No-tillage corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production has been widely accepted in the mid-Atlantic region, favoring establishment of horseweed [Conyza canadensis (L.) Cronq.]. Within 3 yr of using only glyphosate for weed control in continuous glyphosate-resistant soybeans, glyphosate failed to control horseweed in some fields. Seedlings originating from seed of one population collected in Delaware were grown in the greenhouse and exhibited 8- to 13-fold glyphosate resistance compared with a susceptible population. There were no differences between the isopropylamine or diammonium salts of glyphosate.
Summary1. Glyphosate-resistant Conyza canadensis populations now infest more than 44 000 ha of arable land in eastern USA, only 5 years after the first resistant population was reported. Seed dispersal, the expansive use of glyphosate and the lack of tillage are all factors contributing to this high invasion speed. 2. Seed collected from deliberately established C. canadensis source populations were found at the furthest seed traps in our empirical studies, providing one of the most complete dispersal kernels for studying long-distance dispersal. The results indicate that seed regularly disperses at least 500 m from source populations. While a relatively small number of seeds moves long distances, 99% of the seed was found within 100 m of the source. 3. Empirical and mechanistic models were fitted to the data to predict the dispersal in the prevailing wind direction. Both models underestimated seed deposition beyond 200 m but the mechanistic model provided a better fit to the data (lower Akaike information criterion). A two-dimensional analysis examined the correlation between angular directions of wind and seed movement. All trials were anisotropic but only the cumulative wind and seed direction were significantly correlated ( P < 0·05). 4. The empirical model was used to explore the effect of increasing source strength, which would be expected as an infestation of glyphosate-resistant C. canadensis expands on a producer's farm. At infestation levels consistent with a heavy infestation in a 5-ha field, seed dispersed further than 1·5 km, easily affecting 10s to 100s of surrounding farms. 5. Synthesis and applications . Emigration of C. canadensis seed, from a source farm to adjacent farms, means that population dynamics and weed management are dependent on both intra-and interfield dispersal phenomena. Wind-dispersed plants challenge the common practice of single field management as a viable management option for herbicide-resistant weeds. Farms coupled by seed dispersal require proactive management practices by every producer to prevent and minimize the development of glyphosate-resistant infestations of undesirable and alien plants.
N-(phosphonomethyl)glycine (glyphosate) resistance was previously reported in a horseweed [Conyza (=Erigeron) canadensis (L.) Cronq.] population from Houston, DE (P (0) (R) ). Recurrent selection was performed on P (0) (R) , since the population was composed of susceptible (5%) and resistant (95%) phenotypes. After two cycles of selection at 2.0 kg ae glyphosate ha(-1), similar glyphosate rates that reduced plant growth by 50%, glyphosate rates that inflicted 50% mortality in the population, and accumulations of half of the maximum detectable shikimic acid concentration were observed between the parental P (0) (R) and the first (RS(1)) and second (RS(2)) recurrent generations. In addition, RS(1) and RS(2) did not segregate for resistance to glyphosate. This suggested that the RS(2) population comprised a near-homozygous, glyphosate-resistant line. Whole-plant rate responses estimated a fourfold resistance increase to glyphosate between RS(2) and either a pristine Ames, IA (P (0) (P) ) or a susceptible C. canadensis population from Georgetown, DE (P (0) (S) ). The genetics of glyphosate resistance in C. canadensis was investigated by performing reciprocal crosses between RS(2) and either the P (0) (P) or P (0) (S) populations. Evaluations of the first (F(1)) and second (F(2)) filial generations suggested that glyphosate resistance was governed by an incompletely dominant, single-locus gene (R allele) located in the nuclear genome. The proposed genetic model was confirmed by back-crosses of the F(1) to plants that arose from achenes of the original RS(2), P (0) (P) , or P (0) (S) parents. The autogamous nature of C. canadensis, the simple inheritance model of glyphosate resistance, and the fact that heterozygous genotypes (F(1)) survived glyphosate rates well above those recommended by the manufacturer, predicted a rapid increase in frequency of the R allele under continuous glyphosate selection. The impact of genetics on C. canadensis resistance management is discussed.
Horseweed is a winter or summer annual plant, native to North America and distributed worldwide in temperate climates. This plant is considered an important agricultural weed because it can reduce agricultural yields by 90% at high densities and becomes problematic under low-tillage agriculture. Seed production is robust with an estimated 200,000 seeds produced per plant, and seed dispersal is wind-assisted. The confirmation of glyphosate-resistant horseweed in Delaware in 2001 and the rapid spread of the resistant biotype, currently covering more than 44,000 ha, has necessitated a change in the discussion about weed dispersal. Large radio-controlled airplanes were used to sample the lower atmosphere for the presence of horseweed seeds during a 3-d period in early September 2005 in southern Delaware. The collection of multiple seeds at heights ranging from 41 to 140 m above ground level strongly suggests that horseweed seeds are entering the Planetary Boundary Layer (PBL) of the atmosphere, where long-ranged transport of aerial biota frequently occurs. With wind speeds in the PBL frequently exceeding 20 m s−1, seed dispersal can easily exceed 500 km in a single dispersal event.
In this paper, the native and nonnative floras of Boston, New York, Philadelphia, Washington, D.C., Detroit, Chicago, Minneapolis, and St. Louis urban areas are compared, and overall native diversity and nonnative diversity are correlated with a variety of factors. A total of 4,159 species has been reported in the eight urban areas. Of these, 2,708 (65.1%) are native to one or more of the urban areas and 1,451 (34.9%) are nonnative. Only 316 (11.6%) of the native species and only 109 (7.5%) of the nonnative species are common to all of the urban areas. When the similarity of native species is compared, Boston, New York, Philadelphia, and Washington, D.C., form a cluster, as do Detroit, Chicago, and Minneapolis; St. Louis is least similar to the other seven urban areas. Correlating climatic variables (growing season, temperature) and geographical variables (area, latitude, longitude) with species richness showed that nonnative species richness was most strongly correlated with longitude (probably as a function of age of settlement). This is in contrast * Published Online June 24, 2003 with past research on native species showing a strong correlation of native species richness with latitude and elevations due to climatic differences present at different latitudes and elevations. Further studies that incorporate data from additional urban areas are needed to determine if nonnative species richness continues to be strongly correlated with time of a city's settlement.
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