Herbicide resistant weeds have become especially challenging problems in California’s signature cropping systems, which are characterized by little or no crop rotation due to soil limitations or long cropping cycles and relatively few opportunities for mechanical weed control. Although large by specialty crop standards, the approximately 3 million acres devoted to orchard, vineyard and rice production in California is a small market for herbicide manufacturers; thus, herbicide options are somewhat limited. Combined, these factors have led to a high degree of selection pressure for herbicide-resistant weed bio-types as well as weed population shifts to naturally tolerant species . In order to combat complex issues such as herbicide resistance, organized collaborations between weed scientists and other agricultural researchers with a wide array of expertise are required. This includes the activities of UC Cooperative Extension farm advisors and specialists, Agricultural Experiment Station faculty, support scientists, research staff and graduate students, as well as faculty from other universities and agricultural industry representatives . Current herbicide-resistant weed management efforts range from applied research and extension efforts to basic plant biology and evolutionary ecology studies. Although the specifics vary, these efforts can be grouped into three general areas: applied management of herbicide-resistant plants,blueberry container size physiology and mechanisms of resistance and biology, ecology and evolution of herbicide resistance.Many cases of herbicide resistance in weeds are identified after growers, land managers or pest control advisers observe weed control failures with treatments that were once effective.

These weeds are generally brought to the attention of local or statewide Cooperative Extension personnel. If the herbicide application method is ruled out as the cause of poor weed control , researchers often conduct field or greenhouse tests to verify and quantify the level of resistance. Plants from the suspected herbicide-resistant population are treated with the herbicide of interest at rates ranging from below normal doses to doses well above those legally allowed in the field . The response of the putative resistant population is then compared with the response of the known susceptible, or wild-type, population. Resistance is confirmed if the herbicide affects the two populations of the same species in markedly different ways with respect to plant growth and survival. In many cases, an estimate of the level of resistance also is made from these data. For example, if the susceptible population is controlled at one-half the field rate, but the resistant population survives at twice the field rate, it would be described as having a fourfold level of resistance. Identifying and verifying herbicide resistance and developing alternative management strategies provides short-term solutions for weed managers. Researchers often conduct further studies to determine the underlying molecular and physiological causes of resistance and to compare the biology, growth and competitive ability of herbicide-resistant species and bio-types. The mechanism and fitness costs of herbicide resistance can have important ramifications on the selection, spread and competitive ability of herbicide-resistant bio-types, in addition to directly impacting their management. The goal of these efforts is to help growers and pest control advisers recognize the importance of taking a proactive approach to preventing the evolution of a resistant population, rather than a reactive approach to managing herbicide resistance after it occurs. Target-site resistance occurs when the enzyme that is the target of the herbicide becomes less sensitive, or fully insensitive, to the herbicide, often due to a physical change in the target enzyme’s structure.

These physical changes can impair the ability of the herbicide to attach to a specific binding site on the enzyme, thus reducing or eliminating herbicidal activity. Target-site resistance is sometimes evaluated at the tissue level using portions of plants such as leaves, leaf disks or roots . In some cases, a functioning target enzyme can be extracted and its function evaluated in laboratory in vitro experiments in the presence or absence of the herbicide. Recently, overproduction or enhanced activity of the target enzyme has been shown to confer herbicide resistance in certain cases . Several mechanisms of non target-site resistance confer resistance to herbicides in plants without involving the target sites of the herbicides. This can result in unpredictable resistance to unrelated herbicides . Of these, the best-known cases involve resistance in which herbicide-resistant plants have an enhanced ability to metabolically degrade the herbicide to less- or nontoxic forms. Many processes can be involved in metabolic resistance, but the most well-understood cases are due to changes in three groups of isozymes and changes in ATP-binding cassette transporters . This type of resistance is most commonly evaluated using non-herbicidal inhibitors of the various isozymes in the presence or absence of the herbicide and comparing metabolic degradation of the herbicide in laboratory or greenhouse assays.To design effective resistance management strategies for the long term, UC and other scientists are conducting basic research on weed biology and on ecological and evolutionary processes in weed populations. In a few cases, the mechanisms that confer resistance to herbicides have altered the fitness of resistant plants, as compared with susceptible plants of the same species in the absence of herbicide treatment. Differential plant fitness among bio-types can affect the rate at which herbicide resistance can spread. For example, if resistant and susceptible plants have equal fitness, the number of resistant plants in the population would not change relative to the number of susceptible plants during periods when the herbicide was not being applied . In contrast, if resistant plants are less fit than susceptible plants, the number of resistant plants may decrease during periods when herbicide is not applied.

Fitness is usually evaluated by growing resistant and susceptible plants in direct competition with one another, or with the crop of interest, and comparing relative productivity or fecundity. Similar to efforts for other invasive weeds, insects and disease pathogens, surveys are sometimes used to delineate the extent of population growth or the expansion of new herbicide-resistant weed bio-types. Because there often are a few escaped weeds in herbicide-treated fields, herbicide resistance may not be recognized until the resistant biotype makes up a significant portion of the local population . Surveys can help inform growers of emerging herbicide-resistant weed populations while they are still localized; surveys are also often used to encourage adoption of resistance mitigation measures to minimize economic and environmental impacts. Further, surveys combined with population genetic research can determine the evolutionary and geographic origins, and routes of spread, of resistance across an agricultural landscape .Herbicide resistance has been an important management concern in California flooded rice production for several years . Weeds with resistance to the ALS inhibitors , thiocarbamates and ACCase inhibitors are the dominant weed management problems in most of the Sacramento Valley rice production region. In orchards and vineyards, herbicide resistance is a more recent development and is dominated by resistance to the broad-spectrum post emergence herbicide glyphosate. This herbicide is, by far, the most widely used herbicide in the state in perennial crop production systems, as well as in many roadsides, canal banks and residential and industrial areas. Glyphosate-tolerant cotton, alfalfa and corn are becoming widely adopted in the state, which will further increase selection pressure for additional glyphosate-resistant and -tolerant species.Most California rice is produced in monoculture systems due to impeded soil drainage, which limits rotation to other upland crops . Rice fields are kept under continuous flood conditions during the growing season,raspberry planter primarily for the control of grass weeds . Although this system favors sedges and other water-tolerant weeds, selective herbicides such as molinate and bensulfuron provided highly effective weed control for several years. However, in the early 1990s, after repeated use, resistance to the ALS-inhibiting herbicide bensulfuron became widespread among weedy species in rice. By 2000, several additional weed bio-types with resistance to ALS inhibitors, thiocarbamates or ACCase inhibitors had evolved and were causing significant weed management, economic and environmental issues in the rice cropping system. UC researchers, extension personnel and industry partners have devoted considerable efforts to understanding and managing herbicide-resistant weeds in rice. Small flower umbrella sedge and California arrowhead resistance to ALS-inhibiting herbicides was first reported in California rice fields in 1993 following repeated use of bensulfuron . Field research has shown that California arrowhead is a fairly weak competitor in rice systems and that the ALS-resistant bio-types can be adequately controlled with other registered herbicides.

Recently, small flower umbrella sedge bio-types with multiple resistance to the PSII herbicide propanil and to several ALS-inhibiting herbicides were identified in the Sacramento Valley , and research is ongoing to elucidate the mechanisms of resistance and any cross resistance to other rice herbicides. Eared redstem and ricefield bulrush resistance to ALS inhibitor herbicides in rice was reported in 1997. Redstem research has focused on intra and inter specific competition in an effort to develop agronomic solutions to reduce its competition with rice . Studies have shown that California populations of rice field bulrush are resistant to most registered ALS inhibitors, whereas populations from other regions are resistant only to one chemical family, the sulfonylureas, in the ALS inhibitor group . Recently, ricefield bulrush bio-types with multiple resistance to propanil and bensulfuron were identified in the Sacramento Valley . Late water grass populations resistant to ACCase inhibitors, ALS inhibitors and the thiocarbamate herbicides in rice systems were reported in 1998 . This resistance to multiple herbicides within an individual plant indicated that using herbicides with different modes of action would be unlikely to provide satisfactory control of the species in the long term. Further complicating the situation in rice, populations of late water grass and barnyard grass with resistance to both ACCase inhibitors and thiocarbamates, and thus exhibiting multiple resistance, were reported in 2000. Later research confirmed that the mechanisms of multiple resistance to several herbicide classes are due to metabolic degradation of these compounds . Smooth crabgrass resistance to the synthetic auxin herbicide quinclorac was reported in 2002. Detailed research into the mechanisms of resistance suggested that the cause was an altered sensitivity in the auxin response pathway, leading to ACCase activity, ethylene synthesis and enhanced ability to detoxify cyanide . Although crabgrass is not an important rice weed, quinclorac is used in rice systems for control of other weeds, and resistance to it has been reported in Echinochloa species of rice in California and from other regions. Most importantly, the observed changes in ethylene synthesis and production of toxic byproducts may also relate to the plant’s ability to tolerate abiotic stress. Two implications of this finding include the possibilities that quinclorac-resistant smooth crabgrass has the potential to invade a more diverse range of habitats and become an important weed of rice; and adaptation to the abiotic stress of the flooded environments may predispose Echinochloa phyllopogon or other major rice weeds to evolve resistance to quinclorac in the future. Herbicide resistance in orchard and vineyard cropping systems. The first herbicide-resistant weed in orchard cropping systems was perennial ryegrass, Lolium perenne , reported in 1989 . This ALS inhibitor–resistant biotype was selected on roadsides by the use of sulfometuron and, thus far, has not been a major problem in orchards or vineyards because relatively little of this class of herbicides is used in these crops. However, several ALS inhibitors, including rimsulfuron, penoxsulam, halosulfuron and flazasulfuron, are becoming more widely used in tree and vine crops, and selection pressure for ALS inhibitor resistance may increase in the future. The first case of glyphosate resistance in California was reported in a population of rigid ryegrass in 1998 . However, most confirmed glyphosate resistant ryegrass populations have been identified as Italian ryegrass . Glyphosate-resistant ryegrasses have become widespread and are a major weed problem in orchards, vineyards and roadsides of Northern California . Research indicated that resistance in ryegrass is not due to metabolism of the herbicide and is instead due to an altered EPSPS enzyme . Glyphosate resistance in these areas has been largely driven by decreases in grower use of other herbicides, especially those under increasing regulatory pressure because of pesticide contamination of ground or surface water. The use of glyphosate-based herbicide programs also increased when the patent on Roundup expired in 2000 and low-cost, generic glyphosate herbicides became readily available. Today, glyphosate accounts for over 60% of all herbicide-treated acreage in California orchard and vineyard systems .