Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol19.Issue1-2.Feb2001:
Soil Biology & Biochemistry 33 (2001) 137±143
www.elsevier.com/locate/soilbio
Increasing soil temperature to reduce sclerotial viability of
Sclerotium cepivorum in New Zealand soils
K.L. McLean*, J. Swaminathan, A. Stewart
Soil, Plant and Ecological Sciences Division, Plant Sciences Group, P.O.Box 84, Lincoln University, Canterbury, New Zealand.
Received 2 November 1999; received in revised form 13 March 2000; accepted 7 June 2000
Abstract
A preliminary laboratory-based trial indicated Sclerotium cepivorum sclerotial viability could be reduced from .96±10.7% after 28 d at
208C and to 0% after 16 d at 308C. Soil solarisation signi®cantly reduced S. cepivorum sclerotial viability in two separate trials in Canterbury
(Wakanui silt loam soil), New Zealand (to 40.2 and 53.3%, respectively) when soil was covered with clear 50 mm thick polythene for
4 weeks. Sclerotial viability further decreased in two New Zealand sites; Canterbury (to 8.7%) and Blenheim (shallow silt loam soil) (to 0%)
when the soil was solarised for an 8 week period. Solarisation increased the soil temperature by 6±78C in Canterbury, although the highest
temperatures were recorded in Blenheim. Microorganisms isolated from the recovered sclerotia included species of Trichoderma, Verticillium, Fusarium, Mucor, Aspergillus and four unidenti®ed bacterial species. q 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Sclerotium cepivorum; Sclerotia; Solarisation; Soil temperature; New Zealand
1. Introduction
Onion white rot is the most destructive disease of Alliumspecies worldwide and is caused by the soilborne pathogen
Sclerotium cepivorum Berk. In the past, soil applications of
dicarboximide and triazole systemic fungicides have given
adequate control of onion white rot but, in recent years, their
effectiveness has declined due to enhanced microbial degradation of the chemicals (Slade et al., 1992). The build up of
S. cepivorum sclerotia in the soil may also contribute to the
decline in fungicide effectiveness. A control measure such
as soil solarisation could be used to relieve disease pressure
by decreasing the number of viable sclerotia in the soil. Soil
solarisation could be integrated into a disease management
programme for effective control of onion white rot.
Soil solarisation was pioneered in Israel (Katan et al.,
1976) and California (Pullman et al., 1979). The technique
involves levelling the soil with minimal soil compaction
before thorough wetting, which increases the thermal sensitivity of the soil micro¯ora and fauna as well as increasing
heat transfer or conduction in the soil (Mahrer et al., 1984).
The soil is then covered with thin clear polyethylene sheet* Corresponding author. Tel.: 164-3-325-2811, ext. 8157; fax: 164-3325-3843.
E-mail address: [email protected] (K.L. McLean).
ing during the hottest months of the year. Increases in soil
temperature can then eliminate or at least reduce soilborne
pathogen inoculum as well as insects, nematodes and weed
seeds. Clear polythene (50 mm thick) is most commonly
used as coloured polythene tends to absorb heat rather
than allow it to be transmitted into the soil (DeVay, 1991).
Successful reductions in S. cepivorum sclerotial viability
have been reported from Egypt (Satour et al., 1989; 1991),
Spain (Basallote-Ureba and Melero-Vara, 1993) and Australia (Porter and Merriman, 1985). New Zealand has a climate
that is marginal for soil solarisation compared to many parts
of the world where it is practiced. A preliminary laboratorybased soil temperature trial indicated the range of temperatures, which were lethal to S. cepivorum sclerotia. These
results were in agreement with results obtained by Adams
(1987) in the United States of America. When these temperatures were compared with soil temperatures from a soil solarisation weed control trial (Alexander, 1990) in Canterbury,
the results indicated that soil solarisation may be a viable
control option for onion white rot in New Zealand.
The use of soil solarisation to control onion white rot is
novel in New Zealand. This paper reports ®rst on the results
of a preliminary laboratory-based soil temperature trial
examining the in¯uence of constant soil temperatures on
sclerotial viability. Then, results are given of three soil
solarisation trials undertaken in Canterbury (1995, 1996,
0038-0717/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0038-071 7(00)00119-X
138
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
1997) and one trial in Blenheim (1997), which are two of
New Zealand's main onion and garlic growing regions.
2. Methods and materials
2.1. Production of inoculum
Sclerotia of S. cepivorum (E68-isolated from an infected
onion, Pukekohe, Auckland, NZ in 1990 (trial 1) and SC 3isolated from an infected onion, Pukekohe, Auckland, NZ in
November 1996 (trials 2, 3 and 4)) were produced on whole
wheat grains (Alexander and Stewart, 1994) and harvested
after 8 weeks using progressive wet sieving through 850 and
500 mm sieves. Only sclerotia retained on the 500 mm sieve
were used in the trials and the sclerotia were air dried on
sterile Whatman no. 1 ®lter paper for 24 h. A sample of 100
sclerotia were surface sterilised in 0.25% NaOCl for 1 min,
washed in three changes of sterile distilled water (SDW),
touched to Whatman no. 1 ®lter paper using sterile forceps
to absorb excess liquid and placed onto potato dextrose agar
(PDA) droplets in Petri dishes. The sclerotia were incubated
at 208C in the dark and examined daily for 10 d to determine
viability. Sclerotial viability for all trials was recorded as
.96%.
2.2. The in¯uence of constant soil temperatures on sclerotial
viability
Sclerotia were counted into lots of 100. Each lot was
placed in a polyester mesh bag (Scarpa Filtration Ltd.,
Auckland, New Zealand; 85 mm pore size, 10 £ 10 cm2
with a marker attached for location purposes.
Plastic containers 18 £ 18 £ 19 cm3 were ®lled to
within 1 cm of the top with coarsely sieved, air-dried,
unsterile Wakanui silt loam soil (bulk density:
1.01 g cm 23, McLaren and Cameron, 1996). The soil was
wetted to 25% and maintained throughout the course of the
trial to simulate ®eld conditions. Three bags of sclerotia
were buried in each plastic container at a depth of 10 cm.
Although the edges of each bag overlapped, the sclerotia
were positioned in each bag to ensure they were sandwiched
between soil. Seven plastic containers were maintained at
each of the following temperatures: 20, 30, 35, 40, 45 and
508C, a range of temperatures similar to those used by other
researchers (Porter and Merriman, 1983; Adams, 1987). A
reference treatment was included where sclerotia were
maintained at 208C in a glass vial in the dark for the duration
of the trial. For each temperature, three bags of sclerotia
were randomly selected at each of seven assessment
times: 6, 12 h, 1, 2, 8, 16 and 28 d.
The sclerotia were recovered from the bags and surface
sterilised in 0.25% NaOCl for 1 min, washed in ®ve changes
of SDW and blotted dry on sterile Whatman no. 1 ®lter
paper. The sclerotia were then placed individually using
sterile forceps onto isolated PDA droplets in Petri dishes.
The dishes were sealed with polythene wrap and incubated
in the dark at 208C. Sclerotial germination was recorded
every second day for 10 d. The percentage viability of sclerotia, relative to the number buried, was determined. Results
were analysed using an analysis of variance (ANOVA) with
temperature and length of exposure period as variables. The
appropriateness of an ANOVA for this data was checked by
visual inspection of a residual plot. This plot adequately
con®rmed the normality and homogeneity of the variance
of the data. A Fishers Least Signi®cant Difference test was
used for pairwise comparisons. Probit analysis was
performed to determine the exposure period required at
each temperature to reduce sclerotial viability by 95%
(LD95). The data were converted to thermal days
(temperature £ length of exposure period in days) to
compare the effects of short exposure periods at high
temperatures with longer exposure periods at lower
temperatures on sclerotial viability.
2.3. Soil solarisation trial design
For each of the four trials conducted, sclerotia were counted
into lots of 50. Each lot was placed in a polyester mesh bag as
previously described. A reference treatment where sclerotia
were maintained in a glass vial at 208C in the dark for the
duration of the trials was included for each trial.
Trials 1 and 2 each ran for four week periods over two
consecutive summers (Trial 1: 13/12/95±10/1/96, Trial 2:
11/1/97±11/2/97) and were conducted at a ®eld site at
Lincoln University, Canterbury, New Zealand in Wakanui
silt loam soil. In trial 1, each of the eight plots 3 £ 6 m2
contained six bags of sclerotia buried equidistant from each
other. Three of the bags were randomly selected and buried
at a depth of 10 cm and the remaining three bags were
buried at 20 cm. In trial 2, each of the eight plots 2 £
3 m2 contained three bags of sclerotia buried equidistant
from each other at a depth of 10 cm.
Trials 3 and 4 ran for two 8 week periods (10/12/97±2/2/
98 and 15/12/97±12/2/98, respectively). Trial 3 was
conducted at a ®eld site at Lincoln University, Canterbury,
New Zealand and trial 4 was conducted at the Marlborough
Research Centre, Blenheim, New Zealand in shallow silt
loam soil. Each of the eight plots 3 £ 3 m2 contained
three bags of sclerotia buried equidistant from each other
at a depth of 10 cm. The polyester mesh bags contained 20 g
sieved Wakanui silt loam soil (#200 mm particle size) and
20 g quartz sand in addition to the 50 sclerotia.
Following burial of the sclerotia, all plots were irrigated
to saturation and on the following day, 50 mm thick transparent polythene (Permathane Plastics, Auckland, New
Zealand) (Satour et al., 1989) was laid over four randomly
selected plots. The edges of the polythene were buried in the
soil to a depth of 10 cm. The remaining four uncovered plots
were sprinkler irrigated once a week and weeds were
removed by hand.
For trial 1, temperature sensors (Philips KTY83-110)
encased in stainless steel tubes were placed in the soil to a
139
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
Table 1
The mean percentage of viable Sclerotium cepivorum sclerotia recovered from soil at a range of temperatures after selected exposure periods in the laboratory
n 300
Temperature (8C)
Days
0.25
Reference
20
30
35
40
45
50
0.5
100
100
90.3
97.0
31.0
13.7
0
a
a
a
a
a
ef
gh
h
95.0
96.7
99.0
100
3
0
0
1
a
a
a
a
h
h
h
88.3
58.3
46.0
32.0
0
0
0
2
a
cd
e
ef
h
h
h
93.0
70.3
27.0
33.7
2.3
0
0
8
a
bc
fg
ef
h
h
h
91.0
84.0
36.0
0
0
0
0
16
a
ab
ef
h
h
h
h
98.7
47.0
0
0
0
0
0
28
a
de
h
h
h
h
h
98.0
10.7
0
0
0
0
0
a
gh
h
h
h
h
h
a
Mean values followed by the same letter do not differ signi®cantly P , 0:05 within each column and across each row according to a Fishers Least
Signi®cant Difference test, LSD 16.64.
depth of 10 and 20 cm in each of the eight plots. The soil
temperature was recorded every 30 min using a ®eld data
logger (Datataker DT 600) for the duration of the trial. For
trials 2, 3 and 4 a Tiny Tag temperature logger with internal
sensor (Gemini Dataloggers, Chichester, UK) was enclosed
in a plastic container, wrapped in a plastic bag and buried
10 cm deep in one solarised and one non-solarised plot.
Temperatures were recorded every 30 min (trial 2) and
every 3 h (trials 3 and 4).
Sclerotia were retrieved from the bags at trial completion
and assessed for viability as previously described. Sclerotial
germination was recorded every second day and percentage
viability of sclerotia, relative to the number buried, was
determined. Any other microorganisms growing out into
the agar droplets were also noted and identi®ed where possible. For trial 1, the results were analysed using an ANOVA
with solarised and non-solarised treatments and depth of
burial as variables. For trials 2, 3 and 4 the results were
analysed using an ANOVA with solarised and non-solarised
treatments as variables.
3. Results
3.1. The in¯uence of constant soil temperatures on sclerotial
viability
There was a signi®cant reduction P , 0:05 in sclerotial
Table 2
Temperature required at selected exposure periods to reduce Sclerotium
cepivorum sclerotial viability by 95% in laboratory soil (modelled by probit
analysis)
Temperature (8C)
Exposure period (days) (LD95)
47.4
45.2
41.9
40.7
35.0
24.8
24.3
0.25
0.5
1
2
8
16
28
viability when the sclerotia were incubated at different
temperatures and also after varying lengths of exposure
periods (Table 1). There was also a signi®cant interaction
between the soil temperatures and the length of exposure
period. There was no signi®cant reduction P , 0:05 in
sclerotial viability in the reference treatment for the duration
of the trial. At 208C, sclerotial viability was signi®cantly
reduced P , 0:05 after 28 d incubation with only 10.7%
of the sclerotia remaining viable. At 308C, sclerotial viability was reduced P , 0:05 to 0% after incubation for 16 d.
At 358C, 8 d incubation was required to reduce sclerotial
viability to 0%. Incubation at 408C for 6 h was suf®cient to
reduce P , 0:05 sclerotial viability to 31% with a reduction to 0% after only 1 d. At 458C, a 12 h incubation period
reduced P , 0:05 sclerotial viability to zero and at 508C,
sclerotial viability was lost within the ®rst 6 h incubation
period.
Short exposure periods at higher temperatures reduced
sclerotial viability by 95% (LD95) (Table 2). In contrast,
longer exposure periods of 16±28 d were required to reduce
sclerotial viability by 95% at lower temperatures (24.8 and
24.38C, respectively).
When the data were modelled by probit analysis, equivalent thermal day periods did not result in equivalent losses in
sclerotial viability. For example, after exposure to 358C for
8 d, sclerotial viability was reduced to 21% but only to 52%
after exposure to 208C for 14 d, for the same thermal day
period (280). Similarly, after 2 d at 408C (80 thermal days),
only 3% of the sclerotia remained viable, whereas 79% of
the sclerotia remained viable after 4 d at 208C.
3.2. Soil solarisation trials
The mean daily maximum temperatures for solarised and
non-solarised soil for trials 1, 2 and 3 are shown in Figs. 1±
3. There is no soil temperature pro®le for trial 4 as the Tiny
Tag temperature logger in the solarised plot malfunctioned
during the course of the trial and the temperature data could
not be retrieved. The maximum and mean soil temperatures
at 10 cm in the solarised soil ranged from 39.2±42.78C and
140
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
Fig. 1. The mean daily maximum soil temperature for solarised and non-solarised soil at 10 and 20 cm, Canterbury (trial 1).
24.6±28.88C, respectively. In the non-solarised soil, the
maximum and mean soil temperatures at 10 cm ranged
from 27±33.68C and 18.9±23.38C, respectively. Maximum
soil temperatures at 10 cm were higher P , 0:05 in the
solarised soil than the non-solarised soil for trials 1 and 2 but
not for trial 3. The maximum soil temperature at 20 cm for
the solarised and non-solarised soil were also signi®cantly
different P , 0:05 for trial 1. For trial 1, the maximum
temperature at 10 and 20 cm in the solarised plots differed
by a maximum of 48C on any one day (Fig. 1). In the nonsolarised plots, the maximum temperatures differed by a
maximum of 18C on any one day. Soil temperatures of
208C and greater were maintained for 1897, 484 and
777 h in solarised soil compared with 930.5, 279 and
648 h in non-solarised soil for trials 1, 2 and 3, respectively
(Table 3).
Fig. 2. The mean daily maximum soil temperature for solarised and non-solarised soil, Canterbury (trial 2).
141
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
Fig. 3. The mean daily maximum soil temperature for solarised and non-solarised soil, Canterbury (trial 3).
in the solarised plots in all four trials (Table 4). In trial 1 and
4, there was also a signi®cant reduction P , 0:05 in
sclerotial viability between the non-solarised control and
reference treatments.
A number of fungal species emerged from the sclerotia in
all trials when plated onto agar droplets. The fungi were
identi®ed as isolates of Verticillium, Penicillium, Fusarium,
Trichoderma, Aspergillus, Mucor and an isolate of Paecilomyces lilacinus (Thom) Samson. Unidenti®ed bacterial
species were also present. Both the solarised and nonsolarised sclerotia were colonised by the microorganisms
in all trials.
The results for percentage sclerotial recovery and viability for all four trials are presented in Table 4. For trial 1, the
depth at which the sclerotia were buried did not signi®cantly
affect P , 0:05 sclerotial recovery or viability, therefore,
the number of sclerotia recovered at 10 and 20 cm were
combined for analysis.
The percentage of recovered sclerotia was signi®cantly
less (P , 0.05) from the solarised plots than from the reference treatment for all trials except trial 1. In trial 1 there was
no signi®cant difference (P , 0.05) in sclerotial recovery
between the reference, non-solarised control and solarised
treatments. In trial 4, in addition to fewer P , 0:05 sclerotia being recovered from the solarised plots, fewer P ,
0:05 sclerotia were recovered from the non-solarised
control plots compared with the reference treatment.
Fragments of sclerotial rind remained in the bags and
vials, which indicated that the unrecovered sclerotia had
disintegrated.
Sclerotial viability was reduced signi®cantly P , 0:05
4. Discussion
Increases in constant soil temperature in the laboratory
caused a decrease in sclerotial viability. Sclerotia readily
decayed and lost viability when incubated for short periods
Table 3
Total number of hours recorded above selected temperatures and the maximum and mean temperature in solarised and non-solarised soil for four ®eld soil
solarisation trials at 10 cm depth (ND Ð no data)
Temperature (8C)
$ 40
$ 35
$ 30
$ 20
Maximum temperature
Mean temperature
a
Trial 1-Canterbury a
Trial 2-Canterbury
Trial 3-Canterbury
Trial 4 -Blenheim
Solarised
Non-solarised
Solarised
Non-solarised
Solarised
Non-solarised
Solarised
Non-solarised
13
140
551
1897
42.7
27.6
0
0
0
931
27
20.3
0
63
99
484
39.2
24.6
0
0
1
279
27.9
18.9
51
105
240
777
41.3
28.8
0
9
21
648
33.6
21.1
ND
ND
ND
ND
ND
ND
0
0
54
993
33
23.3
Temperatures at 20 cm are not shown.
142
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
Table 4
Percentage Sclerotium cepivorum sclerotial recovery and viability from all treatments within each of the four ®eld soil solarisation trials. Percentage values
followed by the same letter do not differ signi®cantly P , 0:05 within columns (n 300 (trial 1), n 150 (trials 2, 3 and 4))
Sclerotial treatments
% Recovery
Reference
100
Non-solarised control 96.5
Solarised
88.3
a
b
Trial 2-Canterbury 97
Trial 3-Canterbury 97/98 a
Trial 4-Blenheim 97/98 a
% Viability b
% Recovery
% Viability
% Recovery
% Viability
% Recovery
% Viability
100
66.3
40.2
100
94.2
59.0
98.8
88.3
53.3
90.6
74.7
46.0
88.6
70.6
8.7
96.0
61.7
29.0
89.3
49.7
0
Trial 1-Canterbury 95/96
a
a
a
a
b
c
a
a
b
a
a
b
a
a
b
a
a
b
a
b
c
a
b
c
A mixture of soil and sand was added to the bags containing the sclerotia.
Percentage sclerotial viability relative to the number of sclerotia buried.
at temperatures above 408C. Adams (1987) reported a similar result with continuous soil temperatures of 508C for 4 h
and 458C for 12 h, lethal to S. cepivorum sclerotia. The
equivalent thermal day data indicated that although higher
temperatures maintained for short periods of time may have
the same thermal days as lower temperatures maintained for
longer periods, the resulting number of viable sclerotia was
not equivalent. This has important implications in that
higher temperatures are required for a rapid decrease in
sclerotial viability. Temperatures greater than 408C were
recorded in Canterbury and it is probable that soil temperatures reached 408C in Blenheim. However, the signi®cant
reduction in S. cepivorum sclerotial recovery and viability is
more likely due to the effect of ¯uctuating sub-lethal
temperatures rather than high temperatures.
The detrimental effects of ¯uctuating sub-lethal temperatures have been well documented (Katan et al., 1976; Pullman et al., 1979; Porter and Merriman, 1983). It is most
probable that both thermal in¯uence and biological control
activity reduced sclerotial viability in these soil solarisation
trials. Fluctuating temperatures may increase sclerotial
vulnerability to soil microorganisms or increase heat resistant saprophyte populations subsequently increasing the
parasitic and lytic effects on the sclerotia (Katan et al.,
1976). The increased microbial activity could explain why
pieces of sclerotial rind remained in many of the bags.
Increased colonisation of S. cepivorum sclerotia by soil
microorganisms, following treatment with sub-lethal
temperatures has also been reported (Entwistle and Munasinghe, 1990). The presence of soil microorganisms on the
recovered sclerotia may suggest that the sclerotia were in a
weakened state. Aspergillus is reported as a possible secondary colonist of sclerotia (Phillips, 1990) and is tolerant of
high soil temperatures (Dwivedi, 1991). Bacterial species
have also been reported to colonise cracks in the sclerotial
rind of weakened sclerotia (Lifshitz et al., 1983).
With New Zealand's climate, the effects of ¯uctuating
sub-lethal temperatures are more in¯uential in reducing
sclerotial viability than high temperatures. An 8 week solarisation treatment was more effective in reducing sclerotial
viability than a 4 week period as the sclerotia were
subjected to sub-lethal temperatures for twice as long. The
lower viability from the 8 week trials might also have been
related to the addition of soil and sand to the bags. The
polyester mesh bags may have provided an insulating effect
in trials 1 and 2 that was overcome in trials 3 and 4. If an
insulating effect had occurred then it is likely that the loss of
sclerotial viability in natural populations would be higher
than the reported data.
Soil solarisation reduced sclerotial viability by 91.3 and
100% in trial 3 and 4, respectively. All sclerotia were at a
depth of 10 cm. While sclerotia are able to infect Alliumspecies when placed up to 30 cm deep in the soil, it is the
sclerotia in the top 10 cm of soil that mainly contribute to
disease spread (Crowe and Hall, 1980). Reducing sclerotial
viability in the top 10 cm of the soil would therefore ease
disease pressure in Allium crops. In addition, soil temperatures at 10 and 20 cm were almost identical and solarising a
S. cepivorum naturally infested ®eld would still subject
sclerotia at soil depths greater than 20 cm to sub-lethal
temperature ¯uctuations. While the effects would not be
as great deeper in the soil, the sclerotia may still be
weakened and more vulnerable to invasion by antagonistic
microorganisms.
Within New Zealand, the use of soil solarisation to
control onion white rot will be most suited to the garlic
growing regions in Blenheim where soil temperatures and
sunshine hours are generally higher than elsewhere in New
Zealand. Trials are now required to determine the actual
reduction in onion white rot afforded by this technique in
the ®eld. The combination of soil solarisation with biological control agents may provide more effective control of
onion white rot than the use of soil solarisation alone.
Acknowledgements
This research was supported by the Brian Mason Science
and Technical Trust.
References
Adams, P.B., 1987. Effects of soil temperature, moisture and depth on
survival and activity of Sclerotinia minor, Sclerotium cepivorum and
Sporidesmium sclerotivorum. Plant Disease 71, 170±174.
Alexander, B.J.R., Stewart, A., 1994. Survival of sclerotia of Sclerotinia
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
and Sclerotium spp. in New Zealand horticultural soil. Soil Biology &
Biochemistry 26, 1323±1329.
Alexander, R.T., 1990. The effect of solarisation on vegetable crop production. Proceedings of the 43rd NZ Weed and Pest Control Conference,
pp. 270±273.
Basallote-Ureba, M.J., Melero-Vara, J.M., 1993. Control of garlic white rot
by soil solarization. Crop Protection 12, 219±233.
Crowe, F.J., Hall, D.H., 1980. Vertical distribution of sclerotia of Sclerotium cepivorum and host root systems relative to white rot of onion and
garlic. Phytopathology 70, 70±73.
DeVay, J.E., 1991. Use of soil solarization for control of fungal and bacterial plant pathogens including biocontrol. FAO Plant Production and
Protection Paper 109, 70±93.
Dwivedi, S.K., 1991. Effect of solar heating of soil on the dynamics of soil
myco¯ora. Journal of Mycopathological Research. 29, 93±96.
Entwistle, A.R., Munasinghe, H.L., 1990. Evidence for damage in sclerotia
of Sclerotium cepivorum following sublethal heat treatment. In:
Proceedings of the Fourth International Workshop on Allium White
Rot, A.R. Entwistle (Ed.), Section 2, pp. 69±75. Neustudt/Weinstrasse,
Federal Republic of Germany. Biologische bundesanstalt fur Land- und
Forst- wirtschaft, Messeweg 11/12, D-3300 Braunschweig, Federal
Republic of Germany.
Katan, J., Greenberger, A., Alon, H., Grinstein, A., 1976. Solar heating by
polyethylene mulching for control of diseases caused by soil-borne
pathogens. Phytopathology 66, 683±688.
Lifshitz, R., Tabachnik, M., Katan, J., Chet, I., 1983. The effect of sublethal
heating on sclerotia of Sclerotium rolfsii. Canadian Journal of Microbiology 29, 1607±1610.
143
Mahrer, Y., Naot, O., Rawitz, E., Katan, J., 1984. Temperature and moisture regimes in soils mulched with transparent polythene. Soil Science
Society of America Journal 48, 362±367.
McLaren, R.G., Cameron, K.C., 1996. Soil Science, sustainable production
and environmental protection, . 2nd ed.Oxford University Press, Auckland, New Zealand.
Phillips, A.J.L., 1990. The effects of soil solarization on sclerotial populations of Sclerotinia sclerotiorum. Plant Pathology 39, 38±43.
Porter, I.J., Merriman, P.R., 1983. Effects of solarization of soil on nematode and fungal pathogens at two sites in Victoria. Soil Biology &
Biochemistry 15, 39±44.
Porter, I.J., Merriman, P.R., 1985. Evaluation of soil solarization for control
of root diseases of row crops in Victoria. Plant Pathology 34, 108±118.
Pullman, G.S., DeVay, J.E., Garber, R.H., Weinhold, A.R., 1979. Control
of soil-borne fungal pathogens by plastic tarping of soil. In: Schippers,
B., Gams, W. (Eds.). Soil-borne Plant Pathogens, pp. 439±446.
Satour, M.M., Abdel-Rahim, M.F., El-Yamani, T., Radwan, A., Grinstein,
A., Rabinowitch, H.D., Katan, J., 1989. Soil solarization in onion ®elds
in Egypt and Israel: short and long term effects. Acta Horticulturae 255,
151±159.
Satour, M.M., El-Sherif, E.M., El-Ghareeb, L., El-Hada, S.A., El-Wakil,
H.R., 1991. Achievements of soil solarization in Egypt. FAO Plant
Production and Protection Paper 109, pp. 200±212.
Slade, E.A., Fullerton, R.A., Stewart, A., Young, H., 1992. Degradation of
the dicarboximide fungicides iprodione, vinclozolin and procymidone
in Patumahoe clay loam soil, New Zealand. Pesticide Science 35, 95±
100.
www.elsevier.com/locate/soilbio
Increasing soil temperature to reduce sclerotial viability of
Sclerotium cepivorum in New Zealand soils
K.L. McLean*, J. Swaminathan, A. Stewart
Soil, Plant and Ecological Sciences Division, Plant Sciences Group, P.O.Box 84, Lincoln University, Canterbury, New Zealand.
Received 2 November 1999; received in revised form 13 March 2000; accepted 7 June 2000
Abstract
A preliminary laboratory-based trial indicated Sclerotium cepivorum sclerotial viability could be reduced from .96±10.7% after 28 d at
208C and to 0% after 16 d at 308C. Soil solarisation signi®cantly reduced S. cepivorum sclerotial viability in two separate trials in Canterbury
(Wakanui silt loam soil), New Zealand (to 40.2 and 53.3%, respectively) when soil was covered with clear 50 mm thick polythene for
4 weeks. Sclerotial viability further decreased in two New Zealand sites; Canterbury (to 8.7%) and Blenheim (shallow silt loam soil) (to 0%)
when the soil was solarised for an 8 week period. Solarisation increased the soil temperature by 6±78C in Canterbury, although the highest
temperatures were recorded in Blenheim. Microorganisms isolated from the recovered sclerotia included species of Trichoderma, Verticillium, Fusarium, Mucor, Aspergillus and four unidenti®ed bacterial species. q 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Sclerotium cepivorum; Sclerotia; Solarisation; Soil temperature; New Zealand
1. Introduction
Onion white rot is the most destructive disease of Alliumspecies worldwide and is caused by the soilborne pathogen
Sclerotium cepivorum Berk. In the past, soil applications of
dicarboximide and triazole systemic fungicides have given
adequate control of onion white rot but, in recent years, their
effectiveness has declined due to enhanced microbial degradation of the chemicals (Slade et al., 1992). The build up of
S. cepivorum sclerotia in the soil may also contribute to the
decline in fungicide effectiveness. A control measure such
as soil solarisation could be used to relieve disease pressure
by decreasing the number of viable sclerotia in the soil. Soil
solarisation could be integrated into a disease management
programme for effective control of onion white rot.
Soil solarisation was pioneered in Israel (Katan et al.,
1976) and California (Pullman et al., 1979). The technique
involves levelling the soil with minimal soil compaction
before thorough wetting, which increases the thermal sensitivity of the soil micro¯ora and fauna as well as increasing
heat transfer or conduction in the soil (Mahrer et al., 1984).
The soil is then covered with thin clear polyethylene sheet* Corresponding author. Tel.: 164-3-325-2811, ext. 8157; fax: 164-3325-3843.
E-mail address: [email protected] (K.L. McLean).
ing during the hottest months of the year. Increases in soil
temperature can then eliminate or at least reduce soilborne
pathogen inoculum as well as insects, nematodes and weed
seeds. Clear polythene (50 mm thick) is most commonly
used as coloured polythene tends to absorb heat rather
than allow it to be transmitted into the soil (DeVay, 1991).
Successful reductions in S. cepivorum sclerotial viability
have been reported from Egypt (Satour et al., 1989; 1991),
Spain (Basallote-Ureba and Melero-Vara, 1993) and Australia (Porter and Merriman, 1985). New Zealand has a climate
that is marginal for soil solarisation compared to many parts
of the world where it is practiced. A preliminary laboratorybased soil temperature trial indicated the range of temperatures, which were lethal to S. cepivorum sclerotia. These
results were in agreement with results obtained by Adams
(1987) in the United States of America. When these temperatures were compared with soil temperatures from a soil solarisation weed control trial (Alexander, 1990) in Canterbury,
the results indicated that soil solarisation may be a viable
control option for onion white rot in New Zealand.
The use of soil solarisation to control onion white rot is
novel in New Zealand. This paper reports ®rst on the results
of a preliminary laboratory-based soil temperature trial
examining the in¯uence of constant soil temperatures on
sclerotial viability. Then, results are given of three soil
solarisation trials undertaken in Canterbury (1995, 1996,
0038-0717/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0038-071 7(00)00119-X
138
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
1997) and one trial in Blenheim (1997), which are two of
New Zealand's main onion and garlic growing regions.
2. Methods and materials
2.1. Production of inoculum
Sclerotia of S. cepivorum (E68-isolated from an infected
onion, Pukekohe, Auckland, NZ in 1990 (trial 1) and SC 3isolated from an infected onion, Pukekohe, Auckland, NZ in
November 1996 (trials 2, 3 and 4)) were produced on whole
wheat grains (Alexander and Stewart, 1994) and harvested
after 8 weeks using progressive wet sieving through 850 and
500 mm sieves. Only sclerotia retained on the 500 mm sieve
were used in the trials and the sclerotia were air dried on
sterile Whatman no. 1 ®lter paper for 24 h. A sample of 100
sclerotia were surface sterilised in 0.25% NaOCl for 1 min,
washed in three changes of sterile distilled water (SDW),
touched to Whatman no. 1 ®lter paper using sterile forceps
to absorb excess liquid and placed onto potato dextrose agar
(PDA) droplets in Petri dishes. The sclerotia were incubated
at 208C in the dark and examined daily for 10 d to determine
viability. Sclerotial viability for all trials was recorded as
.96%.
2.2. The in¯uence of constant soil temperatures on sclerotial
viability
Sclerotia were counted into lots of 100. Each lot was
placed in a polyester mesh bag (Scarpa Filtration Ltd.,
Auckland, New Zealand; 85 mm pore size, 10 £ 10 cm2
with a marker attached for location purposes.
Plastic containers 18 £ 18 £ 19 cm3 were ®lled to
within 1 cm of the top with coarsely sieved, air-dried,
unsterile Wakanui silt loam soil (bulk density:
1.01 g cm 23, McLaren and Cameron, 1996). The soil was
wetted to 25% and maintained throughout the course of the
trial to simulate ®eld conditions. Three bags of sclerotia
were buried in each plastic container at a depth of 10 cm.
Although the edges of each bag overlapped, the sclerotia
were positioned in each bag to ensure they were sandwiched
between soil. Seven plastic containers were maintained at
each of the following temperatures: 20, 30, 35, 40, 45 and
508C, a range of temperatures similar to those used by other
researchers (Porter and Merriman, 1983; Adams, 1987). A
reference treatment was included where sclerotia were
maintained at 208C in a glass vial in the dark for the duration
of the trial. For each temperature, three bags of sclerotia
were randomly selected at each of seven assessment
times: 6, 12 h, 1, 2, 8, 16 and 28 d.
The sclerotia were recovered from the bags and surface
sterilised in 0.25% NaOCl for 1 min, washed in ®ve changes
of SDW and blotted dry on sterile Whatman no. 1 ®lter
paper. The sclerotia were then placed individually using
sterile forceps onto isolated PDA droplets in Petri dishes.
The dishes were sealed with polythene wrap and incubated
in the dark at 208C. Sclerotial germination was recorded
every second day for 10 d. The percentage viability of sclerotia, relative to the number buried, was determined. Results
were analysed using an analysis of variance (ANOVA) with
temperature and length of exposure period as variables. The
appropriateness of an ANOVA for this data was checked by
visual inspection of a residual plot. This plot adequately
con®rmed the normality and homogeneity of the variance
of the data. A Fishers Least Signi®cant Difference test was
used for pairwise comparisons. Probit analysis was
performed to determine the exposure period required at
each temperature to reduce sclerotial viability by 95%
(LD95). The data were converted to thermal days
(temperature £ length of exposure period in days) to
compare the effects of short exposure periods at high
temperatures with longer exposure periods at lower
temperatures on sclerotial viability.
2.3. Soil solarisation trial design
For each of the four trials conducted, sclerotia were counted
into lots of 50. Each lot was placed in a polyester mesh bag as
previously described. A reference treatment where sclerotia
were maintained in a glass vial at 208C in the dark for the
duration of the trials was included for each trial.
Trials 1 and 2 each ran for four week periods over two
consecutive summers (Trial 1: 13/12/95±10/1/96, Trial 2:
11/1/97±11/2/97) and were conducted at a ®eld site at
Lincoln University, Canterbury, New Zealand in Wakanui
silt loam soil. In trial 1, each of the eight plots 3 £ 6 m2
contained six bags of sclerotia buried equidistant from each
other. Three of the bags were randomly selected and buried
at a depth of 10 cm and the remaining three bags were
buried at 20 cm. In trial 2, each of the eight plots 2 £
3 m2 contained three bags of sclerotia buried equidistant
from each other at a depth of 10 cm.
Trials 3 and 4 ran for two 8 week periods (10/12/97±2/2/
98 and 15/12/97±12/2/98, respectively). Trial 3 was
conducted at a ®eld site at Lincoln University, Canterbury,
New Zealand and trial 4 was conducted at the Marlborough
Research Centre, Blenheim, New Zealand in shallow silt
loam soil. Each of the eight plots 3 £ 3 m2 contained
three bags of sclerotia buried equidistant from each other
at a depth of 10 cm. The polyester mesh bags contained 20 g
sieved Wakanui silt loam soil (#200 mm particle size) and
20 g quartz sand in addition to the 50 sclerotia.
Following burial of the sclerotia, all plots were irrigated
to saturation and on the following day, 50 mm thick transparent polythene (Permathane Plastics, Auckland, New
Zealand) (Satour et al., 1989) was laid over four randomly
selected plots. The edges of the polythene were buried in the
soil to a depth of 10 cm. The remaining four uncovered plots
were sprinkler irrigated once a week and weeds were
removed by hand.
For trial 1, temperature sensors (Philips KTY83-110)
encased in stainless steel tubes were placed in the soil to a
139
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
Table 1
The mean percentage of viable Sclerotium cepivorum sclerotia recovered from soil at a range of temperatures after selected exposure periods in the laboratory
n 300
Temperature (8C)
Days
0.25
Reference
20
30
35
40
45
50
0.5
100
100
90.3
97.0
31.0
13.7
0
a
a
a
a
a
ef
gh
h
95.0
96.7
99.0
100
3
0
0
1
a
a
a
a
h
h
h
88.3
58.3
46.0
32.0
0
0
0
2
a
cd
e
ef
h
h
h
93.0
70.3
27.0
33.7
2.3
0
0
8
a
bc
fg
ef
h
h
h
91.0
84.0
36.0
0
0
0
0
16
a
ab
ef
h
h
h
h
98.7
47.0
0
0
0
0
0
28
a
de
h
h
h
h
h
98.0
10.7
0
0
0
0
0
a
gh
h
h
h
h
h
a
Mean values followed by the same letter do not differ signi®cantly P , 0:05 within each column and across each row according to a Fishers Least
Signi®cant Difference test, LSD 16.64.
depth of 10 and 20 cm in each of the eight plots. The soil
temperature was recorded every 30 min using a ®eld data
logger (Datataker DT 600) for the duration of the trial. For
trials 2, 3 and 4 a Tiny Tag temperature logger with internal
sensor (Gemini Dataloggers, Chichester, UK) was enclosed
in a plastic container, wrapped in a plastic bag and buried
10 cm deep in one solarised and one non-solarised plot.
Temperatures were recorded every 30 min (trial 2) and
every 3 h (trials 3 and 4).
Sclerotia were retrieved from the bags at trial completion
and assessed for viability as previously described. Sclerotial
germination was recorded every second day and percentage
viability of sclerotia, relative to the number buried, was
determined. Any other microorganisms growing out into
the agar droplets were also noted and identi®ed where possible. For trial 1, the results were analysed using an ANOVA
with solarised and non-solarised treatments and depth of
burial as variables. For trials 2, 3 and 4 the results were
analysed using an ANOVA with solarised and non-solarised
treatments as variables.
3. Results
3.1. The in¯uence of constant soil temperatures on sclerotial
viability
There was a signi®cant reduction P , 0:05 in sclerotial
Table 2
Temperature required at selected exposure periods to reduce Sclerotium
cepivorum sclerotial viability by 95% in laboratory soil (modelled by probit
analysis)
Temperature (8C)
Exposure period (days) (LD95)
47.4
45.2
41.9
40.7
35.0
24.8
24.3
0.25
0.5
1
2
8
16
28
viability when the sclerotia were incubated at different
temperatures and also after varying lengths of exposure
periods (Table 1). There was also a signi®cant interaction
between the soil temperatures and the length of exposure
period. There was no signi®cant reduction P , 0:05 in
sclerotial viability in the reference treatment for the duration
of the trial. At 208C, sclerotial viability was signi®cantly
reduced P , 0:05 after 28 d incubation with only 10.7%
of the sclerotia remaining viable. At 308C, sclerotial viability was reduced P , 0:05 to 0% after incubation for 16 d.
At 358C, 8 d incubation was required to reduce sclerotial
viability to 0%. Incubation at 408C for 6 h was suf®cient to
reduce P , 0:05 sclerotial viability to 31% with a reduction to 0% after only 1 d. At 458C, a 12 h incubation period
reduced P , 0:05 sclerotial viability to zero and at 508C,
sclerotial viability was lost within the ®rst 6 h incubation
period.
Short exposure periods at higher temperatures reduced
sclerotial viability by 95% (LD95) (Table 2). In contrast,
longer exposure periods of 16±28 d were required to reduce
sclerotial viability by 95% at lower temperatures (24.8 and
24.38C, respectively).
When the data were modelled by probit analysis, equivalent thermal day periods did not result in equivalent losses in
sclerotial viability. For example, after exposure to 358C for
8 d, sclerotial viability was reduced to 21% but only to 52%
after exposure to 208C for 14 d, for the same thermal day
period (280). Similarly, after 2 d at 408C (80 thermal days),
only 3% of the sclerotia remained viable, whereas 79% of
the sclerotia remained viable after 4 d at 208C.
3.2. Soil solarisation trials
The mean daily maximum temperatures for solarised and
non-solarised soil for trials 1, 2 and 3 are shown in Figs. 1±
3. There is no soil temperature pro®le for trial 4 as the Tiny
Tag temperature logger in the solarised plot malfunctioned
during the course of the trial and the temperature data could
not be retrieved. The maximum and mean soil temperatures
at 10 cm in the solarised soil ranged from 39.2±42.78C and
140
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
Fig. 1. The mean daily maximum soil temperature for solarised and non-solarised soil at 10 and 20 cm, Canterbury (trial 1).
24.6±28.88C, respectively. In the non-solarised soil, the
maximum and mean soil temperatures at 10 cm ranged
from 27±33.68C and 18.9±23.38C, respectively. Maximum
soil temperatures at 10 cm were higher P , 0:05 in the
solarised soil than the non-solarised soil for trials 1 and 2 but
not for trial 3. The maximum soil temperature at 20 cm for
the solarised and non-solarised soil were also signi®cantly
different P , 0:05 for trial 1. For trial 1, the maximum
temperature at 10 and 20 cm in the solarised plots differed
by a maximum of 48C on any one day (Fig. 1). In the nonsolarised plots, the maximum temperatures differed by a
maximum of 18C on any one day. Soil temperatures of
208C and greater were maintained for 1897, 484 and
777 h in solarised soil compared with 930.5, 279 and
648 h in non-solarised soil for trials 1, 2 and 3, respectively
(Table 3).
Fig. 2. The mean daily maximum soil temperature for solarised and non-solarised soil, Canterbury (trial 2).
141
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
Fig. 3. The mean daily maximum soil temperature for solarised and non-solarised soil, Canterbury (trial 3).
in the solarised plots in all four trials (Table 4). In trial 1 and
4, there was also a signi®cant reduction P , 0:05 in
sclerotial viability between the non-solarised control and
reference treatments.
A number of fungal species emerged from the sclerotia in
all trials when plated onto agar droplets. The fungi were
identi®ed as isolates of Verticillium, Penicillium, Fusarium,
Trichoderma, Aspergillus, Mucor and an isolate of Paecilomyces lilacinus (Thom) Samson. Unidenti®ed bacterial
species were also present. Both the solarised and nonsolarised sclerotia were colonised by the microorganisms
in all trials.
The results for percentage sclerotial recovery and viability for all four trials are presented in Table 4. For trial 1, the
depth at which the sclerotia were buried did not signi®cantly
affect P , 0:05 sclerotial recovery or viability, therefore,
the number of sclerotia recovered at 10 and 20 cm were
combined for analysis.
The percentage of recovered sclerotia was signi®cantly
less (P , 0.05) from the solarised plots than from the reference treatment for all trials except trial 1. In trial 1 there was
no signi®cant difference (P , 0.05) in sclerotial recovery
between the reference, non-solarised control and solarised
treatments. In trial 4, in addition to fewer P , 0:05 sclerotia being recovered from the solarised plots, fewer P ,
0:05 sclerotia were recovered from the non-solarised
control plots compared with the reference treatment.
Fragments of sclerotial rind remained in the bags and
vials, which indicated that the unrecovered sclerotia had
disintegrated.
Sclerotial viability was reduced signi®cantly P , 0:05
4. Discussion
Increases in constant soil temperature in the laboratory
caused a decrease in sclerotial viability. Sclerotia readily
decayed and lost viability when incubated for short periods
Table 3
Total number of hours recorded above selected temperatures and the maximum and mean temperature in solarised and non-solarised soil for four ®eld soil
solarisation trials at 10 cm depth (ND Ð no data)
Temperature (8C)
$ 40
$ 35
$ 30
$ 20
Maximum temperature
Mean temperature
a
Trial 1-Canterbury a
Trial 2-Canterbury
Trial 3-Canterbury
Trial 4 -Blenheim
Solarised
Non-solarised
Solarised
Non-solarised
Solarised
Non-solarised
Solarised
Non-solarised
13
140
551
1897
42.7
27.6
0
0
0
931
27
20.3
0
63
99
484
39.2
24.6
0
0
1
279
27.9
18.9
51
105
240
777
41.3
28.8
0
9
21
648
33.6
21.1
ND
ND
ND
ND
ND
ND
0
0
54
993
33
23.3
Temperatures at 20 cm are not shown.
142
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
Table 4
Percentage Sclerotium cepivorum sclerotial recovery and viability from all treatments within each of the four ®eld soil solarisation trials. Percentage values
followed by the same letter do not differ signi®cantly P , 0:05 within columns (n 300 (trial 1), n 150 (trials 2, 3 and 4))
Sclerotial treatments
% Recovery
Reference
100
Non-solarised control 96.5
Solarised
88.3
a
b
Trial 2-Canterbury 97
Trial 3-Canterbury 97/98 a
Trial 4-Blenheim 97/98 a
% Viability b
% Recovery
% Viability
% Recovery
% Viability
% Recovery
% Viability
100
66.3
40.2
100
94.2
59.0
98.8
88.3
53.3
90.6
74.7
46.0
88.6
70.6
8.7
96.0
61.7
29.0
89.3
49.7
0
Trial 1-Canterbury 95/96
a
a
a
a
b
c
a
a
b
a
a
b
a
a
b
a
a
b
a
b
c
a
b
c
A mixture of soil and sand was added to the bags containing the sclerotia.
Percentage sclerotial viability relative to the number of sclerotia buried.
at temperatures above 408C. Adams (1987) reported a similar result with continuous soil temperatures of 508C for 4 h
and 458C for 12 h, lethal to S. cepivorum sclerotia. The
equivalent thermal day data indicated that although higher
temperatures maintained for short periods of time may have
the same thermal days as lower temperatures maintained for
longer periods, the resulting number of viable sclerotia was
not equivalent. This has important implications in that
higher temperatures are required for a rapid decrease in
sclerotial viability. Temperatures greater than 408C were
recorded in Canterbury and it is probable that soil temperatures reached 408C in Blenheim. However, the signi®cant
reduction in S. cepivorum sclerotial recovery and viability is
more likely due to the effect of ¯uctuating sub-lethal
temperatures rather than high temperatures.
The detrimental effects of ¯uctuating sub-lethal temperatures have been well documented (Katan et al., 1976; Pullman et al., 1979; Porter and Merriman, 1983). It is most
probable that both thermal in¯uence and biological control
activity reduced sclerotial viability in these soil solarisation
trials. Fluctuating temperatures may increase sclerotial
vulnerability to soil microorganisms or increase heat resistant saprophyte populations subsequently increasing the
parasitic and lytic effects on the sclerotia (Katan et al.,
1976). The increased microbial activity could explain why
pieces of sclerotial rind remained in many of the bags.
Increased colonisation of S. cepivorum sclerotia by soil
microorganisms, following treatment with sub-lethal
temperatures has also been reported (Entwistle and Munasinghe, 1990). The presence of soil microorganisms on the
recovered sclerotia may suggest that the sclerotia were in a
weakened state. Aspergillus is reported as a possible secondary colonist of sclerotia (Phillips, 1990) and is tolerant of
high soil temperatures (Dwivedi, 1991). Bacterial species
have also been reported to colonise cracks in the sclerotial
rind of weakened sclerotia (Lifshitz et al., 1983).
With New Zealand's climate, the effects of ¯uctuating
sub-lethal temperatures are more in¯uential in reducing
sclerotial viability than high temperatures. An 8 week solarisation treatment was more effective in reducing sclerotial
viability than a 4 week period as the sclerotia were
subjected to sub-lethal temperatures for twice as long. The
lower viability from the 8 week trials might also have been
related to the addition of soil and sand to the bags. The
polyester mesh bags may have provided an insulating effect
in trials 1 and 2 that was overcome in trials 3 and 4. If an
insulating effect had occurred then it is likely that the loss of
sclerotial viability in natural populations would be higher
than the reported data.
Soil solarisation reduced sclerotial viability by 91.3 and
100% in trial 3 and 4, respectively. All sclerotia were at a
depth of 10 cm. While sclerotia are able to infect Alliumspecies when placed up to 30 cm deep in the soil, it is the
sclerotia in the top 10 cm of soil that mainly contribute to
disease spread (Crowe and Hall, 1980). Reducing sclerotial
viability in the top 10 cm of the soil would therefore ease
disease pressure in Allium crops. In addition, soil temperatures at 10 and 20 cm were almost identical and solarising a
S. cepivorum naturally infested ®eld would still subject
sclerotia at soil depths greater than 20 cm to sub-lethal
temperature ¯uctuations. While the effects would not be
as great deeper in the soil, the sclerotia may still be
weakened and more vulnerable to invasion by antagonistic
microorganisms.
Within New Zealand, the use of soil solarisation to
control onion white rot will be most suited to the garlic
growing regions in Blenheim where soil temperatures and
sunshine hours are generally higher than elsewhere in New
Zealand. Trials are now required to determine the actual
reduction in onion white rot afforded by this technique in
the ®eld. The combination of soil solarisation with biological control agents may provide more effective control of
onion white rot than the use of soil solarisation alone.
Acknowledgements
This research was supported by the Brian Mason Science
and Technical Trust.
References
Adams, P.B., 1987. Effects of soil temperature, moisture and depth on
survival and activity of Sclerotinia minor, Sclerotium cepivorum and
Sporidesmium sclerotivorum. Plant Disease 71, 170±174.
Alexander, B.J.R., Stewart, A., 1994. Survival of sclerotia of Sclerotinia
K.L. McLean et al. / Soil Biology & Biochemistry 33 (2001) 137±143
and Sclerotium spp. in New Zealand horticultural soil. Soil Biology &
Biochemistry 26, 1323±1329.
Alexander, R.T., 1990. The effect of solarisation on vegetable crop production. Proceedings of the 43rd NZ Weed and Pest Control Conference,
pp. 270±273.
Basallote-Ureba, M.J., Melero-Vara, J.M., 1993. Control of garlic white rot
by soil solarization. Crop Protection 12, 219±233.
Crowe, F.J., Hall, D.H., 1980. Vertical distribution of sclerotia of Sclerotium cepivorum and host root systems relative to white rot of onion and
garlic. Phytopathology 70, 70±73.
DeVay, J.E., 1991. Use of soil solarization for control of fungal and bacterial plant pathogens including biocontrol. FAO Plant Production and
Protection Paper 109, 70±93.
Dwivedi, S.K., 1991. Effect of solar heating of soil on the dynamics of soil
myco¯ora. Journal of Mycopathological Research. 29, 93±96.
Entwistle, A.R., Munasinghe, H.L., 1990. Evidence for damage in sclerotia
of Sclerotium cepivorum following sublethal heat treatment. In:
Proceedings of the Fourth International Workshop on Allium White
Rot, A.R. Entwistle (Ed.), Section 2, pp. 69±75. Neustudt/Weinstrasse,
Federal Republic of Germany. Biologische bundesanstalt fur Land- und
Forst- wirtschaft, Messeweg 11/12, D-3300 Braunschweig, Federal
Republic of Germany.
Katan, J., Greenberger, A., Alon, H., Grinstein, A., 1976. Solar heating by
polyethylene mulching for control of diseases caused by soil-borne
pathogens. Phytopathology 66, 683±688.
Lifshitz, R., Tabachnik, M., Katan, J., Chet, I., 1983. The effect of sublethal
heating on sclerotia of Sclerotium rolfsii. Canadian Journal of Microbiology 29, 1607±1610.
143
Mahrer, Y., Naot, O., Rawitz, E., Katan, J., 1984. Temperature and moisture regimes in soils mulched with transparent polythene. Soil Science
Society of America Journal 48, 362±367.
McLaren, R.G., Cameron, K.C., 1996. Soil Science, sustainable production
and environmental protection, . 2nd ed.Oxford University Press, Auckland, New Zealand.
Phillips, A.J.L., 1990. The effects of soil solarization on sclerotial populations of Sclerotinia sclerotiorum. Plant Pathology 39, 38±43.
Porter, I.J., Merriman, P.R., 1983. Effects of solarization of soil on nematode and fungal pathogens at two sites in Victoria. Soil Biology &
Biochemistry 15, 39±44.
Porter, I.J., Merriman, P.R., 1985. Evaluation of soil solarization for control
of root diseases of row crops in Victoria. Plant Pathology 34, 108±118.
Pullman, G.S., DeVay, J.E., Garber, R.H., Weinhold, A.R., 1979. Control
of soil-borne fungal pathogens by plastic tarping of soil. In: Schippers,
B., Gams, W. (Eds.). Soil-borne Plant Pathogens, pp. 439±446.
Satour, M.M., Abdel-Rahim, M.F., El-Yamani, T., Radwan, A., Grinstein,
A., Rabinowitch, H.D., Katan, J., 1989. Soil solarization in onion ®elds
in Egypt and Israel: short and long term effects. Acta Horticulturae 255,
151±159.
Satour, M.M., El-Sherif, E.M., El-Ghareeb, L., El-Hada, S.A., El-Wakil,
H.R., 1991. Achievements of soil solarization in Egypt. FAO Plant
Production and Protection Paper 109, pp. 200±212.
Slade, E.A., Fullerton, R.A., Stewart, A., Young, H., 1992. Degradation of
the dicarboximide fungicides iprodione, vinclozolin and procymidone
in Patumahoe clay loam soil, New Zealand. Pesticide Science 35, 95±
100.