Soybeans, Glycine max, are an economically and strategically important crop in South
Africa (SA). In order to meet local demands, large imports of soybeans are required,
e.g., in the 2005/2006 soybean production period, 842 107 tonnes of oilcake were
imported. Due to an increase in soybean production throughout the world, diseases that
affect this crop have also increased in incidence and severity.
Sclerotinia sclerotiorum, the causal organism of sclerotinia stem rot (SSR), is an
important yield limiting disease of soybeans, as well as numerous other crops. The
pathogen was first reported in SA in 1979. However, it was only in 2002 that this fungus
was considered a major pathogen of soybeans in SA.
The research reported in this thesis was conducted to investigate the epidemiology of
S. sclerotiorum and examine numerous potential control methods for this pathogen, i.e.,
resistant cultivars, biocontrol, chemical control and seed treatments. A S. sclerotiorum
isolate was obtained from sunflowers in Delmas, Mpumulanga, SA, in the form of
sclerotia. This isolate was cultured and sent for identification and deposition in the Plant
Protection Research Institute collection. This isolate, in the form of mycelia, was used
for the duration of the study.
For epidemiology studies, the effect of temperature, leaf wetness duration (LWD) and
relative humidity (RH) were examined for their effect on rate of pathogen development.
Twenty four combinations of temperature (19°C, 22°C, 25°C and 28°C), LWD (24, 48
and 72 hr) and RH (85 and 95%) were investigated. No interaction between
temperature, LWD and RH was found. Temperature alone was the only factor that
affected disease development. At 22°C, the rate of pathogen development (0.45 per
unit per day) was significantly higher than all other temperatures, indicating that this
temperature is optimum for disease development.
Thirteen different soybean cultivars, i.e., LS6626RR, LS6710RR, LS666RR, LS555RR,
LS6514RR, LS678RR, Prima 2000, Pan 626, AG5601RR, AG5409RR, 95B33, 95B53
and 96B01B, commercially grown in SA were investigated for their reaction to
S. sclerotiorum. Prima 2000, 96B01B, 95B33 and AG5409RR were considered to be
the least susceptible as they showed a significantly low rate of pathogen development
(0.28, 0.28, 0.24, 0.23 per unit per day, respectively) and produced a significantly low
number of sclerotia (3.03, 3.42, 3.21, 2.38, respectively). LS6626R and LS666RR may
be considered most susceptible because of their significantly high rate of pathogen
development (0.45 and 0.42 per unit per day, respectively) and high sclerotia production
(8.16 and 7.50, respectively). Regression analysis showed a positive correlation
coefficient (R2=0.71) between rate of growth of the pathogen and number of sclerotia
produced, indicating that a higher rate is associated with a higher number of sclerotia.
In vitro dual culture bioassays were performed to identify the biocontrol mechanisms of
the biocontrol agents, EcoT® (a seed treatment) and Eco77® (a foliar treatment), against
hyphae and sclerotia of S. sclerotiorum. Ultrastructural studies revealed that
mycoparasitism is the probable mode of action as initial signs of hyphae of EcoT® and
Eco77® coiling around hyphae of S. sclerotiorum were observed. Surface colonization
of sclerotia by hyphae of EcoT® and Eco77® was also observed.
In vitro antagonism of EcoT® against S. sclerotiorum on soybean seed was performed to
determine pre-emergence and post-emergence disease. There was no significant
difference in percentage germination between seeds treated with EcoT® and plated with
the pathogen, untreated seeds and no S. sclerotiorum, and the control (i.e. no EcoT®
and no pathogen). However, percentage non infected seedlings from seeds not treated
with EcoT® was significantly lower, suggesting that EcoT® may be successfully used as
a seed treatment for the control of SSR. In vivo trials were performed to investigate the
effect of silicon (Si) alone, and in combination with Eco77®, on the effect of the rate of
disease development. Plants treated with Eco77® had a significantly lower rate of
disease development (0.19 per unit per day for plants treated with Eco77® and S.
sclerotiorum and 0.20 per unit per day for plants treated with Eco77®, S. sclerotiorum
and Si), compared to plants not treated with Eco77® (0.29 per unit per day for plants
treated with S. sclerotiorum and 0.30 per unit per day for plants treated with S.
sclerotiorum and Si), regardless of the application of Si. Similarly, plants treated with
Eco77® had a significantly lower number of sclerotia (0.46 for plants treated with Eco77®
and S. sclerotiorum and 0.91 for plants treated with Eco77®, S. sclerotiorum and Si),
compared to plants not treated with Eco77® (3.31 for plants treated with S. sclerotiorum
and 3.64 for plants treated with S. sclerotiorum and Si). The significantly lower rate of
disease development coupled with a significant reduction in sclerotia showed that
Eco77®, and not Si, was responsible for reducing the severity of SSR. A strong positive
correlation between rate of disease development and the number of sclerotia produced
(R2=0.79) was observed.
For the investigation of various fungicides for the control of S. sclerotiorum, in vitro trials
to determine the potential of three different fungicides at different rates, i.e., BAS 516
04F (133 g a.i. ha-1), BAS 516 04F (266 g a.i. ha-1), BAS 512 06F (380 g a.i. ha-1) and
Sumisclex (760 g a.i. ha-1) were initially conducted. The control (non-amended PDA)
had a significantly higher area under mycelial growth curve (243.0) than all fungicides
tested. BAS 516 04F (at both concentrations) and BAS 512 06F completely inhibited
the mycelial growth of S. sclerotiorum. Sumisclex inhibited the fungus by 89.07%. For
in vivo trials, preventative treatments, i.e., BAS 516 04F (133 g a.i. ha-1), BAS 516 04F
(266 g a.i. ha-1), BAS 512 06F (380 g a.i. ha-1), curative treatment, i.e. Sumisclex (760 g
a.i. ha-1) and a combination preventative/curative treatment, i.e., BAS 512 06F (380 g
a.i. ha-1)/Sumisclex (570 g a.i. ha-1) were investigated. No significant difference in
disease severity index (DSI) was found between fungicide treatments and the inoculated
control. BAS 512 06F and BAS 512 06F/Sumisclex had significantly lower grain yields
(6.09 g and 5.96 g, respectively) compared to all other treatments. There was a positive
correlation coefficient (R2=0.76), between DSI and grain yield, indicating that a high DSI
is correlated with low grain yield.
Trials to evaluate the effect of commercially available and currently unregistered seed
treatments for the control of S. sclerotiorum on soybean seeds in vivo and in vitro were
performed. Seed germination tests were performed to determine if seed treatments had
any negative effects on seed germination in vitro. All seed treatments tested, i.e., BAS
516 03F (8, 16 and 32 ml a.i. 100 kg-1 seed), BAS 512 00F (7.5, 15 and 32 ml a.i. 100
kg-1 seed), Celest XL (100, 125, 200 and 250 ml a.i. 100 kg-1 seed), Sumisclex (5 and 10
ml a.i. 100 kg-1 seed), Benomyl (150 g a.i. 100 kg-1 seed), Captan (240 ml a.i. 100 kg-1
seed), Thiulin (180 g a.i. 100 kg-1 seed) and Anchor Red (300 ml a.i. 100 kg-1 seed),
showed no negative effect on seed germination. For in vivo trials, BAS 516 03F (16 and
32 ml a.i. 100 kg-1 seed), BAS 512 00F (7.5, 15 and 32 ml a.i. 100 kg-1 seed), Celest XL
(100, 125, 200 and 250 ml a.i. 100 kg-1 seed), Sumisclex (5 and 10 ml a.i. 100 kg-1
seed), Benomyl and Anchor Red had significantly similar percent germination and
percent seedling survival as the untreated/uninoculated control. These seed treatments
should be recommended for the control of S. sclerotiorum, as they protected seed
during germination and subsequent seedling development. BAS 516 03F (8 ml a.i. 100
kg-1 seed) should not be recommended for the control of SSR, as it gave the lowest
percent germination and percent seedling survival.
The results presented in this thesis have helped to identify optimal environmental
conditions for the development of S. sclerotiorum, which is important for the
development of forecasting models for disease control. The least and most susceptible
cultivars of those tested have been identified. Biocontrol using Eco77® as a foliar
application showed great potential.
The effect of Si needs to be further investigated, including the testing of more frequent
applications and higher concentrations. The fungicides tested in this research did not
show any potential for the control of SSR. However, the spray programme tested is for
the control of soybean rust (Phakopsora pachyrhizi), and was investigated for its
potential for the control of SSR. The spray programme, fungicide application and rating
scale needs to be modified, to determine the true potential of these fungicides for the
control of SSR. Numerous seed treatments have shown potential for the control of seed
infection by SSR. Due to difficulties in producing ascospores, which are the primary
source of inoculum for this pathogen in the field, all studies in this research were
conducted with mycelia and not ascospores. The production, collection and storage of
ascospores needs to be thoroughly investigated, and research conducted with
ascospores. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/3491 |
| Date | January 2007 |
| Creators | Visser, Dael Desiree. |
| Contributors | Caldwell, Patricia May. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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