Abstract
AbstractHorseweed [Conyza canadensis(L.) Cronquist] is a facultative winter annual weed that can emerge from March to November in Michigan. Fall-emergingC. canadensisoverwinters as a rosette, while spring-emergingC. canadensisskips the rosette stage and immediately grows upright upon emergence. In Michigan, primary emergence recently shifted from fall to spring/summer and therefore from a rosette to an upright growth type. Growth chamber experiments were conducted to determine (1) whether bothC. canadensisgrowth types could originate from a single parent and (2) whether common environmental cues can influence growth type. Variations in temperature, photoperiod, competition, shading, and soil moisture only resulted in the rosette growth type in fourC. canadensispopulations originating from seed collected from a single parent of the upright growth type. However, a vernalization period of 4 wk following water imbibition, but before germination, resulted in the upright growth type. Dose–response experiments were conducted to determine whether glyphosate sensitivity differed betweenC. canadensisgrowth types generated from a single parent of the upright growth type. Upright-typeC. canadensisfrom known glyphosate-resistant populations ISB-18 and MSU-18 were 4- and 3-fold less sensitive to glyphosate than their rosette siblings, respectively. Interestingly, differences in glyphosate sensitivity were not observed between growth types from the susceptible population. These results suggest that whileC. canadensispopulations shift from winter to summer annual life cycles, concurrent increases in glyphosate resistance could occur.
Highlights
Horseweed [Conyza canadensis (L.) Cronquist] is a weed species native to North America, where it can emerge in the fall, overwinter, and complete its life cycle the following season or emerge in the spring and complete its life cycle in the same season (Weaver 2001)
Conyza canadensis that emerge in May in Michigan typically skip the rosette stage and immediately grow upright
All other stresses occurring before emergence resulted in all C. canadensis emerging as the rosette type
Summary
Horseweed [Conyza canadensis (L.) Cronquist] is a weed species native to North America, where it can emerge in the fall, overwinter, and complete its life cycle the following season or emerge in the spring and complete its life cycle in the same season (Weaver 2001). To obtain the upright growth type, 0.5 g of seed from each population was surface planted in 28 by 55 by 6 cm trays filled with potting media (Suremix Perlite, Michigan Grower Products), watered, and placed in the MSU Wheat Breeding and Genetics Program’s vernalization room set to 4 C with an 8-h photoperiod for 4 wk. Simultaneous emergence of rosette- and uprighttype C. canadensis generally occurs in July (Figure 1); we hypothesized mimicking July temperatures (27/16 C) and photoperiod (15 h) would result in a split population between growth types.
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