shanking the materials in," Nunez said.

In addition to these alternative products and methods of controlling nematodes, Nunez and other researchers are studying the effects of using organic materials to reduce nematodes, and are looking at different varieties of carrots that are more nematode-resistant.

"Adding a lot of organic material in the offseason and incorporating it into the soil does a lot to help with soil problems," Nunez said.

Some alternative carrot varieties, which include different red, yellow, white and purple varieties, also look promising. The problem is that commercial growers aren't always interested in planting these different colored varieties because they're afraid consumers might not buy them, Nunez said.

"However, there is always the possibility of niche markets," he said.

(Lisa Lieberman is a reporter in Three Rivers. She may be contacted at lisal@thegrid.net.)