The windmill palm is a distinctive outdoor ornamental palm adapted to cooler climates. Weeds pose significant challenges in palm nurseries, particularly during seedling and establishment stages. This research was conducted in a nursery with 5,500 windmill palm seedlings, starting in April 2014, when the palm trees were 3 yr old. Experiments were terminated in October 2018 when weed control was no longer necessary due to the advanced growth of the palm trees. The objectives of this study were to determine the weed composition and diversity, elucidate the effects of mechanical weed management (MWM) on growth rate of palm, and develop a sustainable program to maximize palm tree growth through effective weed management and soil tillage. Few herbicides are registered for nursery use in Türkiye, thus weed control was performed mechanically using garden hoeing machines between rows and hand hoeing for intrarow strips. The most common and dense weeds were purple nutsedge, annual mercury, and common purslane in summer and autumn, and burning nettle in winter and spring. In 2014, weed densities were 100, 127, and 145 weeds m–2 for MWM, hand-weeding (HW), and nontreated (NT) plots, respectively. Transplanted palm seedlings required at least two, ideally three growing seasons of intensive weed control until the palm tree crowns block sunlight and suppress weed growth. The research indicated that palm trees in the MWM treatment had approximately 84 leaves and a height of 210 cm by October 2018, compared with 54 leaves and a height of 136 cm for HW, and 40 leaves and 100 cm height for NT. These results highlight the critical role of MWM in promoting optimal growth of Chinese windmill palms. Effective and sustainable weed management, combining MWM and HW, is essential for producing high-quality palm trees. The research provides valuable insights for nursery managers and contributes to best practices for cultivating windmill palm trees in similar climatic regions.

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.

Weed management in California water-seeded rice (Oryza sativa L.) is challenging due to herbicide-resistant weeds. Research on additional herbicide options is necessary to control herbicide-resistant weeds. Pendimethalin is a dinitroaniline herbicide commonly used in dry-seeded rice; however, it is not registered in water-seeded rice. This study was conducted to determine the pendimethalin fate in water-seeded rice after application to 1-, 3-, and 5-leaf stage rice. Ultra-high performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS) was utilized to quantify pendimethalin and degradants in the water, soil, and rice seedling tissue at 1, 5, and 14 d after treatment (DAT). More than 50% of recovered pendimethalin was observed in the rice tissue and more than 25% in the soil, with the least amounts observed in the water at all application timings and sampling dates. Three pendimethalin degradants were observed at low concentrations: p36, [1-(1-ethylpropyl)-5,6-dimethyl-7-nitro-1H-benximidazole]; p44, [4-[(1-ethylpropyl) amino]-2-methyl-3,5-ditrobenzoic acid]; and p48 [4,5-dimethyl-3-nitro-N2-(pentan-3-yl) benzene-1,2-diamine]. Degradant p36 was observed in all samples and most abundant in the soil. Degradants p36 and p44 increased in concentration in the water by 14 DAT. Degradants p44 and p48 were at low concentrations or below the lowest level of quantification in water, soil, and tissue samples. The pendimethalin parent molecule remained intact and was not readily metabolized in rice tissue. The crown region and shoots of the rice seedlings demonstrated greater pendimethalin concentrations compared with the roots at all rice stages; however, pendimethalin concentrations remained similar across the three sample timings. Rice root and shoot reduction was 16% and 13%, respectively, after the 1-leaf stage application averaged over sample timings, and 6% and 4%, respectively, after the 5-leaf stage application. The results suggest the rice stage at the application timing is an important factor for pendimethalin tolerance; therefore, encouraging early root development can be beneficial for pendimethalin tolerance in water-seeded rice.

Common cattail is a perennial weed that naturally occurs in wet or saturated soils, such as in marshes, lakes, ponds, irrigation and drainage canals, and streams, throughout North America. Recently, common cattail has become an important problem for the drill-seeded rice systems in the Sacramento–San Joaquin River Delta of northern California. This research was conducted in 2022 and 2023 at three sites near Stockton, CA, to evaluate the efficacy of florpyrauxifen-benzyl, a newly registered auxin-mimic herbicide, to control common cattail in drill-seeded rice. Florpyrauxifen-benzyl was applied alone at 40 g ai ha–1 and 80 g ha–1 on 0- to 1-m-tall and 1- to 2-m-tall common cattail and in a sequential application of florpyrauxifen-benzyl at 40 g ha–1 followed by 40 g ha–1 between 14-d intervals on 0- to 1-m-tall and 1- to 2-m-tall common cattail. Triclopyr, another auxin-mimic rice herbicide widely used in California, was applied alone at 420 g ae ha–1 on 0- to 1-m-tall common cattail for comparison. Triclopyr was also applied in combination with florpyrauxifen-benzyl at 40 g ha–1 at the 0- to 1-m growth stage. The injury symptoms on common cattail started within 3 d after treatment (DAT) for the florpyrauxifen-benzyl + triclopyr mixture treatment and within 7 DAT for all other florpyrauxifen-benzyl applied treatments. All florpyrauxifen-benzyl treatments controlled 100% of common cattail at 28 DAT regardless of application rate and timing. Common cattail height and dry biomass at 28 DAT were lower for all treatments compared to the nontreated control. While the common cattail control was excellent for all florpyrauxifen-benzyl applications, rice injury was minimal. This research indicates that common cattail up to 2 m tall can be effectively and rapidly controlled in rice fields with florpyrauxifen-benzyl at 40 g ha–1.

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.

Bearded sprangletop is a problematic native grass weed in California’s rice fields. The widespread and extensive use of acetyl-CoA carboxylase (ACCase)–inhibiting herbicides, such as cyhalofop-p-butyl (cyhalofop), has led to speculation that biotypes of bearded sprangletop have developed herbicide resistance to ACCase. The aim of this study was to evaluate suspected resistant bearded sprangletop biotypes, R1, R2, R3, and the susceptible biotype, S1, in terms of their levels of resistance to three ACCase-inhibiting herbicides and to characterize the molecular mechanisms of resistance. Dose–response experiments suggested that the biotype R1, R2, and R3 had high-level resistance to cyhalofop and to quizalofop-p-ethyl (quizalofop), but not clethodim. The study determined that the resistance to ACCase inhibitors was a target-site mechanism resulting from nucleotide substitution. The carboxyl transferase (CT) domain of the ACCase gene’s sequence analysis revealed the substitutions Trp-2027-Cys for R1 and R2 biotypes and Ile-2041-Asn for the R3 biotype. This study revealed the presence of target-site resistance to cyhalofop and quizalofop in at least two mutation points in representative biotypes of bearded sprangletop in California. This research highlights the significance of careful herbicide selection due to weed species responding quite rapidly to selection pressure, so as to aid in managing bearded sprangletop in rice fields.

The spread of herbicide-resistant weeds is considered a major problem for rice production in California, and there is a need for new herbicides. Tetflupyrolimet is a new herbicide with a novel dihydroorotate dehydrogenase–inhibiting site of action that has strong activity on grasses. Three field studies were conducted at the California Rice Experiment Station in Biggs, CA, in 2022 and 2023 to (1) determine control of watergrass species and bearded sprangletop with tetflupyrolimet, (2) characterize the effects of tetflupyrolimet combined with other herbicides on weed control and rice, and (3) determine the response of rice cultivars to tetflupyrolimet. In the first study, tetflupyrolimet was applied at preemergence (PRE) or at the 1- to 2-leaf stage of rice (POST) at 0.1, 0.125, or 0.15 kg ai ha−1 followed by carfentrazone. Tetflupyrolimet provided ≥99% control of watergrass species and 100% bearded sprangletop control regardless of the rate or application timing, while showing no crop injury symptoms or yield reduction. In the second study, tetflupyrolimet was applied PRE or POST at 0.1 or 0.15 kg ai ha−1 followed by herbicides labeled for use in California rice production. Tetflupyrolimet provided ≥98% control of watergrass species, which was better than the grower standard treatment, and ≥97% control of bearded sprangletop. In the third study, tetflupyrolimet was applied PRE or POST at 0.125, 0.15, 0.25, or 0.3 kg ai ha−1 followed by carfentrazone. The six California rice cultivars evaluated, ‘M-105’, ‘M-206’, ‘M-209’, ‘M-211’, ‘L-208’, and ‘CM-203’, did not show any trend of crop injury caused by tetflupyrolimet. Overall, tetflupyrolimet provided a high level of control of watergrass species and bearded sprangletop without causing visual rice injury or yield reductions, regardless of rice cultivar, when applied alone or in combination with commonly used sedge and broadleaf herbicides in California water-seeded rice.

Currently, a limited number of herbicides is available to treat water-seeded rice in California, with widespread resistance to most of those herbicides. Because no resistant grasses showed resistance to pendimethalin, a series of studies were conducted to evaluate water-seeded rice response to pendimethalin. In a field study conducted at the Rice Experiment Station at Biggs, California, in 2020 and 2021, three pendimethalin formulations, a granule (GR), emulsifiable concentrate (EC), and capsule suspension (CS), were applied at 1.1, 2.3, and 3.4 kg ai ha−1 rates, and at 5, 10, and 15 d after seeding onto water-seeded rice. In addition, a greenhouse study was conducted to examine the response of five common California rice cultivars to GR and CS formulation applications. Echinochloa control levels were reduced at 15 d after seeding after use of EC and CS formulations compared with earlier timings. In both years, rice grain yields were increased by 3,014 kg ha−1 after application of pendimethalin at 3.4 kg ai ha−1 when applied at 15 d after seeding compared with 5 and 10 d after seeding, and similar to 1.1 kg ai ha−1 applications. The GR and CS were safer formulations based on a reduction in injury and an increase in grain yields compared to the EC formulation. Differences in seedling vigor across cultivars appeared to incur an advantage after a pendimethalin application. However, most cultivars evaluated for stand reduction and dry biomass demonstrated tolerance to GR and CS formulation applications only after rice reached the 3-leaf stage. In contrast, an application at 1-leaf stage rice reduced stand up to 68%. Application rate, timing, and formulation are important factors to consider if the use of pendimethalin in water-seeded rice is to be pursued.

Late-season weed emergence in California rice fields complicates decisions concerning the timing of control measures. The objective of this study was to predict the emergence of three problematic weed species in rice using thermal time models. Smallflower umbrella sedge, barnyardgrass, and bearded sprangletop seedlings were counted and removed daily at three locations across the Sacramento Valley rice-growing region in 2018. The accumulation of thermal time (growing degree days; GDD) commenced with the initial flooding of the fields at each location, utilizing the specific base temperatures corresponding to each species. The pattern of emergence as a function of GDD was modeled with a Weibull function. Root-mean-square values for comparing actual and model-predicted cumulative emergence values were 6% to 23%. Percent cumulative emergence initially increased rapidly for smallflower umbrella sedge and reached 90% emergence with accumulation of 13 GDD. Barnyardgrass emerged after smallflower umbrella sedge and reached 90% emergence with an accumulation of 124 GDD. Bearded sprangletop had a delay of 64 GDD compared to barnyardgrass to reach first emergence and reached 90% emergence at 215 GDD. The period of weed emergence at all field sites differed across the three species and led to a continuous spectrum of weed emergence over time. This study characterizes the emergence of three economically important rice weeds and provides useful information for the timing of weed management. Typical herbicide applications on the day of seeding may have less efficacy on the late-emerging weeds, causing reduced weed control. Delayed herbicide application, overlay of residual herbicides, or use of herbicides with longer residual activity are suggested to control late-emerging weeds.

The increasing development of herbicide resistance in weeds found in rice cropping systems has encouraged researchers to evaluate alternate herbicides to prevent and manage herbicide-resistant weed biotypes. Metribuzin is a photosynthetic-inhibiting herbicide that controls various important grass and broadleaf weeds. Several crops, including soybean, wheat, peas, and potato, have shown differential varietal responses to metribuzin. To determine whether rice has differential varietal responses to metribuzin for potential utilization in a rice breeding program, greenhouse experiments were conducted to evaluate the responses of 142 long-, medium-, and short-grain rice genotypes to the herbicide. Metribuzin was applied at 0, 22, 44, 88, 176, and 352 g ai ha−1 when rice plants were in the 3- to 4-leaf stage. Crop response regarding phytotoxicity, height reduction, and biomass reduction was evaluated. Metribuzin caused significant injury to all rice genotypes tested, but short-grain rice genotypes were, on average, more susceptible than medium- and long-grain rice genotypes. Short-grain rice genotypes generally had greater height reduction and produced less biomass than long-grain or medium-grain rice genotypes. Crop visual injury ratings were correlated with plant height reductions and biomass reductions. The results indicate that the level of metribuzin tolerance in rice is inadequate for commercial use; however, further research is needed to develop higher levels of herbicide resistance by mutagenized rice cultivars.

“Weedy” red rice is a problematic weed with phenotypic similarities to cultivated rice. Limited herbicide availability has driven a need for nonchemical control options for managing this pest. One preplanting strategy that is being explored is the stale seedbed methodology, which aims to maximize soil seedbank withdrawals via germination. This technique is adapted in rice by flooding a field, waiting for germination and emergence of weed seedlings, and completing the method with a mechanical or chemical control application. Optimization of this process is dependent on maximizing weed seed germination, which is primarily influenced by both temperature and moisture availability. Germinability across a range of these factors is not well understood in California red rice. Thus, this study aimed to determine germinability of California red rice accessions under various temperature and water potential treatments. Previously described red rice accessions 1, 2, 3, and 5, along with ‘M206’, a common California rice cultivar, were exposed to temperatures from 10 to 40 C in 5 C increments in combination with water potentials of 0, −0.2, −0.4, or −0.8 MPa until either germination or weed seed decay occurred. Statistical analysis indicated a three-way interaction between accession, temperature, and water potential. Germination reached 95% or greater when seeds were exposed to temperatures from 20 to 35 C in combination with 0 or −0.2 MPa. Germination was lowest when seeds were water stressed (−0.8 MPa) and when temperatures were colder than 20 C or warmer than 35 C. The ‘M206’ cultivar was utilized for comparison and demonstrated cold tolerance by germinating at 10 C, whereas weedy accessions 1, 2, and 3 did not. When temperatures were at or above 15 C, however, ‘M206’ germinated less often compared with all weedy accessions. Historical preplant temperatures in this region align with those required for weedy rice germination. Thus, the stale seedbed methodology is a viable strategy in years when ample floodwater is available.

Clomazone is a widely used herbicide in California water-seeded rice for control of bearded sprangletop and watergrass. Generally, clomazone is applied to a flooded rice field at day of rice seeding. However, interest exists among growers to delay the clomazone application. Weather variability may encourage growers to practice Leathers’ method. Leathers’ method is the practice of draining the field 1 to 2 d after air seeding to encourage better and more uniform seedling establishment, then reflooding back to a 10- to 15-cm flood 4 to 7 d later. Therefore the objective of this study was to evaluate grass weed control and rice response at four rates of clomazone, applied at two timings: at day of seeding (DOS) in a continuous 10-cm flood and after Leathers’ method. This study was conducted in 2019 and 2020 at the Rice Experiment Station in Biggs, CA. In 2019, there were no difference across clomazone rates on control of bearded sprangletop independent of application timing used; however, in 2020, bearded sprangletop control with clomazone applied after Leathers’ method was 70% to 71% across clomazone rate by 60 d after treatment (DAT), compared to 92% to 97% in the DOS applications. Watergrass control was 100% in 2019 across clomazone rate and application timing. However, in 2020, watergrass control was greater at the DOS application at 54% to 71%. Clomazone applied at the 0.7 kg ha−1 Leathers’ method resulted in 84% bleaching by 14 DAT and was similar across all Leathers’ method clomazone applications and the 0.7 kg ha−1 DOS application. There was no rice grain yield difference among all clomazone-treated plots, with the exception of the 0.7 kg ha−1 Leathers’ method interaction with the DOS applications.

Water seeding is a common cropping strategy in mechanized rice systems. Water seeding of rice can suppress grass weeds, but it can also encourage aquatic weeds and grass ecotypes that escape deep floodwater. In addition, water seeding prevents many cultural methods of weed control and limits available herbicides. Selection pressure from a limited palette of herbicides has resulted in widespread resistance in rice grown in California. This study examined a novel combination of drill seeding and a stale seedbed (“stale-drill”) as a means of using a nonselective herbicide to manage weeds before rice emergence. In 2016 and 2017, rice cultivar ‘M-206’ was drilled at a rate of 120 kg ha−1 to 1.3-cm, 2.5-cm, and 5.1-cm depths. Planting rice deeper than 1.3 cm delayed emergence by 3 to 4 d. A postplant-burndown (PPB) treatment of glyphosate at 870 g ha−1 was applied just prior to rice emergence. Treatment delays had mixed effects on weed control. PPB treatment was more effective at controlling Echinochloa spp. in 2017, reducing density by 30%, 48%, and 73% at 1.3-cm, 2.5-cm, and 5.1-cm seeding depths, respectively. The greatest overall weed control either year was found with applications of glyphosate + pendimethalin followed by penoxsulam + cyhalofop at 1.3-cm planting depth. Rice stand and yield components were more strongly affected by planting depth in 2017 than in 2016, possibly owing to cool weather immediately after seeding. Yields in 2017 were reduced in deeper plantings by up to 72%. Therefore, if the stale-drill method is implemented with higher-vigor cultivars or higher seeding rates, we see potential in this method as a useful tool for reducing herbicide-resistant weeds in rice fields.

Weedy rice (Oryza sativa f. spontanea Roshev.) has recently become a significant botanical pest in California rice (Oryza sativa L.) production systems. The conspecificity of this pest with cultivated rice negates the use of selective herbicides, rendering the development of nonchemical methods a necessary component of creating management strategies for this weed. Experiments were conducted to determine the emergence and early growth responses of O. sativa spontanea to flooding soil and burial conditions. Treatment combinations of four flooding depths (0, 5, 10, and 15 cm) and four burial depths (1.3, 2.5, 5, and 10 cm) were applied to test the emergence of five O. sativa spontanea accessions as well as ‘M-206’, a commonly used rice cultivar in California, for comparison. Results revealed that burial depth had a significant effect on seedling emergence. A 43% to 91% decrease in emergence between seedlings buried at 1.3 and 2.5 cm depending on the flooding depth and accession and an absence of emergence from seedlings buried at or below 5 cm were observed. Flooding depth did not affect emergence, but there was a significant interaction between burial and flooding treatments. There was no significant difference between total O. sativa spontanea emergence from the soil and water surfaces regardless of burial or flooding depths, implying that once the various accessions have emerged from the soil they will also emerge from the floodwater. Most accessions had similar total emergence compared with M-206 cultivated rice but produced more dry weight than M-206 when planted at 1.3 cm in the soil. The results of this experiment can be used to inform stakeholders of the flooding conditions necessary as well as soil burial depths that will promote or inhibit the emergence of California O. sativa spontanea accessions from the weed seedbank.

California rice (Oryza sativa L.) production has been recently challenged by the early-season nuisance algae blooms. The algal community in rice is a complex of green algae (Nostoc spongiforme Agardh ex Bornet) and cyanobacteria species that can develop a thick algal mat on the surface of the water and interfere with the emergence and establishment of rice seedlings. The objective of this research was to determine the impact of algal infestation level on rice seedling emergence. A mesocosm study was conducted in 57-L tubs. Three levels of algal infestation (low, medium, and high) were produced by adding fertilizer (N:P) into the tubs at 0:0, 75:35, and 150:70 kg−1 ha. Sixty rice seeds (‘M-206’) were soaked for 24 h and spread into tubs filled with water. Photosynthetic active radiation (PAR), chlorophyll a concentration as the quantitative measure of algae, number of emerged rice seedlings, and their dry biomass were studied during the experiment. Results showed that algal infestation can directly change the amount of light received into the water. Minimum, maximum, and mean percentage of PAR inside the water declined as the algal infestation level increased. As a consequence, rice seedling emergence dropped under the high algal pressure. At very high algal infestation (i.e., chlorophyll a concentration of above 500 µg ml−1), rice seedling emergence was reduced up to 90%. Furthermore, rice seedling emergence was delayed under algal infestation. When algal infestation was low, time to 50% rice seedling emergence (t50) ranged between 5 and 10 d, while at high algal infestation, t50 ranged between 12 and 20 d. Moreover, individual rice seedling biomass was reduced from 1 g to 0.01 g as algal infestation increased. The results from this study indicate that uncontrolled algae at the beginning of the rice-growing season could reduce rice seedling emergence, establishment, and rice stand. Given that algal infestation in fields has a patchy pattern, loss of rice stand in these patches could provide empty niches for other weeds to grow.

Late watergrass is a competitive weed of rice that is well adapted to both aerobic and anaerobic environments. Cultural controls such as a stale-seedbed and alternating from wet- to dry-seeding have been proposed as management options. However, the effects of these systems on its emergence and early growth are unknown. The objective of this study was to modify a previously developed population-based threshold model (PBTM) to predict emergence and early growth under field conditions. In 2013, a series of experiments were conducted at the California Rice Experiment Station (CRES) in Biggs, CA, to evaluate emergence and early growth of multiple herbicide–resistant and -susceptible late watergrass at four burial depths (0.5, 2, 4, and 6 cm) under three irrigation regimes: continuously flooded (CF), daily flush (DF), and intermittent flush (IF). Resistant plants emerged at a significantly higher rate under the IF treatment (P < 0.05). Both biotypes showed decreasing emergence with increasing depth, and no plants emerged from the 4- or 6-cm depths in the CF treatment. Using the Gompertz growth curve, resistant plants had greater predicted growth rates (k), lower predicted maximum heights (h max ), and a shorter time to predicted maximum growth rate (t m ) than susceptible plants under the CF and DF treatments. Under the IF treatment, the susceptible plants had greater k, lower h max , and shorter time to predicted t m . Information about burial depth and irrigation was incorporated into a previously developed PBTM for late watergrass, and validated at the CRES in a field with a susceptible late watergrass population in 2013 and 2014, under two irrigation systems, CF and IF. Model fit was best in the CF treatments (average Akaike information criteria [AIC] = 199.05) compared to the IF treatments (average AIC = 208.6).

Weedy rice (Oryza sativa f. spontanea Rosh.) is an emerging weed of California rice (Oryza sativa L.) that has potential to cause large yield losses. Early detection of weedy rice in the field is ideal to effectively control and prevent the spread of this weed. However, it is difficult to differentiate weedy rice from cultivated rice during early growth stages due to the close genetic and phenotypic relatedness of cultivated rice and weedy rice. The objective of this study is to examine phenotypic variation in weedy rice biotypes from California and to identify traits that could be used to visually identify weedy rice infestations at early growth stages for effective management. Greenhouse experiments were conducted in 2017 and 2018 using five phenotypically distinct biotypes of weedy rice found in California, along with diverse cultivated, weedy, and wild rice types in a randomized complete block design. We measured variation for 13 phenotypic traits associated with weedy rice and conducted principal component analysis and factor analysis to identify important weedy traits. Most weedy rice individuals within a biotype clustered together by phenotypic similarity. Pericarp color, hull color, chlorophyll content, grain length, plant height, leaf pubescence, collar color, and leaf sheath color account for most of the observed variation. California weedy rice biotypes are phenotypically distinct from wild rice and from weedy rice from the southern United States in their combinations of seed phenotypes and vegetative characteristics. In comparison with the locally grown temperate japonica cultivars, California weedy rice tends to be taller, with lower chlorophyll content and a red pericarp. Weedy rice biotypes vary in seed shattering and seed dormancy. For weedy rice management, plant height and chlorophyll content are distinct traits that could be used to differentiate weedy rice from the majority of cultivated rice varieties in California during vegetative stages of rice growth.

Weedy rice is an emerging problem of cultivated rice in California. Infestations of weedy rice in cultivated rice result in yield loss and reduced grain quality. In this study, we aimed to evaluate growth and yield components of a widely grown cultivated rice variety in California in response to weedy rice competition. Greenhouse competition experiments in an additive design were conducted in 2017 and 2018 to determine the growth and yield components of ‘M-206’ rice and five weedy rice biotypes found in California at varying weed densities. M-206 rice initially grew at a faster relative growth rate of 0.53 cm−1 wk−1 under competitive conditions compared with 0.47 cm−1 wk−1 in the absence of weedy rice, but absolute and relative growth rates declined more rapidly under competitive conditions as plants approached maturity. At harvest, M-206 plant height was reduced 13% under competitive conditions, and M-206 tiller number was reduced 23% to 49%, depending on the weedy rice biotype it was competing with. Except for 100-grain weight, the growth traits and grain yield components of M-206 rice were reduced with increasing density of weedy rice. At the highest weed density measured, 40 plants m−2, M-206 rice had yield losses of 69% grain yield plant−1, 69% panicle weight, 59% fresh and dry biomass, 55% grain yield panicle−1, and 54% panicle number. The five evaluated weedy rice biotypes varied widely in early growth rates, height, biomass production, and grain yield, indicating differing competitive strategies. Most weedy rice biotypes produce plants with greater plant height, tiller number, panicle number, and above- and below-ground biomass compared with cultivated rice. Weedy rice biotypes produced 45% to 57% higher grain yield per plant than M-206 rice under competitive conditions.

Bearded sprangletop is a problematic weed in California rice production and few herbicides provide effective control. As control of bearded sprangletop has declined, grower suspicion of resistance to clomazone has increased, because of the continuous rice cropping system and herbicide dependence in the region. The objectives of this research were to confirm clomazone resistance in bearded sprangletop populations and determine the level of resistance. Seed from 21 suspected clomazone-resistant populations was collected from the California rice growing region. A greenhouse experiment was conducted to determine population sensitivity to clomazone. Clomazone was applied into the water to emerging seedlings. Plant ht and control of bearded sprangletop were recorded weekly for 3 wk, plants were then harvested, and dry weight was measured. Of the populations tested, 17 were susceptible and four (5%) were resistant to clomazone. A dose-response assay was conducted using eight doses ranging from an eighth of the full rate to 12 times the full rate. The three most resistant populations had resistant-to susceptible ratios of 1.25×, 2×, and 5× the labeled rate of clomazone. The use of clomazone in California rice production is beneficial; however, it should be used at the appropriate timing and as part of an herbicide program to prevent further development of clomazone resistance.