To reveal the effects of herbicide selection on genetic diversity in the outcrossing weed species Schoenoplectus juncoides, six sulfonylurea-resistant (SU-R) and eight sulfonylurea-susceptible (SU-S) populations were analysed using 40 polymorphic inter-simple sequence repeat loci. The plants were collected from three widely separated regions: the Tohoku, Kanto and Kyushu districts of Japan. Genetic diversity values (Nei's gene diversity, h) within each SU-S population ranged from h = 0.125 to h = 0.235. The average genetic diversity within the SU-S populations was H_S = 0.161, and the total genetic diversity was H_T = 0.271. Although the H_S of the SU-R populations (0.051) was lower than that of the SU-S populations, the H_T of the SU-R populations (0.202) was comparable with that of the SU-S populations. Most of the genetic variation was found within the region for both the SU-S and SU-R populations (88% of the genetic variation respectively). Two of the SU-R populations showed relatively high genetic diversity (h = 0.117 and 0.161), which were comparable with those of the SU-S populations. In contrast, the genetic diversity within four SU-R populations was much lower (from h = 0 to 0.018) than in the SU-S populations. The results suggest that selection by sulfonylurea herbicides has decreased genetic diversity within some SU-R populations of S. juncoides. The different level of genetic diversity in the SU-R populations is most likely due to different levels of inbreeding in the populations.

Species distribution models are an important tool to predict potential spread of weeds. While recent progress has improved model performance, there is still concern about the validity of such models, especially when applied to novel geographical regions or climates. This study investigates how different sets of variables influence predicted distributions, considering several measures of model performance and how extrapolation to novel geographical regions may affect results. Potential distributions of three new weeds in New Zealand (Archontophoenix cunninghamiana, Psidium guajava and Schefflera actinophylla) are modelled, by training a model based on global data from native and introduced ranges and projecting it to New Zealand, using Maxent. For each species, four models were calibrated: first with a full set of 19 bioclimatic variables, then with a customised set with selection based on analysis of response curves and finally with two reduced sets of uncorrelated variables. Although AUC across all models was very high (AUC ≥ 0.9), correlations between models ranged between 0.27 and 0.98. Inclusion of all variables predicted larger areas to be suitable in the projected region, with highly unlikely predictions in some areas, especially where bioclimatic variables showed values outside the range of the training data (new environments). Conversely, minimal extrapolation and more realistic predictions of weed distributions were obtained from models including a customised set of variables, and even more so from models including only a reduced set of variables. This study shows that careful selection of variables and investigation into extrapolation are vital in generating more realistic predictions of weed distributions.

Lolium rigidum is an extremely competitive and prevalent grass weed in cereal fields of Mediterranean areas. The proper timing of control measures is a prerequisite to maximising herbicide efficacy, in terms of both improved control and reduced herbicide inputs. The development of models to predict emergence flushes will contribute to this goal. Pooled cumulative emergence data obtained during three seasons from a cereal field were used to develop a Gompertz model. This explained relative seedling emergence from crop sowing onwards as a function of: (i) standard soil thermal time accumulation (TT) with a base temperature of 1.8°C and (ii) soil thermal time accumulation corrected for soil moisture (cTT). For the latter, no thermal time accumulation was computed for days in which the soil water balance within the upper 10-cm soil layer indicated no water available for plants, because evapotranspiration was greater than rainfall plus the stored water remaining from the previous day. The model was validated with six datasets from four different sites and seasons. Compared with TT, the model based on cTT showed better performance in predicting L. rigidum emergence, particularly in predicting the end of emergence. Complemented with in-field observations to minimise deviations, the model may be used as a predictive tool to better control this weed in dryland cereal fields of Mediterranean climate areas.

Parasitic plants have evolved various methods of invading host plants. Some invade aerial parts, whereas others invade the roots to obtain necessary nutrients for their development. Phelipanche and Orobanche spp. (broomrapes) and Cuscuta spp. (dodders) are holoparasitic plants that subsist on roots and shoots, respectively, of a variety of agricultural crops. These weeds are able to connect directly with the vascular system of the host, thereby acquiring the water, minerals and carbohydrates necessary for their own growth and reproduction. This exploitation by parasitic plants often causes severe losses in yield quality and quantity of host crops. The key to an effective means for controlling parasitic plants lies in the development of resistant crops, supported by an improved understanding of broomrape and dodder biology. The haustoria formed at the junctions of parasite and host open the way for translocation of a variety of molecules and macromolecules from the host to the parasite. At the same time, however, the haustoria also open opportunities for the development of methods to control parasitic plants. This review will summarise the current knowledge on translocation of siRNAs, mRNAs, viruses, sugars, proteins and herbicides from host to parasitic plants and the potential significance of such molecules to the parasite. Improved understanding of the molecular exchange between host plants and their parasites is expected to lead to the development of state-of-the-art, effective approaches to parasitic weed management.

Small unmanned aerial systems (UAS) with cameras have not been adopted in weed research, but offer low-cost sensing with high flexibility in terms of spatial resolution. A small rotary-wing UAS was tested as part of a search for an inexpensive, user-friendly and reliable aircraft for practical applications in UAS imagery weed research. In two experiments with post-emergence weed harrowing in barley, the crop resistance parameter, which reflects the crop response to harrowing, was unaffected by image capture altitude in the range from 1 to 50 m. This corresponded to image spatial resolution in the range from 0.3 to 17.1 mm per pixel. This finding is important because spatial resolution is inversely related to sensing capacity. We captured 20 plots comprising a total of about 0.2 ha in one image at 50 m altitude without losing information about the cultivation impacts on vegetation compared with ground truth data. UAS imagery also gave excellent results in logarithmic sprayer experiments in oilseed rape, where we captured 37 m long plots in each image from an altitude of 35 m. Furthermore, perennial weeds could be mapped from UAS images. These first experiences with a small rotary-wing UAS show that it is relatively easy to integrate as a tool in weed research and offers great potential for site-specific weed management.

Orobanche cumana (sunflower broomrape) is found in Spain as an allochthonous species parasitising exclusively sunflower. For many years, it was distributed in the Guadalquivir Valley and Cuenca province, but in recent years, it has spread to new areas. The objective of this research was to study genetic diversity of O. cumana populations from Spain using robust co-dominant molecular markers. Cluster analysis on a set of 50 populations using 15 microsatellite markers revealed the existence of two distant gene pools, one in Cuenca province and another one in the Guadalquivir Valley. Within each gene pool, both inter- and intrapopulation variability were extremely low. This population structure probably reflects a founder effect, with the two genetically distant gene pools deriving from separate introduction events. Different races occurred within the same gene pool, suggesting that current races might have evolved through mutation from a common genetic background. Most of the populations from new areas were identical to the populations from the Guadalquivir Valley. Only a few populations showed larger intrapopulation variation. In these cases, our results suggested the co-existence of both gene pools within the same population, as well as the occurrence of genetic recombination between them. Genetic recombination between distant gene pools is an important mechanism for creating new variation, which might also have an effect on race evolution. These results will contribute to the establishment of improved crop breeding and management strategies for O. cumana control.

Many public authorities rely on the use of non-chemical weed control methods, due to stringent restrictions on herbicide use in urban areas. However, these methods usually require more repeated treatments than chemical weed management, resulting in increased costs of weed management. In order to investigate the efficacy of four non-chemical weed control methods and glyphosate treatment, experiments were carried out on traffic islands in the growing seasons 2005 and 2006. Three trial sites were each divided into six treatment areas, which were either treated with glyphosate, flame, steam, hot air/flame, hot water or left untreated. The treatments were carried out at regular, predetermined intervals throughout the growing season in 2004, whereas in 2005 and 2006 how many treatments that were required to keep weed cover below a predetermined acceptance level of 2% were investigated. Percentage weed cover was measured every second week using a 75 cm × 75 cm quadrat divided into 100 squares. On the control areas, a rapid increase in weed cover was observed, whereas weed cover could be kept below 2% by 2–7 treatments per year, depending on control method. On average, the following numbers of treatments per year were required: glyphosate 2.5, hot water 3, flames 5, hot air/flames 5.5 and steam 5.5 treatments. The results demonstrate that the weed control should be adjusted to the prescribed quality for the traffic islands by regularly assessing the need for weed control. They also show that tailored treatments can reduce the number of required non-chemical treatments per year.

Centaurea stoebe is native to Europe and Western Asia and was introduced into North America in the late 19th century, where it has become highly invasive. In its native range, C. stoebe occurs in two cytotypes, namely diploids (2n = 18) and tetraploids (2n = 36), but only the tetraploid form has been identified in the invaded range. We used special growth pouches to determine whether diploid and tetraploid cytotypes from the native range differed in root growth and architecture. We grew seeds from five populations of each cytotype in growth pouches during a period of sixteen days and measured root growth traits both by hand and using a root-scanning software package (WinRHIZO). Tetraploid cytotypes had significantly larger total root length, taproot length, surface area, root volume, above- and below-ground biomass and root to shoot ratios than diploid cytotypes. We suggest that increased early root growth of tetraploid cytotypes as compared with diploids may be one factor that pre-adapted them towards the colonisation of warmer and drier climates in Europe and North America, where tetraploids are currently expanding and invasive respectively.

A new method for non-destructive monitoring of the subsurface development of Cyperus rotundus (purple nutsedge) is described. A minirhizotron (MR) system was adopted for use to observe vegetative growth of the weed in a net-house and under field conditions. In particular, the key stages of tuber production and sprouting were monitored and quantified. The MR system allowed non-destructive repeated measurements of the key stages in C. rotundus subsurface development, serving as a both quantitative and qualitative tool in the study of C. rotundus. A comparison between the MR method and a destructive method revealed that the former reduces experimental variance, thus providing a more accurate prediction of the sprouting process. The techniques presented in the current study offer advanced methodologies for continuous monitoring of C. rotundus development under the soil surface. The novelty of this method lies in its simplicity and efficiency, allowing continuous observation of specific organs of this noxious weed and potentially of other geophytes.

To suppress weeds in an apple (Malus sp.) orchard, we placed spruce (Picea spp.) bark mulch and cocoa (Theobroma cacao) husk mulch for 3 months in thicknesses of 0, 2.5, 5, 10 and 15 cm. To assess the development of weed cover, an innovative use of log-logistic dose–response models was applied, with mulch thickness as the independent variable. Weed cover was measured by non-destructive image analysis by estimating the relationship between the number of green pixels and the total number of pixels in each experimental plot. The thickness of mulch layer required to attain a 50 and 90% weed suppression (ED50 and ED90) differed significantly within and between mulch types. In all except one instance, the cocoa mulch was superior in suppressing weeds. This method was useful for the evaluation, but further research is needed to give a more general conclusion about the suppression ability of the two mulches under other climatic and growing conditions.

Mazus pumilus is an annual self-pollinating weed that is commonly found in arable land, vegetable gardens and roadsides. This weed harbours insects and pathogens that attack vegetables. The mechanism of resistance to paraquat of M. pumilus found in Ohita, Japan, was studied. Whole plant bioassays revealed that the resistant (R) biotypes were four to six times less susceptible than controls. Chlorophyll destruction of leaf discs by paraquat treatment in R biotypes was 4–20 times lower than those of susceptible (S) biotypes. Ferric reducing antioxidant power (FRAP) values in R biotypes were higher than those of S biotypes before and after paraquat treatments. The activity of superoxide dismutase (SOD) was also higher in R biotypes than those of S biotypes before and after treatment with paraquat, but the activities of ascorbate peroxidase (APX) and catalase (CAT) were not different between R and S biotypes. Change of ascorbate (AsA) contents before and after paraquat treatment was equivalent in both biotypes. These results indicate that the increased SOD activity and antioxidant capacity in R biotypes contribute to the resistance to paraquat of M. pumilus.

Perennial weeds are often controlled by mechanical means, which aim at stimulating axillary and adventitious buds to sprout. This happens when the apical dominance of the main shoot is removed by defoliation or when the underground system is fragmented. By repeating the measures, the result is a depletion of storage compounds, which weakens the plants and reduces their capacity to grow and reproduce. However, timing is critical. Earlier research has indicated that emergence from fragments of Sonchus arvensis cease during a period in autumn, while the seasonal pattern of sprouting in Cirsium arvense appears to be inconsistent. We studied the emergence pattern of defoliated plants with undisturbed root systems, from late summer to early spring. Potted plants grown outdoors were exhumed at regular intervals, put under forcing conditions for 4 weeks, after which shoots above and below soil level were counted and weighed together with the remaining root systems. In both species, the number and weight of emerged shoots decreased during a period in the autumn. In C. arvense, underground shoots were constantly produced during the same period, while fewer underground shoots were present in S. arvensis. For the latter species, apical dominance does not fully explain the effect; thus, endodormancy might be involved. Root weight increased until withering and did not explain the lack of emergence. Our results suggest an impaired sprouting capacity of undisturbed root systems of C. arvense and S. arvensis during September–October, which has implications for the timing and method of control of these species.

In winter wheat systems in the Northern Great Plains of the United States, Bromus tectorum and wheat streak mosaic virus (WSMV) commonly co-occur. While independent effects of these pests on wheat yields have been well documented, to our knowledge, no study has investigated whether WSMV modifies interactions between wheat and B. tectorum. Furthermore, the impact that environmental stressors such as nutrient availability have on these interactions has not been addressed. We conducted a randomised split-plot field study over 2 years to investigate the effects of WSMV and nitrate (N) availability on winter wheat suppression of Bromus tectorum. The study included four N treatments (10–19, 20–31, 31–84, and 85–207 kg ha⁻¹) and two WSMV treatments (mechanically inoculated or control). Increasing soil N increased the susceptibility of wheat to WSMV infection. In 2009, wheat in the lowest and highest N levels had 24% and 65% of plants infected respectively. However, regression analysis indicated that interactive effects of wheat competition, N and WSMV did not play a consistent role in B. tectorum growth. Specifically, the effect of both wheat density and distance from row on B. tectorum biomass remained constant across inoculation treatments, suggesting that wheat inoculated with WSMV suppressed B. tectorum as effectively as healthy wheat. Furthermore, wheat had a greater impact on B. tectorum growth in higher N environments, even though incidence of WSMV infection in wheat was highest. Overall, our results suggest that WSMV infection may not change the ability of wheat to suppress B. tectorum.

Although we know that alterations in crop density, crop spatial pattern and inclusion of more selective weed control can improve weed suppression for organic growers, it is unknown whether these result in changes to the weed seedbank that increase cropping system profitability over time. Data collected from field trials conducted in 2009 and 2010 in Maine, USA, comparing regional grower practices (Standard) with management that aimed to (i) facilitate better physical weed control through the use of wide row spacing and inter-row cultivation (Wide) or (ii) enhance crop–weed competition through increased seeding rate and narrow inter-row spacing (Narrow HD), were used to construct a matrix population model with an economic sub-model. Using field measurements of grain yield and weed survival and fecundity, we investigated the lasting implications of employing alternative organic spring wheat (Triticum aestivum) production practices on Sinapis arvensis population dynamics. In most scenarios, the model indicated that regional production practices were not sufficient to prevent an increase in the weed seedbank, even with excellent weed control. The two alternative methods, on the other hand, were able to limit weed population growth when initial densities were low or cultivation efficacy was >80%. Due to higher seed costs in the Narrow HD system, net returns were still lower after 10 years of simulation in this system compared with wide rows with cultivation, despite a lower weed seedbank.

Conyza bonariensis is a South American native annual Asteraceae that has been introduced to the Mediterranean, where it behaves as a ruderal plant and a weed that is difficult to control in several crops. The development of predictive models can contribute to control measures at early growth stages, but currently there are no studies to predict seedling emergence of Conyza species. Our objectives were to develop and evaluate a model for predicting emergence response of C. bonariensis to the soil hydrothermal environment. A hydrothermal seed germination model was fitted to time course germination data from germination tests carried out at different constant temperatures and water potentials with the aim of establishing the hydrothermal parameters characterising C. bonariensis seed germination. The relationship between cumulative seedling emergence and cumulative hydrothermal time under field conditions was analysed using the Gompertz function. Model development was based on 2 years' data from a field experiment. Base temperature and base water potential for seed germination were estimated at 10.6°C and −0.70 ± 0.151 MPa, respectively. The emergence model showed a very good fit to the experimental data. According to this model, seedling emergence starts at 15 accumulated hydrothermal time (HTT) after sowing, and 50 and 95% emergence is completed at 53 HTT and 105 HTT, respectively. For model evaluation, independent field experiments were carried out in two localities. Cumulative seedling emergence was accurately predicted by the model. Results indicate that this model can be useful as a predictive tool contributing to effective control of C. bonariensis populations.

Management actions are essential for mitigating the potentially harmful changes in biodiversity, ecosystem function and crop/forest productivity caused by invasive species. Species distribution models, if reliable, could be used to design effective management strategies. Although several modelling methods well suited for studying invasive species have been developed for presence-only data, often the size of available data sets for modelling is small and results are not validated with test samples. Moreover, the impact of such methods in practical applications has been overlooked. Here, we evaluated the reliability of the modelling approach based on ecological-niche factor analysis (ENFA) implemented in Biomapper software when applied to environmental weed data in the Azores. Presence-only data sets of two top invasive woody species (Pittosporum undulatum and Acacia melanoxylon) were used. The continuous Boyce curve was used for validation, calculated either in Biomapper (cross-validation) or based on test samples. The species' most habitable areas that should be regarded as management targets were thus estimated from modelling and validation. By imposing size restrictions on the presence-only data sets used in modelling and validation, other habitable areas were defined and compared. The ENFA proved to be a suitable method for modelling environmental weed distributions, regardless of the presence-only dataset size. Moreover, the cross-validation of Biomapper was reliable, although its results should be interpreted with caution as they could potentially lead to statistically different management target areas.