California is nation’s leader in tree nut production. Weed control in orchards is essential for production and the most common form of weed control in commercial production is the use of herbicides. How growers manage their orchards, and weeds, can have an impact on herbicide residues in the environment. This work aims to answer questions that intersect environmental chemistry, weed science, and orchard management practices. We hypothesized that orchard management practices would influence herbicide residue in soil and herbicide transfer to almond kernels. Chapter 1 examines how irrigation water pH and salinity influence the partitioning of three weak acid herbicides between soil and soil solution. A modified method of a traditional Kd experiment was used to quantify herbicide concentrations eluted from treated soil after a series of flushes. We demonstrated that pH and salinity did not have a significant effect on the partitioning of saflufenacil, indaziflam, or penoxsulam out of soil. Chapters 2 and 3 focus on herbicide transfer to almonds via herbicide-bound on soil particles. Low levels of glyphosate and glufosinate residues in almonds have become a concern to the commodity board, growers, and chemical companies because of changing regulations and low maximum residue limits in kernels. Over the course of two field seasons, we measured herbicide residue in unharvested almond fractions, harvested almond fractions, and soil at various timepoints in the field before and during almond harvest as well as at the huller/sheller processing facility. The data addresses the questions of whether residues were coming from herbicide residues in the almonds or from herbicide on soil particles on almonds. It was established that herbicide treated soil could contribute to residues on almonds by being loosely attached to the almond (i.e., dust on almonds). We discovered that a glufosinate metabolite, MPP, is a major contributor to glufosinate residue in almonds and is of concern to the maximum residue limit in processed almond exports to the European Union market. Furthermore, the metabolite was found in unharvested almonds, sampled directly from the tree without contact with soil, at surprisingly elevated levels which leads us to suspect that the translocation of the glufosinate metabolite into the tree is contributing to residue levels. Glyphosate and its common metabolites were not found in kernels at the end stage of processing.

Integrated pest management is a framework for helping farmers use knowledge of weed biology and agroecology to create pest management programs that are less reliant on pesticides. It is imperative that weed scientists engage with other pest management and agricultural scientists to create integrated management solutions that are specifically attuned to each cropping system context. Chapter 1 expounds on these arguments. In this vein, this dissertation examines integrated pest management as it can be applied to weed biology and weed management ecology in nut orchard cropping systems in Central California. Chapter 2 focuses on the reproductive biology of field bindweed (Convolvulus arvensis L.), a perennial weed species that is pernicious in California cropping systems, especially young orchards. We evaluated time to flowering and biomass in field and potted plant experiments in response to orchard weed management practices that selectively affect aboveground and belowground field bindweed development. We found that management practices that affect field bindweed roots, such as systemic herbicides, can delay flowering reliably by over a week, which can have practical implications for planning repeated weed management actions. Chapters 3 and 4 focus on a series of experiments implementing weed-suppressing cover crops in orchards. We found that multifunctional cover crop mixes emerge more consistently than mixes containing functionally-similar species, and we also found that weed-suppressing cover crops can be successfully managed at a range of management intensities, especially with a timely cover crop planting. Weed scientists have a continued responsibility to develop integrated management programs that advance agroecological sustainability, and these results could increase the efficiency of field bindweed management and boost adoption of multispecies cover crops in orchards.

Recent detections of branched broomrape (Phelipanche ramosa) in California tomato (Solanum lycopersicum) fields have led to increased interest in herbicide treatment programs to control this regulated noxious weed. Broomrapes (Phelipanche spp. and Orobanche spp.) are parasitic weeds that pose a significant risk to the processing tomato industry for several reasons that include California’s Mediterranean climate which is similar to that of branched broomrape’s native range, California agronomic practices (wide variety of host species cultivated, successive tomato crops, shared equipment) make the proliferation and spread of broomrape in and among fields highly likely, and broomrape’s phenological development makes it difficult to monitor and inaccessible to conventional weed control practices. In addition, California’s regulatory environment make soil fumigation difficult and costly and herbicides unavailable, while branched broomrape’s regulatory status as quarantine pest does not incentivize accurate reporting. A decision support system and herbicide treatment program, known as PICKIT, was developed over two decades of research in Israel, and has been proven to provide successful management of Egyptian broomrape (P. aegyptiaca) in tomato. The PICKIT system uses a thermal time model to forecast the belowground development of the parasite in order to precisely time the application of ALS inhibitor herbicides to target specific broomrape life stages. Research was conducted in 2019 and 2020 to determine if the PICKIT system could be adapted to manage branched broomrape in California processing tomatoes and to provide herbicide registration support data. Treatment programs based on the PICKIT system herbicides sulfosulfuron and imazapic were evaluated in 2019 and 2020 for crop safety on processing tomato. Treatments included several combinations of preplant incorporated (PPI) sulfosulfuron applications paired with different rates of imazapic either injected into the drip system (chemigation) or applied as foliar treatments. There were no significant differences in phytotoxicity or tomato yield among herbicide treatments in the three experiments. Additionally, a rotational crop study was conducted in which a tomato crop received PICKIT treatments in 2019 and several common rotational crops were planted and evaluated in 2020. Corn planted after the sulfosulfuron treatment suffered chlorosis and stunting, however, safflower, sunflower, melons, and beans were not injured by any of the treatments. After two field seasons, the PICKIT decision support system seems to have reasonable crop safety on processing tomato under California conditions. Rotational crop restrictions will need to be considered if branched broomrape becomes widespread in California and sulfosulfuron becomes part of a broomrape management program. An efficacy study was conducted in 2020 to evaluate the efficacy of a modified PICKIT system in California growing conditions. The study took place in an commercial field near Woodland, CA, reported to be infested with branched broomrape in 2019. This trial examined the efficacy of the PICKIT herbicides sulfosulfuron and imazapic as well as imazapyr, imazethapyr, and imazamox for control of branched broomrape. There were 12 treatments replicated four times, and 47 out of 48 plots (45 m2 ) had broomrape emergence. On average, non-PICKIT treatments had 38 broomrape clusters per plot while PICKIT treatments had 13 clusters per plot. There was a trend in which the PICKIT treatments had fewer broomrape shoots per plot than the non-PICKIT treatments, however, there were no significant differences in the number of broomrape shoots among PICKIT treatments. None of the treatments eliminated broomrape emergence; more studies evaluating PICKIT treatments should be conducted to improve the efficacy of individual treatments. Branched broomrape and Egyptian broomrape differ in phenological development and future research will investigate alternate chemigation times based on this difference. Futhermore, future research will evaluate and focus on imazamox, another imidazolinone herbicide, as this product is already registered in California on alfalfa and has a more promising registration pathway than imazapic.  

This research aimed to evaluate, develop, and refine chemical and cultural controls for branched broomrape in processing tomato, a quarantine pest in California. Branched broomrape is an obligate parasite that can attach to the roots of a wide range of plants, particularly tomato and other agricultural crops. Interest in strategies for management of branched broomrape in processing tomatoes has been growing in California and Chile where tomatoes are major cash crops. Chapter 1 aimed to evaluate and develop herbicide programs based on programs developed in Israel for Egyptian broomrape control in tomato. Crop safety and efficacy studies evaluating preplant incorporated (PPI) sulfosulfuron paired with chemigated imazamox as well as limited treatments including chemigated rimsulfuron were conducted in California and Chile. Chemigated imazamox alone and paired with PPI sulfosulfuron generally reduced broomrape emergence; however, chemigated imazamox resulted in unacceptable crop injury in most trials at rates higher than 9.6 g ai/ha. Chemigated rimsulfuron alone or paired with PPI sulfosulfuron reduced broomrape emergence and did not injure tomatoes. Over several field trials, chemigated imazamox did not have adequate safety in tomato and will not be pursued further In the 2021 and 2022 studies evaluating the crop safety and efficacy of chemigated imazamox, there were differences in crop injury between field sites in California: imazamox-treated tomatoes in the Davis site only experienced minor early season injury while tomatoes at the Woodland site were severely injured or killed. Chapter 2 aimed to investigate the cause of this discrepancy in injury. A study was conducted to investigate imazamox sorption in four California soils to determine if differences in herbicide adsorption played a role in variable crop injury observed in the California field trials. To determine the sorption capacity of imazamox of each soil, a batch equilibrium study was conducted. There were significant differences in sorbed imazamox: the clay soil had the highest adsorption, followed by the sandy loam soil, while the loam soils from the Davis and Woodland trial sites had the lowest adsorption and were not significantly different from one another. The results from this study illustrate only minor differences in imazamox adsorption among the soils tested which suggests that soil type was likely not a major factor contributing to the discrepancy in imazamox injury in the earlier field trials. Chapter 3 aimed to further develop and refine chemigated rimsulfuron treatments. Following the lack of crop safety of chemigated imazamox and positive results of chemigated rimsulfuron in field studies in 2022, field research was conducted in 2023 and 2024 to evaluate various application timings of chemigated rimsulfuron alone, PPI sulfosulfuron paired with chemigated rimsulfuron, as well as foliar maleic hydrazide alone and in combination with PPI sulfosulfuron and chemigated rimsulfuron. In 2023, all treatments with a total of 70 g ai/ha rimsulfuron alone or paired with PPI sulfosulfuron reduced broomrape emergence 77-92% compared to the nontreated control (P<0.0001). In 2024, all rimsulfuron treatments reduced broomrape emergence 68-86% compared to the nontreated control (P<0.0001). In both years, five applications of foliar maleic hydrazide reduced broomrape emergence through at least midseason; however, in 2024 a late flush of broomrape was observed in late summer in these treatments. The 2024 combination treatment of PPI sulfosulfuron, chemigated rimsulfuron, and foliar maleic hydrazide was the best treatment overall, reducing broomrape emergence 96% versus the nontreated control (P<0.0001). Under a recently approved 24c label, growers can currently use three applications of rimsulfuron applied via chemigation to suppress broomrape in known infested fields or to reduce the risk of broomrape in fields of concern for this quarantine pest. Promising results from sulfosulfuron and maleic hydrazide suggest that the registration of additional herbicides could help develop even more robust branched broomrape management programs. Chapter 4 aimed to evaluate integrated cultural control practices for branched broomrape management. Although there are now approved herbicides for in-season management, the development of integrated pest management strategies is needed to reduce the current infestation and mitigate the spread of this highly-regulated parasitic weed. Herbicide efficacy research was conducted in 2020 and 2021 in a field known to be highly infested with branched broomrape; however, during the 2021 experiment no broomrape emerged in any plots. Because tomato cultivar and planting dates differed between the two years, several experiments were conducted to evaluate the impact of cultivar and transplant date on broomrape parasitism in processing tomato. A greenhouse study to evaluate variation in broomrape resistance across 20 cultivars, including the cultivar planted in the 2021 trial (‘SVTM 9024’) and 19 other top commercial cultivars was conducted in 2022 and repeated in 2023 and 2024. Resistance was evaluated based on ability to host broomrape as well as temporal differences in broomrape emergence. In field studies, variation in resistance was also evaluated, with several commercial processing tomato varieties and grafted combinations evaluated during 2022, 2023, and 2024. Planting date studies were also conducted in 2022 and 2024 to evaluate the effects of delayed tomato transplanting on broomrape emergence. Results from the 2022 greenhouse screening indicated that all cultivars, including ‘SVTM 9024’, were susceptible to broomrape parasitism, but that there could be variation in temporal emergence. The 2023 greenhouse studies confirmed this, with significant variation in the timing of broomrape emergence between ‘SVTM 9025’ and ‘H9553’ (P=0.05). The 2024 greenhouse study conducted in mid-summer had no broomrape emergence in any of the cultivars, likely due to hot conditions in the greenhouse leading to secondary dormancy of the preconditioned seed. Field cultivar studies conducted in 2023 and 2024 showed no significant differences in broomrape emergence among conventional or grafted cultivar combinations (P = 0.23, 0.74); results for the non-grafted cultivars were consistent with greenhouse studies. Broomrape was significantly reduced by later planting dates in the 2024 trial (P = <0.001), with no emergence at the latest planting date (June 10, 2024); 2022 planting date study was consistent with these results, with a 52% reduction on emergence at the later date, although differences were not significant (P = 0.21). Taken together, these experiments suggest that there is some variation in resistance among the tested commercial cultivars and delayed transplanting seems to reduce broomrape emergence and future research will seek to confirm this.

Tiafenacil is a protoporphyrinogen oxidase (PPO) inhibitor that blocks chlorophyll biosynthesis and causes protoporphyrin accumulation, a highly photodynamic intermediate. The lipid peroxidation and cell membrane destruction caused by tiafenacil leads to plant death. Glufosinate inhibits glutamine synthetase (GS), a key enzyme for amino acid metabolism and photorespiration. Glufosinate leads to plant death by a massive accumulation of reactive oxygen species (ROS). Herbicide mixtures are commonly used in agriculture to increase weed control spectrum and reduce selection pressure for herbicide resistance. Tiafenacil is registered in the US for preplant use on annual crops such as corn and soybean, but not in orchard crops. Glufosinate is commonly used in orchards with a rate range of 650 to 998 g ai ha-1. Field studies were conducted to determine the crop safety of tiafenacil on young almond, walnut, prune, and pistachio trees, as well as the weed control efficacy on broadleaf and grass weeds relevant to California orchard crops. To evaluate crop safety, tiafenacil was applied at 74 and 148 g ai ha-1 three times per year at the base of prune, walnut, and pistachio trees that were less than one-year-old at the time of the first application. A similar almond experiment also included a 222 g ai ha-1 rate of tiafenacil in the protocol. In all four tree crop experiments, treatments were applied once in the spring of 2020, then reapplied three times during early 2021 and early 2022 so that plots were treated a total of 7 times during a three-year period. There was no visual injury on any of the young trees between 30 and 700 days after initial treatment. Similarly, there were no treatment effects on tree diameter even at the highest rate of tiafenacil. Although no yield measurement was taken because of the age of the trees, the relatively few fruits that formed appeared to be normal. In a separate study on weed control efficacy, tiafenacil at 12 g ai ha-1 performed statistically similarly with tiafenacil plus glufosinate in most instances, but control of both broadleaf and grass weeds numerically improved when tiafenacil was applied in mixture with glufosinate. In a greenhouse study, tiafenacil at 12 g ha-1 alone provided 98-100% control of barnyardgrass and 95-98% control of junglerice. There was no significant difference between tiafenacil alone or tiafenacil plus glufosinate; although in some instances, control of junglerice and barnyardgrass was numerically higher with the tankmix than with glufosinate alone. Most postemergence PPO inhibiting herbicides registered in tree crops have activity only on broadleaf weeds; however, these results indicate that tiafenacil has good activity on broadleaf and grass weeds relevant to California orchard crops and that crop safety was acceptable at up to 2- or 3-fold the expected use rate in tree crops. Although tiafenacil has some activity on grass weeds, mixing tiafenacil with glufosinate may be needed for the most reliable control of both broadleaf and grass weeds.