Evolution of Resistant Echinochloa Spp. in California Rice Claudia E. Marchesi12, CA Greer1, JE Hill1, MA Jasieniuk1, M Canevari1, RG Mutters1, RE Plant1 and AJ Fischer1 1University of California, Davis, USA; 2Instituto Nacional de Investigación Agropecuaria, Uruguay URUGUAY INTRODUCTION Late watergrass (Echinochloa phyllopogon, LWG), Early watergrass (E. oryzoides, EWG) and Barnyardgrass (E. crus-galli, BYG) are the most competitive and difficult weeds to control in rice. Herbicide resistance (R) in some of these Echinochloas (ECH) was confirmed in California in 1998. Factors that shape the evolution and spread of herbicide R weeds can be associated with crop management and landscape properties. RESULTS and DISCUSSION Approximately 39%, 26% and 35% of the samples were EWG, LWG, and BYG, respectively. Resistant accessions were mostly distributed in the northern SV (Fig. 1) . There were no R-ECH in the SJV. Figure 1. Distribution of LWG, EWG and BYG populations with % control by thiobencarb (100% = total control). Water-seeding rice on heavy clay soils where crop rotations are difficult to implement has limited the scope of herbicide options available. Moreover, rice straw incorporation after harvest enriches the weed seed bank in the soil. Alternative tillage and stand establishment methods to break weed cycles and allow for the strategic use of herbicides are being tested as crop management tools for delaying the evolution of R. Resistance gene-flow can compromise the success of crop management and ECH seed dispersal control becomes an additional requisite to delay the evolution of R. LWG has an almost bimodal response to thiobencarb, suggesting that LWG population was highly selected and currently uniform, with distinct highly R or S populations. EWG shows a smooth gradient of % control, suggesting an on-going selection process and/or polygenic resistance. Few BYG populations are R (Fig. 2). This could be associated with the water-seeded rice system of California, which is unfavorable to BYG recruitment. Lack of R-LWG in the SJV and bimodal pattern of responses to thiobencarb (Fig. 2) is in accordance with the hypothesis of R-LWG having spread from one single introduction (founder effect and narrow genetic base of R biotypes). No other variables in the CART study related to R spread in this species (not shown). Therefore, minimizing seed movement between fields should be a priority for delaying or reducing R-LWG evolution and spread. OBJECTIVE: to establish how interactions between management practices, landscape variables and mechanisms of resistance dispersal contribute to define the patterns of ECH resistance in California rice. The CART study for EWG (Fig.3) suggests landscape and management variables are relevant in determining resistance spread patterns. Also, preliminary studies had indicated population genetic structure diversity and resistance endowed by independent mutations in R-EWG populations (Jasieniuk et al. 2008). Thus, reducing selection pressure should be prioritized in EWG. LWG A B Soils: Alfisols Entisols 74.86 n = 29 Molinate use: No Yes 43.4 n = 3 64.2 n = 1 48.6 n = 4 Soils: Vertisols Mollisols 89.73 n = 6 75.68 n = 19 79.06 n = 25 Rice history: short long Cyhalofop use: 81.88 n = 8 71.18 n = 11 Permanent flood: 86.35 n = 2 67.81 n = 9 Late seeding Early seeding 77.2 60.3 n = 5 Rent equipment Own equipment 94.5 71.43 A: EWG, LWG and BYG populations growing in pots at greenhouse, no herbicide treatment; B: Same as A but with 1120 g ai ha-1 thiobencarb, 21 days after treatment. METHODOLOGY Seed samples (n=150), representing populations of either EWG, LWG or BYG, were taken from rice fields along the Sacramento (SV) and San Joaquin Valleys (SJV). For all species, each population along with known R and S controls were screened for resistance against thiobencarb (thicarbamate), fenoxaprop (ACC-ase inhibitor) and bispyribac-Na (ALS inhibitor) in a controlled environment facility. Field histories (herbicide use, tillage, water management, fertilizer, crop rotation and rice cultural practices) were collected via surveys. Samples have been geo-referenced and data was loaded into a geographic information system (ArcInfo 9.2, ESRI, Redlands, CA). Landscape variables (soil type and water districts) were also included. Data was processed using CART (Classification and Regression Trees) to assess relationships between the investigated variables and levels of herbicide resistance. EWG Figure 3. Regression tree explaining the resistance level of EWG to thiobencarb in terms of crop management and landscape variables. For BYG, seed spread by equipment and water management are the main variables associated with the few and less resistant BYG populations observed. Preliminary studies suggest hybridization between EWG and BYG; R gene transfer between these two species is thus also possible. BYG Figure 2. Box and whisker plots of ECH response to thiobencarb (% of control). R-ECH showed variable levels of R to bispyribac and fenoxaprop, with more consistent cross-R patterns in LWG and between thiobencarb and bispyribac.