Dodder Management Strategies in Tomatoes

by Kurt Hembree, Farm Advisor, Fresno County

Dodder is an annual parasitic weed that infests thousands of acres of processing tomatoes yearly in California. Other crops that are also susceptible to attack include alfalfa, carrots, melons, onions, safflower, and sugar beets. Numerous broadleaf weeds also serve as hosts to dodder. In other words, it has many opportunities to thrive within California’s numerous agricultural production systems.

Dodder germinates from seed, begins emerging from the soil as early as February, and continues emerging for several months. The majority of dodder emerges from February through late May. Since dodder is a rootless plant, it must attach to a suitable host plant within 2 to 3 days of emergence or it dies.

As a parasitic weed in tomatoes, it attaches to the plant (usually the stem), penetrates the epidermis, and enters the vascular tissue to obtain water and nutrients for survival. As long as the host plant remains alive, so does the dodder. Feeding off the tomato host plant, it grows rapidly, and envelops the original host and surrounding plants with its wiry orange strands.

Young tomato plants may be killed or severely retarded in growth during a short period of time. Eventually, entire fields may be blanketed with dodder, reducing stand and yields. Dodder produces hard-coated seed that remain dormant in the soil for more than 20 years. Therefore, it is essential that some sort of management be implemented to sustain tomato yields and reduce future dodder outbreaks.

There are no magic bullets for dodder control in tomatoes, but there are options that reduce dodder survival or eliminate the impact dodder has on tomato production. Try using combinations of the following.

• Plant after May 15th to avoid the period when most dodder germinates and emerges. Select varieties that can be planted and grown later in the season to reduce risk of damage caused by high temperatures.

• Use transplants to minimize risk of early stand loss compared to direct-seeded tomatoes which are easily killed by dodder attack. Rapid growth of transplants helps reduce yield loss.

• Plant varieties H9492, H9553, and H9992, which have been previously shown to have a large degree of dodder resistance. These varieties have similar growth and production characteristics to H8892, already commonly grown in California. In most instances, these varieties will significantly reduce the amount of new dodder seed that would normally be produced with non-resistant varieties.

• Use early post-emergence sprays of Shadeout? to help reduce the growth of dodder. While Shadeout? alone will not control dodder, it can delay its growth by 21 days or more. This gives the tomatoes a head start before dodder growth really begins to expand.

• Timely hand removal of infested tomato plants will help reduce dodder populations. Hand-weeding crews should be used when a significant number of attached plants can be seen (usually about the 2-leaf stage). A second walk-thru is needed 7 to 10 days later to pick up the missed plants. Infested plants can be moved to furrow bottoms or hauled from the field in burlap sacks, to be buried or burned. To get the most efficiency for your buck, use weeding crews that are conscientious about chopping out as many infested plants as possible the first time around. Then, come back with the same crew a second time to remove the missed plants. Sending weeding crews through the field later in the season as the only means of dodder removal usually leads to a significant loss of plants, leaving large gaps in the field.

• Practice good weed control techniques in and around known dodder-infested fields. Since dodder readily survives on many weed hosts (including nightshade, pigweed, lambsquarters, and field bindweed), it is imperative that these and other weeds be controlled early in the season. This will reduce the likelihood of supporting dodder survival. Timely herbicide sprays and close cultivation should be used to reduce dodder attachment sites.

• Rotating to non-host crops like cotton or cereals will reduce dodder populations, since they do not attach and produce additional seed. Rotating to host crops like melons, onions, safflower or sugar beets only encourages dodder survival.

While there is no one sure-fire method of controlling dodder in tomatoes, there are several options that can help reduce the amount of new seed produced and the impact on tomato production.

Dodder seed can survive in the soil for many years, so planning for the long-term is the only way of managing dodder effectively. While the old saying goes “nothing is guaranteed in life except death and taxes”, some may argue death, taxes, and dodder.

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