Abstract in English:Abstract Introduction A. hybridus interference in soybeans varies with population size and may be quantified by mathematical models that allow establishing the threshold levels. Objective Determine the interference of A. hybridus escaped from an herbicide program and with multiple resistance to EPSPs and ALS-inhibiting herbicides on soybean yield and its economic threshold (ET). Methods Two experiments were conducted in a randomized block design. The treatments consisted of populations of A. hybridus ranging from 0 to 41 plants m-2 (2018/19) and 0 to 32 plants m-2 (2019/20). The yield components and yield plant-1 of soybean were evaluated. The rectangular hyperbola model was fitted to the soybean yield loss. ET was calculated according to yield potential, price paid for grains, control cost and weed control efficiency Results The increase in populations of A. hybridus has decreased in the number of pods and grains-1 of soybean in both periods. The parameter i was 4.47 and 8.32%, and the ET ranged from 0.16 to 1.21 plants m-2 in 2018/19 and from 0.09 to 0.65 plants m-2 in 2019/20. Conclusions One A. hybridus m-2 may reduce 6.4% in the grain yield of soybean. ET occurs within the range of 0.09 to 1.21 plants m-2.
Abstract in English:Abstract Background Amaranthus spp. are problematic weeds and competitors for nutrients in several crops, especially in soybean and corn. Resistance to glyphosate has been detected in several weed species, and a triple mutation in its EPSPS target gene was detected recently in Amaranthus hybridus. Objective The aim of this work was to develop a simple polymerase chain reaction (PCR) method to detect the EPSPS triple mutation in A. hybridus. Methods Two pairs of primers were designed for PCR-based detection of the EPSPS TAP-IVS triple mutation, which confers resistance to glyphosate, in A. hybridus. Results These sets of allele-specific primers were tested on five Amaranthus species and in 65 different field accessions. The PCR reaction using one set of the primers amplifies the wildtype (TAP) allele while the PCR reaction using the other pair of primers amplifies the triple mutation (IVS) allele. The presence of PCR products in both sets of primers identifies the heterozygous resistant individuals, and PCR product amplified only with the triple mutation set of primers identifies the homozygous resistant individuals. A DNA concentration test was performed and the recommend DNA amount to be used is 100 ng. Conclusions We developed and tested two sets of primers to detect the EPSPS TAP-IVS triple mutation and the results showed a 100% genotypic to phenotypic association. The triple mutation detection assay is easy to use and can be applied in a molecular laboratory with basic equipments. Early detection of resistance helps to better manage and control its spreading.
Abstract in English:Abstract Background Amaranthus hybridus, a common agricultural weed in Brazil, has been associated with complications in the management of major crops. Recently, A. hybridus biotypes with suspected resistance to the herbicide glyphosate have been observed in the state of Paraná. Objective To characterize glyphosate-resistant Amaranthus hybridus biotypes. Methods Three independent experiments were conducted to evaluate four biotypes of A. hybridus. Twelve doses of glyphosate were applied, including 64D, 32D, 16D, 8D, 4D, 2D, D, 1/2D, 1/4D, 1/8D, 1/16D, and a control with no herbicide. The dose D of glyphosate applied was 1080 g ha-1 ae. Four replicates for each treatment and a randomized block design were used. At 28 (DAA), the control (%) and residual dry mass of each plot were analyzed. Results The biotypes with suspected resistance (R1, R2, and R3) had a GR50 in the range of 3019.13 to 3316.45 g ha-1 ae and LD50 in the range of 4081.52 to 4310.29 g ha-1 ae whereas the susceptible biotype had a GR50 of 227.63 g ha-1 ae and LD50 of 196.59 g ha-1 ae. Conclusions Biotypes R1, R2, and R3 are resistant to the herbicide glyphosate.
Abstract in English:Abstract Background Amaranthus palmeri S. Watson is a serious problem in soybean crops from Argentina. This weed has evolved high level of resistance to (ALS)-inhibiting herbicides mainly due to a target-site mechanism by an altered ALS enzyme. In an A. palmeri population from Totoras with cross-resistance to (ALS)-inhibiting herbicides, six allelic versions of the ALS enzyme were identified. Objective The aim of this study was to evaluate plants from that resistant population carrying the ALS substitutions A122S, D376E or A205V, which had not been characterized before for this species. Methods Subpopulations with each substitution were produced by a vegetative cloning procedure or by cross-pollination and dose-response assays and herbicide single-dose tests were performed to evaluate in vivo resistance levels to (ALS)-inhibiting herbicides. Results Dose-response experiments showed that all the resistant subpopulations survived at the highest doses tested (32 X) for imidazolinones, triazolopyrimidines and sulfonylureas, while the susceptible population was completely controlled at considerably lower doses. Furthermore, an analysis of the novel A122S substitution showed that it provides cross-resistance to five classes of (ALS)-inhibiting herbicides, excluding the entire ALS herbicide group as an effective control tool in weed populations carrying this substitution. Conclusions The results indicated that D376E, A205V and A122S substitutions found for the first time in A. palmeri confer cross-resistance to the most used chemical families from herbicide group 2. Interestingly, it was confirmed that the A205V substitution conferred resistance to herbicides in the triazolopyrimidines family. Data generated should be considered in management strategies for delaying the spread of resistance.
Abstract in English:Abstract Background Amaranthus palmeri has emerged as the most widespread weed of agricultural land in large parts of North and South America. Understanding its population dynamics and the influence of meteorological variables becomes important for decision-making in an integrated management context. The hypothesis is that the emergence of A. palmeri is influenced by thermal time and extreme weather events that occurred in the previous 45, 30 or 15 days. Objective The work was aimed to detect the influence of meteorological variables and extreme weather events on the emergence of A. palmeri under field conditions. Methods A field experiment was carried out in order to record seedling emergence of A. palmeri in two growing seasons, 2017/2018 (S1) and 2018/2019 (S2), in Argentina. Associations between weed emergence and thermal time (in growing degree-days GDD), meteorological variables or extreme weather events recorded at 15, 30 and 45 days before to each evaluation time were studied by regression, principal components and multiple correspondence analyses. Results Thermal time was closely associated to the progress of cumulative emergence in both seasons, but the emergence periodicity was conditional with rainfall. The high precipitation during the spring determined a short lag period (121.8 GDD) in S2. Contrarily, the largest lag period (236.6 GDD) was detected in S1 related to a drought that concentrated the emergence in the beginning of the summer when the rainfall increased. Conclusions Thermal time allows the cumulative emergence prediction; however, extreme weather events like drought induce quiescence, concentrating the emergence in a short period.
Abstract in English:Abstract: Background Amaranthus is a genus of Amaranthaceae with ca. 75 species. Many of them are of economic importance as cultivated crops (grains or vegetables) or ornamentals, while around 20 range from significant weeds to minor ones. Objective: List and differentiate the species (and subspecies) of weeds that belong to genus Amaranthus and range in South America. Results: Thirteen species and four subspecies of Amaranthus are considered weeds in South America; six of them are of great importance (A. blitum, A. deflexus, A. hybridus, A. palmeri, A. spinosus, and A. viridis), while the rest cause minor damage. Vernacular names, distribution, diagnoses, notes, and vouchers are given for every taxon. Conclusions: Within the 13 species of weeds that belong to genus Amaranthus in South America, there are two among the worst noxious weeds in the world: A. hybridus (with two subspecies), which ranges in many countries of this continent, and A. palmeri, present up to now only in Argentina. Other four important weeds are widespread in different countries: A. blitum (with two subspecies), A. deflexus, A. spinosus, and A. viridis.
Abstract in English:Abstract: Background: Amaranthus hybridus L. (Amaranthaceae) is an annual, dicotyledonous species that is considered one of the main weed infesting agricultural production systems. Some weeds species are considered host plants to insect pests, serving as “green bridges” for subsequent attacks on the main crop during the season, which can cause significant losses. Recording the occurrence of insects that can reach pest status is important for the deployment of mitigation measures against possible damage and economic losses to crops. The presence of Spodoptera eridania (Cramer) (Lepidoptera: Noctuidae) has not been previously registered in association with A. hybridus plants. Objective: Thus, the present study is the first report to describe the occurrence of S. eridania on A. hybridus in the municipality of Piracicaba, São Paulo, Brazil. Methods: Images of the larvae while feeding on A. hybrids were obtained using an Olympus® E-410 digital camera. The pictures were then sent to the Laboratory of Ecotoxicology and IPM (LEMIP) of the Department of Entomology of Lavras Federal University (UFLA) for species identification. Results: We observed S. eridania larvae larger than 7 cm feeding on the inflorescences and leaves of A. hybridus . Conclusions: This note reports the first occurrence of S. eridania larvae feeding on A. hybridus in the city of Piracicaba, SP, Brazil.