Doctoral Dissertations

Date of Award

5-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plants, Soils, and Insects

Major Professor

Thomas C. Mueller

Abstract

Control of Italian ryegrass is important to Tennessee wheat producers. Control of Italian ryegrass has become more difficult over the years due to diclofop resistance. Italian ryegrass resistance to diclofop has been documented in several countries including the US. Tennessee producers have begun to notice that ryegrass escapes are becoming more prevalent than in years past. The purpose of this research was to use glasshouse methods to screen selected populations of Italian ryegrass for resistance to diclofop and to a more recent wheat herbicide pinoxaden and to utilize field experiments to develop herbicide programs for control Italian ryegrass in the field. Resistance to diclofop was found in eight TN populations. The eight populations did not show cross-resistance to pinoxaden. One population from Union County, NC (R1) was found to be resistant to both diclofop and pinoxaden. The level of resistance to pinoxaden of the R1 population was 14 x that of the susceptible population.Field experiments demonstrated preemergence (PRE) Italian ryegrass control with chlorsulfuron (71 to 94%) and flufenacet + metribuzin (84 to 96%). Italian ryegrass control with pendimethalin applied PRE or delayed preemergence (DPRE) was variable (0 to 85%). Postemergence control of Italian ryegrass was good with pinoxaden, mesosulfuron, flufenacet + metribuzin, and chlorsulfuron + flucarbazone (>80%). Timing of application and herbicide treatment had no effect upon wheat yield, except for diclofop and pendimethalin treatments where Italian ryegrass was not controlled. Pinoxaden is in the phenylpyrazolin herbicide family which offers control of Italian ryegrass but is not toxic to wheat. Pinoxaden has only been on the market for three years yet several wheat producers have suspected resistance in Italian ryegrass biotypes.An experiment was conducted to identify Italian ryegrass biotypes resistant to pinoxaden and to determine the mechanism of resistance using derived Cleaved Amplified Polymorphic Sequence (dCAPS) methods. Two populations were found resistant to pinoxaden, one from the state of Washington (R2) and the other from North Carolina (R1). The substitution of isoleucine by leucine at the 1781 ACCase residue was identified in the R1 biotype but not in the R2. The NC1 biotype is the first known pinoxaden resistant Italian ryegrass population to be documented having the 1781 target-site mutation.

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