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Early season response of tomatoes to differential soil residue levels of preplant herbicide
Over the last two decades, processing tomato planting in the San Joaquin Valley has transitioned to the use of transplants, buried drip irrigation, and shallow tillage from the traditional direct seeded, flood irrigated systems. The use of pre-plant herbicides in tomato production was generally safe and caused no negative effects on plant health, until recent years. In 2009, stunted tomato transplants were observed in fields that were treated with pre-plant herbicides. It was suspected that the pre-plant herbicides may not be breaking down as rapidly in the newer than under the older practices. Therefore, two greenhouse studies were conducted in Fresno, CA in 2014 and 2015 to assess plant response to simulated residues of common pre-plant herbicides. The herbicides tested were trifluralin (Treflan TM), S-metolachlor (Dual Magnum TM), and pendimethalin (Prowl H2O TM) at rates of 0, 0.03, 0.06, 0.12, 0.25 and 0.5 ppm. All herbicides resulted in some reduction of shoot and root biomass of the tomato plants at the higher doses compared to the non-treated plants. Both S-metolachlor and trifluralin caused significant reductions in shoot and root biomass at 0.5 ppm and the potential of the plants to recover from herbicide injury is unknown. Pendimethalin had a lower potential to cause injury than the other two herbicides. However, the extent of reduction in fruit yield by while it is unknown whether harvest yields can be affected by the reductions of plant biomass by S-metolachlor and trifluralin under field conditions need to be ascertained.