WASHINGTON — The parasitic dodder plant doesn’t have a nose, but it knows how to sniff out its prey.

The dodder attacks such plants as tomatoes, carrots, onions, citrus trees, cranberries, alfalfa and even flowers, and is a problem for farmers because chemicals that kill the pesky weed also damage the crops it feeds on.

So discovering how it finds its prey might help lead to a way to block the weed, or for crops to defend themselves, say researchers at Pennsylvania State University.

The question of how dodder finds a host plant has puzzled researchers. Many thought it simply grew in a random direction, with discovery of a plant to attack being a chance encounter.

But the researchers led by Consuelo M. De Moraes found that if they placed tomato plants near a germinating dodder, the parasite headed for the tomato 80 percent of the time.

And when they put scent chemicals from a tomato on rubber, 73 percent of the dodder seedlings headed that way.

“It opens a new avenue” for understanding parasitic plants, lead researcher Consuelo De Moraes said in a telephone interview.

Co-author Mark C. Mescher added, “One of the interesting things we found was that the plants make choices.”

When they gave the dodder seedlings a choice between a tomato plant and a wheat plant, they preferred the tomato.

Dodder will infect wheat if there is no choice, he said, but they discovered that one of the volatile chemicals given off by wheat repels dodder, so it will choose the tomato if allowed to pick.

So, finding one compound that tends to be repellant could lead to ways to either treat crops to resist dodder or even engineer them to produce the compound themselves, Mescher said.

While they were attracted to tomato plants and the chemicals they release, the dodders showed no particular interest when offered a fake tomato plant, a pot of moist soil or vials or red or green colored water.

The plants don’t have a nose, of course, so it’s not clear how they sense the chemicals given off by potential host plants. When the seedlings start out, they tend to rotate in various directions, and they somehow sense the direction where the chemicals are strongest and then grow toward them, Mescher said.

He noted that many plants respond to light by tilting to one direction or another.

And, Mescher added, some research has indicated that plants may be able to sense chemicals emitted by other plants that are attacked by insects.

sted as one of the top 10 noxious weeds by the Department of Agriculture, appears in Friday’s issue of the journal Science. Dodder is also known as strangleweed.

Curtis Swift, an agriculture agent at Colorado State University, said, “Previous to this research the movement of dodder toward a suitable host was thought to be independent of host plant influences. The identification of volatiles responsible for the directed growth of the parasite toward a host plant is very interesting.”

“Even more interesting and possibly more important is the identification of a volatile that serves as a repellent,” he added, urging further study to see if that chemical can be used as a spray to protect crops. Swift was not part of the research team.

Rick Gibson, an extension agent at the University of Arizona, said he was surprised at the discovery.

“That sounds like a fascinating study if it can be replicated,” said Gibson, who was not part of De Moraes team.

The vine-like dodder doesn’t have roots and cannot produce its own food. If it doesn’t find a host within a few days of germinating, it dies.

When it germinates and the shoot finds a victim, it begins growing in a coil around the host plant. It inserts a peg-like tendril into that plant and feeds off it.

De Moraes and her team worked with a type of dodder known as Cuscuta pentagona, but there are several varieties of dodder with similar lifestyles. They are usually yellow or orange and can blanket and kill their hosts. Dodders sprout clusters of small white or pink flowers that produce their seeds.

The research was supported by the David and Lucille Packard Foundation and the Arnold and Mabel Beckman Foundation.