How to interpret test results to evaluate herbicide injury in lettuce

By Barry Tickes, UA Area Agriculture Agent

Three major herbicides have been used in the production of lettuce in Arizona for the past 40 years. Kerb, Balan and Prefar are used alone or in combination on almost every acre of lettuce grown in the state. The three are all pre-emergence herbicides that work by inhibiting root growth in developing seedling weeds.

Under some conditions, all three can produce crop injury that ranges from slight stunting, chlorosis, and death.

Symptoms of crop injury are marginal chlorosis, stunting, irregular development of the cotyledon, early true leaves, and inhibition of the primary and lateral roots.

Other factors can cause crop injury and produce similar symptoms. These include salt and nutrient toxicity, saturated soils, environmental stress from cold, heat, wind, disease and other factors.

Symptoms of herbicide injury are sometimes confused with the symptoms caused by these other sources of injury. The most common diagnostic tool used to evaluate herbicide injury in lettuce is to take soil and or tissue samples from the effected field and send them to a laboratory. Most laboratories use gas chromatographs and liquid chromatographs for analysis. Standard extraction and detection methods are costly but accurate and precise.

Once results are obtained, there have been no guidelines available to interpret what they mean. The method most often used to evaluate if reported levels are toxic has been to compare results from injured and uninjured areas in a field. If levels are significantly different between the two, it can indicate possible herbicide injury. This method is relative, not precise and sometimes misleading.

We conducted a project to establish quantitative guidelines for evaluating herbicide injury in lettuce. To access the project and the results, click on this link:Herbicide Injury In Lettuce.

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

Question of the week - insect management

How do flea beetles damage cole crops?

A - Piercing sucking

B - Total defoliation

C - Ask John (video).