Response of Transplanted Tomatoes to Pre-Plant Herbicides.

Over the last two decades, processing tomato planting in the San Joaquin Valley has transitioned from deeper-tilled furrow irrigation systems to buried drip irrigation systems with shallower tillage.  Initially, the use of pre-plant herbicides in this newer planting system was generally safe and caused no negative effects on plant health.  However, in 2009, stunted tomato plants were discovered in fields that had been treated with pre-plant herbicides.  It was suspected that the breakdown of pre-plant herbicides was facilitated more when deep tillage was done after harvest than under the new system.  Therefore, greenhouse studies were conducted in Fresno, CA in summer 2014 and 2015 to assess plant injury to simulated residues of common pre-plant herbicides used in tomato production.  The objective of these studies was to evaluate above- and below-ground response of potted tomato plants to pre-plant herbicides.  The herbicides used were trifluralin, s-metolachlor, and pendimethalin at doses of 0, 0.5, 1, 2, 4, and 6 ppm in 2014.  Results in the 2014 study showed that the herbicides inhibited root growth at residual rates of 1 ppm.  Therefore, in 2015, the doses were reduced to 0, 0.03, 0.06, 0.12, 0.25, and 0.5 ppm.  It was observed that all three herbicides caused reductions in root and shoot dry biomass at 0.5 ppm rates, with pendimethalin having a lower potential to cause injury than the other two herbicides.  The GR₅₀ for root biomass was estimated as 0.5 ppm for triluralin, s-metolachlor, and pendimethalin, respectively.  In conclusion, this study helped identify soil residue levels of these herbicides that could be potentially cause crop damage.