California Rice Fields Research Articles

Factors influencing shorebird use of post‐harvest flooded rice fields in California's Sacramento Valley

AbstractBecause of the importance of the Central Valley of California, USA, to shorebirds along the Pacific Flyway, conservation investments have been made in the area's agricultural fields and managed wetlands. Increasingly, landowner incentive programs are being used to deliver shorebird habitat, presenting opportunities to answer remaining questions about which implementation strategies and field management practices are most effective at attracting birds. To provide management guidance for these investments, we collected and analyzed 5 years of data (2014–2018) on shorebird abundance in flooded rice fields enrolled in a dynamic habitat incentive program. The program incentivized flooding in fallow and post‐harvest rice fields in seasons when habitat is relatively sparse, specifically the early fall and late spring. Across nearly 9,000 field observations over 5 years, we explored the relationship between abundance and density (birds/ha) of shorebirds and vegetation cover, soil clay, landscape‐level flooding, and local flood timing, duration, and depth. We observed more shorebirds in fields that were approximately 50% flooded, had water depths of 5–10 cm, and had minimal rice straw or stubble cover, with strong or very strong evidence for each of these relationships. We found that the timing of habitat provisioning was important, with moderate evidence that earlier fall flooding and strong evidence that the duration of fall flooding was associated with higher shorebird density. We observed lower shorebird densities in locations with ample flooded rice habitat in surrounding areas, potentially because shorebirds spread out across the landscape. We found very strong evidence that flooding consistency, either at a site that was continually flooded over many months or a site that had been flooded in previous years, was associated with higher shorebird density. Soil clay content was associated with decreased observed shorebird density, potentially through its influence on the ability of shorebirds to forage for invertebrate prey. These results suggest best practices for shorebird habitat creation in agricultural landscapes, providing important information for conservation and population recovery.

Emergence timing of smallflower umbrella sedge (Cyperus difformis), barnyardgrass (Echinochloa crus-galli), and bearded sprangletop (Leptochloa fusca spp. fascicularis) in California water-seeded rice

Abstract 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.

Assessing clethodim spot spraying applications for control of problematic weedy rice and other grasses in California rice fields

AbstractSpot spraying applications offer the opportunity to target specific weeds in a field, while simultaneously reducing herbicide usage and increasing the long‐term efficacy of chemical control options. The study is focused on controlling California weedy rice accessions (Oryza spp.) and problematic grass weeds with a spot spray application of clethodim in a flooded rice system. The efficacy of incorporating nonionic surfactant to clethodim applications was also assessed. Dose‐response experiments were carried out in a greenhouse on five weedy rice accessions, common grass rice weeds, and cultivated rice varieties L207, M105, M206, M209, M211, and S102 to determine the dose needed to affect these populations. Clethodim was applied in a field setting to assess spot spraying efficacy, the possibility of herbicide dispersion in the water, and crop injury. Clethodim successfully controlled weedy rice and grasses in the greenhouse. The effective rates to control 90% of the five test populations (ED90) were between 51 and 74 g ai ha−1 clethodim for weedy rice accessions. Adding nonionic surfactant to clethodim increased its efficacy by 1.6‐ to 1.9‐fold. Cultivated rice varieties did not exhibit any tolerance to clethodim, however, spot spraying applications at 150 g ai ha−1 clethodim did not cause any dispersion in the field. Clethodim spot spray application was effective both at the three‐ to four‐leaf growth stage and tillering growth stage for weedy rice.

Assessment of potassium soil balances and availability in high yielding rice systems

Plant demand for K in rice is comparable to nitrogen. With yields and management practices changing, refining K fertility management and decision-making tools is necessary. Our objectives were to determine (1) how soil K balances affect soil K indices, (2) the best soil test indicator of plant K availability, and (3) the relationships between plant and soil K indices. We assessed soil (plow layer) and flag-leaf samples from 55 commercial rice fields in California. Growers provided historical information on K fertility practices, straw management, and yields to develop a soil K balance. A soil K balance average of − 15 kg K ha−1 yr−1 (range: − 72 to 47) suggested an increased likelihood of K infertility; however, K balance was not correlated with soil K indices or flag-leaf K. This, plus the potential for K losses and fixation of surplus K, suggests that attempting to build up soil K may not be an effective strategy. Soil K indices were poorly correlated with plant K, but 1 N NH4OAc-extractable K (Kext) was the best of the K indices tested. Soils that were low in Kext had low clay content, CEC, and K saturation, a high Ca + Mg:K ratio, and showed evidence of K fixation. Soil K varied regionally and may be related to irrigation water and soil parent material. The critical level for Kext (currently 60 mg K kg−1) may need to be revised based on our findings, as there was evidence of K deficiencies above this concentration.

Reconciling fish and farms: Methods for managing California rice fields as salmon habitat.

Rearing habitat for juvenile Chinook Salmon (Oncorhynchus tshawytscha) in California, the southernmost portion of their range, has drastically declined throughout the past century. Recently, through cooperative agreements with diverse stakeholders, winter-flooded agricultural rice fields in California’s Central Valley have emerged as ecologically functioning floodplain rearing habitat for juvenile Chinook Salmon. From 2013 to 2016, we conducted a series of experiments examining methods to enhance habitat benefits for fall-run Chinook Salmon reared on winter-flooded rice fields in the Yolo Bypass, a modified floodplain managed for flood control, agriculture, and wildlife habitat in the Sacramento River Valley of California. Investigations included studying the effect of 1) post-harvest field substrate; 2) depth refugia; 3) duration of field drainage; and 4) duration of rearing occupancy on in-situ diet, growth and survival of juvenile salmon. Post-harvest substrate treatment had only a small effect on the lower trophic food web and an insignificant effect on growth rates or survival of rearing hatchery-origin, fall-run Chinook Salmon. Similarly, depth refugia, created by trenches dug to various depths, also had an insignificant effect on survival. Rapid field drainage yielded significantly higher survival compared to drainage methods drawn out over longer periods. A mortality of approximately one third was observed in the first week after fish were released in the floodplain. This initial mortality event was followed by high, stable survival rates for the remainder of the 6-week duration of floodplain rearing study. Across years, in-field survival ranged 7.4–61.6% and increased over the course of the experiments. Despite coinciding with the most extreme drought in California’s recorded history, which elevated water temperatures and reduced the regional extent of adjacent flooded habitats which concentrated avian predators, the adaptive research framework enabled incremental improvements in design to increase survival. Zooplankton (fish food) in the winter-flooded rice fields were 53-150x more abundant than those sampled concurrently in the adjacent Sacramento River channel. Correspondingly, observed somatic growth rates of juvenile hatchery-sourced fall-run Chinook Salmon stocked in rice fields were two to five times greater than concurrently and previously observed growth rates in the adjacent Sacramento River. The abundance of food resources and exceptionally high growth rates observed during these experiments illustrate the potential benefits of using existing agricultural infrastructure to approximate the floodplain wetland physical conditions and hydrologic patterns (shallow, long-duration inundation of cool floodplain habitats in mid-winter) under which Chinook Salmon evolved and to which they are adapted.

Quantum Yield for the Aqueous Photochemical Degradation of Chlorantraniliprole and Simulation of Its Environmental Fate in a Model California Rice Field.

The photochemical degradation of chlorantraniliprole (3-bromo-N-[4-chloro-2-methyl-6-(methylcarbamoyl)phenyl]-1-(3-chloro-2-pyridine-2-yl)-1H-pyrazole-5-carboxamide; CAP) was characterized under simulated solar light with 2-nitrobenzaldehyde (2NB) actinometry. Overall, aqueous CAP degraded quickly via direct photodegradation with no significant difference observed between high-purity water and filtered rice field water. The 24-h average half-life normalized to summer sunlight using 2NB was 34.5 ± 4.0 h (jCAP,env = 0.020 ± 0.0023 h-1 , n = 3), and the calculated apparent quantum yield in simulated sunlight was 0.0099 ± 0.00060. These new values were used-alongside previously characterized data for air/and soil/water partitioning, degradation in soil, and hydrolysis-in the Pesticides in Flooded Applications Model to simulate CAP dissipation in a model California (USA) rice field. The model estimates an environmental half-life of 26 d in the aqueous phase, but the bulk of applied CAP remains in the benthic zone and degrades, with estimated half-lives of 29 and 92 d in flooded and drained fields, respectively. Environ Toxicol Chem 2020;39:1929-1935. © 2020 SETAC.

The contribution of percolation to water balances in water-seeded rice systems

Rice (Oryza sativa) has one of highest applied water footprints of any crop, due in many cases to high percolation and lateral seepage rates in flooded rice fields. Better understanding the magnitude and variability of these subsurface water flows and their contribution to the rice field water balance is critical for efforts to reduce the water footprint of rice production and to limit the transport of pollutants to surface water and groundwater. Percolation was directly measured in eight direct-seeded California rice fields that ranged from 20 to 61% clay and a complete water balance was developed for three of these fields. For these latter fields, directly measured percolation rates were compared to percolation calculated with Darcy’s law, and combined percolation and lateral seepage calculated as the residual of a water balance was compared to directly measured values. Across eight fields, cumulative percolation over the growing season ranged from 0.04 to 6.95 cm season−1. The mean cumulative percolation for the three water balance fields was 2.1 cm based on direct measurements compared to 3.2 cm based on Darcy’s law calculations. Combined percolation and lateral seepage calculated as the residual term of a water balance for the three fields was 17.1 cm, compared to 4.4 cm based on direct measurements, corresponding to 13.2 % and 3.4 % of water inputs, respectively (inputs were 98–99 % from irrigation). Based on these results, water management strategies that remove floodwater (e.g. alternate wetting and drying) would have limited potential to reduce water inputs in California rice production (0.7–2.7 cm season-1). Furthermore, using the most conservative (largest) estimates for each component of the water balance, we conclude that the average water requirement for California rice fields is approximately 108 cm season-1.

Effect of Rice Winter Cultural Management Practices on the Size of the Hatching Population of Triops longicaudatus (Notostraca: Triopsidae) in California Rice Fields.

The tadpole shrimp [Triops longicaudatus (Leconte)] has emerged as a significant pest of rice grown in California in recent decades. The change in T. longicaudatus' pest status has coincided with changes in cultural management of residual rice straw postharvest. Policy changes have reduced the postharvest burning of fields from nearly 95% to less than 10%, promoting increased use of winter flooding as a means of accelerating straw decomposition. Field and laboratory trials were conducted from 2015 to 2017 at the Rice Experiment Station in Biggs, CA and in greenhouses at the University of California (UC) Davis to evaluate the effects of burning, flooding, and a fallow control on T. longicaudatus population dynamics. Experiments demonstrated that burning of rice straw failed to suppress densities of hatching T. longicaudatus and actually had the reverse effect, causing a 51% increase in numbers hatching, perhaps as a result of burning triggering termination of multiyear T. longicaudatus egg dormancy. Winter flooding had no measurable effect on T. longicaudatus hatch. Thus, these changes in winter cultural practices do not appear to be responsible for the emergence of T. longicaudatus as a major rice pest.

Influence of pH and Divalent Metals Relevant to California Rice Fields on the Hydroxide-Mediated Hydrolysis of the Insecticide Chlorantraniliprole

The hydrolysis of chlorantraniliprole (3-bromo-N-[4-chloro-2-methyl-6-(methylcarbamoyl)phenyl]-1-(3-chloro-2-pyridine-2-yl)-1H-pyrazole-5-carboxamide; CAP) was investigated over the pH range of 6-10, reflective of California rice field conditions, with variable additions of Cu2+, Zn2+, Mn2+, or Ni2+. Dissipation accelerated as pH increased with half-lives ranging from 26.9 to 2.2 days with slight inhibition in rice field water. The addition of divalent metals was not observed to catalyze the hydrolysis of CAP at pH 6, indicating that the insecticide is likely to remain recalcitrant to hydrolysis in neutral or acidic surface waters. However, Mn2+ and Ni2+ were observed to inhibit hydrolysis at pH 8 and 9. Attenuated total reflectance Fourier transform infrared analysis supports the conclusion that divalent metals may withdraw electron density from the amide nitrogen via interaction with the amide oxygen, though additional quantum chemical modeling is necessary to provide further mechanistic insights. Overall, the hydrolysis of CAP in California rice fields and their surrounding surface waters will be dominated by pH and inhibited by dissolved metal species.

Mechanism of clomazone resistance in Leptochloa fusca spp. fasicularis to clomazone

Bearded sprangletop (Leptochloa fusca spp. fasicularis) is a problematic weed in California rice (Oryza sativa) production. Historically, bearded sprangletop is controlled with clomazone in California rice fields. The continuous use of clomazone and lack of crop rotation in rice fields resulted in resistance to clomazone in several bearded sprangletop. The objective of this research was to determine the clomazone mechanism of resistance of two bearded sprangletop populations in California rice by investigating clomazone absorption, translocation, and metabolism under controlled environmental conditions in two resistant (CRBS1 and CRBS2) and one susceptible (S) populations. Absorption and translocation of 14C-clomazone were similar in R and S. Clomazone metabolism, as determined by inhibition of cytochrome P450 enzymes with malathion and determining clomazone metabolites profile, was different between S and R 3 days after treatment. Bearded sprangletop pretreated with malathion was 2-fold more sensitive to clomazone than when treated with clomazone alone, suggesting that cytochrome P450-mediated clomazone metabolism might be involved in the mechanism of resistance. An HPLC-MS/MS analysis revealed differences in clomazone metabolism between R and S biotypes. Hydroxymethylclomazone was the most abundant metabolite found in R plants with three and five-fold more abundant in CRBS1 and CRBS2, respectively, when compared to S plants. 5-ketoclomazone, the known toxic metabolite of clomazone, accumulated 2-fold more in S plants than in R plants at 72 h after treatment. This research shows that clomazone is metabolized differently between R and S populations of bearded sprangletop and that P450 monooxidation is likely involved in the mechanism of resistance.

Modeling germination of smallflower umbrella sedge (Cyperus difformisL.) seeds from rice fields in California across suboptimal temperatures

AbstractSmallflower umbrella sedge is a prolific C3weed commonly found in rice fields in 47 countries. The increasing infestation of herbicide-resistant smallflower umbrella sedge populations threatens rice production. Our objectives for this study were to characterize thermal requirements for germination of smallflower umbrella sedge seeds from rice fields in California and to parameterize a population thermal-time model for smallflower umbrella sedge germination. Because the use of modeling techniques is hampered by the lack of thermal-time model parameters for smallflower umbrella sedge seed germination, trials were carried out by placing field-collected seeds in a thermogradient table set at constant temperatures of 11.7 to 41.7 C. Germination was assessed daily for 30 d, and the whole experiment was repeated a month later. Using probit regression analysis, thermal time to median germination [θT(50)], base temperature for germination (Tb), and SD of thermal times for germination [σθT(50)] were estimated from germination data, and model parameters were derived using the Solver tool in Microsoft Excel®. Germination rates increased linearly below the estimated optimum temperatures of 33.5 to 36 C. EstimatedTbaveraged 16.7 C, whereasθT(50)equaled 17.1 degree-days and σθT(50)was only 0.1 degree-day. The estimatedTbfor smallflower umbrella sedge is remarkably higher than that ofjaponicaandindicatypes of rice, as well asTbof important weeds in theEchinochloacomplex. Relative to the latter, smallflower umbrella sedge has lower thermal-time requirements to germination and greater germination synchronicity. However, it would also initiate germination much later because of its higherTb, given low soil temperatures early in the rice growing season in California. When integrated into weed growth models, these results might help optimize the timing and efficacy of smallflower umbrella sedge control measures.

Influence of Flooding, Salinization, and Soil Properties on Degradation of Chlorantraniliprole in California Rice Field Soils.

Chlorantraniliprole (3-bromo-N-[4-chloro-2-methyl-6-(methylcarbamoyl)phenyl]-1-(3-chloro-2-pyridine-2-yl)-1H-pyrazole-5-carboxamide; CAP) was granted supplemental registration for use in rice cultivation in California through December, 2018. Previous work investigated the partitioning of CAP in California rice field soils; however, its degradation in soils under conditions relevant to California rice culture has not been investigated. The degradation of CAP in soils from two California rice fields was examined under aerobic and anaerobic conditions with varying salinity via microcosm experiments. Results indicate that soil properties governing bioavailability may have a greater influence on degradation than flooding practices or field salinization over a typical growing season. Differences between native and autoclaved soils (t1/2 = 59.0-100.2 and 78.5-171.7 days) suggest that biological processes were primarily responsible for CAP degradation; however, future work should be done to confirm specific biotic processes as well as to elucidate abiotic processes, such as degradation via manganese oxides and formation of nonextractable residues, which may contribute to its dissipation.

The magnitude and variability of lateral seepage in California rice fields

Lateral seepage can be an important water loss pathway at the field level and a conduit for the discharge of pesticides and nutrients from rice fields. However, few studies have directly measured lateral seepage rates in flooded rice fields. This study sought to characterize the magnitude and variability of lateral seepage in California rice fields, and to explore the relationship between lateral seepage and soil properties or hydrologic conditions. Lateral seepage was measured during the growing season at 50 locations spread across six rice fields using a methodology that operates analogously to a double-ring infiltrometer. Lateral seepage rates varied over four orders of magnitude (0.05–33.11 cm2 h−1 or 0.0011–1.05 cm h−1), though even the highest rate measured was small compared to typical water inputs in California rice fields, and lateral seepage corresponded to only 1.0%–1.9% of water inputs at sites where irrigation inputs were measured. Lateral seepage was negatively correlated with levee width (p = 0.025) and the relative water height in the adjacent field, supply canal, or drainage ditch (p = 0.045), though this relationship explained little of the variation in lateral seepage rates (marginal r2 = 0.129). Contrary to expectations, not all supply canals or higher flooded fields were a source for lateral seepage into fields, as unsaturated zones present in most levees served as a sink for lateral seepage from both sides of the levee. Future research should examine the prevalence of preferential flow pathways in rice field levees and the potential for transport of nutrients or agrochemicals through these flow pathways.

Environmental fate and impact assessment of thiobencarb application in California rice fields using RICEWQ

Thiobencarb is a commonly used herbicide in Northern California rice fields. Released paddy water containing thiobencarb may pose ecological risks to non-targeted organisms. In this research, the Rice Water Quality Model (RICEWQ) is equilibrium tested and then calibrated using monitoring data at field level. Then it is employed to assess the environmental fate and impacts of thiobencarb in the Colusa Basin, and the effects of different management practices on water use and thiobencarb exposures. The model predicted thiobencarb concentrations from rice fields for multiple years throughout the Basin, using input from California Pesticide Use Reporting (PUR) database, and assessed both the temporal/spatial distribution of thiobencarb exposure and potential acute toxicity on non-target organisms.Our study indicated that RICEWQ can accurately reflect the initial partitioning of thiobencarb in both paddy water and soil phases and capture the dynamics of thiobencarb at field level after calibration. Mandatory water holding is critical for reducing thiobencarb exposure in released paddy water. A thirty-day holding time reduces thiobencarb concentrations by 64% relative to a 6-day holding practice. The geo-spatial pattern of exposure in the study domain indicates the differing extents of pollutant levels and their distribution over space. “Risk zones” for different species were identified based on the geospatial patterns of thiobencarb exposure and the species-specific susceptibilities of various non-target species to thiobencarb.

Aqueous Photolysis of Benzobicyclon Hydrolysate.

Benzobicyclon [3-(2-chloro-4-(methylsulfonyl)benzoyl)-2-phenylthiobicyclo[3.2.1]oct-2-en-4-one] is a pro-herbicide used against resistant weeds in California rice fields. Persistence of its active product, benzobicyclon hydrolysate, is of concern. As an acidic herbicide, the neutral species photolyzed faster than the more predominant anionic species ( t1/2 = 1 and 320 h, respectively; natural sunlight), from a >10-fold difference in the quantum yield. Dissolved organic matter in natural waters reduced direct photolysis and increased indirect photolysis compared to high-purity water. Light attenuation appears significant in rice field water and can slow photolysis. These results, used in the pesticides in flooded applications model with other experimental properties, indicate that a floodwater hold time of 20 days could be sufficient for dissipation of the majority of initial aqueous benzobicyclon hydrolysate prior to release. However, soil recalcitrance of both compounds will keep aqueous benzobicyclon hydrolysate levels constant months after benzobicyclon application.

Increased Human Incidence of West Nile Virus Disease near Rice Fields in California but Not in Southern United States.

Anthropogenic land use change, including agriculture, can alter mosquito larval habitat quality, increase mosquito abundance, and increase incidence of vector-borne disease. Rice is a staple food crop for more than half of the world's population, with ∼1% of global production occurring within the United States (US). Flooded rice fields provide enormous areas of larval habitat for mosquito species and may be hotspots for mosquito-borne pathogens, including West Nile virus (WNV). West Nile virus was introduced into the Americas in 1999 and causes yearly epidemics in the US with an average of approximately 1,400 neuroinvasive cases and 130 deaths per year. We examined correlations between rice cultivation and WNV disease incidence in rice-growing regions within the US. Incidence of WNV disease increased with the fraction of each county under rice cultivation in California but not in the southern US. We show that this is likely due to regional variation in the mosquitoes transmitting WNV. Culex tarsalis was an important vector of WNV in California, and its abundance increased with rice cultivation, whereas in rice-growing areas of the southern US, the dominant WNV vector was Culex quinquefasciatus, which rarely breeds in rice fields. These results illustrate how cultivation of particular crops can increase disease risk and how spatial variation in vector ecology can alter the relationship between land cover and disease.

Water and air temperature impacts on rice (Oryza sativa) phenology

Air temperature (Ta) is commonly used for modeling rice phenology. However, since the growing point of rice is under water during the vegetative and the early part of the reproductive period, water temperature (Tw) is likely to have a greater influence on crop developmental rates than Ta during this period. To test this hypothesis, we monitored Tw, Ta, and crop phenology in three commercial irrigated rice fields in California, USA. Sampling locations were set up on along a transect from the water inlet into the field. (Water warms up as it moves into the field.) Ta averaged 22.7 °C across sampling locations within each field, but average seasonal Tw increased from 22 °C near the inlet to 23.4 °C furthest away from the inlet. Relative to Tw furthest from the inlet, low Tw near the inlet delayed time to panicle initiation (PI 5 days) and heading (HD 8 days) and the appearance of one yellow hull on the main stem panicle (R7 9 days). Using Tw instead of Ta when the active growing point is under water until booting (midway between PI and HD) in a thermal time model improved accuracy (root-mean-square error, RMSE) for predicting time to PI by 2.5 days and HD by 1.6 days and R7 by 1.8 days. This model was further validated under more typical field conditions (i.e., not close to cold water inlets) in six locations in California. Under these conditions, average Tw was 2.6 °C higher than Ta between planting and booting, primarily due to higher daily maximum Tw values. Using Tw in the model until booting improved RMSE by 1.2 days in predicting time to HD. Using Tw instead of Ta during this period could improve the accuracy of rice phenology models.

Impact of Simulated California Rice-Growing Conditions on Chlorantraniliprole Partitioning.

Chlorantraniliprole (3-bromo-N-[4-chloro-2-methyl-6-(methylcarbamoyl)phenyl]-1-(3-chloro-2-pyridine-2-yl)-1H-pyrazole-5-carboxamide, CAP; water solubility 1.023 mg·L-1) was recently registered for application on California rice fields. Air- and soil-water partitioning of CAP were investigated under simulated California rice field conditions through calculation of KH and ΔawH and a batch equilibrium method following OECD 106 guidelines, respectively. KH and ΔawH were determined to be 1.69 × 10-16 - 2.81 × 10-15 atm·m3·mol-1 (15-35 °C) and 103.68 kJ·mol-1, respectively. Log(Koc) ranged from 2.59 to 2.96 across all soil and temperature treatments. Log(KF) ranged from 0.61 to 1.14 across all soil, temperature, and salinity treatments. Temperature and salinity increased sorption significantly at 35 °C (P < 0.05) and 0.2 M (P < 0.0001), respectively, while soil properties impacted sorption across all treatments. Overall results, corroborated using the Pesticides in Flooded Applications Model, indicate that volatilization of CAP is not a major route of dissipation and sorption of CAP to California rice field soils is moderately weak and reversible.

Dissipation of the Herbicide Benzobicyclon Hydrolysate in a Model California Rice Field Soil.

The herbicide benzobicyclon (BZB; 3-(2-chloro-4-(methylsulfonyl)benzoyl)-2-phenylthiobicyclo[3.2.1]oct-2-en-4-one) has recently been approved for use on California rice fields by the United States Environmental Protection Agency (U.S. EPA). Hydrolysis of BZB rapidly forms the active compound, benzobicyclon hydrolysate (BH), whose fate is currently not well understood. A model California rice soil was used to determine BH soil dissipation. The pKa and aqueous solubility were also determined, as experimental values are not currently available. Sorption data indicate BH does not bind tightly, or irreversibly, with this soil. Flooding resulted in decreased BH loss, indicating anaerobic microbes are less likely to transform BH compared to aerobic microorganisms. Temperature increased dissipation, while autoclaving decreased BH loss. Overall, dissipation was slow regardless of treatment. Further investigation is needed to elucidate the exact routes of loss in soil, though BH is expected to dissipate slowly in flooded rice field soil.

Genetic Structure of the Rice Blast Pathogen (Magnaporthe oryzae) over a Decade in North Central California Rice Fields

Rice blast, caused by the ascomycete Magnaporthe oryzae, is one of the most destructive rice diseases worldwide. Even though the disease has been present in California since 1996, there is no data for the pathogen population biology in the state. Using amplified fragment length polymorphisms and mating-type markers, the M. oryzae population diversity was investigated using isolates collected when the disease was first established in California and isolates collected a decade later. While in the 1990 samples, a single multilocus genotype (MLG) was identified (MLG1), over a decade later, we found 14 additional MLGs in the 2000 isolates. Some of these MLGs were found to infect the only rice blast-resistant cultivar (M-208) available for commercial production in California. The same samples also had a significant decrease of MLG1. MLG1 was found infecting the resistant rice cultivar M-208 on one occasion whereas MLG7 was the most common genotype infecting the M-208. MLG7 was identified in the 2000 samples, and it was not present in the M. oryzae population a decade earlier. Our results demonstrate a significant increase in genotypic diversity over time with no evidence of sexual reproduction and suggest a recent introduction of new virulent race(s) of the pathogen. In addition, our data could provide information regarding the durability of the Pi-z resistance gene of the M-208. This information will be critical to plant breeders in developing strategies for deployment of other rice blast resistance genes/cultivars in the future.