EVALUATION OF BRASSICA COVER CROPS FOR CONTROL OF SOILBORNE
PEST AND DISEASES ON SUBSEQUENT ZUCCHINI
J. A. J. Desaeger, A. S. Csinos, K. W. Seebold
Plant Pathology
University of Georgia
P O Box 748
Tifton, GA 31793
Introduction
Many plants produce compounds called allelochemicals that directly or indirectly impact
their biological environment. Glucosinolates (GSLs) are allelochemicals that occur throughout
the agronomically important Brassicaceae (Cruciferae) family. There is sufficient evidence to
suggest that glucosinolates contained in Brassicaceae tissues produce a variety of
allelochemicals that are effective pesticides. Glucosinolate degradation products such as
isothiocyanates (ITC's) have broad-spectrum biocidal activity, including insecticidal,
nematicidal, fungicidal, antibiotic and phytotoxic effects. For example, methylisothiocyanate
(MITC) is used as a soil fumigant and is the active pesticidal agent produced from the
degradation of synthetic dithiocarbamates (e.g., metam sodium) and diazines (e.g., dazomet).
Because isothiocyanates have pesticidal activities and are dominant products formed from
glucosinolates in soil, the use of Brassica species and other glucosinolate-producing species to
control soilborne plant pests could be a valuable component of a methyl bromide alternatives
program.
In this test we evaluated the effect of different Brassica species to control soilborne pests
and diseases on a subsequent zucchini plasticulture crop.
Materials and Methods
The study was located at the Blackshank Farm, CPES, Tifton, GA. The area had a history
of soybeans, tobacco, and assorted vegetables. The area was prepared using all current
University of Georgia Extension Service recommendations. The plot design was a split plot
design with fumigants as main treatments and mustard cover crop and fallow plots as
sub-treatments. Plots were arranged in randomized complete blocks consisting of single bed
plots replicated five times. Each plot was 30 feet long and 30 in wide
Brassica crops are: two cultivars of mustard greens (B. juncea 'FL broadleaf' and B.
juncea 'Curly leaf'), two turnip cultivars (B. rapa 'Purple Top White Globe' and B. rapa 'White
Egg'), Rutabaga (B. napus, cv. ‘American Purple Top’), Kale (B. oleracea, cv. ‘Dwarf Blue
Curled Scotch’), radish (Raphanus sativus, cv. Scarlet Globe’) and a non-Brassica, spinach. Two
more treatments consist of a natural weed cover, with and without a metam sodium application
prior to each plastic-mulch vegetable crop. Brassica’s were planted on 25 February 2003 with a
Stanhay planter. On 11 June, 2003, Brassica covers were cut with a Flail mower, fertilizer (1010-10) applied and plots were rototilled. Beds were shaped and all beds were covered with 1 mil
black polyethylene with drip tape in the center of the bed approximately 1in. deep. The
following day, metam sodium was drip-applied at 50 gal/A (Table1).
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Zucchini seedlings were produced in nutrient tray system to the 4-leaf stage. A single
plant was transplanted using a mechanical type transplanter, which cuts holes in the plastic just
ahead of the planters in the center of the plastic bed adjacent to the drip tape on 7 July. Plant
spacing was 12 in.
As per the recommendation of the University Of Georgia Extension service, all plots
received 500 lbs of fertilizer prior to mustard planting and 700 lbs. of fertilizer (10-10-10) prior
to plastic laying. Additional fertilizer on squash was added in the form of liquid fertilizer (NPK
20-20-20 and 8-0-4 alternated) injected through the irrigation tubing during the growing season.
All squash plots were sprayed on a 4 to 7 day interval with Manex with Zinc (2.4 qt/A) plus
Kocide LF (0.5 gal/A) and Bravo (2 pts/A) for control of foliar diseases, and Ambush (10 oz./A)
alternating with Pounce 3.2 (6 oz./A), Asana XL (6 oz./A) and Avaunt (3 oz./A) for insect
control.
Stand counts and plant vigor ratings were done on 21 and 28 July. Plant vigor was rated
on a 1 to 10 scale, 10 representing live and healthy plants and 1 representing dead plants.
Twelve cores of soil, 2.5-cm-diam × 25-cm-deep, were collected from the center of each
plot before planting brassica’s (24 February), at harvest of brassica’s (11 June), and at planting
(7 July) and harvest (25 August) of zucchini. Nematodes were extracted from a 150-cm3 soil
sub-sample using a centrifugal sugar flotation technique, except at planting when they were
extracted in Baermann pans (to capture only active nematodes). On 1 August (at flowering stage)
an early root gall evaluation was done on three plants per plot using a 0 to 10 scale, whereby, 0 =
no galls, 1 = very few small galls, 2 = numerous small galls, 3 = numerous small galls of which
some are grown together, 4 = numerous small and some big galls, 5 = 25 % of roots severely
galled, 6 = 50 % of roots severely galled, 7 =75 % of roots severely galled, 8 = no healthy roots
but plant is still green, 9 = roots rotting and plant dying, 10 = plant and roots dead. Again
following final harvest on 25 August ten plants per plot were evaluated for root galls using that
same scale.
Soil fungal assays were done using a subsample from the samples taken for nematode
assessments. Aliquots of soil were removed from each subsample and air dried for 24 hours.
Five grams of soil were added to 100 ml of 0.3% water agar and mixed thoroughly. Immediately
afterward, 1 ml of soil/agar was removed and mixed with 20 ml of 0.3% water agar. One ml of
the first preparation was dispensed and spread evenly onto a petri plate containing an oomyceteselective medium (pimaricin-ampcillin-rifampicin-PCNB) for isolation of Pythium and
Phytophthora spp. One ml of the second preparation was dispensed and spread onto a petri plate
containing a Fusarium-selective medium (peptone-PCNB). Treatment efficacy was also
evaluated by placing propagules (fungus-infested wheat seed and/or toothpicks) of Fusarium
solani and Rhizoctonia solani AG-4 into nylon mesh bags (sachets) and burying them in plots
prior to application of treatments. Sachets were removed at transplanting and survival of the
fungal propagules was evaluated on semi-selective media, whereas nutsedge germination was
evaluated on wet tissue paper.
All zucchini fruits were hand-harvested from the 15-ft center area of each bed (15 plants
per plot). Each harvest was separated into marketable and cull fruits, counted, and weighed.
There were a total of four harvests, on 4, 7, 13 and 18 August.
All data collected was analyzed with an analysis of variance (P = 0.05) and means were
separated using Duncan's Multiple range test.
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Summary
Many Brassica covers crops showed poor growth (Fig. 1). Mustard ‘Curly Leaf’, turnip
‘White Egg’ and spinach were therefore excluded from the remainder of the test. High build-up
of root-knot nematodes in the soil was observed following turnip, kale and mustard (Table 2).
Wheat showed significantly lower nematode soil populations than the former three Brassica’s.
Root-knot nematode populations at plant of zucchini, after incorporation of Brassica cover crops,
were significantly greater for turnip, kale, mustard and rutabaga as compared to radish and wheat
+ vapam (Table 3). Stubby root nematodes were highest following rutabaga and free-living
nematodes were lowest following wheat + vapam. By harvest of zucchini, soil populations of
root-knot and free-living nematodes were still significantly less following wheat + vapam (Table
4). Radish had lowest populations among Brassica cover crops.
High root-knot nematode pressure was noted on zucchini and root gall indices (0-10)
ranged from 0.4 to 7.9 at 3 weeks after planting (WAP) and from 2.3 to 9.3 at 7 WAP (Table 5).
Highest gall indices were noted following turnip and kale, and lowest following vapam and
radish (Table 5).
Plant vigor at 1 and 2 WAP was significantly less following turnip and kale (Table 5).
Zucchini yields were significantly affected by root-knot nematode. Lowest yields were recorded
following kale and turnip and highest yields following wheat + vapam and radish (Table 6).
Fusarium solani and Rhizoctonia solani propagules in sachets showed lowest survival
following wheat + vapam, wheat, radish and mustard (the latter for Fusarium only) (Fig. 2). Soil
populations of fungi (Fusarium, Pythium and total fungi) were greatest following kale and
lowest following wheat + vapam (Table 7).
Acknowledgments
The authors wish to thank Jimmy Laska, Chris Williamson, Tonya Jo Cravens, Unessee
Hargett, Don Hickey, Lewis Mullis and Bryan Horton for technical support.
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Table 1. Populations of plant-parasitic and free-living nematodes before planting of Brassica’s, spring 2003,
Black Shank Farm Tifton, GA.
Plant-parasitic nematode soil populations
Cover crop
Mustard FL Broadleaf
Root-knot
70
92
Total
72
108
Bacteriovores
536 a
Fungiovores
108
12
Stubby
2
4
502 ab
Turnip Purple Top White G
152
8
0
160
Turnip White Egg
142
2
10
72
8
Rutabaga
Kale
42
122
Spinach
Wheat
Mustard Curly leaf
Radish
Ring
0
Free-living nematode soil populations
86
Omniovores
68 ab
156 a
Total
712
744
534 ab
78
100 ab
712
154
560 ab
86
142 ab
788
2
82
352 b
48
50 b
450
4
8
0
2
46
132
526 ab
596 a
90
58
146 a
104 ab
762
758
100
12
4
116
500 ab
72
168 a
740
140
3
2
145
610 a
108
92 ab
810
Root-knot nematode (Meloidogyne spp.); Ring nematode (Criconemoides); Stubby root nematode (Trichodoridae); Free-living
nematodes = non-parasitic nematodes.
Data are means of five replications. Means in the same column followed by the same letter are not different (P = 0.05)
according to Duncan’s multiple range test. No letters indicate non-significant difference.
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Table 2. Populations of plant-parasitic and free-living nematodes at harvest of Brassica’s, spring 2003,
Black Shank Farm Tifton, GA.
Plant-parasitic nematode soil populations
Cover crop
Mustard FL Broadleaf
Root-knot
458 ab
896 a
Ring
32
Radish
Free-living nematode soil populations
Total
514
966
Bacteriovores
346 a
Fungiovores
34 a
14
Stubby
24 b
56 ab
344 a
116 bc
54
44 ab
214
Rutabaga
144 bc
42
130 a
Kale
552 ab
12
99 c
14
Turnip Purple Top White G
Wheat
10 b
Omniovores
174 ab
126 ab
Total
554 ab
480 bc
396 a
20 ab
90 b
506 abc
316
414 a
20 ab
160 ab
594 ab
114 a
678
524 a
20 ab
220 a
764 a
25 b
138
226 b
17 ab
128 b
388 c
Root-knot nematode (Meloidogyne spp.); Ring nematode (Criconemoides); Stubby root nematode (Trichodoridae); Free-living
nematodes = non-parasitic nematodes.
Data are means of five replications. Means in the same column followed by the same letter are not different (P = 0.05)
according to Duncan’s multiple range test. No letters indicate non-significant difference.
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Table 3. Populations of plant-parasitic and free-living nematodes at planting of zucchini following incorporation of different
Brassica cover crops, spring 2003, Black Shank Farm Tifton, GA.
Plant-parasitic nematode soil populations
Cover crop
Mustard FL Broadleaf
Turnip Purple Top White G
Radish
Rutabaga
Kale
Wheat
Wheat + vapam
Ring
3
Stubby
5b
Free-living nematode soil populations
Bacteriovores
415 ab
Fungiovores
52
70
Omniovores
25 a
10 ab
Total
492 ab
616 a
536 a
Root-knot
403 ab
558 a
0
14 b
Total
411 ab
572 a
58 cd
6
8b
72 c
420 ab
68
24 ab
512 ab
285 ab
470 a
92 bc
8
0
4
78 a
4b
2b
371 ab
474 ab
98 bc
550 a
376 ab
402 ab
65
18
84
30 ab
16 ab
50 a
645 a
410 ab
536 a
22 d
2
0b
24 d
126 b
24
4b
154 b
Root-knot nematode (Meloidogyne spp.); Ring nematode (Criconemoides); Stubby root nematode (Trichodoridae); Free-living nematodes = non-parasitic
nematodes.
Data are means of five replications. Means in the same column followed by the same letter are not different (P = 0.05) according to Duncan’s multiple
range test. No letters indicate non-significant difference.
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Table 4. Populations of plant-parasitic and free-living nematodes at harvest of zucchini following incorporation of
different Brassica cover crops, spring 2003, Black Shank Farm Tifton, GA.
Plant-parasitic nematode soil populations
Cover crop
Root-knot
Ring
Free-living nematode soil populations
Stubby
Total
Bacteriovores
Fungiovores
Omniovores
Total
Mustard FL Broadleaf
2073 ab
15
17
2105
1287 a
32 ab
57 b
1376 a
Turnip Purple Top White G
3448 a
2
4
3454
1318 a
6b
66 ab
1390 a
742 b
18
6
766
1324 a
42 a
62 b
1428 a
Rutabaga
1663 ab
23
13
1699
725 ab
10 b
80 ab
815 ab
Kale
3274 ab
2
8
3284
1036 ab
4b
70 b
1110 ab
Wheat
1716 ab
12
6
1734
1516 a
14 ab
166 a
1580 a
60 c
0
5
65
465 b
10 b
37 b
512 b
Radish
Wheat + vapam
Root-knot nematode (Meloidogyne spp.); Spiral nematode (Helicotylenchus spp.); Stubby root nematode (Trichodoridae); Free-living nematodes = nonparasitic nematodes. Data are means of five replications. Means in the same column followed by the same letter are not different (P = 0.05) according to
Duncan’s multiple range test. No letters indicate non-significant difference.
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Table 5. Plant vigor and root-gall indices of zucchini following incorporation of different Brassica cover crops, spring 2003,
Black Shank Farm Tifton, GA.
Plant vigor a
Gall index b
(1-10)
(0-10)
Cover crop
Mustard FL Broadleaf
Stand count
25
Turnip Purple Top White Globe
25
At 1week
7.0 bc
6.2 c
Radish
25
8.1 ab
8.9 abc
3.3 d
5.1 b
Rutabaga
Kale
26
26
8.1 ab
6.1 c
8.4 abc
6.0 de
3.8 cd
6.9 ab
7.8 ab
8.6 a
Wheat
24
8.2 ab
9.0 ab
4.9 bcd
7.9 ab
Wheat + vapam
26
9.4 a
9.5 a
0.4 e
2.1 c
a
b
At 2 weeks
6.9 cde
At 3 weeks
6.0 abcd
At 8 weeks
8.0 ab
5.4 e
7.9 a
9.3 a
Vigor was done a 1-10 scale with 10= live and healthy plants and 1=dead plants.
Root Gall Index 0-10 scale whereby, 0 = no galls, 1 = very few small galls, 2 = numerous small galls, 3 = numerous small galls of which some are grown
together, 4 = numerous small and some big galls, 5 = 25 % of roots severely galled, 6 = 50 % of roots severely galled, 7 =75 % of roots severely galled, 8
= no healthy roots but plant is still green, 9 = roots rotting and plant dying, 10 = plant and roots dead.
Data are means of five replications. Means in the same column followed by the same letter are not different (P = 0.05) according to Duncan’s multiple
range test. No letters indicate non-significant difference.
b
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Table 6. Marketable yield of zucchini following incorporation of different Brassica cover crops, spring 2003,
Black Shank Farm Tifton, GA.
Number of marketable fruits
Cover crop
Yield 1
Yield 2
Mustard FL Broadleaf
2 cd
6 ab
Turnip Purple Top White G
1d
Radish
Yield 4
Total
Yield 1
Yield 2
Yield 3
4 abc
6 ab
18 bc
2.2 bcd
8.9 ab
5.5 ab
6.7 abc
23.3 ab
4b
3 bc
8 ab
16 c
0.4 d
4.6 b
4.3 ab
7.4 abc
16.7 b
6 abc
9 ab
7 ab
9 ab
31 a
7.2 ab
10.0 ab
7.9 a
9.2 ab
34.3 a
Rutabaga
6 abc
8 ab
7a
8 ab
29 a
5.5 abc
7.5 b
9.1 a
9.0 ab
31.0 a
Kale
2 cd
5b
2c
6 ab
15 c
1.2 cd
5.7 b
2.4 b
5.9 bc
15.2 b
Wheat
5 abc
8 ab
5 abc
8 ab
27 ab
6.3 ab
10.0 ab
7.3 ab
7.8 abc
31.4 a
9 ab
5 abc
6 ab
31 a
9.8 a
10.7 ab
6.7 ab
5.5 bc
32.8 a
Wheat + vapam
10 a
Yield 3
Weight of marketable fruits (lbs)
Yield 4
Total
* per 15 ft bed length
Data are means of five replications. Means in the same column followed by the same letter are not different (P = 0.05) according to Duncan’s multiple
range test. No letters indicate non-significant difference.
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Table 7. Effects of Brassica cover crops on fungal soil populations (CFU/g soil), spring-summer 2003,
Black Shank Farm Tifton, GA
At planting Brassica’s (Feb. 25)
Cover crop
At planting Zucchini (July 7)
Pythium
Fusarium
Total
Pythium
Fusarium
Total
Mustard FL Broadleaf
20
6976
14768
23 ab
10420 b
13900 bc
Turnip Purple Top White Globe
14
9
5072
4992
11008
13 b
7360 c
12080 c
14576
26 a
10304 b
17040 bc
9
6608
13648
13 b
14300 a
19620 ab
Kale
14
5072
11008
17 ab
15568 a
22624 a
Wheat
12
6576
14900
23 ab
8848 bc
13456 c
-
-
-
16 ab
2700 d
4040 d
Radish
Rutabaga
Wheat + vapam
Data are means of five replications. Means in the same column followed by the same letter are not different (P = 0.05) according to Duncan’s multiple
range test. No letters indicate non-significant difference.
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Fig. 1. Soil cover (%) of different Brassica cover crops, June 2003, Black Shank Farm Tifton, GA.
% soil cover
100
80
60
40
20
0
Turnip
'White
Egg'
Mustard Mustard Rutabaga Turnip
'Curly
'FL BL'
'PTWG'
Leaf'
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Kale
Radish
Fig. 2. Effects of Brassica cover crops on fungal propagule survival following bio- and chemical
fumigation, spring-summer 2003, Black Shank Farm Tifton, GA
F U S A R IU M S O L A N I
Propagule survival (%)
60
a
a
50
a
40
30
20
10
b
b
b
b
0
T u r n ip
K a le
M u s ta r d
R u ta b a g a
R a d is h
W he a t
W he a t +
va p a m
R H IZ O C T O N IA S O L A N I
100
Propagule survival (%)
90
80
a
a
a
ab
70
b
60
b
50
b
40
30
20
10
0
T u r n ip
K a le
M u s tar d
Ru ta b a g a
-31-
R a d is h
W h e at
W h e at +
vap am