Bradley D. Hanson

Branched broomrape was presumed to have been eradicated in California in the 1980s but has reemerged as a threat to the processing tomato industry. Although chemicals for suppressing branched broomrape have been used in the past few years as an emergency response, limited research has been conducted on cultural practices that could be incorporated into integrated pest management programs. Three laboratory-scale studies were conducted in 2024 and 2025 to evaluate the responses of branched broomrape seeds to nitrogen fertilization and flooding. The fertilizer study evaluated the effects of diammonium hydrogen phosphate, calcium nitrate tetrahydrate, potassium chloride, potassium nitrate, ammonium nitrate, ammonium sulfate, and urea on branched broomrape seed germination and radicle elongation at various concentrations (0, 1.56, 3.125, 6.25, 12.5, 25, and 50 mM) and at various seed germination stages (preconditioning, germination stimulation, and radicle elongation). The flooding studies evaluated the effects of flooding duration (3, 7, 14, 28, 42, and 56 d) and temperature (10 C and 28 C) on branched broomrape seed germination and the number of seed attachments to the tomato host plant. Results from the fertilizer study showed that fertilizers containing diammonium hydrogen phosphate, ammonium nitrate, and urea inhibited branched broomrape seed germination and radicle elongation, and this effect was observed only when fertilizers were applied during the germination stimulation stage. In the flooding studies, branched broomrape seed germination dropped to 20% after 14 d of flooding at 28 C. Flooding at both 10 C and 28 C reduced broomrape seed attachment to tomato plants by 50% within 7 d in the greenhouse study. These laboratory-scale data suggest that fertilization management and flooding have the potential to control branched broomrape infestation as cultural management strategies, but the results should be validated at the field scale in future experiments.

The root parasitic weed branched broomrape [ Phelipanche ramosa (L.) Pomel] spends most of its life cycle underground, complicating early detection and control. A predictive model based on growing degree days (GDD) can improve management by anticipating key developmental stages. This study aimed to characterize P. ramosa phenology and develop a thermal time model to inform more effective control strategies. Two greenhouse experiments using rhizotrons were conducted over 2 yr (2021 to 2022), along with a complementary pot experiment in 2022 at Davis, CA. The P. ramosa development was classified into four phenological stages: attachment, elongation, emergence, and full flowering. We recorded the thermal timing (GDD accumulation from transplanting) and number of individuals at each phenological stage and modeled the development dynamics using an inverse Weibull function. Of the 255 P. ramosa attachments recorded across two rhizotron runs, 87% advanced to elongation, 63% reached emergence, and only 49% successfully developed to full flowering. Model estimates indicated that 5% of attachments occurred at 299 GDD and 95% by 730 GDD, marking a critical control window. After the initial attachment, on average, it took about 85 GDD for an attachment to initiate the elongation process, and 385 and 815 GDD to reach 5% and 95% elongation, respectively. Emergence followed, with 5% to 95% occurring from 520 to 1,100 GDD. Full flowering occurred roughly 181 GDD after emergence, spanning 700 (5%) to 1,280 (95%) GDD after transplanting. Model validation using data from related field experiments conducted in 2022 and 2023 in a P. ramosa –infested tomato ( Solanum lycopersicum L.) field in Yolo County, CA, confirmed the model’s accuracy in predicting full flowering with root mean-square error (RMSE) of 142 GDD. This thermal time model offers a valuable decision support tool to optimize the timing of P. ramosa management, particularly in-season soil-applied herbicide programs, in tomato cropping systems.

Quinclorac is a selective postemergence herbicide with activity on both grasses and broadleaf weeds, including the problematic field bindweed, making it useful in systems with mixed weed populations. Field studies conducted from 2021 to 2022 in California, Michigan, New Jersey, New York, and Oregon assessed both crop safety and weed control efficacy for potential registration in grapes. Quinclorac was applied as a directed spray at 0.42 and 0.84 kg ai ha ⁻¹ at three timings: 90 d before harvest (Timing A), 60 d before harvest (Timing B), and post-harvest (Timing C). No grapevine injury was observed in California or Oregon. However, injury occurred in Michigan, New Jersey, and New York, with maximum ratings up to 32%, though severity varied by year and location. Mixed-effects modeling showed that cumulative rainfall and soil clay-plus-organic matter (COM) content explained 52% of the variation in injury (P < 0.001), with injury increasing as rainfall and COM content rose. Despite visible symptoms at some low-COM, high-rainfall sites, quinclorac had no significant effect on yield, berry size, or dormant pruning weights. Field bindweed control was consistently improved by quinclorac, particularly when a post-harvest application was included. Under the evaluated rates and timings, quinclorac posed an unacceptable risk of grapevine injury in many environments and is not currently suitable for use in grapes. Future work should focus on identifying management practices to mitigate the potential for injury.

Branched broomrape management is of increasing concern to California processing tomato growers. Field research was conducted in 2023 and 2024 to evaluate various application timings of chemigated rimsulfuron alone, preplant-incorporated (PPI) sulfosulfuron paired with chemigated rimsulfuron, and foliar maleic hydrazide alone and paired with PPI sulfosulfuron and chemigated rimsulfuron. In 2023, all treatments with 70 g ai ha⁻¹ rimsulfuron, alone or paired with PPI sulfosulfuron, reduced broomrape emergence 77% to 92% compared to the nontreated control. In 2024, broomrape pressure was higher, and all rimsulfuron treatments reduced broomrape emergence 68% to 86% compared to the control. In both years, five applications of foliar maleic hydrazide reduced broomrape emergence through at least mid-season. The 2024 experiment included a combination treatment of PPI sulfosulfuron, chemigated rimsulfuron, and foliar maleic hydrazide, which resulted in −1. In a 2024 grower-scale demonstration trial, two application regimes totaling 70 g ai ha⁻¹ of chemigated rimsulfuron reduced broomrape emergence 83% to 89% compared to the control. Overall, chemigated rimsulfuron applied at various timings and rates totaling 70 g ai ha⁻¹ reduced broomrape emergence by two-thirds or more compared to the nontreated plots. No crop injury was observed in trials with rimsulfuron, sulfosulfuron, or maleic hydrazide treatments in small-plot trials or with rimsulfuron in the grower-scale demonstration trial. Under a recently approved 24(c) Special Local Need label, California growers can use three applications of rimsulfuron applied via chemigation to suppress broomrape in known infested fields or to reduce the risk of broomrape establishment in fields of concern for this quarantine pest. Promising results from sulfosulfuron and maleic hydrazide suggest that registering additional herbicides could help develop even more robust branched broomrape management programs.

Off-target herbicide drift away from rice is historically a concern in California, where susceptible crops such as orchards and vineyards are nearby. Tetflupyrolimet is a potent inhibitor of dihydroorotate dehydrogenase that provides excellent grass weed control in rice cropping systems. In efforts to steward tetflupyrolimet before its registration in California, this research was conducted to compare the onset of foliar symptoms from tetflupyrolimet applications onto almond, grapevine, peach, pistachio, plum, and walnut. Tetflupyrolimet was applied to these tree and vine crops at fractional rates of 1/200×, 1/100×, 1/33×, and 1/10× of the 125 g ai ha^–1 recommended use rate on rice. Almond, pistachio, and walnut trees also received 1× of the use rate. Tetflupyrolimet treatments were applied on one side of 3- to 4-yr-old almond, peach, pistachio, plum, and walnut trees, and on one side of 25- to 26-yr-old grapevines in 2022 and 2023. Visible injury ratings were carried out weekly to assess symptomology throughout the growing seasons and at leaf-out the following springs. Tree trunk diameter was recorded before and after herbicide applications. No injury was observed to any tested crops, regardless of the tetflupyrolimet application rate. In all orchard crops, tree trunk diameter was not affected by tetflupyrolimet treatments. Likewise, grape yield was not reduced even at the 1/10× tetflupyrolimet fractional rate. Since no injury symptoms were recorded, this research suggests that tetflupyrolimet can be safely used on nearby rice fields and might be a target for future registration consideration for use on orchard and vineyard crops.

Off-target movement of herbicides is a concern in California rice production, where sensitive crops are often grown nearby. Florpyrauxifen-benzyl and triclopyr are auxin mimics that are commonly used in rice systems. To steward florpyrauxifen-benzyl around the time of its initial registration in the state, research was conducted to compare the onset of foliar symptoms from simulated florpyrauxifen-benzyl and triclopyr drift onto grapevine, peach, and plum. The use rates on rice were 1/200×, 1/100×, 1/33×, and 1/10× of 29.4 g ai ha^–1 florpyrauxifen-benzyl; and 1/200×, 1/100×, and 1/33× of 420.3 g ae ha^–1 triclopyr. Herbicides were applied on one side of 1- to 2-year-old peach and plum trees and one side of established grapevines in 2020 and 2021. The general symptoms from applications of florpyrauxifen-benzyl and triclopyr were similar and included chlorosis, leaf curling, leaf distortion, leaf malformation, leaf crinkling, and necrosis. The symptoms from herbicides were observed on both sides of the grapevine canopy, whereas florpyrauxifen-benzyl symptoms on peach and plum were mostly observed on the treated side of the tree. Florpyrauxifen-benzyl and triclopyr symptoms were observed 3 d after treatment (DAT) for grapevines and 7 DAT for peach and plum. In all crops, most symptoms persisted through 42 DAT. Some grape clusters showed deformation and dropping of berries. All treated crops gradually recovered during the season regardless of application rates. Because symptoms in peach and plum were relatively minor, this research suggests that application precautions to reduce off-site drift are likely to minimize the occurrence of significant injury. However, grapevines were more sensitive and showed injury symptoms of up to 71% at 14 DAT with a simulated drift rate of 1/10× florpyrauxifen-benzyl. Therefore, extra precautions, such as using drift-management agents and closely monitoring wind speed conditions at the time of florpyrauxifen-benzyl applications may be necessary if vineyards are nearby.

Rice herbicide drift poses a significant challenge in California, where rice fields are near almond, pistachio, and walnut orchards. This research was conducted as part of a stewardship program for a newly registered rice herbicide and specifically aimed to compare the onset of foliar symptoms resulting from simulated florpyrauxifen-benzyl drift with residues in almond, pistachio, and walnut leaves at several time points after exposure. Treatments were applied to one side of the canopy of 1- and 2-yr-old trees at 1/100X and 1/33X of the florpyrauxifen-benzyl rice field use rate of 29.4 g ai ha^–1 in 2020 and 2021. Symptoms were observed 3 d after treatment (DAT) for pistachio and 7 DAT for almond and walnut, with peak severity at approximately 14 DAT. While almond and walnut symptoms gradually dissipated throughout the growing season, pistachio still had symptoms at leaf out in the following spring. Leaf samples were randomly collected from each tree for residue analysis at 7, 14, and 28 DAT. At 7 DAT with the 1/33X rate, almond, pistachio, and walnut leaves had florpyrauxifen-benzyl at 6.06, 5.95, and 13.12 ng g^–1 fresh weight (FW) leaf, respectively. By 28 DAT, all samples from all crops treated with the 1/33X drift rate had florpyrauxifen-benzyl at less than 0.25 ng g^–1 FW leaf. At the 1/100X rate, pistachio, almond, and walnut residues were 1.78, 2.31, and 3.58 ng g^–1 FW leaf at 7 DAT, respectively. At 28 DAT with the 1/100X rate, pistachio and almond samples had florpyrauxifen-benzyl at 0.1 and 0.04 ng g^–1 FW leaf, respectively, but walnut leaves did not have detectable residues. Together, these data suggest that residue analysis from leaf samples collected after severe symptoms may substantially underestimate actual exposure due to the relatively rapid dissipation of florpyrauxifen-benzyl in nut tree foliage.

Tiafenacil is registered in the United States for use in annual crops such as corn and soybean, but not on orchard crops. Field studies were conducted to determine orchard crop safety and efficacy of tiafenacil on important California orchard weeds. To evaluate crop safety, tiafenacil was applied at 74, 148, and 222 g ai ha⁻¹ alone and with 38 g ai ha⁻¹ of tolpyralate three times per year at the base of almond, pistachio, prune, and walnut trees. The first treatment was applied 2 mo after the trees had been transplanted. In all four tree crop experiments, treatments were applied once in May 2020, then three times again during the winter of 2021 and 2022 at 21-d treatment intervals. There were no visual foliar injury symptoms or treatment-related effects on tree trunk diameter change even at the highest tested rate of tiafenacil applied seven times over three growing seasons. In a separate study of weed control, in most instances, tiafenacil applied at 12 g ai ha⁻¹ performed similarly to that of tiafenacil plus glufosinate. Control of glyphosate-resistant hairy fleabane with tiafenacil applied alone at 25 g ai ha⁻¹ was 65% by 14 d after treatment. Tiafenacil applied at 50 g ai ha⁻¹ to hairy fleabane performed similarly to glufosinate plus glyphosate. In a greenhouse study, tiafenacil applied at 12 g ha⁻¹ provided 95% to 100% control of barnyardgrass and junglerice, and there was no significant difference between tiafenacil applied alone or with glufosinate. Saflufenacil applied alone or in a mixture with glufosinate was not as effective as the tiafenacil treatments for grass weed control. Based on experiments conducted over three growing seasons in four tree fruit and tree nut crops, tiafenacil crop safety appeared to be acceptable even at up to 2- or 3-fold the expected use rate.

Weed management in cantaloupe and other melon crops is important to maximize fruit yield; however, there are few registered herbicides available in California. Several independent herbicide trials were conducted at University of California field stations in Davis (Yolo County), Five Points (Fresno County), and Holtville (Imperial County) from 2013 to 2019 to evaluate both registered and unregistered herbicides and incorporation methods (sprinklers, cultivation, or none) for crop safety and weed control in melons. Although specific treatments varied among locations depending on local practice and research objectives, ethalfluralin and halosulfuron were used in all experiments, and bensulide and S-metolachlor were evaluated in 4 of 6 site-years. Additional herbicides included clethodim, clomazone, DCPA, napropamide, pendimethalin, sethoxydim, and sulfentrazone. Among registered herbicides, halosulfuron, halosulfuron + ethalfluralin, and ethalfluralin + bensulide combinations provided consistently beneficial weed control across all site-years compared to the nontreated control. S-metolachlor performed as well as the best of the registered herbicides tested at each site-year; although moderate injury was noted at the Davis location, this did not reduce melon yield. The method used to incorporate preplant herbicides had a significant impact on weed control efficacy but varied by location. Mechanical incorporation of preplant herbicides resulted in improved weed control and yield compared to sprinklers. Early-season weed control, whether by herbicides or hand weeding, resulted in significant yield increase in most site-years.

Threespike goosegrass ( Eleusine tristachya ) is a difficult-to-control perennial grass of increasing concern for orchard production systems in the Central Valley of California, USA. This grass has a bunch-type growth habit when tillered, which can interfere with orchard operations, particularly nut pickup from the ground at harvest. From 2016 to 2019, herbicide efficacy on threespike goosegrass was evaluated in a walnut ( Juglans regia ) orchard in Chico, CA, USA; an almond ( Prunus dulcis ) orchard in Livingston, CA, USA; and a prune ( Prunus domestica ) orchard in Orland, CA, USA. At each location, two independent experiments were conducted to evaluate 12 preemergent (PRE) herbicide treatments and eight postemergent (POST) treatments over several years, for a total of 16 trials. PRE herbicides were applied in January according to the region’s typical winter orchard management practices. One treatment included an additional sequential application in March to extend residual activity later into the warm season when threespike goosegrass germinates or resumes growth. In separate studies, POST control of established stands of threespike goosegrass was evaluated in May and June of each year. Each trial was conducted in a randomized complete block design with four replications. Threespike goosegrass control was visually estimated monthly for 5 months after the PRE treatments or at weekly intervals for 5 weeks following POST treatments. The most effective PRE treatment was a sequential application of indaziflam in January, followed by a March application of pendimethalin, providing 90% or greater control of threespike goosegrass 5 months after treatment across all sites and all years. Of the POST treatments, the three graminicides outperformed the other treatments with 73% to 91% control overall sites and years at 5 weeks after treatment. Fluazifop had the highest control ratings (85% to 91%) among the graminicide herbicides but was not always statistically better than clethodim or sethoxydim (74% to 83% control). Glyphosate alone resulted in unacceptable control (33% to 51%) regardless of rates tested, experimental sites, or years. Together, these results confirm grower reports of poor glyphosate performance on threespike goosegrass but suggest that effective herbicide programs can be developed to manage threespike goosegrass using PRE herbicides and POST graminicides registered in California orchard crops.