Potato response to simulated drift of imazamethabenz, imazethapyr, or imazapyr was evaluated in field studies conducted near Aberdeen, ID, in 1989 and 1990. Herbicides were applied at rates corresponding to 0.02, 0.1, and 0.5 times the typical use rate (x rate) of each herbicide. Simulated drift treatments were applied at potato emergence, tuber initiation, or tuber bulking. Foliar injury symptoms varied with herbicide, rate, and potato growth stage at the time of application, but were generally most severe with imazapyr and least severe with imazamethabenz. Yield losses also varied with herbicide, rate, and potato growth stage at the time of application, but generally were greater when drift occurred at tuber initiation or tuber bulking than at potato emergence. Losses in U.S. Number 1 (highest quality) yield were greater than losses in total yield, indicating that tuber quality was affected more by the herbicides than was tuber biomass accumulation. U.S. Number 1 yield was not reduced by simulated drift of imazamethabenz at 0.02x but was reduced 25% by the 0.1x rate applied at tuber bulking. At the 0.5x rate, U.S. Number 1 yield was reduced 36, 85, or 92% when drift occurred at potato emergence, tuber initiation, or tuber bulking, respectively. For imazethapyr, U.S. Number 1 yield losses ranged from 0 to 68% for the 0.02x rate; 19 to 98% for the 0.lx rate; and 64 to 100% with the 0.5x rate, depending on potato growth stage at the time of application. Losses were least when drift occurred at potato emergence. Simulated drift of imazapyr at the 0.02x rate applied as potatoes were emerging reduced U.S. Number 1 yield 79%. All other imazapyr treatments caused ≥99% loss in U.S. Number 1 yield.

Pathogenicity of four Colletotrichum coccodes isolates to potato, eastern black nightshade, and hairy nightshade was evaluated in greenhouse studies. Three Idaho isolates, AC-14, AC-28, and AC-44, and an isolate obtained from Mycogen Corporation, AC-Myc, produced lesions on potato foliage. Plants inoculated with AC-14, AC-28, and AC-44 also showed wilt symptoms similar to those produced by Verticillium dahliae as potatoes were beginning to senesce. Infection by AC-44 reduced tuber fresh weight and specific gravity. All isolates colonized eastern black and hairy nightshade, although some isolates caused little or no symptom development. Thus, both species may serve as sources of primary inoculum for C. coccodes isolates that are pathogenic to potatoes. Only AC-Myc substantially injured the leaves of both nightshade species. AC-Myc was more pathogenic to eastern black than to hairy nightshade, and to plants inoculated at the 3- to 4-leaf versus the 7- to 8-leaf stage.

Field studies were conducted to evaluate potato injury and weed control with postemergence-directed (PDIR) bromoxynil, glufosinate, or monocarbamide dihydrogen sulfate (MCDS). All three herbicides controlled hairy nightshade, common lambsquarters, redroot pigweed, and kochia ≥ 85%, depending on rate, with either mid- or late-PDIR treatment. Bromoxynil or MCDS caused 5 to 6% more initial injury with mid- than with late-PDIR treatment, but total yield and yield of U.S. #1 tubers were not reduced by either herbicide. PDIR glufosinate caused moderate injury at 0.84 kg ha−1, the rate needed for acceptable (≥ 85%) weed control. Total yield was reduced 10% and U.S. #1 yield was reduced 30% by PDIR glufosinate at 0.84 kg ha−1. Thus bromoxynil and MCDS showed potential for use as PDIR treatments in potatoes, but glufosinate did not.

Field studies were conducted to examine hairy nightshade control with, and potato tolerance to, bentazon or bentazon plus nonionic surfactant (NIS), petroleum oil concentrate (POC), methylated seed oil (MSO), BCH-815, or urea-ammonium nitrate (UAN). Bentazon treatments were applied early or late POST. Hairy nightshade control with bentazon at 0.56 kg ha–1 plus POC, MSO, BCH-815, or UAN was equal to or better than bentazon alone at 1.12 kg ha–1 regardless of time of application. Potato injury with bentazon plus additives applied late POST was sometimes greater than with bentazon alone, but use of additives did not reduce potato yield or quality. Yield of US ♯1 tubers was lower when bentazon was applied late POST than when applied early POST in one of two years.

Fall-planted rapeseed and sudangrass were evaluated for weed control in potato during a two-year study. Rapeseed incorporated in the spring in a loamy sand soil reduced weed density 85 and 73% in 1992 and 1993, respectively, and reduced weed biomass 96 and 50% in 1992 and 1993, respectively, in following potato crops compared to potato after fallow. Potato following rapeseed yielded 25% and 17% more total tuber weight than potato following sudangrass in 1992 and fallow in 1993, respectively. In greenhouse trials, rapeseed tissue added to a loamy sand soil at 20 g fresh weight per 400 g dry soil reduced biomass of hairy nightshade and longspine sandbur by 90 and 83%, respectively. Similarly, white mustard tissue added at 20 g fresh weight per 400 g dry soil reduced biomass of hairy nightshade and green foxtail by 83 and 70%, respectively.

Field studies were conducted to determine the effect of green foxtail density and location on the yield of potato under dryland conditions. Total and marketable tuber yields were reduced by green foxtail densities of less than 75 plants m-2. Marketable yields were affected more by increasing green foxtail density than were total yields. Increasing weed density reduced both the number and size (tuber weight) of marketable tubers but not the specific gravity or the sucrose content of the tubers at harvest. Green foxtail growing between crop rows was as competitive with potato as plants growing within the row. The effect of weed competition on total and marketable tuber yields was more pronounced in drought years than years with normal precipitation.

Reduction of initial top growth of quackgrass with haloxyfop at 0.13 kg ai ha-1 and above in the greenhouse was comparable at the 2-, 4-, and 6-leaf stages. Quackgrass control was not affected by rhizome length in plants grown from 2-node and 10-node rhizome pieces. Removal of quackgrass shoots 6 h after treatment resulted in increased initial topgrowth, regrowth, and rhizome weight at all application rates when compared to treatments with no removal of shoots; however, growth decreased with increasing rates of haloxyfop. Later removal at 24 or 96 h after treatment did not affect quackgrass control, indicating rapid translocation. In field-grown potato, haloxyfop applied at 0.25 kg ha-1 at the 4- to 5-leaf stage controlled more than 80% of quackgrass. Lower rates controlled less than 80% when used alone; but adding cultivation improved control. No adverse effect on potato yield was noted with haloxyfop.

Volunteer potatoes are difficult to control in onions and can greatly reduce onion growth and yield. Herbicides and cultivation were evaluated for control of simulated volunteer potatoes in onions in 1996 and 2000. Three interrow cultivations did not control potatoes in the onion row and the remaining plants reduced onion yield 50 and 73% compared with the hand-weeded checks. Three applications of oxyfluorfen (0.2 + 0.17 + 0.17 kg ai/ha) or bromoxynil plus oxyfluorfen (0.2 + 0.17 kg ai/ha) at the two-, three-, and four- to five-leaf stages of onions followed by a cultivation after each application reduced potato tuber weight 69 to 96% and tuber number 32 to 86% compared with cultivation alone and prevented onion yield loss associated with potatoes. Ethofumesate applied preemergence at 0.6 kg/ha followed by postemergence ethofumesate plus bromoxynil and cultivation reduced potato tuber weight 90% and tuber number 68% compared with cultivation alone, and onions yielded equal to hand-weeded checks. Two applications of fluroxypyr (0.3 kg ai/ha) plus bromoxynil (0.2 kg ai/ha) at the two- and three-leaf stages of onions followed by a cultivation after each application reduced potato tuber weight by greater than 90%, but onion yields were reduced 38 to 66%.

Management of volunteer potato is difficult and requires an integrated approach. Soil fumigation is one tactic known to reduce population densities of certain weeds and may be a method to improve the management of volunteer potato. The effect of 1,3-dichloropropene (1,3-D) and metham sodium on potato tuber viability was tested in sealed glass jars at various doses, incubation temperatures, and times of exposure. Tuber viability data were fitted to a logistic model, and I 90 doses (90% suppression) were calculated for each combination of temperature and time of exposure. I 90 doses for 1,3-D ranged from 41 to 151 kg/ha and from 96 to over 480 kg/ha metham sodium. Both nondormant and dormant tubers were injured by exposure to metham sodium. Soil fumigation with 1,3-D and metham sodium has the potential to greatly reduce the number of viable potato tubers.

Sulfentrazone was applied POST at 13, 26, or 53 g ai/ha alone or in combination with metribuzin at 280 or 420 g ai/ha in field trials conducted with ‘Russet Burbank’ potatoes in 2002 to 2004. Sulfentrazone alone provided less than 84% redroot pigweed, common lambsquarters, and kochia control, although control usually improved to 90% or greater when metribuzin was included. Hairy nightshade control reached 90% only when the highest rates of both herbicides were applied in combination. Sulfentrazone alone did not provide any volunteer oat control, whereas control was 85% when the highest metribuzin rate was included. Potato crop injury, consisting of chlorosis, interveinal blackening of the leaves, eventual necrosis, leaf malformation, and plant stunting, increased as the sulfentrazone rate increased. In contrast, injury decreased as metribuzin rate increased from 0 to 420 g/ha, when averaged across sulfentrazone rates. Reduction in injury levels and increased weed control translated to improved tuber yields as metribuzin rate increased. However, when sulfentrazone was combined with the highest metribuzin rate, potato injury was still relatively high at 26 and 18% at 1 and 4 wk after treatment, and acceptance of sulfentrazone applied POST with metribuzin by potato growers is unlikely.

Volunteer potato is a perennial weed that is difficult to control in crop rotations. Field studies were conducted near Paterson, WA, in 2001 and 2002 to evaluate the control of volunteer potato with carfentrazone-ethyl and dicamba in field corn. When potatoes were not controlled corn yield was reduced 23 and 62% in 2001 and 2002, respectively. Single postemergence (POST) applications of carfentrazone-ethyl at 9 g/ha killed exposed foliage of potato, but new shoots continued to emerge both years and reduced corn yield in 2002. The most effective treatments tested were a single mid-postemergence application of carfentrazone-ethyl plus dicamba (9 + 280 g/ha), two applications of carfentrazone-ethyl alone at early postemergence and late postemergence, and three POST applications of carfentrazone-ethyl, which controlled volunteer potato 77 to 87% in early June, reduced weight of tubers produced by 76 to 96% compared with nontreated checks, and prevented corn yield loss compared with hand-weeded checks. Herbicide treatments reduced potato tuber weight more than tuber number.

Field trials were conducted with sulfentrazone at 53, 80, and 105 g ai/ha combined with 0, 420, and 560 g ai/ha metribuzin in a 3 by 3 factorial arrangement. Sulfentrazone and metribuzin combinations improved control of redroot pigweed, common lambsquarters, hairy nightshade, and volunteer oat compared with sulfentrazone applied alone. Kochia control was more than 90% regardless of metribuzin presence in the treatment. Sulfentrazone at 105 g/ha was needed for greater than 90% hairy nightshade control. Sulfentrazone alone or in combination with metribuzin did not provide greater than 89% volunteer oat control. Potato crop injury was less than 5% and total tuber yields increased as metribuzin rate increased from 0 to 560 g/ha, reflecting improved weed control as metribuzin rate increased.

In 2004 and 2005, field research was conducted in Idaho to compare tank mixtures of flumioxazin at the 53 g ai/ha potato use rate with comparable tank mixtures of rimsulfuron at 26 g ai/ha for broad-spectrum weed control. Flumioxazin in two-way tank mixtures with metribuzin, EPTC, pendimethalin, S-metolachlor, or ethalfluralin provided greater than 90% hairy nightshade control, which was comparable with control by similar rimsulfuron two-way tank mixtures. Flumioxazin plus metribuzin or rimsulfuron were the only two-way mixtures with flumioxazin consistently providing 90% or greater redroot pigweed, common lambsquarters, and green foxtail control. Control of these weeds by any of the rimsulfuron two-way mixtures was almost always greater than 90%. Three-way tank mixtures containing flumioxazin or rimsulfuron controlled hairy nightshade, redroot pigweed, and common lambsquarters similarly, and control ranged from 89 to 100%. When metribuzin was not included with flumioxazin in three-way mixtures, control was 80 to 97% and not always comparable with the 89 to 100% control by similar rimsulfuron mixtures. Green foxtail control by flumioxazin or rimsulfuron three-way mixtures usually was similar and greater than 90%.

‘Russet Burbank’ potato tolerance to dimethenamid-p applied preemergence at 0.7, 1.4, or 2.9 kg ai/ha was assessed in field studies conducted at Aberdeen, ID and Ontario, OR. Although crop injury was evident approximately 2 wk after treatment, most injury had diminished by row closure 2- to 3-wk later. Initial injury did not translate to yield loss and U.S. No. 1 and total tuber yields in dimethenamid-p–treated plots were similar to yields in the untreated, weed-free control.

The effects of postemergence rimsulfuron, metribuzin, and adjuvant combinations on potato crop safety and weed control were evaluated in field studies conducted at the University of Idaho Aberdeen Research and Extension Center in 1999 and 2000. Rimsulfuron at 26 g ai/ha plus metribuzin at 0, 140, or 280 g ai/ha was combined with nonionic surfactant (NIS), crop oil concentrate (COC), or methylated seed oil (MSO) in a 3 by 3 factorial with two controls. Under cool, cloudy conditions in 1999, initial ‘Russet Burbank’ potato injury was greater when metribuzin was included in the tank mixture than when rimsulfuron was applied alone, regardless of adjuvant. Under warmer conditions in 2000, however, adding MSO or COC to the tank mixture caused more injury than adding NIS. Rimsulfuron did not provide acceptable season-long common lambsquarters control in 1999 (76%) or in 2000 (88%), regardless of adjuvant. Rimsulfuron combined with metribuzin at 140 or 280 g/ha provided ≥95% common lambsquarters control both years, regardless of adjuvant. Among adjuvants, using MSO (1999 and 2000) or COC (2000) in the spray mixture improved common lambsquarters control compared with using NIS. Tuber yield and quality were not reduced as a result of metribuzin rate or adjuvant treatments either year compared with the weed-free control.

Volunteer potato can be a host of serious pest problems in potato and could provide a source of inoculum for subsequent potato crops. Volunteer potato can also be difficult to control in many rotational crops. Potato shoots were removed once, twice, and throughout the growing season, beginning at early and late tuberization. Compared with no shoot removal, two or more shoot removal treatments reduced the number of tubers 42% or more. A single shoot removal treatment at early tuberization reduced tuber biomass 37%, compared with 65% when shoot removal was initiated several weeks later. Regardless of timing, a single shoot removal increased the number and biomass of small tubers (≤ 57 g each). Control tactics that remove or kill volunteer potato shoots require repeated application or integration with other management practices to suppress the weed.