Early silique-shedding wild radish (Raphanus raphanistrum L.) phenotypes persist in a long-term harvest weed seed control managed field in Western Australia.

BACKGROUND: This study introduces a wild radish population collected from Yelbeni in the Western Australian grainbelt that evolved an early silique abscission (shedding) trait to persist despite long-term harvest weed seed control (HWSC) use. In 2017, field-collected seed (known herein as Yelbeni) was compared to surrounding ruderal and field-collected populations in a fully randomized common garden study. RESULTS: The Yelbeni population exhibited a higher rate of silique abscission when compared to the ruderal populations collected from the site before wheat (Triticum aestivum L.) harvest (assessed at soft dough stage, Zadoks 83). A similar common garden study was conducted in the subsequent season (2018) using progeny reproduced on a single site without stress. The HWSC-selected progeny (Yelbeni P) shed 1048 (±288) siliques before wheat maturity at the soft dough stage (Zadoks 83) compared to 25 (±7) siliques from the pooled control populations. The Yelbeni P population only flowered 6 days earlier (FT50 as determined by log-logistic analysis) than pooled control populations, which is unlikely to fully account for the increased rate of silique abscission. The Yelbeni P population also located its lowest siliques below the lowest height for harvest interception (10 cm), which is likely to increase HWSC evasion. The mechanism inducing early silique-shedding is yet to be determined; however, wild radish is known for its significant genetic variability and has demonstrated its capacity to adapt to environmental and management stresses. CONCLUSION: This study demonstrates that the repeated use of HWSC can lead to the selection of HWSC-avoidance traits including early silique-shedding before harvest and/or locating siliques below the harvest cutting height for interception. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Distribution, frequency and molecular basis of clethodim and quizalofop resistance in brome grass (Bromus diandrus).

BACKGROUND: Brome grass (Bromus diandrus Roth) is prevalent in the southern and western cropping regions of Australia, where it causes significant economic damage. A targeted herbicide resistance survey was conducted in 2020 by collecting brome grass populations from 40 farms in Western Australia and subjecting these samples to comprehensive herbicide screening. One sample (population 172-20), from a field that had received 12 applications of clethodim over 20 years of continuous cropping, was found to be highly resistant to the acetyl-CoA carboxylase (ACCase)-inhibiting herbicides clethodim and quizalofop, and so the molecular basis of resistance was investigated. RESULTS: All 31 individuals examined from population 172-20 carried the same resistance-endowing point mutation causing an aspartate-to-glycine substitution at position 2078 in the translated ACCase protein sequence. A wild-type susceptible population and the resistant population had similar expression levels of plastidic ACCase genes. The level of resistance to quizalofop, either standalone or in mixture with clethodim, in population 172-20 was lower under cooler growing conditions. CONCLUSION: Target-site resistance to ACCase-inhibiting herbicides, conferred by one ACCase mutation, was selected in all tested brome plants infesting a field with a history of repeated clethodim use. This mutation appears to have been fixed in the infesting population. Notably, clethodim resistance in this population was not detected by the farmer, and a high future incidence of quizalofop resistance is anticipated. Herbicide resistance testing is essential for the detection of evolving weed resistance issues and to inform effective management strategies. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Assessing the Capability and Potential of LiDAR for Weed Detection.

Conventional methods of uniformly spraying fields to combat weeds, requires large herbicide inputs at significant cost with impacts on the environment. More focused weed control methods such as site-specific weed management (SSWM) have become popular but require methods to identify weed locations. Advances in technology allows the potential for automated methods such as drone, but also ground-based sensors for detecting and mapping weeds. In this study, the capability of Light Detection and Ranging (LiDAR) sensors were assessed to detect and locate weeds. For this purpose, two trials were performed using artificial targets (representing weeds) at different heights and diameter to understand the detection limits of a LiDAR. The results showed the detectability of the target at different scanning distances from the LiDAR was directly influenced by the size of the target and its orientation toward the LiDAR. A third trial was performed in a wheat plot where the LiDAR was used to scan different weed species at various heights above the crop canopy, to verify the capacity of the stationary LiDAR to detect weeds in a field situation. The results showed that 100% of weeds in the wheat plot were detected by the LiDAR, based on their height differences with the crop canopy.

Farming without Glyphosate?

Recent statements from scientific organisations and court decisions have resulted in widespread public interest and concern over the safety of glyphosate, the most popular and effective herbicide used worldwide. Consequently, glyphosate-based products are under intense scrutiny from governments at all levels. Some jurisdictions have already banned or restricted its use, which will adversely impact international trade in bulk grain commmodities if glyphosate residues are detected. The possibility of farming without glyphosate is becoming an important issue facing the agri-food research and development sector. Contingency plans need to be formulated if that scenario becomes a reality. In this review, we briefly summarize international events that have led to this possible situation, describe current glyphosate usage in major agronomic field crops worldwide, outline possible alternatives to glyphosate in two agroregions and perform bioeconomic model scenarios of southern Australian broadacre cropping systems without the herbicide. Model predictions suggest that we can farm profitably without glyphosate by consistently utilizing key non-herbicidal weed management practices combined with robust pre-emergence soil residual herbicide treatments. However, maintaining low weed seed banks will be challenging. If the social license to use glyphosate is revoked, what other pesticides will soon follow?

Pyroxasulfone efficacy for annual ryegrass control is affected by wheat residue height, amount and orientation.

BACKGROUND: Pre-emergent herbicides play an important role in conservation agriculture, however, crop residues on the soil surface in these systems can intercept a considerable amount of herbicide during application. Cutting crops relatively high at harvest has some advantages, such as allowing faster harvest, and this also means that there is less horizontal residue on the soil surface. This field study tested the impact of standing wheat residue height and amount of horizontal residue on the interception, leaching and weed-control efficacy of the pre-emergent herbicide pyroxasulfone in the 2015 and 2016 growing seasons. RESULTS: Spray coverage of pyroxasulfone declined from 14.6% to 7.5% with increasing amounts (0 to 4 t ha-1 ) of horizontal wheat residue. Horizontal wheat residue at 1 t ha-1 had 10.3% spray coverage (more herbicide interception) compared with 15.4% for the equivalent amount of standing residue. Greater amounts of horizontal residue also significantly reduced the efficacy of pyroxasulfone in controlling ryegrass in the field and decreased pyroxasulfone concentrations in the soil. Rainfall after herbicide application increased herbicide efficacy for all residue amounts. Generally, cutting standing residue higher resulted a relatively small decrease in spray coverage at the soil surface and weed control efficacy, and this was significant only between nil stubble and 0.3 m cut height. CONCLUSION: Cutting residue relatively high, leaving less on the surface, improves spray coverage and herbicide efficacy compared with having more horizontal residue. This research may assist farmers and advisors to maximize the efficacy of pre-emergent herbicide in no-tillage systems. © 2019 Society of Chemical Industry.

Herbicide Resistance Management: Recent Developments and Trends.

This review covers recent developments and trends in herbicide-resistant (HR) weed management in agronomic field crops. In countries where input-intensive agriculture is practiced, these developments and trends over the past decade include renewed efforts by the agrichemical industry in herbicide discovery, cultivation of crops with combined (stacked) HR traits, increasing reliance on preemergence vs. postemergence herbicides, breeding for weed-competitive crop cultivars, expansion of harvest weed seed control practices, and advances in site-specific or precision weed management. The unifying framework or strategy underlying these developments and trends is mitigation of viable weed seeds into the soil seed bank and maintaining low weed seed banks to minimize population proliferation, evolution of resistance to additional herbicidal sites of action, and spread. A key question going forward is: how much weed control is enough to consistently achieve the goal of low weed seed banks? The vision for future HR weed management programs must be sustained crop production and profitability with reduced herbicide (particularly glyphosate) dependency.

Rainfall affects leaching of pre-emergent herbicide from wheat residue into the soil.

No-tillage with stubble retention is a widely used cropping system for its conservation and yield benefits. The no-tillage farming system in southern Australia relies heavily on herbicides for weed management, but heavy crop residues may have a negative impact on the activity of pre-emergent herbicides applied. Any herbicide intercepted by the crop residue may not reach the soil surface without timely rainfall and may dissipate due to volatilisation, photo-degradation and/or microbial activity. Two experiments were carried out to investigate the interception of prosulfocarb, pyroxasulfone, and trifluralin herbicides by wheat residue and retention following simulated rainfall. For the first experiment, there were four simulated rainfall amounts (0, 5, 10, and 20 mm), three intensities (5, 10, and 20 mm h-1) and five application times (immediately after spraying herbicide, 6 h, 1, 7, and 14 days after spraying). In the second experiment, 20 mm of rainfall was applied at 10 mm h-1 in either 4 × 5 mm rainfall events over two days, 2 × 10 mm rainfall events over one day, or a single 20 mm rainfall event, with a no-rainfall control treatment. Bioassays were used to assess the herbicide activity/availability in the soil and remaining on the residue, using cucumber (Cucumis sativus L.) and Italian ryegrass (Lolium multiflorum Lam.) as indicator plants. At higher rainfall amounts, most of the herbicide leached from the stubble into the soil soon after application; more so with rain in one event rather than multiple events. However, the intensity of rainfall had no effect. Pyroxasulfone leached easily from the residue to the soil to potentially offer good weed control, prosulfocarb had an intermediary leaching effect, while only a small amount of trifluralin leached from stubble after rain. Therefore, in no-tillage situations with large amounts of crop residue present on the soil surface, herbicides that leach easily from the residue should be considered, like pyroxasulfone.

Effect of crop residues on interception and activity of prosulfocarb, pyroxasulfone, and trifluralin.

Crop residue retention on the soil surface in no-tillage system can intercept pre-emergent herbicides and reduce their efficacy. Three experiments were conducted to investigate the effect of crop residue amount (0, 1, 2 and 4 t ha-1), moisture (wet versus dry), type (wheat, barley, canola, chickpea and lupin) and age (fresh or aged for one year) on the interception and subsequent leaching of prosulfocarb, pyroxasulfone, and trifluralin from the residue into soil. Bioassays, using cucumber and annual ryegrass as indicator plants, were used to assess herbicide activity/availability in the soil and on the residue. Herbicide interception increased considerably as residue quantity increased from 2 to 4 t ha-1. After simulated rainfall, which washed herbicide into the soil, complete control of ryegrass occurred for trifluralin with 0 t ha-1 residue, for prosulfocarb with 0 and 1 t ha-1 residue, and for pyroxasulfone with all residue rates. Therefore, with rain or irrigation, pyroxasulfone was the herbicide least affected by high residue loads. Less chemical leached from the crop residue into the soil after rainfall, when prosulfocarb and trifluralin were applied to wet residue compared with dry residue, but the initial moisture condition had no effect on the leaching of pyroxasulfone from residue. If practically possible, farmers should minimise spraying prosulfocarb and trifluralin onto wet crop residue. Barley and wheat residues intercepted more herbicide than an equivalent mass of canola, chickpea or lupin residue, which was largely due to the increased ground cover with cereal residues. The effect of residue age on herbicide interception and leaching was relatively small and variable. Overall, more herbicide reached the soil when sprayed on one-year old residue than new residue, which was largely due to reduced ground cover with aged residue. A strong positive linear relationship existed between ground cover percentage and growth of bioassay species (r2 = 0.75). This means that there was little difference in the ability of residue to adsorb and retain herbicide between crop residue types and ages, such that farmers can simply use the ground cover of the crop residue to assess interception.

Spatially-Optimized Sequential Sampling Plan for Cabbage Aphids Brevicoryne brassicae L. (Hemiptera: Aphididae) in Canola Fields.

The cabbage aphid is a significant pest worldwide in brassica crops, including canola. This pest has shown considerable ability to develop resistance to insecticides, so these should only be applied on a "when and where needed" basis. Thus, optimized sampling plans to accurately assess cabbage aphid densities are critically important to determine the potential need for pesticide applications. In this study, we developed a spatially optimized binomial sequential sampling plan for cabbage aphids in canola fields. Based on five sampled canola fields, sampling plans were developed using 0.1, 0.2, and 0.3 proportions of plants infested as action thresholds. Average sample numbers required to make a decision ranged from 10 to 25 plants. Decreasing acceptable error from 10 to 5% was not considered practically feasible, as it substantially increased the number of samples required to reach a decision. We determined the relationship between the proportions of canola plants infested and cabbage aphid densities per plant, and proposed a spatially optimized sequential sampling plan for cabbage aphids in canola fields, in which spatial features (i.e., edge effects) and optimization of sampling effort (i.e., sequential sampling) are combined. Two forms of stratification were performed to reduce spatial variability caused by edge effects and large field sizes. Spatially optimized sampling, starting at the edge of fields, reduced spatial variability and therefore increased the accuracy of infested plant density estimates. The proposed spatially optimized sampling plan may be used to spatially target insecticide applications, resulting in cost savings, insecticide resistance mitigation, conservation of natural enemies, and reduced environmental impact.

Recurrent selection with reduced 2,4-D amine doses results in the rapid evolution of 2,4-D herbicide resistance in wild radish (Raphanus raphanistrum L.).

BACKGROUND: When used at effective doses, weed resistance to auxinic herbicides has been slow to evolve when compared with other modes of action. Here we report the evolutionary response of a herbicide-susceptible population of wild radish (Raphanus raphanistrum L.) and confirm that sublethal doses of 2,4-dichlorophenoxyacetic acid (2,4-D) amine can lead to the rapid evolution of 2,4-D resistance and cross-resistance to acetolactate synthase (ALS)-inhibiting herbicides. RESULTS: Following four generations of 2,4-D selection, the progeny of a herbicide-susceptible wild radish population evolved 2,4-D resistance, increasing the LD50 from 16 to 138 g ha-1 . Along with 2,4-D resistance, cross-resistance to the ALS-inhibiting herbicides metosulam (4.0-fold) and chlorsulfuron (4.5-fold) was evident. Pretreatment of the 2,4-D-selected population with the cytochrome P450 inhibitor malathion restored chlorsulfuron to full efficacy, indicating that cross-resistance to chlorsulfuron was likely due to P450-catalysed enhanced rates of herbicide metabolism. CONCLUSION: This study is the first to confirm the rapid evolution of auxinic herbicide resistance through the use of low doses of 2,4-D and serves as a reminder that 2,4-D must always be used at highly effective doses. With the introduction of transgenic auxinic-herbicide-resistant crops in the Americas, there will be a marked increase in auxinic herbicide use and therefore the risk of resistance evolution. Auxinic herbicides should be used only at effective doses and with diversity if resistance is to remain a minimal issue. © 2016 Society of Chemical Industry.

Directional selection for flowering time leads to adaptive evolution in Raphanus raphanistrum (Wild radish).

Herbicides have been the primary tool for controlling large populations of yield depleting weeds from agro-ecosystems, resulting in the evolution of widespread herbicide resistance. In response, nonherbicidal techniques have been developed which intercept weed seeds at harvest before they enter the soil seed bank. However, the efficiency of these techniques allows an intense selection for any trait that enables weeds to evade collection, with early-flowering ecotypes considered likely to result in early seed shedding. Using a field-collected wild radish population, five recurrent generations were selected for early maturity and three generations for late maturity. Phenology associated with flowering time and growth traits were measured. Our results demonstrate the adaptive capacity of wild radish to halve its time to flowering following five generations of early-flowering selection. Early-maturing phenotypes had reduced height and biomass at maturity, leading to less competitive, more prostrate growth forms. Following three generations of late-flowering selection, wild radish doubled its time to flowering time leading to increased biomass and flowering height at maturity. This study demonstrates the potential for the rapid evolution in growth traits in response to highly effective seed collection techniques that imposed a selection on weed populations within agro-ecosystems at harvest.

Nonrandom Distribution of Cabbage Aphids (Hemiptera: Aphididae) in Dryland Canola (Brassicales: Brassicaceae).

Characterization of spatial distribution patterns of pests in large-scale agricultural fields is important because these patterns affect the sampling effort needed to accurately detect and estimate their population density. In this study, we conducted experimental releases of alate cabbage aphids (Brevicoryne brassicae L.) into centers of small plots of canola (Brassica napus L.), and their gradual spread over a 7-wk period was characterized. The small-plot experiment demonstrated gradient effects from plot centers and a nonrandom vertical distribution, with initial colonization occurring on the abaxial side of lower canopy leaves and, later, highest numbers of cabbage aphids occurring on racemes. We also conducted large-scale distribution analyses of cabbage aphid infestations in two commercial canola fields, using visual inspection and sweep net sampling. We used canola plant phenological and landscape features as explanatory variables of the spatial distribution of cabbage aphid counts. These large-scale experiments showed strong edge effects with negative associations between cabbage aphid counts and distance to crop edges, including tree lines and contour banks. Cabbage aphid distribution was more effectively displayed using logistic regression than ordinary regression, Spatial Analysis by Distance IndicEs, or both. Based on the study findings, a nonrandom or optimized inspection approach is proposed to focus monitoring efforts on canola plants within 20 m from field edges with particular attention to the abaxial side of lower-canopy leaves. Detection of advanced cabbage aphid infestations should target the racemes within 20 m from field edges.

Identification of the first glyphosate-resistant wild radish (Raphanus raphanistrum L.) populations.

BACKGROUND: In Australia, glyphosate has been used routinely to control wild radish (Raphanus raphanistrum L.) for the past 40 years. This study focuses on two field-evolved glyphosate-resistant populations of wild radish collected from the grainbelt of Western Australia. RESULTS: Two wild radish biotypes were confirmed to be glyphosate resistant by comparing R/S of two suspected populations. Based on R/S from dose-response curves, the R1 and R2 populations were 2.3 and 3.2 times more resistant to glyphosate respectively. Dose response on glyphosate-selected progeny (>1080 g ha(-1)) demonstrated that the glyphosate resistance mechanism was heritable. When compared with the pooled mortality results of three known susceptible populations (S1, S2 and S3), the R1 and R2 subpopulations were 3.4-fold and 4.5-fold more resistant at the LD50 level respectively. Both populations were found to have multiple resistance to the phytoene desaturase inhibitor; diflufenican, the synthetic auxin; 2,4-D and the ALS inhibitors; chlorsulfuron, sulfometuron-methyl, imazethapyr and metosulam. CONCLUSIONS: This is the first report confirming glyphosate resistance evolution in wild radish and serves to re-emphasise the importance of diverse weed control strategies. Proactive and integrated measures for resistance management need to be developed to diversify control measures away from glyphosate and advance the use of non-herbicidal techniques.