Dithiopyr and dinitroanilines are preemergence-applied, mitotic-inhibiting herbicides used to control goosegrass [Eleusine indica (L.) Gaertn.]) in turfgrass. A suspected resistant E. indica population was collected from a golf course putting green and was evaluated for possible resistance to dithiopyr and prodiamine. After dose-response evaluation, the α-tubulin gene was sequenced for known target-site mutations that have been reported to confer resistance to mitotic-inhibiting herbicides. A mutation was discovered that resulted in an amino acid substitution at position 136 from leucine to phenylalanine (Leu136-Phe). Previous research has indicated that Leu136-Phe does confer resistance to dinitroaniline herbicides. The level of resistance indicated by regression models and I50 values indicates that there is a 54.1-, 4.7-, >100-, and >100-fold resistance to dithiopyr, prodiamine, pendimethalin, and oryzalin, respectively when compared to the susceptible population based on seedling emergence response and 88.4-, 7.8-, >100-, and >100-fold resistance to dithiopyr, prodiamine, pendimethalin, and oryzalin, respectively when compared to the susceptible population based on biomass reduction response. This is the first report of less resistance to prodiamine compared to pendimethalin or oryzalin due to a target-site α-tubulin mutation and the first report of a target-site α-tubulin mutation associated with dithiopyr resistance.
Advanced sequencing techniques have improved the ability to identify and understand target-site resistance in herbicide-resistant species. Despite innovations in sequencing, polyploid species can still face issues that are typically not seen in diploid species, often because of the presence of conflicting subgenomes. Further confounding the difficulties of polyploidy is the α-tubulin gene, which has subgenomic duplication of gene family copies. Poa annua L., an allotetraploid, is a persistent weed in turfgrass that has developed resistance to multiple herbicide modes of action, including mitotic-inhibiting herbicides. Sequencing α-tubulin cannot be performed by simple Sanger sequencing to identify target-site mutations because of frequent nucleotide conflictions between the numerous α-tubulin copies present on both subgenomes, which was illustrated by vector cloning in combination with Sanger. Improved sequencing is needed to understand resistance to mitotic-inhibiting herbicides. Amplicon sequencing via Illumina technology was used to identify targetsite mutations. Eighty-two populations resistant to mitotic-inhibiting herbicides were sequenced via next-generation sequencing. Seventy-five populations presented with the known single nucleotide polymorphism, Thr239Ile. The ability to successfully sequence and analyze α-tubulin data provides a vehicle for further insight into herbicide resistance.
The goal of weed science extension efforts are to encourage and accelerate adoption of diverse, effective, and economical management tactics. To be most successful and efficient, extension personnel need to be aware of growers’ preferred information sources, delivery format, and areas of focus for future research. To this end, surveys were distributed at crop and forage extension meetings in Virginia. The results from 249 responses indicate that both crop and forage producers have similar influences as well as preferences for both programming and future research. Agribusiness personnel (e.g. co-ops, suppliers, vendors, crop consultants, sales reps) had the greatest influence on herbicide-purchasing decisions and the primary source of information for weed management decisions, and thus should be a target audience of extension. Respondents said that economic assessments, weed control data, and yield data are most likely to influence changes in management and that they would prefer to get that information through traditional extension formats (presentations, publications, and on-farm demonstrations). Generally, respondents also indicated that they wanted extension to focus on evaluating new herbicides for weed control and crop safety in the future over alternative non-herbicidal weed control methods. Therefore, extension is likely to be more successful by including herbicides in the integrated weed management approach rather than solely non-chemical approaches.
Mitotic-inhibiting herbicides, like prodiamine and dithiopyr, are used to control annual bluegrass (Poa annua L.) preemergence in managed turfgrass; however, resistance to mitotic-inhibiting herbicides has evolved due to repeated applications of herbicide from a single mechanism of action. Three suspected resistant populations (R1, R2, and R3) were collected in Alabama and Florida and screened for resistance to prodiamine. Part of the α-tubulin gene was sequenced for known target-site mutations. Target-site mutations were reported in all three R populations, with each of them containing an amino acid substitution at position 239 from threonine to isoleucine (Thr239-Ile). Previous research has indicated that the Thr239-Ile mutation confers resistance to dinitroaniline herbicides in other species. Dose response screens using prodiamine and dithiopyr were conducted and I50 values were calculated for R1, R2, and R3 using regression models based on seedling emergence. For prodiamine, I50 values for R1, R2, and R3 were 35.3, 502.7, and 91.5 g ai ha-1, respectively, resulting in 2.9-, 41.9-, and 7.6-fold resistance, respectively, when compared to a susceptible (S) population. For dithiopyr, I50 values for R1, R2, and R3 were 154.0, 114.2, and 190.1 g ai ha-1, respectively, resulting in 3.6-, 2.7-, and 4.5-fold resistance, respectively, when compared to a S population. When comparing I90 values to the highest labeled use rates, R2 had a 2.9-fold level of resistance to prodiamine and R1, R2, and R3 had a 2.4-, 2.0-, and 3.2-fold level of resistance to dithiopyr, respectively. This is the first report of a variable response in P. annua to prodiamine despite each R population possessing the same mutation.
Poa annua L. (annual bluegrass) is a common weed in turfgrass and has been confirmed resistant to twelve different herbicide sites of action, with various combinations of multiple‐herbicide resistance having been identified. In an effort to quantify the extent of herbicide resistant P. annua, the ResistPoa Project (resistpoa.org) surveyed 1349 P. annua populations for resistance to nine sites of action and one plant growth retardant. Herein we report results from sequencing of known target site mutations found in EPSPS, ALS, psbA, and 𝛼‐tubulin genes. Populations were sequenced using either capillary or amplicon sequencing (AmpSeq), depending on the complexity of the gene, and were analyzed for target‐site resistance. After additional resistance screening, a total of 389 suspected resistant populations were sequenced—131 for ALS, 83 for EPSPS, 93 for psbA, and 82 for 𝛼‐tubulin. From the resistant populations, 64 displayed resistance to multiple sites of action. After sequencing, it was determined that target‐site resistance was the common form of resistance for all sites of action outside of psbA with 65.6% of ALS populations, 73.5% of EPSPS, 39.8% of psbA, 91.5% of 𝛼‐tubulin having presented a target‐site mutation.This article is protected by copyright. All rights reserved
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