Progress made with nematode-resistant carrot cultivars

As most growers know, one of the biggest challenges farmers face in growing carrots is dealing with root knot nematodes. As government regulations on nematicides continue to grow tighter, it's likely that growers will face even more challenges in coming years.

That's why scientists like Phil Simon, a U.S. Department of Agriculture Agricultural Research Service genetic researcher and professor from the University of Wisconsin, are working so diligently to develop nematode-resistant genes in various carrot cultivars to be bred into commercially viable varieties that will appeal to consumers and be economically viable for growers, especially organic growers.

Simon and other researchers are also focusing their efforts into making these nematode-resistant carrots more palatable to the American consumer.

In terms of nematodes, there are two main types that growers must deal with—the Meloidogyne incognita and the M. javanica. In general, M. incognita is the main problem for Kern County farmers, where there are heavy plantings of cotton that serve as good hosts for M. incognita nematodes. The M. javanica species is more prevalent in areas like the Coachella and Imperial valleys. With that said, however, there are still a number of carrot-growing regions that have both types of root knot nematodes.

Dealing with nematode problems is important, not only because of the aesthetic damage nematodes create, but also because of secondary issues that pop up, such as problems with soft rot and even alternaria leaf blight, which researchers said seems to be showing up more often on carrot leaves where there are problems with nematodes.

Simon and other researchers found that out of hundreds of trials, a handful of cultivars from such countries as Brazil, China and Syria showed resistance to nematodes. Right now, the germoplasm from Brazil seems to show the most promise.

The problem, though, with these cultivars, is that while they might grow fine in their respective countries, they tend to bolt too early in the season in California. They also tend to create short, fat roots, as opposed to the thin shapes and rounded shoulders Americans prefer. Regardless of whether these cultivars show nematode-resistant qualities, it won't matter if the carrots come in a form that consumers don't want to eat.

Although they may be a few years off, Simon said at a recent carrot conference that he had some examples of some of the very first carrots that show nematode resistance and which are also long, have good flavor and are orange.

"We are having some success with material that shows good nematode resistance as well as other characteristics that will be vital to carrot growers and hopefully appealing to consumers," Simon said. "In a few years, we would really like to see companies like Grimmway and Bolthouse be able to use these carrots that are both good tasting and nematode resistant."

In terms of the current research that should eventually make these goals a reality, Phil Roberts, a professor of nematology at the University of California, Riverside, who works with Simon, says that there is a gene in the Brasilia called MJ-1 that appears to be effective with the M. javinica, but not the M. incognita species.

"We've been trying to spend a lot of time trying to sort out why that is, and what we've come to realize is that there looks like there's a second gene in the Brasilia that confers resistance to the M. incognita as well," Roberts said.

Roberts said he believes that somewhere in the breeding lines, there is material that has good resistance for both types of root knot nematodes. The trick is finding a way to create molecular markers to select for the presence of the MJ-1 gene, and the new, as-yet-unnamed gene that provides resistance to M. incognita.

"What we're trying to do is map its genetic location to see where the resistance gene lies in the carrot genome and to find which chromosome it's on and where on the chromosome it is, so we can develop molecular markers to allow us to select for the presence of that gene," Roberts said.

Continually growing carrots in the fields or greenhouses to check for nematode resistance is time consuming and ultimately not as effective as simply being able to find genetic markers in the carrots to find both genes that are resistant to both types of root knot nematodes.

It's important, Roberts said, to find a comprehensive resistance type of material for both types of nematodes, because often growers don't know what type of root knot nematode they're dealing with.

"Some fields have a mix of the two types of root knot nematode species, so obviously it would be better to have carrot varieties that are resistant to both types," Roberts said.

Simon and Roberts said they believe that finding these nematode-resistant molecular markers is just a matter of time because in other crops, such as cotton or black-eye beans, there are good genetic markers that show nematode resistance.

Cotton and black-eye beans are larger commodities, though, which have had the benefit of more private funding from larger breeding companies, the researchers said.

"From the standpoint of carrots, we're a little further behind, but we're still making progress," Roberts said.

Finding these gene markers and interbreeding them with commercially viable carrots will be a huge step for the carrot sector, Roberts said.

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