Characterization and Management of Selected Herbicide Resistant Weed Populations

Weed populations or biotypes expressing resistance to glyphosate and other herbicides have become more wide spread and in some cases more difficult to manage in agricultural systems. Determining genotypic and phenotypic variation among populations can increase knowledge that may be useful in developing weed management strategies. Assessment of genetic diversity among and within eight Palmer amaranth biotypes collected from North Carolina and Georgia using Amplified Fragment Length Polymorphism (AFLP) markers revealed levels of genetic diversity to be high, ranging in value from 0.0773-0.8605. Cluster and Principal Coordinate (PCO) analyses grouped individuals mostly by geographic origin irrespective of either resistance or susceptibility to glyphosate or gender of individuals. Analysis of Molecular Variance (AMOVA) results indicated that within biotype contribution towards total variation was always higher and significant (P ≤ 0.0001) than among biotypes. Resistance in a Palmer amaranth biotype from North Carolina was found to be incompletely dominant, nuclear inherited, and might not be consistent with a single gene mechanism of inheritance.;Recovery of fifteen Palmer amaranth biotypes collected across North Carolina following exposure to different periods of moisture stress indicated that percent reduction in height for the glyphosate resistant (GR) group was less compared with acetolactate synthase resistant (ALSR) and susceptible (ALSS/GS) groups 24 DAE. Dry weight of both ALSR and GR groups was affected less by moisture stress than the ALSS/GS group. Early season interference of corn, cotton, peanut, snap bean, and soybean growth by eight Palmer amaranth biotypes (4 GR and 4 GS) in greenhouse revealed that when pooled over crops the percent reduction for fresh weight was 26% in presence of GS biotypes and 20% in presence of GR biotypes. Differential interference of soybean by seven Palmer amaranth biotypes (4 GR and 3 GS) in the field was observed.;Research was conducted to determine if differences in control existed among Palmer amaranth biotypes when treated with herbicides with different modes of action. In the greenhouse, when GR and GS biotypes were grouped, minor differences in control were observed with herbicides other than glyphosate. No clear conclusion could be made relative to differences in control of GR and GS biotypes of Palmer amaranth by herbicides representing major modes of action.;Populations of Italian ryegrass collected from six fields from two counties in North Carolina where mesosulfuron was no longer effective were used to confirm resistance to ALS-inhibiting herbicides and to determine if resistance to the acetyl-CoA carboxylase (ACCase) inhibitors diclofop and pinoxaden was present in these populations. All populations were resistant to diclofop and cross resistant to pinoxaden. Five of the six populations displayed multiple resistance to the ACCase inhibitors and the ALS inhibitor. An additional study with two biotypes confirmed cross resistance to the ALS inhibitors imazamox, mesosulfuron, and pyroxsulam. Resistance to mesosulfuron was shown to be heritable.;Research was conducted in order to confirm common ragweed resistance to the ALS-inhibiting herbicide diclosulam in a field with suspected resistance and to compare herbicide programs in that field designed to control ALS-resistant common ragweed in corn, cotton, peanut, and soybean. In greenhouse experiments, I50 values following postemergence application of diclosulam for mortality of plants, visual estimates of percent control, and percent reduction in plant fresh weight were 557 to 653 fold higher for the suspected ALSresistant biotype compared with a suspected ALS-susceptible biotype. Herbicides with modes of action different from ALS-inhibiting herbicides, including: atrazine, dicamba, and glyphosate in corn; fomesafen, glyphosate, MSMA, and prometryn in cotton; bentazon, flumioxazin, and lactofen in peanut; and flumioxazin, glyphosate, and lactofen in soybean controlled common ragweed more effectively in many instances than programs relying on cloransulam (soybean), diclosulam (peanut), thifensulfuron (corn), and trifloxysulfuron (cotton) that typically control common ragweed populations not resistant to ALS-inhibiting herbicides. Applying tank-mix or sequential applications of herbicides with different modes of action were effective in controlling ALS-resistant common ragweed in all crops.