Paraquat-Resistant Horseweed Identified in the Mid-Atlantic States

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Mark J. VanGessel, Professor and Extension Specialist, Barbara A. Scott, Research Associate, and Quintin R. Johnson, Extension Associate, Department of Plant and Soil Science, University of Delaware, Georgetown 19947

Abstract

In 2003, a small percentage of horseweed (Conyza canadensis) plants survived sequential paraquat applications in a research trial conducted in a commercial field. This research project focused on determining if this horseweed population was resistant to paraquat. The susceptible biotype required a paraquat application of 0.03 lb ai/acre to reduce biomass by 50%, while the suspected-resistant biotype required 0.7 lb ai/acre. This consists of a 22-fold difference in response between the two biotypes. This is the first documented case of paraquat-resistant horseweed in the Mid-Atlantic states and the second reported case in the United States for this species.

Introduction

Horseweed (or marestail, Conyza canadensis [syn. Erigeron canadensis]) is native to North America, and is classified as a fall and spring emerging annual plant. It has been documented worldwide, but primarily exists in temperate climates. Horseweed is found in all field settings where tillage has been reduced or eliminated, and can thrive on a wide range of soil types (6,14). It often grows in association with other winter annuals. However, as an early succession species, horseweed can develop essentially a monoculture in recently abandoned agricultural fields.

Horseweed develops as a rosette shortly after emergence. Elongated flower stalks (bolts) develop in late spring, with flowering occurring in mid-July, and seed produced in early August. It is reported in the literature that a mature plant can reach 10 ft in height and can produce more than 200,000 seeds that are wind-born with the aid of a pappus (5,14).

Numerous herbicides can effectively control this species if treated in the rosette stage. However, there are reports of horseweed having developed resistance to broad-spectrum herbicides including triazines (3,4,8,14), paraquat (4,10), acetolactate synthase (ALS) inhibitors (4), and glyphosate (13). These represent various herbicide families that are principally used for control or suppression of horseweed in no-till production systems and non-cropped areas. At least four horseweed populations have exhibited resistance to more than one mode of action (multiple-resistance) (4,9).

In 2003, research plots on a private farm near Georgetown, DE had a few horseweed plants survive sequential paraquat applications, while most plants were controlled. This research project focused on determining if this surviving horseweed population was resistant to paraquat.

Greenhouse Evaluation for Paraquat Resistance

Horseweed seed was collected at maturity from a soybean field located near Georgetown, DE from plants surviving sequential paraquat (Gramoxone Max) applications of 0.84 followed by 0.63 lb ai/acre. Paraquat applications were made 1 week apart. Seed was also collected from horseweed plants growing at the University of Delaware Research and Education Center (UD-REC), near Georgetown, DE. The field at UD-REC had not been treated with paraquat in the past 5 years. Seed from UD-REC was then chosen as a susceptible check.

Seed was planted 0.1-inch deep in flats containing commercial potting mix and placed in a greenhouse. Plants were grown at a temperature range of 80 to 95°F and daylength was extended to 15 h using high-pressure sodium lights. Two-week-old seedlings were transplanted to pots (3.5 by 3.5 by 4 inches) with one plant per pot. Plants were irrigated twice a day. Seedlings were treated 4 weeks after planting, with paraquat at rates of 0.015, 0.030, 0.0625, 0.125, 0.25, 0.5, 1.0, 2.0, and 4.0 lbs ai/acre. All herbicide treatments included a non-ionic surfactant at 0.25% v/v. An untreated check was also included. Herbicide treatments were applied with a compressed air cabinet sprayer calibrated to deliver 25 g/acre at 30 psi.

The experimental design was a randomized complete block with a two factor factorial arrangement of treatments. The first factor was biotype, suspected resistant from Georgetown (R) or susceptible (S) from UD-REC. The second factor was paraquat rate. Each treatment was replicated four times and the experiment was repeated.

Plants were visually evaluated for percent control at weekly intervals (0 = no apparent reduction in plant biomass and 100 = dead horseweed). Three weeks after treatment, plants were clipped at the soil surface, dried at 110°F for 4 days, and weighed.

A reduction in average dry biomass per plant was expressed as a percentage of the untreated control plants and a square root transformation was used to achieve homogeneity of variance. Nonlinear regression parameters were predicted using the sigmoid model as described by SigmaPlot (SigmaPlot 2000 for Windows, Version 6.00; SPSS Inc., Chicago, IL) that relates dry biomass as a percentage of the control (Y) to the herbicide rate (x):

In this equation, a is the upper response limit, GR₅₀ is the herbicide rate that results in a 50% reduction in biomass, and b is the slope of the curve around the GR₅₀ value. The level of resistance was determined by calculating a Resistant/Susceptible (R/S) ratio (GR₅₀ for the resistant biotype divided by GR₅₀ for the susceptible biotype).

Biotypes Respond Differently to Paraquat Rate

Visual horseweed control and dry biomass were best described by a sigmoidal function of the paraquat rate for both the Georgetown and UD-REC biotypes. Rate by biotype interaction was highly significant (P = 0.0001). The interaction indicates that the Georgetown and UD-REC biotypes responded differently to paraquat rates (Fig. 1). To achieve greater than 90% control by visual rating, the UD-REC biotype required 0.125 lb/acre, but the Georgetown biotype required 4.0 lb/acre (data not presented). Biomass differences were detected between the two biotypes at each paraquat rate (P = 0.0001), except 0 and 4.0 lb/acre (Fig. 2).

Fig. 1. Photo of paraquat resistant (top row) and susceptible (bottom row) horseweed biotypes. Paraquat rates (from left to right) are 0, 0.03, 0.5, and 1.0 lb/acre. Photo was taken 3 weeks after herbicde application.

Fig. 2. Response of resistant (R) and susceptible (S) horseweed biotypes to varying rates of paraquat. Biomass reduction is expressed as a percentage of the untreated control. The predicted line is described by

(see text for description of the equation).

The UD-REC biotype required a paraquat application of 0.03 lb/acre to reduce biomass by 50% (Fig. 2). However, the Georgetown biotype required 0.7 lb/acre to achieve at least 50% reduction in biomass. Based on GR₅₀ values, a 22-fold difference in response to paraquat occurred between the two biotypes.

A similar level of paraquat resistance was reported with horseweed in Ontario, Canada (10). The 22-fold difference is a lower level of resistance than other reports for Conyza canadensis (2,11,12). Other researchers have indicated that levels of resistance can change with size or stage of the plants and can change with methodology of determining resistance (10,12). The mechanism of paraquat resistant has not been fully elucidated, although research to date indicates it is due to a reduction in movement, due to a sequestration mechanism (7).

Considerations for Management of a Wind-Dispersed Species

Paraquat resistance has been reported in 23 species worldwide (4). However, the only terrestrial dicot plants reported as paraquat-resistant in North America are horseweed, Virginia pepperweed (Lepidium virginicum), and American black nightshade (Solanum americanum). The cases of paraquat resistance in Ontario were selected for by multiple applications per year in orchards. The Georgetown biotype has not been subjected to intense selection pressure. The field had been conventionally tilled prior to corn or soybeans and had received no paraquat applications before 2003.

Due to the aerial dissemination of horseweed seed, the history of the field where the resistant plants were identified may have little impact on selection of resistance. The reproduction, dispersal, and germination ecology for horseweed are characteristics that increase the likelihood that horseweed will infest adjacent and distant fields (5,14). Anderson (1) measured settling velocities of wind-dispersed seed from 19 asteraceae species and found horseweed seed to have the lowest settling velocity of those measured. Settling velocity in association with the height of seed release indicates significant potential for wind-dispersal of horseweed over considerable distances. Field observations have revealed that horseweed in adjacent fields or nearby fields is a serious threat for colonization. This adds a new dynamic to management of a herbicide-resistant weed.

Summary

Horseweed biotypes have been documented as resistant to various triazines, bipyridyliums, sulfonylureas, and glyphosate. This is the first case of paraquat resistance in the Mid-Atlantic states, for any species, and only the second report of paraquat-resistant horseweed in the USA. However with the potential for wind-dispersal of horseweed seed, paraquat-resistant horseweed is likely more wide-spread than this one observation. Additional research is needed to identify the mechanism of resistance in horseweed and to determine genetic similarities with other paraquat-resistant biotypes.

Literature Cited

1. Anderson, M. C. 1993. Diaspore morphology and seed dispersal in several wind-dispersed asteraceae. Am. J. Botany. 80:487-492.

2. Fuerst, E. P., and Vaughn, K. C. 1990. Mechanisms of paraquat resistance. Weed Technol. 4:150-156.

3. Gressel, J., Ammon, H. U., Fogelfors, H., Gasquez, J., Kay, Q. O. N., and Kees, H. 1982. Herbicide Resistance in Plants. H. M. LeBaron and J. Gressel, eds. John Wiley and Sons, Inc. New York, NY.

4. Heap, I. 2005. International survey of herbicide resistant weeds. Online. HRAC, NAHRAC, and WSSA.

5. Holm, L., Doll, J., Holm, E., Pancho, J., and Herberger, J. 1997. Pages 226-235 in: World Weeds: Natural Histories and Distribution. John Wiley and Sons, Inc., New York, NY.

6. Leroux, G. D., Benoit, D. L., and Banville, S. 1996. Effect of crop rotations on weed control, Bidens cernua and Erigeron canadensis populations, and carrot yields in organic soils. Crop Prot. 15:171-178.

7. Norman, M. A., Fuerst, E. P., Smeda, R. J., and Vaughn, K. C. 1993. Evaluation of paraquat resistance mechanisms of Conyza. Pest. Biochem. Physiol. 46:236-249.

8. Pölös, E., Laskay, G., Szigeti, Z., Pataki, Sz., and Lehoczki, E. 1987. Photosynthetic properties and cross-resistance to some urea herbicides of triazine-resistant Conyza canadensis Cronq (L.). Z. Naturforsch. 42c:783-793.

9. Pölös, E., Mikulás, J., Szigeti, Z., Matkovics, B., Hai, D. Q., Párducz, Á., and Lehoczki E. 1988. Paraquat and atrazine co-resistance in Conyza canadensis (L.) Cronq. Pest. Biochem. Phys. 30:142-154.

10. Smisek, A., Doucet, C., Jones, M., and Weaver, S. 1998. Paraquat resistance in horseweed (Conyza canadensis) and Virginia pepperweed (Lepidium virginicum) from Essex County, Ontario. Weed Sci. 46:200-204.

11. Szigeti, Z., and Lehoczki, E. 2003. A review of physiological and biochemical aspects of resistance to atrazine and paraquat in Hungarian weeds. Pest. Management Sci. 59:451-458.

12. Turcsanyi, E., Darko, E., Borbely, G., and Lehoczki, E. 1998. The activity of oxyradical-detoxifying enzymes is not correlated with paraquat resistance in Conyza canadensis. Pestic. Biochem. Physiol. 60:1-11.

13. VanGessel, M. J. 2001. Glyphosate-resistant horseweed from Delaware. Weed Sci. 49:703-705.

14. Weaver, S. E. 2001. The biology of Canadian weeds. 115. Conyza canadensis. Can. J. Plant Sci. 81:867-875.