The development of a linuron-free weed management strategy for carrot production is essential as a result of the herbicide reevaluation programs launched by the Pest Management Regulatory Agency in Canada for herbicides registered before 1995 and the discovery of linuron-resistant pigweed species in Ontario. Field trials were conducted in one of Ontario’s main carrot-growing regions on high organic soils in 2016 and 2017. Pigweed species seedlings were effectively controlled with PRE treatments of prometryn, pendimethalin, S-metolachlor, or glufosinate. POST treatments of pyroxasulfone and metribuzin followed by predetermined biologically effective dose (≥90% control of pigweed seedlings) of acifluorfen, oxyfluorfen, fluthiacet-methyl, and fomesafen achieved excellent crop selectivity and commercially acceptable pigweed species seedling control under field conditions. Carfentrazone-ethyl or fomesafen applied PRE severely reduced seedling emergence and yield in the wet growing season of 2017. This study demonstrated clearly that an alternative linuron-free strategy can be developed for carrots. The strategy of exploring the potential to use the biologically effective dose of selected herbicides to achieve crop selectivity and control of pigweed species seedlings was verified.

A soilless unit was devised in which host plant (carrot, tomato) roots can be readily infected with broomrape. The simple, inexpensive and reusable unit consists of two glass plates, synthetic cloth, and foam rubber. The host plants are placed (or grown) between the glass plates, being held against one of the plates by foam rubber. The unit provides proper growing conditions, convenient visibility of host roots and parasite from an early stage of infection, and nondestructive access to the roots and parasite. Exposure of the broomrape seeds to a synthetic germination stimulant (GR 24) prior to their application to the host roots, generally, but not always, caused some increase in the number of parasitic infections and/or the rate of broomrape growth.

The study was undertaken to compare the level of total phenolics and the allelopathic potential of Pluchea lanceolata in soils cultivated once or twice a year. Thirty-five cultivated fields were sampled from seven sites in three different seasons. The leaf area, leaf weight, height, and density of P. lanceolata plants were measured. Leaves of P. lanceolata were analyzed for nine chemical characteristics: leaf ash, total phenolics, phosphate, Cu, Zn, Na, K, Mg, and Ca. Weed-infested topsoil and subsoil were analyzed for pH, electrical conductivity, organic carbon, chloride, bicarbonate, total carbonate, phosphate, total phenolics, Zn, Na, K, Mg, and Ca. Fields cultivated twice a year were designated as Group I, while those cultivated once a year were designated as Group II. Biotic and chemical characteristics of plants of Group I and II were not different. Topsoil and subsoil of Group II had higher values for Ca, while topsoil of Group I had higher values for total phenolics. The higher phenolic content of Group I topsoil was probably due to greater incorporation of weed plant parts into the soils as compared to Group II fields. Seedling growth of crop plants such as radish, carrot, maize, mustard, tomato, turnip, and wheat was affected more by soil from P. lanceolata-infested fields that were cultivated twice a year than those cultivated only once. A correlation between higher total phenolic content of P. lanceolata-infested topsoil with two cultivations, and growth response of crop plants was recorded.

Field research was conducted to evaluate the effectiveness of herbicides and carrot mowing for swamp dodder control. Herbicide evaluation indicated the highest carrot yield and lowest crop injury with pendimethalin compared to the industry standard linuron. Swamp dodder control with pendimethalin was greater than 80% at 56 and 70 d after planting (DAP). Other herbicides controlled swamp dodder, but crop injury was unacceptable. Carrot yield was greater where pendimethalin or s-metolachlor was applied compared to all other herbicides. Carrot mowing once 72, 86, or 100 DAP and mowing twice (72 plus 100 DAP) reduced the percentage of carrots infected with swamp dodder. Carrot infection level was least when mowed 100 DAP, and mowing did not increase yield compared to the non-treated check. These results suggest that the integration of pendimethalin for early-season swamp dodder control, followed by mowing 100 DAP, could reduce the impact of swamp dodder on carrots.

Some herbicides applied to corn or soybean can carry over and injure vegetable crops grown the following year. Injury and yield effects on vegetable crops from carryover of nicosulfuron and flumetsulam 1 yr after application to corn and from carryover of imazethapyr and imazamox 1 yr after application to soybean were evaluated. Experiments were conducted at Oakes, ND, on sandy loam soils with pH 7.7 in 1994 and pH 7.3 in 1996. Nicosulfuron and flumetsulam were applied to corn, and imazethapyr was applied to soybean at a standard herbicide rate (1×) (35, 56, and 70 g ai/ha, respectively) or 2× rates in 1994 and at 1×, 2×, or 4× rates in 1996. Imazamox was also applied to soybean at 1× (35 g ai/ha), 2×, or 4× rates in 1996. Cabbage, carrot, potato, onion, acorn squash, and tomato were planted in 1995, 1997, and 1998. In 1995, residual imazethapyr delayed tomato maturity but did not reduce tomato yield. Other vegetable crops were not injured by herbicide residues. In 1997, flumetsulam carryover injured plants and reduced yields of cabbage and squash; nicosulfuron carryover injured cabbage and onion plants and reduced onion yield; and imazethapyr carryover injured cabbage, onion, and tomato plants and reduced tomato yield. Most crop injury occurred only at the 2× or 4× herbicide rates. Potato and carrot were not injured by herbicide carryover. Herbicides applied to corn or soybean in 1996 did not carry over to injure any vegetable planted in 1998. The low rate of injury to vegetable crops in this study was mostly due to adequate soil moisture and warm summer soil temperatures, which enhanced microbial degradation of these herbicides and hastened their dissipation. Low clay and moderately low soil organic matter, and slightly alkaline soil pH, also promoted dissipation of some herbicides.

Increasing concerns about pesticide use and a steadily increasing conversion to organic farming have been major factors driving research in physical and cultural weed control methods in Europe. This article reviews some of the major results achieved with nonchemical methods and strategies, especially adapted for row crops (e.g., corn, sugar beet, onion, leek, and carrot) and small-grain cereals (e.g., barley and wheat). In row crops, intrarow weeds constitute a major challenge, and research has mainly aimed at replacing laborious hand-weeding with mechanization. A number of investigations have focused on optimizing the use of thermal and mechanical weeding methods against intrarow weeds, such as flaming, harrowing, brush weeding, hoeing, torsion weeding, and finger weeding. And new methods are now under investigation such as robotic weeding for row crops with abundant spacing between individual plants and band-steaming for row crops developing dense crop stands. The strategic use of mechanical weed control methods in small-grain cereals has been another area of considerable interest. Weed harrowing and interrow hoeing provide promising results when they are part of a strategy that also involves cultural methods such as fertilizer placement, seed vigor, seed rate, and competitive varieties. Although research in preventive, cultural, and physical methods have improved weed control in row crops and small-grain cereals, effective long-term weed management in low external input and organic systems can only be achieved by tackling the problem in a wider context, i.e., at the cropping system level. Basic principles of this approach, examples of cover crop and intercropping use for weed suppression, and an application in a 2-yr rotation are presented and discussed.

A Powell amaranth population suspected to be resistant (R) to linuron was discovered in a carrot field in Keswick, Ontario, Canada, in 1999. Dose–response analysis with different herbicides and DNA sequencing of the psbA gene encoding the D1 protein of photosystem II were done to confirm the resistance and identify its basis. A calculated resistance factor indicated a 12-fold increased resistance when linuron was applied to an R population compared with a susceptible (S) population. Moreover, the R population showed 6.4- and 3.1-fold greater resistance to two other phenylurea herbicides (diuron and monolinuron), 1.8- and 1.4-fold greater resistance to two triazine herbicides (metribuzin and prometryn), and 2.6-fold greater resistance to the triazinone metribuzin. R population was also cross-resistant to bentazon and bromoxynil when compared with S population, with a calculated resistance factor of 1.4 and 2.2, respectively. The partial nucleotide sequence of the psbA gene of R populations differed at two locations when compared with S populations. The first mutation coded for a Val219Ile substitution in the deduced amino acid sequence of the D1 protein, and the second mutation was silent and encoded for a proline at position 279 in both R and S populations. The Val219Ile substitution in the psbA gene is most likely the cause of this Powell amaranth population resistance to linuron and other PSII inhibitors. This is the first recorded instance of a Val219Ile substitution in an Amaranthus species.

The chlorophyll-lacking holoparasite Egyptian broomrape is a major threat for many field crops in Israel. In carrot, a high-value crop that is grown year round in Israel, heavy infestation with broomrape can cause severe damage and even total yield loss. The objective of this study was to determine, under field conditions, selective herbicides that would effectively control Egyptian broomrape without damaging the carrots. Ten field experiments were performed between the years 2010 and 2013. The acetolactate synthase inhibitor herbicides imazapic and imazamox caused deformation of carrot taproots at low doses, and significantly reduced yield amount and quality. Glyphosate was found to be the safest herbicide for broomrape management in carrot. Carrot selectivity and broomrape control efficacy were examined with three sequential applications of nine glyphosate doses. A nonlinear log-logistic curve described the response of noninfested carrot taproot biomass to glyphosate. No significant reduction in taproot biomass was observed when glyphosate was applied at up to 149 g ae ha−1. When glyphosate was applied in an Egyptian broomrape-infested carrot field, a hormetic effect was observed, perhaps due to Egyptian broomrape control. A two-parameter exponential decay curve described the broomrape response to glyphosate. Three sequential foliar applications of glyphosate, at 108 g ha−1, completely controlled Egyptian broomrape. Our results demonstrate that glyphosate applied sequentially at a low dose on Egyptian broomrape-infested carrot can control this parasitic weed.

Mesotrione is an effective herbicide for volunteer potato control but is not selective in several crops, including onion and carrot. Studies were conducted in 2006 to evaluate the effectiveness of wiper-applied mesotrione for control of volunteer potato in a potato crop. Surprisingly, nontreated potatoes growing adjacent to mesotrione-treated plants exhibited bleaching symptoms resembling mesotrione injury. Additional field trials confirmed injury to nontreated plants growing within 26 cm of potatoes treated with mesotrione applied with a wiper at 0.25, 0.5, and 1% (v/v) solutions. Greenhouse bioassays confirmed that mesotrione applied to potato leaves moved down through the plant and was exuded into perlite potting medium in sufficient quantities to injure potato plants that were exposed to the leachate from the perlite pots. In tracer studies, 52% of 14C-labeled mesotrione applied to potato leaves was absorbed into the potato plant by 15 d and 15% of the absorbed 14C-mesotrione was exuded into the soil and soil leachate. Mesotrione applied to potato by the wick application method has potential to injure neighboring susceptible plants from root uptake of exuded mesotrione.

Trials were established in 2007, 2008, and 2009 in Ontario, Canada, to determine the effect of soil residues of saflufenacil on growth, yield, and quality of eight rotational crops planted 1 yr after application. In the year of establishment, saflufenacil was applied PRE to field corn at rates of 75, 100, and 200 g ai ha−1. Cabbage, carrot, cucumber, onion, pea, pepper, potato, and sugar beet were planted 1 yr later, maintained weed-free, and plant dry weight, yield, and quality measures of interest to processors for each crop were determined. Reductions in dry weight and yield of all grades of cucumber were determined at both the 100 and 200 g ha−1 rates of saflufenacil. Plant dry weight, bulb number, and size and yield of onion were also reduced by saflufenacil at 100 and 200 g ha−1. Sugar beet plant dry weight and yield, but not sucrose content, were decreased by saflufenacil at 100 and 200 g ha−1. Cabbage plant dry weight, head size, and yield; carrot root weight and yield; and pepper dry weight, fruit number and size, and yield were only reduced in those treatments in which twice the field corn rate had been applied to simulate the effect of spray overlap in the previous year. Pea and potato were not negatively impacted by applications of saflufenacil in the year prior to planting. It is recommended that cabbage, carrot, cucumber, onion, pepper, and sugar beet not be planted the year after saflufenacil application at rates up to 200 g ha−1. Pea and potato can be safely planted the year following application of saflufenacil up to rates of 200 g ha−1.

Swamp dodder continues to spread through Wisconsin carrots, reducing crop yield and quality. Greenhouse research was conducted to evaluate the effect of swamp dodder infection timing on carrot development and to evaluate carrot cultivar tolerance to the parasite. Data collection included measurement of carrot processing qualities such as root length, root diameter, and fresh weight, as well as quantification of carrot leaf biomass and swamp dodder biomass. Carrot processing qualities were reduced the most in carrots infected 14 d after emergence (DAE). Carrots infected 70 DAE were similar to the noninfected carrots for all processing qualities. Carrot cultivars differed in tolerance of swamp dodder parasitism. All parameters for cultivar tolerance were significantly reduced by swamp dodder infection; however, ‘Enterprise’, ‘Sweet Bites’, ‘Sugar Snax 54’, ‘Topcut 93’, and ‘Mokum’ were most tolerant of parasitism and exhibited minimal reductions in root length, diameter, and fresh weight compared with other cultivars. ‘7626B’, ‘Bolero’, ‘Legend’, and ‘Imperial Cuts’ cultivars grew poorly when parasitized.

Few herbicides are used in carrot production in the United States, and none suppress volunteer potato, a serious weed where the two crops are grown in rotation. Hand-weeding is the primary method of controlling emerged volunteer potato within carrot. The objective of this work was to evaluate carrot tolerance and volunteer potato control with single or sequential applications of prometryn, prometryn plus fluroxypyr, and ethofumesate. The treatment with fluroxypyr resulted in malformed carrots with numerous root hairs and reduced carrot yield. Treatments with prometryn, either as single or sequential POST applications at 2.23 kg ai/ha, were safe on carrot and frequently controlled volunteer potato similar to the hand-weeded treatment. Ethofumesate applied as single or sequential PRE or POST at 2.2 kg ai/ha proved safe on carrot, but higher rates reduced yield. Ethofumesate applied POST or PRE followed by POST consistently reduced volunteer potato tuber mass. If registered for use in carrot, prometryn and ethofumesate would help modernize weed management in carrot and reduce or eliminate the need for hand-weeding volunteer potato.

The degree of resistance to linuron of a common ragweed biotype was investigated. Suspected linuron-resistant plants collected from a carrot field near Sherrington, Québec, were subjected to increasing rates of linuron under glasshouse conditions. Resistance to linuron of the common ragweed biotype was suspected because 33% of plants survived to reproduction after they were sprayed at a rate of 4.5 kg ai/ha, two times the dose rate recommended for linuron in carrots, and also because 3% of plants survived to reproduction after they were sprayed at a rate of 22.5 kg ai/ ha, 10 times the recommended dose. Susceptible plants collected from a field with no prior history of linuron use were all killed when sprayed at the lowest dose rate recommended, 1.125 kg ai/ha. The herbicide-resistance ratio was 29.0 for linuron, and for cross-resistance to atrazine, the ratio was 1.3, indicating that these plants exhibit greater resistance to linuron than to atrazine.

In northern U.S. vegetable cropping systems, attempts at no-till (NT) production have generally failed because of poor crop establishment and delayed crop maturity. Strip tillage (ST) minimizes these problems by targeting tillage to the zone where crops are planted while maintaining untilled zones between crop rows, which foster improvements in soil quality. ST has been shown to maintain crop yields while reducing energy use and protecting soils in vegetable crops, including sweet corn, winter squash, snap bean, carrot, and cole crops. Despite potential benefits of ST, weed management remains an important obstacle to widespread adoption. Increased adoption of ST in cropping systems for which effective, low-cost herbicides are either limited (e.g., most vegetable crops) or prohibited (e.g., organic systems) will require integration of multiple cultural, biological, and mechanical approaches targeting weak points in weed life cycles. Weed population dynamics under ST are more complex than under either full-width, conventional tillage (CT) or NT because weed propagules—as well as factors influencing them—can move readily between zones. For example, the untilled zone in ST may provide a refuge for seed predators or a source of slowly mineralized nitrogen, which affects weed seed mortality and germination in the tilled zone. Greater understanding of such interzonal interactions may suggest manipulations to selectively suppress weeds while promoting crop growth in ST systems. Previous studies and recent experiences in ST vegetable cropping systems suggest a need to develop weed management strategies that target distinct zones while balancing crop and soil management tradeoffs. For example, in untilled zones, optimal management may consist of weed-suppressive cover crop mulching, combined with nitrogen exclusion and high-residue cultivation as needed. In contrast, weed management in the tilled zone may benefit from innovations in precision cultivation and flame-weeding technologies. These short-term strategies will benefit from longer-term approaches, including tillage-rotation, crop rotation, and cover cropping strategies, aimed at preventing seed production, promoting seed predation and decay, and preventing buildup of problematic perennial weeds. However, a concerted research effort focused on understanding weed populations as well as testing and refining integrated weed management strategies will be necessary before ST is likely to be widely adopted in vegetable cropping systems without increased reliance on herbicides.

Trials were established in 2003, 2004, and 2005 in Ontario to determine the effects of residues of mesotrione, atrazine, and mesotrione plus atrazine 1 and 2 yr after application on broccoli, carrot, cucumber, onion, and potato. One yr after mesotrione application, injury was 43, 37, 18, 24, and 0% in broccoli, carrot, cucumber, onion, and potato, respectively. The addition of atrazine to mesotrione in the year before planting increased injury to 55, 53, 30, 42, and 3% in broccoli, carrot, cucumber, onion, and potato, respectively. Plant dry weight and yield were also decreased by mesotrione residues the year after application in all crops except potato. The addition of atrazine to mesotrione accentuated the reduction in dry weight and yield in broccoli, carrot, cucumber, and onion. There was no injury, or reductions in dry weight or yield in any crop planted 2 yr after application of mesotrione alone or in tank mix with atrazine. A recropping interval of 2 yr is recommended following applications of mesotrione or mesotrione plus atrazine for broccoli, carrot, cucumber, and onion. Potato can be safely planted the year following application of mesotrione plus atrazine.