Yellow nutsedge (Cyperus esculentus) and nightshade (both black and hairy, Solanum nigrum and S. sarrachoides) are two dominant weed problems for tomato growers.

One of the available herbicides for both of these weeds is metalochlor (Dual Magnum), which received late registration in 2003. While there are a few other herbicides registered to control these weeds, two relatively new chemicals that target these species specifically are rimsulfuron (formerly Shadeout, now marketed under the trade name Matrix) and halosulfuron-methyl (trade name Sandea). Post-emergence sprays of Matrix target nightshades, whereas Sandea is almost exclusively a nutsedge herbicide. Efficacy for both is improved through the use of a non-ionic surfactant or crop oil concentrate. Furthermore, tank-mixes of Sandea plus Matrix have given exceptionally good weed control of both nutsedge and nightshades.

One disadvantage with Sandea is potential crop phytotoxicity, especially with certain varieties. This sensitivity is exacerbated with the addition of Matrix. In trials on processing tomatoes in 2004, certain varieties showed up to 80 percent phytotoxicity symptoms with a Sandea plus Matrix combination. Yields were not significantly affected, but fruit quality was not evaluated.

In 2005, eight fresh market tomato varieties were screened for sensitivity to various post-application herbicides. In processing tomatoes, nightshade and nutsedge control were evaluated with several different pre and post application materials. In both locations, the standard herbicide was Dual Magnum. The objective of these trials was to compare efficacy and crop sensitivity to various herbicides that suppress nutsedge and nightshade in tomatoes.

In production fields

The trials were located in commercial production fields near Gustine (fresh market) and Firebaugh (processing). Plots were furrow irrigated and managed similarly as the rest of the field with the exception that mechanical cultivation and hand weeding were not performed. At the Firebaugh location, the pre-plant herbicides were incorporated with sprinklers, whereas at the Gustine site Dual Magnum was incorporated with a disk. Post emergent herbicides were applied over-the-top when the crop was near first bloom. Following herbicide application, plots were evaluated for weed control on a 0 to 10 scale, where 0 equals no weed growth and 10 would indicate complete weed coverage.

At the Gustine location, Dual Magnum, Sandea, and a tank-mix of Sandea plus Matrix did the best job controlling nutsedge, especially by the latest evaluation date on July 18. At this time, all herbicide treatments provided significantly better control of both nutsedge and broadleaf weeds than untreated check plots. Sencor did not perform as well as the other herbicides on controlling nutsedge, but did significantly reduce broadleaf weeds as compared to the untreated control.

The main weeds in this trial were purslane (Portulaca oleracea) and yellow nutsedge, and as a result Matrix alone had significant less nutsedge control than Sandea, Dual, or Sandea plus Matrix (Matrix post emergent is predominantly a nightshade control material). Matrix did significantly reduce purslane as compared to the untreated control. Dual Magnum, however, did not suppress purslane as well as the other weeds, especially later in the season. There were few grass weeds in this location, though there was a trend for more grassy weeds in the untreated plots.

No phytotoxicity

No herbicide treatment was found to cause phytotoxicity problems with any of the varieties used in this test. Furthermore, there was no impact on yield or fruit maturity.

Early season weed growth at the processing tomato trial was dominated by nutsedge. Prior to transplanting at the Firebaugh location, all pre-plant herbicides significantly reduced nutsedge growth as compared to the untreated control treatment, though Dual Magnum did better than Matrix. As a post-emergence herbicide, Matrix is mainly effective on nightshades, but as a pre-emergence offers some suppression of nutsedge as well.

At the July 19 rating, all herbicide treatments significantly reduced nutsedge compared to the untreated control, though there was no significant difference between pre-plant or post-emergence. There was a trend for reduced broadleaf weeds (mainly nightshade and purslane) as compared to the check plots, but this was not significant. Overall best weed control was observed with V-10142 at 0.5 lbs a.t. (unregistered herbicide from Valent), Dual Magnum, and the Sandea plus Matrix (post) tank mix.

Like the fresh market trial, no crop phytotoxicity was observed (field variety was H9665). Yield was not measured at this location.

Summary: In the trials conducted in 2005 in commercial tomato fields, yellow nutsedge was a greater problem than nightshade. At both locations, Dual Magnum pre-plant incorporated significantly reduced nutsedge as compared to not applying any herbicide. In plots without pre-plant herbicides, best weed control was seen with the Sandea plus Matrix tank mix. In three years of trials in various tomato production fields, a tank-mix of Sandea plus Matrix has consistently provided excellent weed control as a post-emergence herbicide treatment.

A few processing varieties have been found to be sensitive to this mix, but in general most tomato varieties tolerate this tank-mix well and yield nor fruit development are significantly impacted.