Lettuce is in the composite or sunflower family which is the largest family of vascular plants. 34 of the 156 weeds in the Illustrated Guide to Arizona Weeds by Kittie Parker are in this family. It is remarkable that we are able to control almost all of the weeds in this and other plant families selectively in lettuce with a minimal amount of crop injury. We are able to do this with the same 3 herbicides, Prefar, Kerb and Balan that have been used for the last 45 years. Injury does occasionally occur, however, and when it does it can be difficult to diagnose. All three herbicides are mitotic inhibitors that are used preemergence to inhibit root growth in developing seedlings. They are often used in combination and produce the same injury symptoms. Symptoms of crop injury include marginal chlorosis, stunting, irregular development of the cotyledon and first true leaves and inhibition of both the primary and lateral roots. Many other factors can cause these same symptoms and crop injury is difficult to predict and diagnose. Thousands of dollars are spent every season to have laboratories analyze soil and tissue samples that are difficult to interpret. Laboratories commonly use a chromatograph to run samples which cost from $100 to $200 per sample to run. Herbicide levels as low as 0.1 ppm can be detected using this technology. Results are very precise but can be difficult to interpret.

We conducted a project to establish a correlation between detected levels of Kerb, Balan and Prefar in soil and lettuce tissue and potential crop injury. Quantitative guidelines using the amount detected in the plant tissue and soil were established. You can access these guidelines on this link.

A less precise but more direct technique used to predict and diagnose herbicide injury is to conduct a herbicide bioassay in the greenhouse. The crop or other sensitive plants are grown in pots containing soil from the questionable field. Although this technique is less precise, it can be more accurate in predicting and diagnosing crop injury. We will be using the new greenhouses at the Valley Agriculture Center to conduct herbicide bioassays for anyone wishing to use this technique to predict or diagnose injury. There will be no charge for this service. You can contact myself (928-580-9902) or Marco Peña (928-782-5871) for instructions on how to collect and drop off soil samples.

To contact Barry Tickes go to: btickes@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.