California Pepper Commission

2005-2006

Preemergence Weed Control Trials in Peppers

California Pepper Commission – Final Report, 2005
Title: Preemergence Weed Control Trials in Peppers

Principle Investigators:

Richard Smith
Farm Advisor, Monterey County
1432 Abbott Street
Salinas, CA 93901
(831) 759–7350; (831) 758–3018 FAX
rifsmith@ucdavis.edu

Michelle LeStrange
Farm Advisor, Tulare County
4437 S. Laspina St., Ste. B
Tulare, CA, 93274
(559) 685–3309 Ext. 220; Fax: (559) 685–3319
mlestrange@ucdavis.edu

Timeline: April 30, 2005 to November 30, 2005

Summary: The Central Coast trial provided evidence that Goal Tender applied to shaped beds prior to transplanting (and subsequently not worked prior to transplanting) provided acceptable safety to the peppers and good weed control. This use pattern could provide an alternative "at planting" treatment and can provide weed control for the first 30 days following transplanting. Outlook was applied over–the–top in both trials, but was more damaging to the peppers in the Fresno trial. This material did not reduce yields in the Central Coast trial and should be further examined as a pretransplant and layby application. Both trials showed that flumioxazin impregnated on fertilizer has promise as a post transplant application on peppers. The Fresno Trial showed that Dual Magnum, Outlook and Dacthal all provided good layby weed control. Dacthal is already registered for this use, but the Dual Magnum label would need to be adjusted to allow this use. In summary, these trials showed promise for developing a weed control system to provide early and late season weed control for peppers.

Background: Peppers are long-season vegetables that have several weed control challenges: They compete weakly with weeds for the first 40 to 60 days following transplanting. They are a long-season crop in many production districts that can be subject to flushes of both winter and summer weeds over the course of their growing cycle. The preemergence herbicides registered for peppers have gaps in the spectrum of weeds that they control (Smith et al., 2003). As a result, growers may spend from $200 to $350/acre (Klonsky et al., 1997) on weed management. Field selection, field sanitation, cultivation and the use of plastic mulches are cultural practices that reduce weed pressure in production fields. Fumigation provides substantial weed control and is frequently used in conjunction with plastic mulches which improves the level of weed control provided by both techniques. Goal Tender was registered in California in 2004 for use with plastic mulch and provides control of Little Mallow (Malva parviflora) which is only partially controlled by fumigants and other preemergence herbicides registered for use on peppers. However, many acres of peppers are not grown with plastic mulch, and weed control is a challenge. Devrinol, Prefar and Treflan are registered preemergence herbicides in peppers. Dual Magnum is registered under a 24C and provides good control of hairy nightshade (Solanum sarrachoides) and yellow nutsedge (Cyperus esculentus) which are not controlled by the other preemergent materials. Late season weed control is also an important issue in this crop. The objective of these studies was to examine at transplant and layby herbicide combinations for peppers that can provide long–term and economical weed control for peppers grown without plastic mulch. The herbicides tested included: Dual Magnum 7.62 (s–metolachlor), Goal Tender 4F (oxyfluorfen), Outlook 6.0 (dimethenamid), flumioxazin (Chateau) impregnated on fertilizer, and Dacthal 75W (DCPA).

Objectives:

  1. Examine the transplant weed control options (Goal Tender 4F, Dual Magnum, flumioxazin impregnated fertilizer and Outlook) followed by combinations of layby herbicides (Dacthal – standard, Dual Magnum, and Outlook). Evaluations of weed control, crop safety and yield will be conducted.
  2. Conduct the above trials in the Coastal and Central Valley production districts.

METHODS

Field trials were conducted on the Central Coast (Santa Clara County) and Fresno County in 2005 to provide an evaluation of the test herbicides over a wider range of growing conditions and weed spectra.

Central Coast study: The trial was conducted with a cooperating grower in Gilroy. Goal Tender treatments were applied onto shaped beds two weeks prior to transplanting the peppers on April 28. The field was transplanted on May 13. The at–planting treatments were applied over–the–top of the plants immediately following transplanting. Sprinkler irrigation was started 5 hours following transplanting applying 0.38 inch of water. Layby applications were made on June 16 (34 days after treatment – DAT) and the material was incorporated with the last sprinkler irrigation before the field was switched to drip irrigation. The plots were evaluated and weeding evaluations conducted on June 3 (21 DAT). The plots were again evaluated for weeds and weeded on July 1 (48 DAT) which evidenced newly sprouted weeds following the layby application. The plots were not cultivated prior to the July 1 weed evaluation. Each plot was one 40-inch bed wide by 25 feet long and replicated four times in a randomized complete block design (RCBD). All spray treatments were applied to the entire bed in 74 gallons of water per acre with two passes of a one nozzle wand with an 8008E teejet nozzle at 30 psi. Flumioxazin on fertilizer granules was spread by hand on the bed top immediately following transplanting. Soil type was Pacheco silt loam and the variety was Baron (planted in greenhouse on March 17).

Fresno County Trial: The field trial was conducted on a Panoche clay loam soil at the UC West Side Research and Extension Center (WSREC) near Five Points. On June 7 the bell pepper variety "Jupiter" was transplanted in single rows into 40" beds. Within row plant spacing was 10". Plot size was two 40–inch beds x 70 feet of row length and replicated 4 times in a RCBD. Preplant applications of Goal Tender were made onto shaped beds on May 10, 28 days prior to transplanting the peppers and incorporated with 0.50 inches of rainfall. The at–planting treatments of Dual Magnum, Outlook and flumioxazin were applied over the top of the plants and the field was sprinkler irrigated applying 0.50 inches of water immediately following transplanting. Sprinkler irrigation continued as needed for a few weeks and then switched to furrow irrigation. On July 25 the field was machine cultivated before layby applications of the herbicides were made as a directed spray to the base of the plants. These applications were incorporated by sprinkler irrigation. All sprayed treatments were applied to the entire plot in 30 gallons of water per acre using a CO2 backpack sprayer @30 psi and a 2 nozzle boom with 8003evs tips. Flumioxazin on fertilizer granules was hand broadcast over the top of the peppers. There were two untreated checks: one was handweeded twice in addition to layby cultivation and the other was allowed to grow weedy all season.

Plots were evaluated for phytotoxicity to the peppers and weed control on July 1, July 22, and August 12. Pepper stand counts were made on July 13. A portion of each plot (25 row) was hand harvested on August 23 (west bed) and on September 8 (east bed) and the yields were combined for total yield. See tables 2 and 3 for treatments and evaluations.

RESULTS

Central Coast: Hairy Nightshade was the dominant weed at this site. The best weed control was provided by Outlook, then by Dual Magnum and Goal Tender 21 days after treatment (DAT) (Table 1). Flumioxazin impregnated on fertilizer provided good weed control in two treatments, but not in the other. This may be due to problems with obtaining an even distribution of this dry granular material on the top of the bed. Devrinol was at a distinct disadvantage at this site because Hairy Nightshade was the main weed at this site and it does not control this weed. Outlook caused stunting of the plants at 21 DAT and less at 28 DAT, but stunting was still noticeable 48 DAT. It is clear that this material is not appropriate for an over-the-top application. There was no difference in the stand among treatments, but there were some instances of burned pepper plants in the flumioxazin treatment, presumably where a prill of the material lodged against the stem of a plant. All herbicides except Devrinol reduce time to weed the plots, but Goal and flumioxazin on fertilizer tended to take more time than Dual Magnum and Outlook. The Devrinol and untreated treatments had the most weeds following the layby treatments 48 DAT and there were no differences among the other weed control treatments (Table 2). These results indicate that the at transplanting treatments had lingering effects that were measured 48 DAT. Unfortunately, few weeds emerge after this date and this test did not provide a good opportunity to evaluate the long-term weed control system for peppers. There were no significant differences in tonnage and individual fruit weight among the treatments (Table 2). There were some differences in number of fruit per acre with Dual Magnum having the lowest yield of the transplanting treatments. There was no impact of layby treatments on yield.

Fresno County Trial: Because of excessive rainfall many weed seeds germinated in the untreated area of the field after the preplant treatments of Goal Tender but before the peppers were transplanted. Goal Tender was extremely effective in controlling all of the weeds (see photo). However, prior to transplanting it was necessary to cultivate and reshape the beds, thus destroying the herbicide layer and the effectiveness of Goal Tender. Weed control ratings on July 1 and July 22 (a few days before layby) showed how Goal Tender was no longer effective (Table 2).

Weeds were vigorous and abundant throughout the season and included several broadleaf species and virtually no grasses except for occasional jungle rice (Echinochloa colonum). The major broadleaf weeds were prostrate, tumble, and redroot pigweeds (Amaranthus blitoides, A. albus, and A. retroflexus); primarily black nightshade (Solanum nigrum), but also some hairy nightshade (S. sarrachoides) and lanceleaf groundcherry (Physalis lanceifolia); common lambsquarters (Chenopodium album); and purslane (Portulaca oleracea). Mustards, shepherds-purse (Capsella bursa-pastoris) and London rocket (Sisymbrium irio), were initially present prior to layby, but were taken out with the layby cultivation and were not serious competitors. Puncturevine (Tribulus terrestris) was also scattered throughout the experimental site but was not included in the weed counts because its populations were too random.

At planting applications: Although weed control was initially excellent, Outlook really hurt the peppers with an over the top application and many plants remained stunted for the entire season. Pepper yields were extremely reduced (Table 3). As the season progressed weeds germinated and the small pepper plants offered little competition. Flumioxazin provided good weed control and only slight pepper phytotoxicity was observed using the dry granular formulation, although some care was given to try to keep the prills off of the pepper plants during the broadcast application. Weed control is probably compromised by this method of application due to the difficulty of obtaining uniform coverage. Dual Magnum provided the best weed control. A little damage was seen on the peppers, but yields were not affected.

Layby applications: After layby there was not a lot of new weeds that germinated however, weeds that were missed by cultivations continued to grow. Dual, Outlook, and Dacthal all provided good to excellent weed control when applied at layby. All of the Goal Tender preplant plots and the flumioxaxin at planting plots were improved with the layby applications. Dual, Outlook, and flumioxazin were effective on nightshades, and reduced pigweeds, purslane, and lambsquarters populations to varying degrees, although none of these products provided complete control of these weeds in this experiment. Still a hand weeding crew would have been able to clean up the field in a relatively short time, if the pepper field had been treated with almost any of these combinations.

Table 1. Central Coast Trial. Post transplant evaluations: Number of weeds (per 6ft2), and phytotoxicity (two dates), stand count and time of weed evaluations.

TRT
Code
ApplicationsLbs a.i./AMaterial/ANightshade

21 DAT
Total
Weeds
21 DAT
Phyto

21 DAT
Phyto

28 DAT
Plants
per plot
21 DAT
Time to weed
(hrs/A)
21 DAT
1Dual Magnum 7.62
Fb* Dual Magnum 7.62
1.43
1.43
1.50 pts
1.50 pts
0.50.50.30.036.81.6
2Dual Magnum 7.62
Fb Outlook 6.0
1.43
0.60
1.50 pts
0.80 pt
0.80.80.80.335.51.3
3Dual Magnum 7.62
Fb Dacthal 75W
1.43
7.00
1.50 pts
9.3 lbs
1.31.30.30.136.01.3
4Goal Tender 4F1
Fb Dual Magnum 7.62
0.50
1.43
1.00 pt
1.50 pts
2.33.30.80.435.33.6
5Goal Tender 4F1
Fb Outlook 6.0
0.50
0.60
1.00 pt
0.80 pt
2.52.80.30.035.03.3
6Goal Tender 4F1
Fb Dacthal 75W
0.50
7.00
1.00 pt
9.3 lbs
2.32.30.50.035.82.8
7Outlook 6.0
Fb Dual Magnum 7.62
0.60
1.43
0.80 pt
1.50 pts
0.30.31.01.334.81.2
8Outlook 6.0
Fb Outlook 6.0
0.60
0.60
0.80 pt
0.80 pt
0.30.31.50.835.31.2
9Outlook 6.0
Fb Dacthal 75W
0.60
7.00
0.80 pt
9.3 lbs
0.50.51.30.836.51.1
10Flumioxazin
impregnated on fertilizer
Fb Dual Magnum 7.62

0.094
1.43

188 lbs
1.50 pts
1.01.31.31.035.02.2
11Flumioxazin
impregnated on fertilizer
Fb Outlook 6.0

0.094
0.60

188 lbs
0.80 pt
1.01.01.51.035.52.2
12Flumioxazin
impregnated on fertilizer
Fb Dacthal 75W

0.094
7.00

188 lbs
9.3 lbs
4.04.00.80.536.53.4
13Devrinol
Fb Dacthal 75W
1.50
7.00
3.0 lbs
9.3 lbs
11.812.00.00.035.88.4
14Untreated------11.813.30.00.036.37.7
 LSD (0.05)  3.83.71.41.0NS2.5

1 applied 16 days prior to transplanting. * Fb = Followed by

Table 2. Central Coast Trial. Post transplant evaluations: Number of weeds (per 6ft2), and phytotoxicity (two dates), stand count and time of weed evaluations.

TRT
Code
ApplicationsLbs a.i./AMaterial/ANightshade

48 DAT
Phyto

48 DAT
Time to weed
(hrs/A)
48 DAT
Fruit
Per acre
X 1000
Fruit weight
Tons/A
Mean
fruit weight
lbs
1Dual Magnum 7.62
Fb* Dual Magnum 7.62
1.43
1.43
1.50 pts
1.50 pts
0.30.01.2148.726.20.35
2Dual Magnum 7.62
Fb Outlook 6.0
1.43
0.60
1.50 pts
0.80 pt
0.30.01.3153.128.10.36
3Dual Magnum 7.62
Fb Dacthal 75W
1.43
7.00
1.50 pts
9.3 lbs
1.30.01.8157.929.70.37
4Goal Tender 4F1
Fb Dual Magnum 7.62
0.50
1.43
1.00 pt
1.50 pts
3.00.32.4187.534.10.36
5Goal Tender 4F1
Fb Outlook 6.0
0.50
0.60
1.00 pt
0.80 pt
3.30.02.0182.733.70.37
6Goal Tender 4F1
Fb Dacthal 75W
0.50
7.00
1.00 pt
9.3 lbs
2.30.01.9169.229.70.35
7Outlook 6.0
Fb Dual Magnum 7.62
0.60
1.43
0.80 pt
1.50 pts
0.00.71.2148.727.10.36
8Outlook 6.0
Fb Outlook 6.0
0.60
0.60
0.80 pt
0.80 pt
0.00.31.2174.933.30.38
9Outlook 6.0
Fb Dacthal 75W
0.60
7.00
0.80 pt
9.3 lbs
0.01.01.2158.729.60.37
10Flumioxazin
impregnated on fertilizer
Fb Dual Magnum 7.62

0.094
1.43

188 lbs
1.50 pts
0.30.01.3183.529.30.33
11Flumioxazin
impregnated on fertilizer
Fb Outlook 6.0

0.094
0.60

188 lbs
0.80 pt
0.70.01.4145.627.80.38
12Flumioxazin
impregnated on fertilizer
Fb Dacthal 75W

0.094
7.00

188 lbs
9.3 lbs
4.70.73.1167.931.30.37
13Devrinol
Fb Dacthal 75W
1.50
7.00
3.0 lbs
9.3 lbs
9.30.03.1185.134.90.38
14Untreated------8.70.03.4184.029.30.32
 LSD (0.05)  4.20.91.330.4NSNS

1 applied 16 days prior to transplanting. * Fb = Followed by

Table 3. Fresno Trial. Weed control ratings and Weed counts.

ApplicationsWeed CONTROL Ratings *Weed Counts per plot
TRTPreemergence ApplicationsLbs a.i.Material ----- all broadleaf weeds -----August 12, 2005* (67 DAT)TOTAL
CodeHerbicidesper Acreper Acre24 DAT45 DAT67 DATPigNightPursLambBrdlvs
1Dual Magnum 7.62
Fb Dual Magnum
1.43
1.43
1.5 pt
1.5 pt
9.8a9.2a9.6ab10.70.06.72.019.3ab
2Dual Magnum
Fb Outlook
1.43
0.60
1.5 pt
.75 pt
10.0a9.5a8.5bc15.00.33.73.022.0ab
3Dual Magnum
Fb Dacthal
1.43
7.00
1.5 pt
9.5 lb
10.0a9.5a9.7a13.00.33.70.017.0a
4Goal Tender 4F1
Fb Dual Magnum
0.50
1.43
1 pt
1.5 pt
1.3c4.0c8.7abc13.77.02.30.723.7ab
5Goal Tender1
Fb Outlook
0.50
0.60
1 pt
.75 pt
1.5c6.2b9.3abc6.02.32.71.712.7a
6Goal Tender1
Fb Dacthal
0.50
7.00
1 pt
9.5 lb
1.0c4.0c8.8abc13.72.72.00.318.7ab
7Outlook 6.0
Fb Dual Magnum
0.60
1.43
.75 pt
1.5 pt
10.0a9.0a8.3c32.30.05.34.041.7bc
8Outlook
Fb Outlook
0.60
0.60
.75 pt
.75 pt
10.0a9.6a8.5bc21.70.03.75.731.0ab
9Outlook
Fb Dacthal
0.60
7.00
.75 pt
9.5 lb
9.7a9.5a7.0d37.01.09.78.356.0c
10Flumioxazin
impregnated on fertilizer
Fb Dual Magnum

0.094
1.43

150 lbs
1.5 pt
7.7b8.3a9.0abc15.00.03.04.722.7ab
11Flumioxazin
Fb Outlook
0.094
0.60
150 lbs
.75 pt
9.7a8.7a8.8abc20.70.03.35.029.0ab
12Flumioxazin
Fb Dacthal
0.094
7.00
150 lbs
9.5 lb
8.2b8.3a8.8abc18.31.34.72.026.3ab
13Untreated - weeded--
--
--
--
0.7c8.3a6.7d32.319.73.31.757.0c
14Untreated - weedy--
--
--
--
1.0c1.7d0.7e64.324.024.717.0130.0d
 LSD (0.05)  1.0 1.5 1.2 20.54.05.36.524.4

1- Applied 28 days prior to transplanting. * July 1=24 DAT; July 22=45 DAT; Aug 12=67 DAT. August 12 evaluations are 18 days post layby application.

Table 4. Fresno Trial. Pepper yield, Stand counts, and Phytotoxicity ratings (two dates)*.

TRTApplicationsLbs a.i.MaterialPepper Yield lbs/plotPeppers/plotPhytotoxicity
Codeper Acreper AcreGoodSmallSunburnTotal36 DAT24 DAT67 DAT*
1Dual Magnum 7.62
Fb Dual Magnum
1.43
1.43
1.5 pt
1.5 pt
77.6ab18.715.9112.2ab186.0bcd1.3b0.0d
2Dual Magnum
Fb Outlook
1.43
0.60
1.5 pt
.75 pt
71.3ab18.413.9103.5ab184.5cd1.5b3.5bcd
3Dual Magnum
Fb Dacthal
1.43
7.00
1.5 pt
9.5 lb
61.0ab16.911.789.5ab194.5abcd1.0b0.0cd
4Goal Tender 4F1
Fb Dual Magnum
0.50
1.43
1 pt
1.5 pt
62.6ab20.714.397.6ab186.5bcd0.2b0.3d
5Goal Tender1
Fb Outlook
0.50
0.60
1 pt
.75 pt
91.6a12.611.8116.0a196.5abc0.5b0.0d
6Goal Tender1
Fb Dacthal
0.50
7.00
1 pt
9.5 lb
66.0ab12.910.289.1ab200.0ab0.2b0.0d
7Outlook 6.0
Fb Dual Magnum
0.60
1.43
.75 pt
1.5 pt
47.2bc8.99.565.5bc190.5abcd3.7a6.0ab
8Outlook
Fb Outlook
0.60
0.60
.75 pt
.75 pt
41.8bc10.713.766.2bc187.0bcd3.7a2.0abc
9Outlook
Fb Dacthal
0.60
7.00
.75 pt
9.5 lb
21.0c7.13.731.9c188.0bcd5.2a6.5a
10Flumioxazin
impregnated on fertilizer
Fb Dual Magnum

0.094
1.43

150 lbs
1.5 pt
57.5abc15.511.384.4ab182.0cd1.0b1.0cd
11Flumioxazin
Fb Outlook
0.094
0.60
150 lbs
.75 pt
79.4ab15.417.6112.3ab194.0abcd0.5b0.5d
12Flumioxazin
Fb Dacthal
0.094
7.00
150 lbs
9.5 lb
65.3ab16.37.389.0ab203.5a1.5b2.0bcd
13Untreated weeded--
--
--
--
78.4ab19.210.0107.5ab181.5d0.3b0.3d
14Untreated weedy--
--
--
--
20.9c6.24.531.5c182.5cd0.0b0.0d
 LSD (0.05)  39.8 8.89.147.4 14.6 1.9 2.8

1- Applied 28 days prior to transplanting. * July 1=24 DAT; July 13=36 DAT; Aug 12=67 DAT. August 12 evaluations are 18 days post layby application.


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