Resumo em Inglês:

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.

Resumo em Inglês:

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.

Resumo em Inglês:

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.

Resumo em Inglês:

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.

Resumo em Inglês:

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.

Resumo em Inglês:

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.

Resumo em Inglês:

Abstract Background Species of Amaranthus genus are common in agricultural areas of Brazil. Such weeds are problematic, and they bring complexity to the management mainly due to herbicide resistance. Thus, the monitoring and mapping of chlorimuron-resistant Amaranthus spp. is necessary to detect resistance in different locations of Brazil. Objective Elaborate discriminating dose of distinction between Amaranthus spp. populations that are susceptible and resistant to chlorimuron and monitor the resistance dispersal throughout five crops. Methods 33 pigweed populations from the main grain producing properties in Brazil by means of dose-response curves were evaluated. For the D dose, chlorimuron dose of 20 g ha-1 ai was considered. Once the discriminating dose was identified, a monitoring screening of the dispersal of resistance of Amaranthus spp. to chlorimuron was conducted with 226 samples between the 2016 and 2020 crops. Results The discriminating dose (“base line”) considered ideal to control susceptible plants was 20 g ha-1 of chlorimuron. Among 226 pigweed samples evaluated in the five years of monitoring, 74% of populations were considered susceptible (S), while those classified as resistant (R) and segregating (r) did not exceed 26.0%. Conclusions By comparing susceptible biotypes of Amaranthus spp. with international scientific literature standards and leaflet averages, it could be safely concluded that the discriminating dose of chlorimuron is 20 g ha-1. Resistance of Amaranthus spp. to chlorimuron in Brazil is present in the main soybean producing regions evaluated, with a frequency of 26% of the total samples evaluated.

Resumo em Inglês:

Abstract: Plants of the genus Amaranthus are important agricultural weeds that compromise food production worldwide. Several biological characteristics make these plants thrive in the environment and cause significant yield losses in many crops. Among the seven most important Amaranthus species in the Americas, four have populations with resistance to more than one mode of action (A. hybridus, A. palmeri, A. retroflexus, and A. viridis). While multiple herbicide-resistance in Amaranthus species is widespread, chemical control remains as one of the most important tools against those weeds. In this review, we compiled data from multiple sources on the efficacy of different herbicides across the most common modes of action that are used in Amaranthus management. Both PRE and POST herbicides are discussed, as well as the key factors to be considered when using each one of them. Residual PRE herbicides bring several advantages when managing Amaranthus species. These herbicides can avoid weed interference in the initial stages of crop development and provide a more favorable situation for weed control in POST. In addition, including PRE herbicides allows for the addition of alternative modes of action that are not available as POST treatments. Most POST herbicides have limitations regarding weed size and herbicide resistance status. Applying POST herbicides at the early growth stage of weeds is crucial to obtain efficacy. Finally, weed management sustainability depends on herbicides. Therefore, herbicide use should be combined with other weed control methods to avoid herbicide resistance evolution.

Resumo em Inglês:

Abstract Herbicide resistance in weeds is an evolutionary process. Although there is a great global diversity of weeds, independent origins of herbicide resistance evolution have been shown to converge into similar molecular and physiological resistance mechanisms in geographically distant weed populations. Amaranthus species have shown an extraordinary ability to evolve herbicide resistance and invade new environments at a global scale, which represents an opportunity for identifying adaptive evolutionary patterns. The most frequent cases of herbicide-resistant Amaranthus species have been identified in North America, where A. hybridus, A. palmeri, A. tuberculatus and A. retroflexus comprise more than 90% of them. Meanwhile, A. retroflexus, A. hybridus and A. palmeri have been the most reported species in South America. Around 70% of the cases of herbicide-resistant Amaranthus species have been identified in global soybean and corn crops. The higher fecundity and adaptability of plants to a broad range of environments would make populations more likely to persist and be selected for herbicide resistance. Co-evolution of multiple herbicide resistance mechanisms at the plant and/or population level is evident in weed species. For Amaranthus spp., resistance cases highlight evolutionary responses to herbicide use with clear patterns of selection for multiple herbicide resistance in particular regions and spread to new areas within and between global cropping systems. Seed-mediated gene flow is an important component to the spread of herbicide resistant Amaranthus spp. populations. Reduction of the intensity of herbicide selection by combining diverse and integrated weed control practices should be a common goal in weed management programs.

Resumo em Inglês:

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.