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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