Genetic markers have been used in horticulture and conservation biology to identify breeding lines, assess genetic diversity, and examine gene flow. In weed science, analysis of genetic markers is not as common, as the focus often lies in the development of control methods. This is unfortunate, because advances in genetic marker techniques may lead to innovative methods in controlling weedy plants. Microsatellites, random amplified polymorphic DNA (RAPDs), intersimple sequence repeats (ISSRs), amplified fragment length polymorphisms (AFLPs), DNA sequences, single nucleotide polymorphisms (SNPs), and derived cleaved amplified polymorphic sequences (dCAPS) have been used in studying genetics of weedy plants. Beyond assessing genetic diversity of weeds, markers have been used to examine gene flow, patterns of dispersal, ploidy levels, and relationships of weedy and nonweedy species, as well as identifying founder populations. Identification of closely related species may indicate the potential for hybridization, cross reactivity of chemical applications, or nontarget biological control effects. Furthermore, markers have been used in identification of mutations conferring herbicide resistance and identification of regions correlating fitness of weedy and hybrid plants in new habitats. Knowledge from genetic markers provides fundamental information that can be used in advancing chemical and biological control in weed management. This article serves as a review of these marker types and their application to weed science.

Purple deadnettle is an obligate winter annual weed and an alternate host of soybean cyst nematode (SCN). Field experiments were conducted using microplots to determine (1) the effect of purple deadnettle planting date and (2) the effect of purple deadnettle removal time on SCN egg population density in continuous no-tillage soybean. A population change factor (PCF) to assess treatment effects on SCN population densities was calculated by dividing the SCN population density at each sampling time (Pf) by the initial population density before treatments were applied (Pi). In the planting date experiment, purple deadnettle seeded on October 3, 11, and 18 resulted in PCF values ranging from 7.28 to 11.41, which were three- to fivefold higher than the PCF values for the weed-free control or purple deadnettle seeded on September 6, 20, and 27. Self-thinning of purple deadnettle seeded in September may have resulted in higher levels of SCN mortality compared to later seeding dates. In the removal time experiment, purple deadnettle removal at 2 and 4 wk after emergence (WAE) resulted in PCF values of 1.19 and 1.54, respectively, which were similar to the PCF for the weed-free control. In contrast, PCF values for purple deadnettle removal times of 6 WAE or later ranged from 2.46 to 5.44. Field and greenhouse experiments provided evidence that completion of the SCN life cycle on purple deadnettle was prevented if the weed was removed before the accumulation of 380 soil degree days within the 5 to 30 C range. Results from the removal study also suggested that completion of the first SCN generation on purple deadnettle in the fall was the primary factor causing increases in egg population density, since PCF values did not continue to increase significantly beyond the 6 WAE removal time.