Management Strategies for Lettuce
Drop in Arizona and California
Barry Pryor
Department of Plant Pathology
University of Arizona
Lettuce Drop
The Pathogens:
Sclerotinia minor and S.
sclerotiorum
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•
•
•
over 1500 recorded hosts
cottony rot, white mold, watery rot on
fruits, flowers, leaves, stems, and roots
growth is favored by moist conditions
both species produce long-lived resting
structures known as sclerotia
S. sclerotiorum
S. minor
Lettuce drop
disease cycle
Eruptive Germination
S. sclerotiorum
S. minor
Lettuce drop
disease cycle
Apothecium discharging
ascospores
Integrated disease
management of lettuce drop
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host resistance
cultural practices
chemical control
biocontrol
Host Resistance
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No commercially acceptable cultivar exist
Most breeding programs have focused on
S. minor
-“slow-dying” phenotypes have been
described
Little progress has been made on
resistance to S. sclerotiorum
Cultural Practices
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„
Crop rotation
-broccoli rotation for control of S. minor
-fallow rotation for control of S. sclerotiorum
Irrigation
water management is key to reduced disease
-furrow…sprinkler…subsurface drip
Chemical control of lettuce drop
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„
iprodione (Rovral)
vinclozolin (Ronilan)
dicloran (Botran)
boscolid (Endura)
2003 Yuma Fungicide Trials
Treatment
Rovral 4F
Ronilan 50DF
Endura 70WG
Serenade AS
Pristine 38WG
Botran 5F
Endura 70WG
Pristine 38WG
Botran 5F
Switch 62.5WG
Endura 70WG
Switch 62.5WG
Contans
Contans
Non-treated control
Rate
lb a.i./A
S. minor S. slerotiorum
1.0
1.0
0.35
4.0 qt prod.
0.4
1.87
0.45
0.45
3.75
0.43
0.35
0.56
4.0 lb prod.
2.0 lb prod.
--------
6.4
9.8
10.0
10.0
10.2
10.8
11.0
11.0
11.0
11.6
11.8
12.4
12.6
16.8
23.2
29.2
21.2
16.6
27.4
27.8
24.6
17.2
19.8
28.2
27.6
37.2
30.0
12.8
18.2
37.8
*each plot contained approx. 55 plants
Benefits of biocontrol strategies
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complement other disease management
strategies
development of resistance unlikely
cost effective
few restrictions on use
worker and environmental safety
development of specialty markets
Objectives 2002-2003
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•
•
evaluate the efficacy of commercially
available biocontrol agents for the control
of lettuce drop
evaluate the survival of biocontrol agents
in the field
evaluate the sensitivity of biocontrol
agents to Botran, Rovral, and Ronilan
Evaluate the efficacy of
commercially available biocontrol
agents for the control of lettuce drop
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„
„
Contans - Coniothyrium minitans
Soilgard - Gliocladium virens
Trichodex, Supresivit, TRI 002 -
„
Trichoderma harzianum
Companion – Bacillus subtilis
„
Sporidesmium sclerotivorum
Evaluate the efficacy of
commercially available biocontrol
agents for the control of lettuce drop
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„
„
RCBD with 4 blocks
biocontrol treatments plus one
fungicide treatment plus control
two trials - high and low density of
sclerotia- S. minor and S. sclerotiorum
two application schedules - pre-plant
application and pre-plant plus side
application
Evaluate the efficacy of
commercially available biocontrol
agents for the control of lettuce drop
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„
record incidence of lettuce drop
record number and weight of marketable
lettuce heads
tr
ol
ta
ns
co
m
so
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pa
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on
ar
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ev
it
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pr
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ie
ld
20
su
40
sh
60
pl
an
t
80
ta
ns
120
co
n
Yuma S. Sclerotiorum Trial 2003
vr
al
Tu
sa
Tr
l
ic
ho
de
x
So
ilg
ar
C
om
d
pa
ni
on
Su
pr
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ev
it
Yuma S. sclerotiorum trial 2002
ro
0
R
ov
ra
l
C
on
ta
Pl
ns
an
ts
hi
el
R
d
oo
ts
hi
el
d
20
ol
100
tr
100
on
tr
ol
40
C
120
Healthy heads/50' bed
60
B
la
nk
R
120
co
n
100
healthy heads/ 50' bed
on
tr
la
nk
ol
ov
ra
l
C
on
ta
Pl
an ns
ts
hi
el
R
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hi
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Tu
sa
Tr
l
ic
ho
de
x
So
i lg
ar
C
om
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pa
Su nio
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pr
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ev
it
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B
healthy heads/50' bed
80
bl
an
k
pl
an
ts
hi
el
d
su
pr
es
ev
it
so
ilg
ua
rd
co
m
pa
ni
on
co
n
ro
vr
al
co
n
bl
an
k
healthy heads/ 50' bed
Yuma Biocontrol Trials
Yuma S. minor trial 2002
80
60
40
20
0
Yuma S. Minor Trial 2003
120
100
80
60
40
20
0
Imperial Valley Biocontrol Trials
Holtville S. minor trial 2003
140
120
120
healthy heads/50' bed
140
100
80
60
40
20
100
80
60
40
20
pa
ni
on
co
m
pl
an
ts
hi
el
d
ta
ns
co
n
ro
vr
al
tr
ol
co
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pa
ni
on
co
m
pl
an
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hi
el
d
ta
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co
n
ro
vr
al
tr
ol
co
n
bl
an
k
0
0
bl
an
k
healthy heads/50' bed
Holtville S. sclerotiorum trial 2003
Blank
Untreated control
Rovral
Contans
Blank
Untreated control
Rovral
Contans
Long-term Objective
To develop biocontrol strategies for
desert winter lettuce production that
are compatible with chemical and
cultural management strategies