Genetic diversity and differentiation in populations of invasive Eurasian (Myriophyllum spicatum) and hybrid (Myriophyllum spicatum × Myriophyllum sibiricum) watermilfoil

Population genetic studies of within- and among-population genetic variability are still lacking for managed submerged aquatic plant species, and such studies could provide important information for managers. For example, the extent of within-population genetic variation may influence the potential for managed populations to locally adapt to environmental conditions and control tactics. Similarly, among-population variation may influence whether specific control tactics work equally effectively in different locations. In the case of invasive Eurasian watermilfoil (Myriophyllum spicatum L.), including interspecific hybrids with native northern watermilfoil (Myriophyllum sibiricum Kom.), managers recognize that there is genetic variation for growth and herbicide response. However, it is unclear how much overall genetic variation there is, and how it is structured within and among populations. Here, we studied patterns of within- and among-lake genetic variation in 41 lakes in Michigan and 62 lakes in Minnesota using microsatellite markers. We found that within-lake genetic diversity was generally low, and among-lake genetic diversity was relatively high. However, some lakes were genetically diverse, and some genotypes were shared across multiple lakes. For genetically diverse lakes, managers should explicitly recognize the potential for genotypes to differ in control response and should account for this in monitoring and efficacy evaluation and using pretreatment herbicide screens to predict efficacy. Similarly, managers should consider differences in genetic composition among lakes as a source of variation in the growth and herbicide response of lakes with similar control tactics. Finally, laboratory or field information on control efficacy from one lake may be applied to other lakes where genotypes are shared among lakes.

Population Variability in the Response of Ripgut Brome (Bromus diandrus) to Sulfosulfuron and Glyphosate Herbicides

Ripgut brome has become a problematic weed in Spain both as a consequence of the continuous cropping of winter wheat through minimal tillage systems and its difficult control with selective herbicides. Ripgut brome populations collected in the regions of Castilla-León and Cataluña, two main cereal-growing areas in Spain, were screened in the greenhouse for response to sulfosulfuron, a selective herbicide for the control of brome grasses in wheat, and to glyphosate, often used as a pre-plant knockdown to control bromes in no-till systems. The fresh weight percentage relative to the untreated controls was calculated for each ripgut brome population and herbicide and was used as a measure of the herbicide response. Results showed variation in fresh weight response to both herbicides among populations. Fresh weight of the populations after sulfosulfuron was applied at the two-leaf stage at a rate of 20 g ai ha−1varied from 3% in the most susceptible population to 35% in the most resistant; the response was similar (6 to 38%) when the herbicide dose was reduced to half. For glyphosate at 800 g ae ha−1, fresh weight varied from 2 to 25% among populations, but the range of variation in fresh weight response increased as herbicide dose decreased to one half, with rates of from 4% to 90% among populations. The location of the collection site (inside the field or in-margin) showed no differences in response to both herbicides, but there was a statistically significant, geographically correlated differentiation for glyphosate response, with a greater resistance in the populations from Castilla-León. Undamaged plants were found after treatments with both herbicides, showing differences in resistance among plants. The study shows inter- and intrapopulation variability for the response of ripgut brome to sulfosulfuron and glyphosate. The implications for resistance development are discussed within the framework of relationships of the structure of the populations relative to their herbicide response.

Effects of Preemergence Herbicides on Soybean (Glycine max): Weed Competition

Glasshouse experiments were conducted to evaluate the effects of preemergence applications of metribuzin, pendimethalin, alachlor, and imazaquin on the competitive relationship between soybean and sicklepod, tall morningglory, or common cocklebur. An equal-ratio series design was employed where the proportion of species in a mixture remained constant and total plant density was varied. The ratio of soybean:sicklepod fresh weights was altered by alachlor and imazaquin while the ratio of soybean:tall morningglory fresh weights was altered only by imazaquin. The ratio of soybean:common cocklebur fresh weights was altered by metribuzin and imazaquin. Increasing herbicide rates resulted in higher soybean:weed fresh weight ratios and higher herbicide response coefficients.

Associations between Discrete Genetic Loci and Genetic Variability for Herbicide Reaction in Plant Populations

Genetic variability for loci governing enzyme/morphological variants and for herbicide response was determined in 10 populations of the slender wild oat (Avena barbataPott. ex Link ♯ AVEBA), six populations of wild oat (Avena fatuaL. ♯ AVEFA), and three populations of godetia (Clarkia williamsoniiLewis & Lewis). The enzyme loci were identified by starch gel electrophoresis and included peroxidase, 6-phosphogluconate dehydrogenase, esterase, and leucine aminopeptidase for the slender wild oat; peroxidase, esterase, leucine aminopeptidase, and malate dehydrogenase for the wild oat; and esterase, phosphoglucoisomerase, leucine aminopeptidase, acid phosphatase, and glutamate oxaloacetate transaminase for godetia. Morphological loci included lemma and leaf sheath hairiness for the oats. For both the enzymatic and morphological loci, levels of genetic variation for each population were quantified by a polymorphic index. The herbicide barban (4-chloro-2-butynyl 3-chlorophenylcarbamate) was used on the wild oats; bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) was used on godetia. Genetic variation for herbicide response was based on genetic variances calculated from phytotoxicity scores. Populations were ranked from highest to lowest for the polymorphic indices and the genetic variances. Concordance between the rankings was tested by rank correlation. Statistically significant relationships were found between the enzyme/morphological characters and herbicide response in the slender wild oat and the wild oat. For some species, the level of genetic variation for response to herbicides appears to be associated with genetic variation for enzymatic and morphological loci.

Control of Johnsongrass (Sorghum halepense) in Soybeans (Glycine max) with Foliar-Applied Herbicides

Field and greenhouse experiments were conducted to evaluate factors affecting johnsongrass [Sorghum halepense(L.) Pers. # SORHA] control in soybeans [Glycine max(L.) Merr. ‘Essex’] with three foliarly-applied herbicides. In 1980, johnsongrass height at treatment time had little effect on control due to inadequate soil moisture. In 1981, a wet year, best control was achieved on 15- or 40-cm-tall johnsongrass. BAS-9052 {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.6 kg/ha gave good to excellent control at all growth stages. Two applications of 0.3 kg/ha of BAS-9052 in both years and PP-009 butyl {2-[4-[5-(trifluoromethyl-2-pyridinyloxy)] phenoxy] propanoate} in 1981 increased johnsongrass control compared to only one treatment. Mixing BAS-9052 with other herbicides significantly reduced johnsongrass control. Greenhouse studies to determine the effect of soil water on herbicide response indicated that drier soils decreased johnsongrass regrowth with PP-009 and increased regrowth with BAS-9052.

Herbicide Response and Morphology of Interspecific Sowthistle Crosses

Interspecific crosses were made between perennial sowthistle (Sonchus arvensisL.) and annual sowthistle (Sonchus oleraceusL.). Flower head size, leaf type, and creeping root growth of the hybrid were intermediate to those same morphological characteristics of the parents. Backcrosses to the parents produced plants which resembled annual or perennial sowthistle more closely than did the F1hybrids. Annual sowthistle and backcrosses to annual sowthistle were more tolerant to (2,4-dichlorophenoxy)acetic acid (2,4-D) and 3,6-dichloro-o-anisic acid (dicamba) than were perennial sowthistle or backcrosses to perennial sowthistle. However, annual and perennial sowthistle were similar to each other in response to 4-amino-3,5,6-trichloropicolinic acid (picloram). Natural crosses between annual and perennial sowthistles may account for some of the variability in herbicide response found in perennial sowthistle.

Stomata Variations in Canada Thistle and Response to Herbicides

Canada thistle(Cirsium arvense(L) Scop.) includes many regional races that are heritable, distinct ecotypes. When cultured at Bozeman, Montana a group of these ecotypes varied in phenological, morphological, and anatomical characteristics. These ecotypes also responded differently to (2,4-dichlorophenoxy)acetic acid (2,4-D) and 3-amino-s-triazole (amitrole). Stomatal frequency and area on leaves also differed among ecotypes studied. Although stomatal frequency and stomatal area differed among the ecotypes studied, there was no correlation with herbicide response. Apparently, stomata were not a significant portal of entry of 2,4-D into the upper surface of Canada thistle leaves.