Distribution and Characterization of Herbicide-Resistant Italian ryegrass and Palmer amaranth in Virginia

Abstract

Weed infestation is the major reason for economic losses in agriculture. Italian ryegrass and Palmer amaranth are some of the most troublesome weed species in Virginia. These species are strong competitors with crops for growth resources which eventually leads to significant yield losses in absence of adequate control. One of the challenges for the control of these species is development of herbicide-resistant populations. There is a limited knowledge about the distribution of resistant populations of Italian ryegrass and Palmer amaranth in Virginia. Palmer amaranth resistance to glyphosate was confirmed in 2011 and Italian ryegrass resistance to diclofop was confirmed in 1993. These are the only two confirmed cases of herbicide resistance in Virginia but based on control failure reports, resistance to these species is suspected to be more widespread in Virginia. To investigate the distribution and levels of resistance in populations from Virginia there is a necessity for more updated surveys. A total of 32 populations of Italian ryegrass were collected. Plants were grown in the greenhouse to test for sensitivity to herbicides commonly used for burndown or in-crop control of Italian ryegrass: pinoxaden, diclofop, glyphosate, mesosulfuron, pyroxsulam, and pyroxasulfone. At 21 days after the herbicide treatments, visible injury ratings were recorded on a scale 0 to 100%, where 0 indicates no control and 100 represents complete plant necrosis. Populations exhibiting ≤49% control were suspected to be resistant. Based on this criteria, 10, 27, 0, 14, 0, and 7 populations were found to be resistant to pinoxaden, diclofop, glyphosate, mesosulfuron, pyroxasulfone, and pyroxsulam, respectively. Following the initial screening, dose-response assays with pinoxaden, diclofop, mesosulfuron and pyroxsulam were conducted. Resistance indices (R/S ratios), calculated based on GR50 (herbicide dose that reduced biomass by 50%) values for resistant and susceptible populations, were 20 for pinoxaden, 87 for mesosulfuron, and 161 for pyroxsulam. The R/S value for diclofop could not be determined because even the highest tested dose could not achieve 50% growth reduction in the resistant population. Cross and multiple resistance was observed in this study and 6% of populations were found resistant to pinoxaden, diclofop-methyl, mesosulfuron, and pyroxsulam. A total of 68 Palmer amaranth populations were collected from corn, soybean and cotton fields across Virginia. Palmer amaranth seedlings grown in the greenhouse were treated with: trifloxysulfuron, 2,4-D, fomesafen, atrazine, mesotrione, glyphosate, glufosinate and dicamba. Visible control ratings were recorded on a 0 to 100% scale, where populations with up to 49% injury were considered resistant. Upon testing the populations, resistance was found in 46, 1, 3, 7, 3, 50, 0 and 0 populations to trifloxysulfuron, 2,4-D, fomesafen, atrazine, mesotrione, glyphosate, glufosinate and dicamba, respectively. Dose-response assay for glyphosate revealed that GR50 value for resistant population was 1,238 g ae ha-1, however R/S value could not be calculated as susceptible population was not available. The R/S values for trifloxysulfuron, fomesafen and atrazine were 47, 14 and 18, respectively. Approximately 69% of the populations showed multiple resistance to two or more herbicide sites of action. Overall, findings from these statewide surveys provide critical insights into the current herbicide resistance status for both Italian ryegrass and Palmer amaranth in Virginia. This information will help growers better understand the effectiveness of commonly used herbicides and make more informed management decisions.