Dodder has been a difficult to control weed in this area for as long as crops have
been grown. It was kept in check for many years but seems to have been getting worse
in recent years. It is listed as one of the 10 most problematic weeds in the U.S.
by the Department of Agriculture. There are more than 150 species of dodder. Fifteen
of these can be found in Arizona, 3 are common and one, field dodder (Cuscuta campestris),
is an economic problem in the lower Colorado River region. It can be a problem on
many crops grown during the summer months including melons, safflower, asparagus,
sugar beets, beans, alfalfa and others. Dodder is an unusual weed. It is a parasitic
annual that has no leaves or roots after it is attached to a host. It obtains all
of its energy from the plants it attaches to. Dodder germinates near the soil surface
and lives off of food reserves in the seed. It must attach to a host within 5 to
10 days or it will die. After it has found a suitable host, it produces adventitious
roots called haustoria that grow into the vascular system of the host plant. After
it is attached, the lower portion of the dodder seedling dies and all contact with
the soil ends. Dodder seed is small, about 1/16 inch in diameter, and germinates
in the spring or summer once the soil temperature reaches about 60 degrees. It is
very prolific and each plant produces several thousand seeds. The seed is hard and
less than a third can germinate in the first season. The remainder can remain dormant
but viable for 20 years or more. The seed is small and light enough to float in
water and blow in the wind. Most of the movement is probably from contaminated crops,
seed and equipment. Controlling this weed starts by reducing seed movement into
an area. Most states and countries have laws that prohibit the import of dodder
seed. A major cause for the spread of dodder in Arizona has probably been in seed.
Once it is present in a field, annual crops that are poor hosts can reduce the spread
of this weed. Poor host crops include grasses, grains, and other monocots. Other
weeds can serve as hosts and should be eliminated. Preemergent herbicides can be
effective in keeping this weed from becoming established. Postemergence herbicides
need to at least temporarily destroy the host. Perennial crops, like alfalfa, can
be temporarily burned to the ground with contact herbicides like Gramoxone, Chateau
or Scythe plus an adjuvant. Dodder can be selectively killed with glyphosate in
Roundup Ready Alfalfa. The most effective control is with preemergent herbicides.
These include trifluralin, pendimethalin, benefin, Dacthal and others. When the
level of these herbicides has dropped to sub-lethal amounts, dodder seed will germinate
and survive. Repeated applications are normally required
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