Resistance of Echinochloa crus-galli

Three Echinochloa crus-galli (barnyardgrass) populations from rice fields in Arkansas (AR1 and AR2) and Mississippi (MS1), USA, were recently confirmed to be resistant to imazethapyr. Experiments were conducted to characterize cross-resistance to acetolactate synthase- (ALS-) inhibiting herbicides and determine if malathion, a known cytochrome P450 monooxygenase (CYP) inhibitor, would overcome resistance. The AR1 and MS1 populations were cross-resistant to bispyribac-sodium; however, AR2 was sensitive to bispyribac-sodium. The AR1, AR2, and MS1 populations were >94, >94, and 3.3 times, respectively, more resistant to imazamox; >94, 30, and 9.4 times, respectively, more resistant to penoxsulam; and 15, 0.9, and 7.2 times, respectively, more resistant to bispyribac-sodium compared to a susceptible population. Addition of malathion to penoxsulam reduced dry weight of all populations and increased mortality of AR2 and MS1 populations compared to penoxsulam alone. Addition of malathion to imazethapyr and bispyribac-sodium increased the mortality of MS1 population in mixture with imazethapyr and AR1 population in mixture with bispyribac-sodium compared to treatments with imazethapyr and bispyribac-sodium applied alone. Synergism of ALS-inhibiting herbicides with malathion indicates increased herbicide degradation by CYP as partial mechanism of resistance to penoxsulam in all resistant populations and probably to imazethapyr in MS1 and bispyribac-sodium in AR1 populations

Weed resistance to synthetic auxin herbicides

Herbicides classified as synthetic auxins have been most commonly used to control broadleaf weeds in a variety of crops and in non-cropland areas since the first synthetic auxin herbicide (SAH), 2,4-D, was introduced to the market in the mid-1940s. The incidence of weed species resistant to SAHs is relatively low considering their long-term global application with 29 broadleaf weed species confirmed resistant to date. An understanding of the context and mechanisms of SAH resistance evolution can inform management practices to sustain the longevity and utility of this important class of herbicides. A symposium was convened during the 2nd Global Herbicide Resistance Challenge (May 2017 in Denver, CO, USA) to provide an overview of the current state of knowledge of SAH resistance mechanisms including case studies of weed species resistant to SAHs and perspectives on mitigating resistance development in SAH-tolerant crops

EST-SSR development from 5 Lactuca species and their use in studying genetic diversity among L. serriola biotypes

Prickly lettuce (Lactuca serriola L.) is a problematic weed of Pacific Northwest and recently developed resistance to the auxinic herbicide 2,4-D. There are no publically available simple sequence repeat (SSR) markers to tag 2,4-D resistance genes in L. serriola. Therefore, a study was conducted to develop SSR markers from expressed sequence tags (ESTs) of 5 Lactuca species. A total of 15,970 SSRs were identified among 57,126 EST assemblies belonging to 5 Lactuca species. SSR-containing ESTs (SSR-ESTs) ranged from 6.23% to 7.87%, and SSR densities ranged from 1.28 to 2.51 kb(-1) among the ESTs of 5 Lactuca species. Trinucleotide repeats were the most abundant SSRs detected during the study. As a representative sample, 45 ESTs carrying class I SSRs (≥ 20 nucleotides) were selected for designing primers and were also searched against the dbEST entries for L. sativa and Helianthus annuus (≤ 10(-50); score ≥ 100). In silico analysis of 45 SSR-ESTs showed 82% conservation across species and 68% conservation across genera. Primer pairs synthesized for the above 45 EST-SSRs were used to study genetic diversity among a collection of 22 L. serriola biotypes. Comparison of the resultant dendrogram to that developed using phenotypic evaluation of the same subset of lines showed limited correspondence. Taken together, this study reported a collection of useful SSR markers for L. serriola, confirmed transferability of these markers within and across genera, and demonstrated their usefulness in studying genetic diversity

Echinochloa crus-galli Populations to Acetolactate Synthase-Inhibiting Herbicides

Three Echinochloa crus-galli (barnyardgrass) populations from rice fields in Arkansas (AR1 and AR2) and Mississippi (MS1), USA, were recently confirmed to be resistant to imazethapyr. Experiments were conducted to characterize cross-resistance to acetolactate synthase-(ALS-) inhibiting herbicides and determine if malathion, a known cytochrome P450 monooxygenase (CYP) inhibitor, would overcome resistance. The AR1 and MS1 populations were cross-resistant to bispyribac-sodium; however, AR2 was sensitive to bispyribac-sodium. The AR1, AR2, and MS1 populations were >94, >94, and 3.3 times, respectively, more resistant to imazamox; >94, 30, and 9.4 times, respectively, more resistant to penoxsulam; and 15, 0.9, and 7.2 times, respectively, more resistant to bispyribac-sodium compared to a susceptible population. Addition of malathion to penoxsulam reduced dry weight of all populations and increased mortality of AR2 and MS1 populations compared to penoxsulam alone. Addition of malathion to imazethapyr and bispyribac-sodium increased the mortality of MS1 population in mixture with imazethapyr and AR1 population in mixture with bispyribac-sodium compared to treatments with imazethapyr and bispyribac-sodium applied alone. Synergism of ALS-inhibiting herbicides with malathion indicates increased herbicide degradation by CYP as partial mechanism of resistance to penoxsulam in all resistant populations and probably to imazethapyr in MS1 and bispyribac-sodium in AR1 populations