By Barry Tickes, UA Area Agriculture Agent, Yuma

Lettuce is in the composite or sunflower family which is the largest family of vascular plants. Thirty-four 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 weeds in this and other plant families selectively in lettuce with a minimal amount of crop injury.

We are able to accomplish this with three herbicides: Prefar, Kerb and Balan, which have been used for the last 45 years. Injury does occasionally occur. When this happens it can be difficult to diagnose.

These herbicides are mitotic inhibitors that are used pre-emergence to inhibit root growth in developing seedlings. The herbicides 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 the inhibition of the primary and lateral roots. Many other factors can cause the 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-$200/sample to run. Herbicide levels as low as 0.1 parts per million 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.

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.

Contact us for instructions on how to collect and drop off soil samples.

Contact Tickes: (928) 580-9902 or btickes@ag.arizona.edu.