“Bid amaranthus all his beauty shed, And daffodils fill their cups with tears, To stew the laurate hearse where Lycid lies.” Paradise Lost, John Milton (1667)“Immortal amaranth, a flower which once In Paradise, fast by the Tree of Life, Began to bloom, but soon for Man's offence, To Heav'n remov'd, where first it grew, there grows, And flours aloft shading the Fount of Life.” Paradise Lost, John Milton (1667)“There are no fields of amaranth on this side of the grave” Imaginary Conversations “Aesop and Rhodope” in works of Walter Savage Landor (1846)“Weeds never die” Danish proverb There are nearly 75 species in the genus Amaranthus, part of the Amaranthaceae family, worldwide. In this large genus, there is a distinct group of 10 species that are dioecious (separate male and female plants). In contrast to the monoecious Amaranthus spp. which are represented by species endemic to every continent, the dioecious Amaranthus spp. are all native to North America. The Amaranthus spp. have a long documented history of being fellow travelers with humans. In recent years, three dioecious Amaranthus spp.: Palmer amaranth (Amaranthus palmeri S. Wats.), common waterhemp (Amaranthus rudis Sauer), and tall waterhemp [Amaranthus tuberculatus (Moq) Sauer] have become major weeds to row crops in North America. Palmer amaranth has become a very troublesome weed to cotton (Gossypium hirsutum L.), corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production in large parts of the southern United States, and the waterhemp complex is now a major weed pest in corn and soybean production in the midwestern United States (Horak and Loughin 2000; King 1966; Mabberly 1997; Robertson 1981; Sauer 1950, 1955, 1972; Wax 1995).
Germination of weed seed and time of emergence are greatly affected by temperature. The effects of temperature on seed germination of tumble pigweed, prostrate pigweed, smooth pigweed, Palmer amaranth, Powell amaranth, spiny amaranth, redroot pigweed, common waterhemp, and tall waterhemp were examined under constant and alternating temperature regimens at 5, 10, 15, 20, 25, 30, and 35 C. Averaged over all temperatures, alternating temperature regimens increased total germination of all species, except Powell amaranth, which germinated similarly under both constant and alternating temperatures. In addition, Powell amaranth seed exhibited the highest total germination across all temperatures compared with the other amaranth species. Prostrate pigweed seed demonstrated the lowest total germination. Optimal temperatures for maximum germination were greater than 20 C for all species, except prostrate pigweed. The alternating temperature regimen centering at 30 C was used to compare the germination rates of the nine species. Palmer amaranth and smooth pigweed attained complete germination on the first day. The rate of germination for these species was much more rapid than the otherAmaranthusspp., which took 3 to 8 d to reach 50% germination.
Knowing the interference potential of common waterhemp in corn could be beneficial in planning waterhemp management strategies. In 2000, 2001, and 2002, field studies were conducted to examine both early- and late-season common waterhemp interference in corn. Early-season interference was determined by removing common waterhemp at the VE (vegetative emergence), V4 (four visible leaf collars), V6, V8, V10, V12, and V14 growth stages of corn for the entire season, and late-season interference was determined by allowing common waterhemp to emerge and compete from the VE, V4, V6, V8, V10, V12, and V14 corn growth stages. The interference potential of common waterhemp varied between the year 2000 and the combined years of 2001–2002. This is probably due to differences in precipitation in May and June in these two environments (297 mm in 2000 compared with 198 mm in 2001–2002). An excess of 590 g m−2 of dry matter and 13,000 and 1,200 seeds per female plant were produced when common waterhemp emerged at V4 and V6 corn, respectively, the 2 yr that corn was drought stressed. When corn was not moisture stressed, common waterhemp that emerged at V4 and V6 corn produced less than 220 g m−2 and less than 500 seeds per female plant. Season-long common waterhemp interference reduced corn yield 74% in 2 yr of the study and 11% in the third. Early-season common waterhemp interference began at V6 corn, with a 4 and 23% yield loss in 2000 and 2001–2002, respectively. Common waterhemp interference from late-season emergence reduced corn yield when emergence occurred before the V8 corn growth stage. Taking into account early- and late-season common waterhemp interference. the critical common waterhemp–free period was around the V6 corn stage to optimize corn yield.
Field studies were conducted in 2000, 2001, and 2002 at Urbana, IL, to examine the interference potential of common waterhemp that emerged at soybean growth stages VE, V2-V3, V4-V5, R1-R2, and R3-R4 in 19- and 76-cm row soybean. Soybean row width and common waterhemp emergence timing significantly influenced common waterhemp density, biomass, seed production, mortality, and soybean yield loss. Common waterhemp density declined as emergence timings were at later soybean growth stages. This decline happened at earlier growth stages in narrow-row soybean. Significant reductions in common waterhemp biomass and seed production occurred at the V2-V3 and V4-V5 emergence timings for the narrow- and wide-row soybean, respectively. Common waterhemp seed production was more than 23,000 seeds per plant at the VE emergence timing for both soybean row widths. Survival of common waterhemp that emerged after the V4-V5 soybean growth stage was less than 20% in both row widths. Common waterhemp interference reduced soybean seed yield at the VE, V2-V3, and the V4-V5 emergence timings. Row width affected the magnitude of yield reductions at these interference timings, with reductions being less in narrow-row soybean. This research suggests that control measures need to be implemented to common waterhemp plants that emerge before V4-V5 soybean to protect soybean yield and reduce common waterhemp seed production.
Two new mutations of PPX2 (R98G, R98M) likely confer resistance to PPO-inhibitors in A. palmeri, and can be rapidly identified using a dCAPS assay. © 2017 Society of Chemical Industry.
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