Destruction of sclerotia of Sclerotinia
minor and S. sclerotiorum in wet soil
Michael E. Matheron
Extension Plant Pathologist
Yuma Agricultural Center
Sclerotinia drop of lettuce
Sclerotinia minor S. sclerotiorum
Conditions that favor
Sclerotinia drop of lettuce
• High population of sclerotia in soil
• Moist soil
• Sclerotia of both species can survive up to 8-10
years in soil
– Sclerotium germination decreases with time
and depth of burial
• Both fungi grow from 50 to 77 F and optimally
at 68 F
Incorporation of crop residue and
sclerotia into soil
Management of Sclerotinia
drop in lettuce
• Sclerotia are the survival structures of the
pathogen, which remain dormant in soil
until activated by the presence of lettuce
• Disease control measures for Sclerotinia
drop focus on destroying or inactivating
these sclerotia
Infection zone for Sclerotinia minor
3 inches
1 inch
Ascospores of S. sclerotiorum
How can sclerotia be destroyed
or inactivated?
• Destruction in wet soil
Laboratory studies: Effect of temperature
and moisture on viability of sclerotia
• Sclerotia of S. minor or S. sclerotiorum were
buried in a dry field soil (7-56-37 sand-silt-clay)
in a series of containers 3 inches in diameter
and 4 inches deep
• Sclerotia in soil were incubated at 58, 68, 77,
86, 95 and 104 F for 1 to 4 weeks
• Soil in containers was either kept dry or
saturated with water
• After burial in soil for 1, 2, 3, or 4 weeks,
sclerotia were tested for viability after surfacesterilizing with bleach and alcohol by plating
onto acidified potato dextrose agar
Laboratory studies
Mycelial growth from sclerotia on PDA
Germination of sclerotia in wet soil
(Average for the 1 to 4 week study period)
100
Constant temperature incubator study
80
60
40
20
0
Cont.
15 58F 20 68F
S. minor
25 77F 30 86F
S. sclerotiorum
35 95F
40 104F
Germination of sclerotia in dry soil
(Average for the 1 to 4 week study period)
100
Constant temperature incubator study
80
60
40
20
0
Cont.
15 58F 20 68F 25 77F
S. minor
30 86F
S. sclerotiorum
35 95F
40 104F
Field studies: Effect of temperature
and moisture on viability of sclerotia
• Sclerotia of S. minor or S. sclerotiorum were placed
at a depth of 0 or 2 inches ( 5 cm) within furrows
• Soil was either irrigated every 7 to 14 days or
maintained in a dry state
• Sclerotia were collected after 2, 4, 6 and 8 weeks,
surface-sterilized and tested for ability to
germinate on potato dextrose agar
• This test was performed when mean soil
temperature was 26 C (79 F) and 33 C (91 F)
Field studies: wet and dry soil
Effect of mean soil temperature and
soil depth on germination of sclerotia
(After 8 weeks)
45
40
35
30
25
20
15
10
5
0
Field trials - irrigated
S.m. 79F
S.m. 91F
Soil surface
S.s. 79F
5 cm depth
S.s. 91F
S. minor: germination of sclerotia in dry vs.
irrigated soil (7-14 day interval)
(after 8 weeks)
100
Field trials – 2 inch depth
80
60
40
20
0
cont.
Control
2 wk
4 wk
Dry soil 100F
6 wk
8 wk
Irrigated soil 88-91F
S. sclerotiorum: germination of sclerotia in
dry vs. irrigated soil (7-14 day interval)
(after 8 weeks)
80
Field trials - 2 inch depth
70
60
50
40
30
20
10
0
cont.
Control
2 wk
4 wk
Dry soil 100F
6 wk
8 wk
Irrigated soil 88-91F
Conclusions
• In irrigated soil, sclerotia of S. minor are
inactivated at a greater rate than S.
sclerotiorum
• Sclerotia of both pathogens survive much
better in dry soil than in irrigated soil
Effect of irrigation frequency on
germination of sclerotia
Once per week
compared to
3 times a week
Effect of irrigation frequency on
germination of sclerotia
(after 8 weeks)
90
80
70
60
50
40
30
20
10
0
Field trials – Temp (73 to 80 F)
cont.
S.m. 0cm
Control
S.m. 5cm
Irrigated once/wk
S.s. 0cm
S.s. 5cm
Irrigated 3x/wk
Effect of soil flooding on germination
of sclerotia
Soil flooding experiments
Effect of soil flooding on sclerotia germination at
soil depths of 0, 10 and 20 cm
(after 8 weeks)
Germination of control sclerotia was 90%
14
12
Field trials
Temp (86 to 91F)
10
8
6
4
2
0
S.m. 0
S.m. 10
S.m. 20
1 week
S.s. 0
2 weeks
S.s. 10
3 weeks
S.s. 20
Land preparation activities in July
and August prior to lettuce seeding