Feb 24, 2012
New nursery studies nematodes, cavity spot in carrots

Root knot nematodes and cavity spot are two of the biggest problems with carrots in Kern County, Calif. – one of the largest carrot growing centers of the world.

Researchers recently opened up a specialized 40-acre nursery for studying carrot cavity spot and nematodes at a field station in Shafter, Kern County. Up until recently, the main nematode research nursery had been in Parlier, about 100 miles away from where the bulk of commercial carrots are grown.

The new carrot research center is one of the few of its kind in the United States.

One big advantage of having a university-run nursery dedicated solely to studying nematodes and cavity spot is that these plots of land are isolated and researchers have complete control over the inputs added to the soil.

This is an especially important study, because the state Department of Pesticide Regulations has been imposing stricter rules on the use of soil fumigants in the past several years, restricting their usage, putting caps on the amount of fumigants allowed in certain townships and instituting new buffer zones.

“The buffer zones can be anywhere from 300 to 500 feet around a field,” said Joe Nunez, a University of California (UC) Cooperative Extension adviser in Kern County. “And that means in conventional fields that’s how much ground is being untreated.”

Nunez has already experimented with new chemicals that might be good alternatives to Metam Sodium and Telone, including three new products called Nema-Q, Ecozin and MeloCon.

“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.

The Metam Sodium plot did the best, with Ecozin and Nema-Q Coming in a close second and third.

Nunez’s goal in the new nursery is to continue experimenting with alternative controls. Normally, the problem growers have with nematodes and cavity spot is that they appear in patches in the fields. This makes it difficult to isolate certain parts of a grower’s field to effectively test different types of treatments. At Nunez’s nursery, his first objective is to infest various sections with as many nematodes and as much cavity spot as possible. Once this is done, Nunez will be able to test which treatments work best for both problems.

For the cavity spot trials, Nunez is using varying doses of Ridomil, which was originally registered for that disease in the 1980s. Ridomil has worked relatively well over the past 20-plus years.

“Over the past two or three years, cavity spot has become more widespread. It affects all carrots in California” Nunez said.

Some outbreaks of cavity spot – which causes small lesions in the carrots, rendering them commercially unsalable – have been so bad, growers have walked away from whole fields, Nunez said.

Growers typically apply small amounts of Ridomil three or four times throughout the season, which appears to have worked relatively well – up until recently. Over the past two or three years, Ridomil seems to have grown less effective, however. This could pose a serious problem for carrot growers. Other chemicals have been registered in the past few years for cavity spot, but none of them seem to be as effective as Ridomil, Nunez said.

“It could be that the treatment with Ridomil isn’t working as well as it was in the past, because of biodegradation,” he said. “The microbes are probably chewing up the material before the plant has a chance to pick it up.”

Nunez is planning an experiment this year: Instead of using small doses of Ridomil, he’ll use one big shot of it shortly before harvest, so the plants have time to pick up the chemical and fight the fungus.

“If the plant has a chance to pick up enough Ridomil, it will kill the fungus if it tries to attack the plant,” Nunez said.

In addition to experimenting with Ridomil, other chemicals and IPM treatments to fight cavity spot and nematodes, Nunez’s goal is to build up the microbial content of the soil at the nursery as much as possible. A similar study was done at the Kearney field station in Parlier for three years, he said.

“We were able to suppress problems by building up the soil, but it didn’t last very long. It was so warm in the valley that the organic matter broke down and reverted back to its natural state.”

Now, in a new part of the valley where the soil is sandier and coarser, Nunez is trying different techniques to build up the soil and organically fight problems like nematodes and cavity spot. One plan is to add 30 tons of green manure and 30 tons of compost to one of the plots to help build up the microbial content. This can’t be duplicated in a conventional grower’s field, due to cost.

“But we want to see if we can do it, and then how much we can cut it back so that it will still work in an actual grower’s field,” Nunez said.

In addition to cavity spot and root-knot nematodes, Nunez will be studying carrot alternaria, another big problem for carrot growers in California.

By Lisa Lieberman, VGN Correspondent

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