Herbicide resistance in California is currently dominated by two patterns of resistance that differ somewhat from the rest of the United States. The first major category is multiple-resistance in a number of sedge and grass weeds of the rice production region of the Sacramento Valley. Rice in these areas is typically grown under flooded conditions and is often in long-term rice monoculture due to soil and water conditions in the area which makes them heavily reliant on herbicides for weed control. These cases tend to be due to non-target site mechanisms or, in some cases, combinations of target site and non-target site mechanisms of resistance, and present a serious challenge to growers. The second broad category is glyphosate-resistant weeds in orchards, vineyards, and associated non-crop areas such as roadsides, canal banks, and field margins. Glyphosate has been the most important herbicide in these areas for several years due to efficacy, economical cost, and a favorable environmental and toxicological profile. In the past decade, several new cases of glyphosate resistance have been documented. The most widespread of these appears to be hairy fleabane which can be found nearly anywhere in the agricultural areas of the state. One important distinction between California and other regions of the US is that our cases of glyphosate-resistance are not primarily associated with Roundup Ready cropping systems but instead with non-crop uses or directed spray applications in woody perennial crops. Roundup Ready crops (cotton, maize, and recently alfalfa) are grown in California and do have some challenges with glyphosate-resistant weeds; however, tillage and irrigation practices seem to have reduced or delayed this problem in comparison to the reduced tillage row crop systems in other parts of the country.

During the last couple of years we are running a survey to try and understand the magnitude of herbicide resistance in the southern part of Israel where wheat is the most common crop. We have identified some false positive herbicide resistant reports due to miss application of herbicides, especially ACCase inhibitors. Over all, the cases found are mostly based on NTS mechanism but laboratory experiments are still in progress to base those assumptions. Our Conyza canadensis have developed resistance due to two target site mutations in the ALS and PSII genes in the same individual plants (real multiple resistances). Avena sterilis also showed mutation in the ACCase gene and this population is still under investigation. After several field trials that have been conducted this year in wheat fields; we believe that the answer for the resistance in grass weeds might be pre-em treatments with VLCFA herbicides. Our goal is to convince our farmers to do so in the next few years.

Best regards, Maor and Baruch