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Brassicaceae Cover-Crop Effects on WeedManagement in Plasticulture TomatoSanjeev K. Bangarwaa & Jason K. Norsworthya

a Department of Crop, Soil, and Environmental Sciences, Universityof Arkansas, Fayetteville, Arkansas, USAPublished online: 28 Mar 2014.

To cite this article: Sanjeev K. Bangarwa & Jason K. Norsworthy (2014) Brassicaceae Cover-CropEffects on Weed Management in Plasticulture Tomato, Journal of Crop Improvement, 28:2, 145-158,DOI: 10.1080/15427528.2013.858381

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Journal of Crop Improvement, 28:145–158, 2014Copyright © Taylor & Francis Group, LLCISSN: 1542-7528 print/1542-7536 onlineDOI: 10.1080/15427528.2013.858381

Brassicaceae Cover-Crop Effects on WeedManagement in Plasticulture Tomato

SANJEEV K. BANGARWA and JASON K. NORSWORTHYDepartment of Crop, Soil, and Environmental Sciences, University of Arkansas,

Fayetteville, Arkansas, USA

Weed control options are limited in plasticulture vegetables in theabsence of methyl bromide. Field and laboratory studies were con-ducted to test the efficacy of soil amendment with Brassicaceaecover crops in combination with low-density polyethylene (LDPE)and virtually impermeable film (VIF) mulches for weed control infresh-market tomato (Solanum lycopersicum). Three Brassicaceaecover crops, ‘Seventop’ turnip (Brassica rapa), ‘Pacific Gold’ ori-ental mustard (Brassica juncea), and ‘Caliente’, a blend of brownmustard (Brassica juncea) and white mustard (Sinapis alba), weretested along with methyl bromide for yellow nutsedge (Cyperusesculentus) and johnsongrass (Sorghum halepense) control intomato. Glucosinoate (GSL) analysis indicated that ‘Caliente’ mus-tard, ‘Pacific Gold’ oriental mustard, and ‘Seventop’ turnip pro-duced GSLs totaling 26,399, 16,798, and 18,847 µmol m−2, respec-tively, prior to termination. The VIF mulch was neither effective inincreasing weed control nor in improving tomato yield over LDPEmulch. Regardless of mulch type, Brassicaceae cover crops providedmarginal control of yellow nutsedge (≤39%) and johnsongrass(≤46%) at 2 weeks after transplanting (WATP). Moreover, weedcontrol in cover-crop plots declined to ≤20% at ≥4 WATP. Soilamendment with Brassicaceae cover crops did not injure tomatoplants. Although soil amendment improved weed control andmarketable yield over fallow plots, none of the amended plots pro-duced marketable yield equivalent of methyl bromide (59 t ha−1).Therefore, soil amendment with Brassicaceae cover crops cannotbe used as a practical alternative to methyl bromide, but it can be

Received 18 June 2013; accepted 19 October 2013.Address correspondence to Sanjeev K. Bangarwa, Department of Crop, Soil, and

Environmental Sciences, University of Arkansas, 1366 W. Altheimer Dr., Fayetteville, Arkansas72704, USA. E-mail: sanjeev_hau@yahoo.com

145

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146 S. K. Bangarwa and J. K. Norsworthy

combined with other strategies in an integrated pest managementprogram.

KEYWORDS allelopathy, biofumigation, biological weed control,integrated weed management, methyl bromide alternative

INTRODUCTION

Soil fumigation with methyl bromide has been a common practice for con-trolling a wide array of soil-borne pests, including weeds, in plasticulturevegetable production (Duniway 2002). In a commercial production sys-tem, methyl bromide is injected pre-plant into the soil and covered witha low-density polyethylene (LDPE) mulch. Plastic mulch is used to minimizegaseous losses of methyl bromide and thereby maximize efficacy againstpests. However, LDPE mulch is not completely impermeable and gaseouslosses of methyl bromide escape into the atmosphere. As a result, methylbromide causes stratospheric ozone-depletion and therefore is being phasedout of U.S. agriculture and is only available in limited quantity under criticaluse exemption (U.S. Environmental Protection Agency 2008).

Weed management will be challenging for tomato producers in theabsence of methyl bromide. Plastic mulches suppress some weeds by pro-viding a physical barrier and by altering light quantity and quality aswell as increasing the heat beneath the mulch. However, purple nutsedge(Cyperus rotundus) and yellow nutsedge (Cyperus esculentus) species canreadily puncture the plastic mulches (Patterson 1998). In addition, an under-ground network of tubers and rhizomes makes nutsedge species the mostdifficult-to-control weeds and a major cause of significant yield reduction invegetable crops (Morales-Payan et al. 1996, 1997a, 1997b, 1998; Santos et al.1998; Webster 2005a, b; Bangarwa et al. 2008). Therefore, methyl bromidealternatives are urgently needed.

One cultural alternative is biofumigation, which is basically suppressionof soil-borne pests by naturally produced allelopathic volatile compounds(Gimsing and Kirkegaard 2009). Examples of such volatile compounds areisothiocyanates (ITCs) produced by Brassicaceae plants. Plants belongingto the Brassicaceae family contain a variety of glucosinolates (GSLs) intheir cell vacuoles, which on enzymatic hydrolysis are converted into ITCs(Brown and Morra 1995). The ITCs are biocidal compounds that are inher-ently volatile and therefore can serve as a fumigant (Brown and Morra 1995,1996). The simplest form of ITC is methyl ITC, which is the active ingredi-ent in the commercial fumigant Vapam (AMVAC 2011). The ITCs reportedlyreduce yellow nutsedge emergence and shoot growth in purple andyellow nutsedge (Norsworthy and Meehan 2005b; Norsworthy et al. 2006).

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Brassica Covers crops for weed control in tomato 147

In another study, soil amendment with wild radish (Raphanus raphistrum), aBrassicaceae weed, reduced yellow nutsedge tuber production in amendedsoil (Norsworthy and Meehan 2005a). In the Pacific Northwest, soil amendedwith rapeseed (Brassica napus) successfully reduced weed biomass up to96% in the following potato (Solanum tuberosum) crop (Boydston and Hang1995). Malik et al. (2008) reported that soil amendment with wild radish com-bined with half the label rates of herbicides could provide optimum largecrabgrass (Digitaria sanguinalis) control in sweet corn (Zea mays). Similarly,incorporation of Brassicaceae green manure into soil before planting a com-mercial crop supplemented herbicidal control of johnsongrass (Uremis et al.2009). Therefore, soil amendment with Brassicaceae crops can provide acompetitive advantage to the preceding crop by suppressing weeds early inthe season.

The magnitude of weed suppression from soil amendment is dependentupon the concentration and toxicity of ITCs released in treated soil and thelength of exposure of ITCs to target weed species (Teasdale and Taylorson1986). However, the high volatility of ITCs makes them vulnerable to gaseouslosses from amended soils under field conditions (Brown and Morra 1996;Peterson et al. 2001; Morra and Kirkegaard 2002). Volatilization losses ofITCs in the field can be minimized by covering the amended soil with low-permeability plastic mulches immediately after tissue incorporation (Priceet al. 2005). Under laboratory experiments, virtually impermeable film (VIF)mulch was more effective than LDPE mulch in reducing methyl ITC lossesfrom the treated soil (Austerweil et al. 2006). Therefore, VIF mulch mayprovide higher retention of ITCs and in turn improve weed control overLDPE mulch. The objective of this research was to evaluate the weed controlin plasticulture tomato using biofumigation from Brassicaceae cover cropsunder LDPE or VIF mulches and compare it with a standard methyl bromideprogram.

MATERIALS AND METHODS

Location and Soil Type

Weed control potential of Brassicaceae cover crops was evaluated in fieldand laboratory experiments conducted in 2007 at the Arkansas AgriculturalResearch and Extension Center at Fayetteville, Arkansas. The soil at the fieldtest site was a silt loam (fine-loamy, mixed, active, non-acid) with 1.7%organic matter content and a pH of 6.1. The test site was fallow in 2006 butwas tilled twice in spring and fall. The test site contained a natural populationof yellow nutsedge and johnsongrass. It was thoroughly tilled in mid-Marchto remove any vegetation and prepare a smooth seedbed for cover cropplanting.

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148 S. K. Bangarwa and J. K. Norsworthy

Experimental Design

The experiment was organized in a split-plot design with four replica-tions. Main plots consisted of two mulch types, LDPE and VIF, and thesubplot consisted of fallow (nontreated), methyl bromide, and three covercrops: ‘Seventop’ turnip, ‘Pacific Gold’ oriental mustard, and ‘Caliente’ mus-tard. A standard treatment of methyl bromide included a mixture of methylbromide (67%) and chloropicrin (33%) applied at 390 kg ha−1.

Trial Procedure

All cover crops were drill-seeded in 7.6-m-long plots with an 18-cm-rowspacing. Seeding rate varied with individual cover crop: 5.6 kg ha−1 for‘Seventop’ turnip and 9 kg ha−1 for ‘Pacific Gold’ oriental mustard and‘Caliente’ mustard. The fertilization program included pre-plant incorpora-tion of 40 kg ha−1 of nitrogen (N) followed by a top dressing of 40 kg ha−1

N and 30 kg ha−1 sulfur (S) at 5 weeks after planting (WAP). Cover cropsreceived no additional irrigation other than rainfall (total monthly precipi-tation was: March, 102 mm; April, 130 mm; and May, 100 mm). No pestmanagement practices were followed in cover crops.

At 50%–80% pod-fill stage, all the cover crops were flail mowed andincorporated into the top 7.5 cm of the soil using a roto-tiller. Prior tomowing, cover crop densities were recorded by randomly placing 0.5-m2

quadrat in each cover-crop plot. Subsequently, cover-crop shoots and rootswere harvested from this 0.5-m2 area and freeze-dried for biomass andGSL quantification. Immediately after incorporation, 70-cm wide raised bedswere formed on 1.8-m centers and covered with a black LDPE mulch orblack/white VIF mulch using a tractor-mounted plastic layer. Simultaneously,a single row of drip tape was placed in the bed center under the plas-tic mulch. Treatments were separated by cutting the plastic and coveringwith soil to avoid volatile ITC movement across the treatments. Similarly,raised beds were formed and covered with plastic mulch in fallow plots.Weed densities were taken in fallow (untreated) plots before forming beds.Averaged across eight plots, the untreated checks had 68 shoots/m2 of yellownutsedge and 3 plants/m2 of johnsongrass. Transplant holes were punchedin the plastic 1 wk following incorporation of cover crops and laying ofplastic mulch. A single row of seven ‘Amelia’ tomato transplants (5 wkold) was planted into beds at 60-cm spacing. Additional methyl bromidetreatments were applied under LDPE and VIF mulches at 3 weeks priorto transplanting. Standard production and management practices were fol-lowed as recommended for drip-irrigated fresh-market plasticulture tomatoes(Holmes and Kemble, 2010). Bare-ground row-middles between the bedswere kept clean by directed application of S-metolachlor (Dual Magnum7.62 EC) and paraquat (Gramoxone Inteon 2 EC).

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Brassica Covers crops for weed control in tomato 149

Data Collection

Weed control and crop injury were visually rated at 2, 4, and 6 WATP on a0 to 100 scale, where 0 = no injury to crop or no weed control and 100 =crop death or complete weed control. Visual injury rating was taken by eval-uating all tomato plants in each plot or bed. Visual weed control rating wastaken from central 7-m length of the plot or bed (70-cm wide bed). Visualratings were based on symptoms, such as chlorosis, necrosis, and stand lossand stunting of tomato or weeds. In addition, tomato fruit-yield data wererecorded based on seven tomato harvests from whole plot during the season.Tomato fruits were harvested and graded into six categories (jumbo, extra-large, large, medium, small, and culls) based on USDA standards for freshmarket tomato (U.S. Department of Agriculture 1997). For GSLs analysis inshoots and roots of each cover crop, freeze-dried cover crop tissues wereground to powder and passed through a 1-mm screen. In laboratory exper-iment, GSLs from 0.3-g freeze-dried tissue were extracted and analyzed byhigh-performance liquid chromatography according to published procedures(Gardiner et al. 1999, Norsworthy et al. 2007).

Statistical Analysis

Mean values and associated standard errors of means were calculated forbiomass and GSL content for each cover crop. Data on crop injury, weedcontrol, and tomato yield were subjected to analysis of variance with a split-plot structure using PROC GLM in SAS (version 9.2; SAS Institute, Cary, NC).Whole-plot treatment (two mulch type levels) was considered in a random-ized complete block, and cover crops/fumigants were the split-plot factor(five levels). Treatment means were separated by Fisher’s protected LSD atα = 0.05.

RESULTS AND DISCUSSION

Cover Crop Biomass

Prior to terminatin, ‘Caliente’ mustard, ‘Pacific Gold’ oriental mustard, and‘Seventop’ turnip produced a total dry biomass of 695 (± 59.1), 707 (±71.3),and 584 (±52.1) g m−2, respectively. Total biomass production in each covercrop was mainly contributed by shoot biomass, which ranged from 88% to90% of the total biomass (data not shown). In previous cover crop studies,total biomass produced by ‘Caliente’ mustard, ‘Pacific Gold’ oriental mus-tard, and ‘Seventop’ turnip ranged from 272 to 752 g m−2, 670 to 920 g m−2,and 598 to 684 g m−2, respectively (Hartz et al. 2005; Norsworthy et al.2005; Bangarwa et al. 2011a). This variation in biomass production in pre-vious trials could be attributed to variation in agronomic and environmental

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150 S. K. Bangarwa and J. K. Norsworthy

conditions, time of establishment (fall vs. spring), and stage of terminationof the cover crop.

Glucosinolate Production

The type and amount of GSL varied among cover crops as well as betweenshoots and roots of each cover crop (Table 1). Three types of aliphatic[(2R)-2-hydroxybut-3-enyl, 2-propenyl, and but-3-enyl] and three types ofaromatic GSLs (p-hydroxybenzyl, benzyl, and 2-phenylethyl) were detectedin either shoots or roots of Brassicaceae cover crops. The major shoot GSL in‘Caliente’ and ‘Pacific Gold’ mustards was 2-propenyl, whereas benzyl and2-phenylethyl were dominant shoot GSLs in ‘Seventop’ turnip. In ‘Caliente’mustard roots, 2-propenyl, p-hydroxybenzyl, and 2-phenylethyl were domi-nant GSL, whereas 2-propenyl was major root GSL in ‘Pacific Gold’ mustard.‘Seventop’ turnip roots contained a high amount of 2-phenylethyl GSL. TotalGSL concentration in the shoots was 2.8- and 2.6-fold higher than that inroots of ‘Caliente’ mustard and ‘Pacific Gold’ mustard, respectively (Table 1).Conversely, root GSL concentration was 2.8-fold higher than that of the shootin ‘Seventop’ turnip. However, total GSL production per unit area was mainlycontributed by the shoot GSLs because 88 to 90% of total biomass was con-tributed by shoots (Table 1). Total (shoot plus root) GSL production per unitarea for ‘Caliente’ mustard, ‘Pacific Gold’ oriental mustard, and ‘Seventop’turnip was estimated to be 26,399, 16,798, and 18,847 μmol/m2, respectively(Table 1). In previous studies, total GSL content in ‘Caliente’ mustard, ‘PacificGold’ oriental mustard, and ‘Seventop’ turnip varied from 8,600 to 26,700μmol m−2, 9,800 to 11,300 μmol m−2, and 12,365 to 22,844 μmol m−2, respec-tively (Hartz et al. 2005; Norsworthy et al. 2005; Bangarwa et al. 2011b).

Weed Control

There was no significant main effect of mulch type or any cover crop bymulch interaction for yellow nutsedge and johnsongrass control at any rat-ing. However, weed control was influenced by the main effect of covercrop/fumigant (Table 2), indicating that VIF mulch did not improve weedcontrol over LDPE mulch in fallow, cover crop, and methyl bromide plots.Thus, VIF mulch was no more effective than LDPE mulch in retaining higherconcentration of ITCs in treated soil and providing effective weed control.In previous research, VIF mulch failed to increase methyl ITC retention com-pared with an LDPE mulch (El Hadiri et al. 2003). Likewise, Haar et al.(2003) tested methyl ITC efficacy against little mallow (Malwa parviflora L.)seeds under VIF and standard mulch and found that viable seed density oflittle mallow was not statistically different under standard and VIF mulch.In another study, Bangarwa et al. (2011c) reported that herbicidal efficacyof allyl ITC was not improved by replacing LDPE mulch with VIF mulch.

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TAB

LE1

Typ

ean

dco

nce

ntrat

ion

ofgl

uco

sinola

tes

det

ecte

din

the

shootan

dro

ottis

sues

ofB

rass

icac

eae

cove

rcr

ops

and

estim

ated

gluco

sinola

tes

pro

duct

ion

per

unit

area

prior

tote

rmin

atio

nofco

ver

cropsz

y.

‘Cal

iente

’m

ust

ard

Pac

ific

Gold

’m

ust

ard

‘Sev

ento

p’

turn

ip‘C

alie

nte

’m

ust

ard

Pac

ific

Gold

’m

ust

ard

‘Sev

ento

p’

turn

ipPla

ntpar

tan

dgl

uco

sinola

tety

pe

μm

olg−1

μm

olm

−2

Shoot

(2R)-

2-hyd

roxy

but-3-

enyl

nd

nd

2.9

(0.2

)nd

nd

1516

.6(1

02.6

)

2-pro

pen

yl25

.1(0

.9)

25.1

(1.3

)nd

1548

4.3

(546

.7)

1568

2.1

(818

.2)

nd

p-hyd

roxy

ben

zyl

7.3

(2.6

)nd

nd

4518

.1(1

602.

7)nd

nd

But-3-

enyl

nd

0.2

(<

0.05

)nd

nd

126.

6(1

0.9)

nd

Ben

zyl

5.8

(0.2

)nd

10.9

(0.7

)35

53.7

(120

.4)

nd

5695

.8(3

56.5

)2-

phen

ylet

hyl

1.8

(0.8

)0.

3(<

0.05

)13

.4(9

.8)

1131

.8(4

72.4

)19

6.2

(23.

7)70

17.7

(51.

7.5)

Tota

l40

.0(3

.2)

25.6

(1.4

)27

.2(9

.6)

2468

8.0

(198

0.4)

1600

4.9

(850

.4)

1423

0.2

(501

8.4)

Root

(2R)-

2-hyd

roxy

but-3-

enyl

nd

nd

3.8

(0.1

)nd

nd

234.

3(8

.7)

2-pro

pen

yl8.

1(0

.1)

9.2

(0.3

)nd

622.

3(5

.8)

762.

3(2

7.1)

nd

p-hyd

roxy

ben

zyl

7.6

(0.1

)nd

nd

583.

3(7

.3)

nd

nd

But-3-

enyl

nd

nd

nd

nd

4.0

(3.2

)nd

Ben

zyl

0.6

(0.1

)nd

10.8

(0.4

)42

.6(4

.3)

nd

656.

1(2

3.1)

2-phen

ylet

hyl

6.0

(0.1

)0.

3(0

.1)

61.1

(2.0

)46

2.9

(8.4

)27

.1(6

.3)

3726

.6(1

24.2

)To

tal

22.2

(0.3

)9.

6(0

.3)

75.7

(2.6

)17

11.0

(25.

8)79

3.3

(24.

8)46

16.9

(155

.8)

zSt

andar

der

ror

ofea

chm

ean

isin

cluded

inpar

enth

esis

.ynd

=non-d

etec

ted.

151

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TAB

LE2

AN

OVA

table

show

ing

effe

ctof

mulc

hty

pe

and

fum

igat

ion

with

met

hyl

bro

mid

e/so

il-am

endm

ent

with

Bra

ssic

acea

eco

ver

crops

on

yello

wnuts

edge

and

johnso

ngr

ass

control

at2,

4,an

d6

WATP.

Tre

atm

ents

mea

ns

are

pro

vided

tosh

ow

the

effe

ctof

fum

igat

ion

with

met

hyl

bro

mid

e/so

il-am

endm

entw

ithB

rass

icac

eae

cove

rcr

ops

on

yello

wnuts

edge

and

johnso

ngr

ass

controlat

2,4,

and

6W

ATP,

aver

aged

ove

rm

ulc

hty

pe

(LD

PE

and

VIF

).

AN

OV

Ata

ble

z

Yel

low

nuts

edge

(%co

ntrol)

Johnso

ngr

ass

(%co

ntrol)

Sourc

edf

2W

ATP

4W

ATP

6W

ATP

2W

ATP

4W

ATP

6W

ATP

Rep

licat

ion

3N

SN

SN

SN

SN

SN

SM

ulc

h1

NS

NS

NS

NS

NS

NS

Cove

rCro

p4

∗∗∗

∗∗∗

∗∗∗

∗∗∗

∗∗∗

∗∗∗

Cove

rCro

p×

Mulc

h4

NS

NS

NS

NS

NS

NS

Tre

atm

ent

mea

ns

aver

aged

ove

rm

ulc

hty

pey

Bra

ssic

acea

eco

ver

crop/

fum

igan

tYel

low

nuts

edge

(%co

ntrol)

Johnso

ngr

ass

(%co

ntrol)

2W

ATP

4W

ATP

6W

ATP

2W

ATP

4W

ATP

6W

ATP

Met

hyl

bro

mid

ex96

a93

a89

a98

a96

a95

a‘C

alia

nte

’must

ard

32b

19b

8bc

38bc

26b

17b

‘Pac

ific

Gold

’must

ard

39b

22b

13b

46b

26b

20b

‘Sev

ento

p’t

urn

ip24

c11

c2

cd34

c22

b12

bFa

llow

0d

0d

0d

0d

0c

0c

z∗ ,

∗∗,

∗∗∗

den

ote

sign

ifica

nce

atth

e5%

,1%

,an

d0.

1%pro

bab

ility

leve

ls,re

spec

tivel

y.N

Sden

ote

snotsi

gnifi

cant.

yTre

atm

entm

eans

with

ina

colu

mn

follo

wed

by

the

sam

ele

tter

are

notdiffe

rentbas

edon

Fish

er’s

pro

tect

edLS

Dat

P<

0.05

.xM

ethyl

bro

mid

e=

met

hyl

bro

mid

e:ch

loro

pic

rin

(67:

33%

)at

390

kgha−1

.A

bbre

viat

ions:

WATP

=w

eeks

afte

rtran

spla

ntin

g,LD

PE

=bla

cklo

wden

sity

poly

ethyl

ene

mulc

h,VIF

=bla

ck/w

hite

virtual

lyim

per

mea

ble

film

mulc

h.

152

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Brassica Covers crops for weed control in tomato 153

In contrast, effective nutsedge (mixture of purple and yellow nutsedge) con-trol was achieved in a previous study at 25% of the labeled rate of methylbromide by using VIF compared with a full rate under LDPE mulch (Motiset al. 2003). In another study, Santos et al. (2007) reported that VIF mulchcould retain 1.8 to 3.7 times more methyl bromide than LDPE or HDPEmulches, and, therefore, methyl bromide rates could be cut down to half thelabel rates without losing nutsedge-control efficacy.

In the present study, no mulch effect was caused by little differentialretention of ITCs under LDPE and VIF mulches, which could be attributed toweak fumigation activity of ITCs produced under the mulch compared withother strong volatile fumigants like methyl bromide used in previous studies.This can be explained by the huge difference in vapor pressure of ITCs (e.g.,allyl ITC vapor pressure = 0.53 kPa at 20◦C; methyl ITC vapor pressure =2.8 kPA at 20◦C) and methyl bromide (vapor pressure = 188 kPa at 20◦C).Thus, we think that there was not enough fumigation activity from ITCs torecognize differential retention and in turn herbicidal activity between thetwo mulches. No effect of mulch in the methyl bromide treatments could beexplained by the high efficacy of the methyl bromide: chloropicrin mixturealone when applied at 390 kg ha−1.

Yellow nutsedge and johnsongrass control from the methyl bromidetreatment ranged from 89 to 96% and 95 to 98%, respectively at all ratings(Table 2). In cover crop plots, maximum control of yellow nutsedge andjohnsongrass control was 39 and 46%, respectively, at 2 WATP. However,weed control in cover-crop plots declined later in the season, with no morethan 13 and 20% control of yellow nutsedge and johnsongrass, respec-tively at 6 WATP. Similar results were reported in previous studies whereBrassicaceae cover crops provided marginal early-season weed control,which declined later in the season (Norsworthy et al. 2007; Bangarwa et al.2011a). Although variable among cover crops, the level of weed controlprovided by Brassicaceae cover crops in the present study would not becommercially acceptable, and therefore, Brassicaceae cover crops cannotbe used as a stand-alone replacement of methyl bromide. Marginal weedcontrol in cover-crop plots was attributed to low ITC concentration in theamended soil based on the ITC production by the plants. Weed suppressionis a function of concentration and length of exposure of ITCs to target species(Teasdale and Taylorson 1986). Based on the GSL content, the biofumigationpotential of each Brassicaceae cover crops can be calculated by estimatingpotential ITC production after tissue incorporation. For example, ‘Caliente’mustard, ‘Pacific Gold’ oriental mustard, and ‘Seventop’ turnip in the presentstudy can potentially contribute a maximum of 26,399, 16,798, and 18,847μmol m−2 of total ITCs, respectively (assuming 100% conversion of eachGSL to its respective ITC). A comparison of these ITC levels to the commer-cial fumigant Vapam, which is a methyl ITC generator (AMVAC ChemicalCorp. 2011), can be used to estimate the potential effectiveness of the

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154 S. K. Bangarwa and J. K. Norsworthy

ITCs produced, assuming efficacy is similar among ITCs. When applied ata label rate of 358 kg ha−1, Vapam can produce up to 249,700 μmol m−2

of methyl ITC (assuming 90% conversion). Therefore, Vapam can gener-ate 9.5 to13.2 times more ITC (molar basis) than the amount potentiallyproduced by any cover crops used in this study, which can explain themarginal weed efficacy in cover-crop plots. Moreover, soil adsorption, leach-ing, volatilization, and microbial degradation could further reduce the ITCconcentration in amended soil (Matthiessen and Kirkegaard 2006; Gimsingand Kirkegaard 2009). Hence, regardless of mulch type, soil amendmentwith Brassicaceae cover crops alone is not a practical option for effective,season-long weed control.

Crop Tolerance

No tomato injury was observed in any treatment at any rating (data notshown). This indicated that tomato was tolerant of the ITCs produced, atleast at the concentrations present in soil at 1 wk after incorporation ofcover crops. The short persistence of ITCs in amended soil might also con-tribute to the tolerance of tomato (Brown and Morra 1995). Similarly, noinjury has previously been reported, where tomatoes were transplanted insoil amended with each of these three cover crops (Bangarwa et al. 2010).

Crop Yield

There was no significant main effect of mulch type or any cover crop bymulch type interaction on grade-wise and total marketable tomato yield.However, yield was affected by the main effect of cover crop/fumigant.Therefore, yield data are presented for various cover crops/fumigant treat-ments, averaged across mulch types (Table 3). Most of the marketable yieldwas contributed by jumbo-size fruits. Maximum marketable fruit yield of59 t ha−1 occurred in plots treated with methyl bromide, whereas mini-mum yield was recorded in fallow (nontreated) plots (32.9 t ha−1). Tomatofrom cover-crop plots produced intermediate marketable yield. However,none of the cover-crop treatments provided marketable yield equivalent tomethyl bromide. The yield response was most likely related to the levelof yellow nutsedge and johnsongrass control in the plots. Other possiblefactors for improved yield in cover crop plots over that of fallow plotsmight be improved soil structure, nutrient recycling, and suppression of othersoil-borne pests (Haramoto and Gallandt 2004; Matthiessen and Shackleton2005), which were not evaluated in this research.

In summary, soil amendment with Brassicaceae cover crop can pro-vide marginal early-season weed control that would not be acceptable inmost commercial production systems. Therefore, soil amendment shouldbe supplemented with other weed control strategies (e.g., herbicides,

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TAB

LE3

AN

OVA

table

show

ing

effe

ctofm

ulc

hty

pe

and

fum

igat

ion

with

met

hyl

bro

mid

e/so

il-am

endm

entw

ithB

rass

icac

eae

cove

rcr

ops

on

fruit

yiel

dof

bel

lpep

per

.Tre

atm

ents

mea

ns

are

pro

vided

tosh

ow

the

effe

ctof

fum

igat

ion

with

met

hyl

bro

mid

e/so

il-am

endm

ent

with

Bra

ssic

acea

eco

ver

crops

on

fruit

yiel

dofbel

lpep

per

,av

erag

edove

rm

ulc

hty

pe

(LD

PE

and

VIF

).

AN

OV

Ata

ble

z

Tom

ato

fruit

yiel

d(t

ha−1

)y

Sourc

edf

Jum

bo

Ext

ra-lar

geLa

rge

Med

ium

Smal

lM

arke

table

Rep

licat

ion

3N

SN

SN

SN

SN

SN

SM

ulc

h1

NS

NS

NS

NS

NS

NS

Cove

rCro

p4

∗∗∗

∗∗

NS

NS

∗∗

Cove

rCro

p×

Mulc

h4

NS

NS

NS

NS

NS

NS

Tre

atm

ent

mea

ns

aver

aged

ove

rm

ulc

hty

pex

Bra

ssic

acea

eco

ver

crop/

fum

igan

tTo

mat

ofr

uit

yiel

d(t

ha−1

)

Jum

bo

Ext

ra-lar

geLa

rge

Med

ium

Smal

lM

arke

table

Met

hyl

bro

mid

ew35

.7a

5.6

a14

.0a

2.4

a1.

2a

59.0

a‘C

alia

nte

’must

ard

26.1

b3.

9ab

12.5

ab1.

7a

0.9

a45

.2b

‘Pac

ific

Gold

’must

ard

28.5

b3.

5b

11.7

abc

2.1

a0.

9a

46.7

b‘S

even

top’t

urn

ip23

.7bc

3.5

b9.

0bc

1.7

a0.

9a

38.9

bc

Fallo

w18

.5c

3.5

b8.

4c

1.8

a0.

7a

32.9

c

z∗ ,

∗∗,

∗∗∗

den

ote

sign

ifica

nce

atth

e5%

,1%

,an

d0.

1%pro

bab

ility

leve

ls,re

spec

tivel

y.N

Sden

ote

snotsi

gnifi

cant.

yTo

mat

ofruit

yiel

d=

fres

hw

eigh

tofto

mat

ofruits

per

cate

gory

.M

arke

table

yiel

dis

the

sum

ofju

mbo,ex

tra-

larg

e,la

rge,

med

ium

,an

dsm

allca

tego

ryfruit

wei

ghts

.xTre

atm

entm

eans

with

ina

colu

mn

follo

wed

by

the

sam

ele

tter

are

notdiffe

rentbas

edon

Fish

er’s

pro

tect

edLS

Dat

P<

0.05

.wM

ethyl

bro

mid

e=

met

hyl

bro

mid

e:ch

loro

pic

rin

(67:

33%

)at

390

kgha−1

.A

bbre

viat

ions:

LDPE

=bla

cklo

wden

sity

poly

ethyl

ene

mulc

h,VIF

=bla

ck/w

hite

virtual

lyim

per

mea

ble

film

mulc

h.

155

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014

156 S. K. Bangarwa and J. K. Norsworthy

solarization, mechanical, etc.) to provide effective season-long weed control.Introduction of Brassicaceae cover crop with high biomass production andhigh GSL content could improve early-season weed suppression. Consideringpractical implementation, soil amendment with Brassicaceae alone is unlikelyto be used as replacement of methyl bromide. However, Brassicaceae covercrop can be used as a component of IPM program to suppress soil bornepests as well as improve soil health and fertility.

ACKNOWLEDGEMENTS

We extend our appreciation to crewmembers of the Weed Science programat the University of Arkansas for technical support.

FUNDING

Funding for this research was provided by Southern Integrated PestManagement Center.

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