Resistance to acetyl-Coenzyme A carboxylase (ACCase)-inhibiting herbicides in Bromus spp. in Australia

Peter Boutsalis and Christopher Prestonhttp://www.weedinfo.com.au/bk_15awc.htm

Herbicide cross resistance in Bromus diandrus and B. rigidus populations across southeastern Australia

Herbicide resistance to Group A (AC Case-inhibiting herbicides) and B herbicides (ALSinhibiting herbicides) in Bromus diandrus and B. rigidus is becoming more common in southeastern Australia but there is limited information available on its regional distribution in either species. At present it is unclear if resistant brome populations differ in their response to different herbicides within the same herbicide group. Another unresolved question is related to the herbicide dose response of resistant brome populations. This study identified differences between Group A herbicides in their activity on resistant brome. However, increasing herbicide dose only marginally improved weed control in most cases. Varying degrees of Group B resistance was also detected in four brome populations. These initial findings suggest that increasing herbicide dose is unlikely to improve brome control but some Group A and B herbicides are more effective than others in controlling resistant populations.Peter Boutsalis, Christopher Preston and Gurjeet Gil

Control of rigid ryegrass in australian wheat production with pyroxasulfone

Abstract in English and SpanishIn Australia, most wheat is sown in a no-till system without prior cultivation where herbicides are applied prior to sowing and incorporated by the planter. Trifluralin has been the most widely used PRE herbicide to control rigid ryegrass. The objective of this research was to determine crop safety and efficacy of alternative mechanism of action PRE herbicides for rigid ryegrass control in no-till wheat production. Pyroxasulfone achieved 98% control with PRE applications. The alternative PRE herbicides tested alone and in mixtures occasionally resulted in a significant reduction in wheat emergence but not crop yield. Trifluralin treatments failed at sites having trifluralin-resistant rigid ryegrass. Pyroxasulfone and prosulfocarb plus S-metolachlor were effective for control of rigid ryegrass across all trials with control ranging from 64 to 94%. This research demonstrated that PRE applications of herbicides other than trifluralin such as pyroxasulfone and prosulfocarb plus S-metolachlor can be safely and effectively used to control rigid ryegrass in no-till wheat.=En Australia, la mayoría del trigo se siembra en un sistema de labranza cero sin cultivo previo donde los herbicidas son aplicados antes de la siembra e incorporados con la sembradora. Trifluralin ha sido el herbicida PRE más ampliamente usado para el control de Lolium rigidum. El objetivo de esta investigación fue determinar la seguridad para el cultivo y la eficacia de herbicidas PRE con mecanismos de acción alternativos para el control de L. rigidum en producción de trigo en labranza cero. Pyroxasulfone alcanzó 98% de control con aplicaciones PRE. Los herbicidas PRE alternativos evaluados solos y en mezclas ocasionalmente resultaron en una reducción significativa en la emergencia del trigo pero no del rendimiento del cultivo. Los tratamientos de trifluralin fallaron en sitios que tenían L. rigidum resistente a trifluralin. Pyroxasulfone y prosulfocarb más S-metolachlor fueron efectivos para controlar L. rigidum en todos los ensayos con un control que fluctuó entre 64 y 94%. Esta investigación demostró que aplicaciones PRE de herbicidas diferentes a trifluralin, tales como pyroxasulfone y prosulfocarb más S-metolachlor pueden ser usados en forma segura y efectiva para el control de L. rigidum en trigo en labranza cero.Peter Boutsalis, Gurjeet S. Gill, and Christopher Presto

Distribution of herbicide-resistant acetyl-coenzyme A carboxylase alleles in Lolium rigidum across grain cropping areas of South Australia

Resistance to the acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides in Lolium rigidum is widespread in grain cropping areas of South Australia. To better understand the occurrence and spread of resistance to these herbicides and how it has changed with time, the carboxyl transferase (CT) domain of the ACCase gene from resistant L. rigidum plants, col- lected from both random surveys of the mid-north of Southern Australia over 10 years as well as stratified surveys in individual fields, was sequenced and target site mutations characterised. Amino acid substitutions occurring as a consequence of these target site muta- tions, at seven positions in the ACCase gene previously correlated with herbicide resistance, were identified in c. 80% of resistant individuals, indicating target site mutation is a common mechanism of resistance in L. rigidum to this herbicide mode of action. Individu- als containing multiple amino acid substitutions (two, and in two cases, three substitutions) were also found. Substitutions at position 2041 occurred at the highest frequency in all years of the large area survey, while substitutions at position 2078 were most common in the single farm analysis. This study has shown that target site mutations leading to amino acid substitu- tions in ACCase of L. rigidum are widespread across South Australia and that these mutations have likely evolved independently in different locations. The results indicate that seed movement, both within and between fields, may contribute to the spread of resis- tance in a single field. However, over a large area, the independent appearance and selection of target site mutations conferring resistance through herbicide use is the most important factor.J. M. Malone, P. Boutsalis, J. Baker, C. Presto

Management of trifluralin resistance in annual ryegrass (Lolium rigidum Gaudin) in southern Australia

Peter Boutsalis, Christopher Preston and John Brosterhttp://www.weedinfo.com.au/bk_15awc.htm

Resistance to very-long-chain fatty-acid (VLCFA)- inhibiting herbicides in multiple field-selected rigid ryegrass( Lolium rigidum) populations

Five populations of rigid ryegrass (LoliumrigidumGaudin) fromfields across cropping regions in southern Australia were suspected of having resistance to thiocarbamates, chloroacetamides, and sulfonylisoxazoline herbicides. Resistant (R) populations 375-14, 198-15, 16.2, EP162, RAC1, and A18 and two susceptible (S) populations (SLR4 and VLR1) were included in a dose–response study. All suspected R populations expressed resistance to one or all herbicides (thiocarbamates, chloroacetamides, and pyroxasulfone). Population 198-15 exhibited the highest LD50 to triallate (44.7-fold), prosulfocarb (45.7-fold), S-metolachlor (31.5-fold), and metazachlor (27.2-fold) compared with the S populations. Populations 198-15 and 375-14 were also resistant to pyroxasulfone (13.5- and 14.9-fold) compared with the S populations, as was population EP162. This study documents the first case of field-evolved resistance to thiocarbamate, chloroacetamide, and sulfonylisoxazoline herbicides in L. rigidum.David J. Brunton, Peter Boutsalis, Gurjeet Gill and Christopher Presto

Glyphosate resistance in barnyard grass (Echinochloa colona)

Echinochloa colona is an important summer-growing weed species in northern Australian cropping regions. As a result of the intensive use of glyphosate in summer fallows, glyphosate resistant populations of E. colona have evolved, with the number of resistant populations identified rapidly growing. This study identified glyphosate resistance in E. colona collected from different locations in Australia and investigated the mechanism of glyphosate resistance. Pot trials conducted on populations ofE. colona collected from northern Australia identified resistance to glyphosate in 11 populations of this weed species. The level of resistance varied among the populations from 2- to 11-fold. Sequencing of the target-site (EPSPS) identified a mutation at position 106 leading to a change from proline to serine in the most resistant population A533.1 only. With the range of resistance levels identified, it is expected that different mechanisms of resistance will be present among the rest of the resistant populations.Hoan Nguyen Thai, Jenna Malone, Peter Boutsalis and Christopher Prest

Growth, development, and seed biology of feather fingergrass (chloris virgata) in southern Australia

Feather fingergrass is a major weed in agricultural systems in northern Australia and has now spread to southern Australia. To better understand the biology of this emerging weed species, its growth, development, and seed biology were examined. Under field conditions in South Australia, seedlings that emerged after summer rainfall events required 1,200 growing degree days from emergence to mature seed production and produced 700 g m−2 shoot biomass. Plants produced up to 1,000 seeds panicle−1 and more than 40,000 seeds plant−1, with seed weight ranging from 0.36 to 0.46 mg. Harvested seeds were dormant for a period of about 2 mo and required 5 mo of after-ripening to reach 50% germination. Freshly harvested seed could be released from dormancy by pretreatment with 564 mM sodium hypochlorite for 30 min. Light significantly increased germination. Seed could germinate over a wide temperature range (10 to 40 C), with maximum germination at 15 to 25 C. At 20 to 25 C, 50% germination was reached within 2.7 to 3.3 d, and the predicted base temperature to germinate was 2.1 to 3.0 C. The osmotic potential and NaCl concentration required to inhibit germination by 50% were −0.16 to −0.20 MPa and 90 to 124 mM, respectively. Seedling emergence was highest (76%) for seeds present on soil surface and was significantly reduced by burial at 1 (57%), 2 (49%), and 5 cm (9%). Under field conditions, seeds buried in the soil persisted longer than those left on the soil surface, and low spring–summer rainfall increased seed persistence. This study provides important information on growth, development, and seed biology of feather fingergrass that will contribute to the development of a more effective management program for this weed species in Australia.The D. Ngo, Peter Boutsalis, Christopher Preston and Gurjeet Gil