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A parasitic weed...

California Growers Facing Dodder Challenge

The Tomato Magazine
June 2005

By Lisa Lieberman

“ Dodder is like a car accident. It’s a curiosity when you see it in someone else’s field, but it’s a real problem when it’s in yours,” a tomato weed specialist once said.

The parasitic plant forms a yellow or orange thread-like vine that winds around the above ground parts of affected plants. These shoots produce pegs that penetrate into the plant to absorb nutrients. The vine spreads to adjacent plants as it continues to grow.

Dodder has become a growing problem in recent years in many California tomato fields. About 30,000 acres in California are infested with dodder, researchers say. Left uncontrolled, dodder can reduce tomato yields by over 75 percent and produce thousands of seeds that can remain viable for up to 20 years.

“ Years ago, two to three percent of tomato acreage had some level of dodder. Now it’s up to five to 10 percent of the acreage,” notes Tom Lanini, a weed specialist at the University of California, Davis. “Dodder is not a big problem for people who don’t have it in their fields, but for those growers who do, it’s their most serious problem.”

Dodder wraps its long tentacles around tomato plants like mistletoe does around trees; it literally sucks the life out of the plants. Because of that, unlike nightshade and other weeds, there’s no way to remove dodder from the field without removing entire tomato plants, Lanini points out.

Matrix and a few other chemicals can be used to suppress dodder, but the longevity has been for only a few weeks at a time.

More Suppression Than Control
“ We can’t control dodder as much as suppress it,” Lanini says. “Dodder doesn’t have any roots, so when it germinates, there’s nothing to absorb the herbicide. Therefore, it pretty well escapes control. The best option is to try to avoid it altogether.”

Since most dodder germinates before May 15, growers can avoid the weed by planting in mid-May or by using transplants. Transplants are harder for dodder to infest, Lanini says, since the weed has a tougher time attaching itself to larger sized plants.

Jeremy Hughes of Hughes Farms in West Fresno County says he likes to use direct seeding for two reasons: first, it is generally cheaper than transplants, and second, direct seeding generally gives him a seven to 10 day jump on the early season market.

Hughes says he is considering using transplants next year, however, because he can use pre-emergents to fight dodder on the transplants that he can’t with direct seeding. Transplants would also give Hughes the option to pre-irrigate the fields before putting in the transplants. By allowing the water to soak in before planting, he can keep the top four inches of his soil dry, thereby making it more difficult for dodder to germinate.

Hughes claims he’s seen more dodder in his fields in the past two years than ever before.

“It used to be that nightshade was our biggest problem, but over the past couple of years, dodder has become much more challenging,” he says.

Part of the reason for the extra dodder pressure might be that in recent years, the months of February and March were warmer than usual. This allowed more time for dodder to germinate.

Another factor to consider in dodder management is irrigation. When Hughes switched from furrow to buried drip irrigation two years ago, that helped improve productivity and yield. At the same time, it may also have exacerbated his dodder problems, he says. While the tape makes irrigation, itself, much more efficient, the faster growing tomato plants spread across the rows rapidly from side to side which also enables dodder to spread more rapidly.

Like Giving Them Steroids
“ Using drip irrigation on them is like giving them steroids,” Hughes says. “The tomatoes grow bigger, and they spread faster across the rows which means the dodder is also spreading faster. A healthy plant is also a healthy host.”

Hughes has been participating in a University of California experiment to test Maverick, an herbicide normally used on wheat. So far, Maverick has been relatively effective against dodder. The problem, though, is that Maverick isn’t registered for use on tomatoes, and the challenge will be getting chemical companies interested in spending the money to get it registered.

Right now, the best way to control dodder is to remove it as soon as it starts attaching itself to tomato plants, Lanini says. The sooner the weed is removed from the field, the quicker it can be stopped from spreading and destroying other plants. Growers also need to send hand crews into the fields regularly to destroy the plants since it’s easy to miss infestations.

“ We’ve been working on sending in labor crews earlier in the season, and we walk the field every 10 days because the dodder is easy to miss,” Hughes says.

Since Dodder, if left in close proximity to living tomato plants, can reattach itself to a new host, infected plants need to be moved at least six inches away from remaining tomato plants, the researcher advises. New host plants must be out of reach.

To keep dodder from spreading from field to field, growers are warned to wash their equipment regularly, Lanini cautions.

“As more custom operations are performed, the chances for dodder spread increase,” Lanini warns. “After leaving an infested tomato field and entering a new one, always wash your equipment. If new dodder patches are detected, eradicate them before they have a chance to produce seeds.”

In the absence of a legally registered, effective herbicide for dodder control, Lanini points out that some tomato varieties have more resistance against the parasite than others. Past field trails have shown that four Heinz varieties have dodder resistance: H9492, H9553, H9992 and H9888. Additionally, CDX 233, H1100, H9997 and SVR 024 20665 have shown good dodder resistance in greenhouse studies.

“ Some dodder is able to attach and survive on these varieties, but, generally, tomato yields are not reduced and dodder seed production is very low or non-existent,” the researcher says.

If dodder is a potential threat, varieties to avoid include: AB2, ENP 113, APT 410, CDX 222, H2501, H2601, H8892 and H9665, Lanini recommends. All are highly sensitive to dodder.

© 2005 Columbia Publishing

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