The following body of research provides a detailed overview of the interactive effects of
biocontrol agents and environmental factors and how these influence both the host plant
and pathogen populations within hydroponic systems.
Pythium and other zoosporic fungi are pathogens well suited to the aquatic environment
of hydroponics. Motile zoospores facilitate rapid dispersal through fertigation water,
resulting in Pythium becoming a yield reducing factor in most hydroponic systems and
on most crops. With increasing trends away from pesticide use, biocontrol is becoming
an ever more popular option. Unfortunately, much of our knowledge of biocontrol agents
and their formulation can not be directly transferred to the widely differing environments
of hydroponic systems. Paulitz (1997) was of the opinion that if biocontrol was to be
successful anywhere, it would be in hydroponics. This is primarily due to the increased
ability, in hydroponics, to control the growing environment and to differentiate between
the requirements of the pathogen versus those of the host plant and biocontrol agent.
Key environmental factors were identified as soil moisture, root zone temperature, form
of nitrogen and pH.
A review of the literature collated background information on the effects of biocontrol
agents and environmental manipulation on plant growth and disease severity in
hydroponic systems.
A commercial formulation of Trichoderma (Eco-T(R1)) was used as the biocontrol agent
in all trials. Dose responses in Pythium control and plant growth stimulation in lettuce
were first determined using a horizontal trough system (closed system). In such systems
optimum application rates were found to be lower than in field application (1.25x10[to the power of 5]
spores/ml). This is probably because Trichoderma conidia are not lost from the system,
but re-circulate until being transported into the root zone of a host plant. No significant
growth stimulation was observed, although at high doses (5x10[to the power of 5] and 2.5x10[to the power of 5] spores/ml)
a significant reduction in yield was recorded. Possible reasons for this growth inhibition
are suggested and a new theory is proposed and investigated later in the thesis. In an
open system of cucumber production (drip irrigated bag culture) no statistically
significant results were initially obtained, however, general trends still showed the
occurrence of positive biocontrol activity. The initial lack of significant results was mostly
due to a poor knowledge of the horticulture of the crop and a lack of understanding of
the epidemiology behind Trichoderma biocontrol activity. These pitfalls are highlighted
and, in a repeat trial, were overcome. As a result it could be concluded that application
rates in such systems are similar to those used in field applications.
Management of soil moisture within artificial growing media can aid in the control of
Pythium induced reductions in yield. A vertical hydroponic system was used to
determine the interactive effects of soil moisture and Trichoderma. This system was
used because it allowed for separate irrigation regimes at all 36 stations, controlled by
a programmable logic controller (PLC). With lettuce plants receiving optimum irrigation
levels, no significant reduction in yield was observed when inoculated with Pythium.
However, after Pythium inoculation, stresses related to over- or under-watering caused
significant yield losses. In both cases, Trichoderma overcame these negative effects
and achieved significant levels of disease control, especially under higher soil moisture
levels. Growth stimulation responses were also seen to increase with increasing soil
moisture. Similar results were obtained from strawberry trials. These results show that
Pythium control is best achieved through the integration of Trichoderma at optimum soil
moisture. However, where soil moisture is above or below optimum, Trichoderma serves
to minimize the negative effects of Pythium, providing a buffering capacity against the
effects of poor soil moisture management.
Pythium, root zone temperature and form of nitrogen interact significantly. In
greenhouse trials using horizontal mini troughs with facilities for heating or cooling
recirculating water, nitrate fertilizer treatments resulted in statistically significant results.
Lettuce growth was highest at 12°C, although no significant differences in yield were
observed between 12-24°C. Pythium was effective in causing disease over the same
temperature range. Pythium inoculation did not result in yield reduction at 6 and 30°C.
Trichoderma showed a slight competitive advantage under cooler temperatures (i.e., 12 degrees C), although significant biocontrol occurred over the 12-24 degrees C range. Ammonium
fertilizer trials did not generate statistically significant data. This is possibly due to
complex interactions between root temperature, ammonium uptake, and competitive
exclusion of nitrification bacteria by Trichoderma. These interactions are difficult to
replicate over time and are probably influenced by air temperature and available light
which are difficult to keep constant over time in the system used. However, the data did
lead to the first clues regarding the effects of Trichoderma on nitrogen cycling as plants
grown with a high level of ammonium at high temperatures were seen to suffer more
from ammonium toxicity when high levels of Trichoderma were added.
In further trials, conducted in the recirculating horizontal mini trough system, it was
determined that Trichoderma applications resulted in an increase in the percentage
ammonium nitrogen in both the re-circulating solution and the growing medium. This
was a dose-related response, with the percentage ammonium nitrogen increasing with
increasing levels of Trichoderma application. At the same time an increase in
ammonium in the root tissue was observed, corresponding with a decrease in leaf
nitrate levels and an increase in levels of Cu, Na, Fe and P in leaf tissue. In independent
pot trials, populations of nitrifying bacteria in the rhizosphere were also seen to
decrease with increasing Trichoderma application rates. This led to the conclusion that
the increase in ammonium concentration was as a result of decreased nitrification
activity due to the competitive exclusion of nitrifying bacteria by Trichoderma. The
possibility that Trichoderma functions as a mycorrhizal fungus and so increases the
availability of ammonium for plant uptake is not discarded and it is thought that both
mechanisms probably contribute.
Water pH provides the most powerful tool for enhancing biocontrol of Pythium by
Trichoderma. Trichoderma shows a preference for more acidic pHs while Pythium
prefers pHs between 6.0 and 7.0. In vitro tests showed that Trichoderma achieved
greater control of Pythium at pH 5.0, while achieving no control at pH 8.0. In greenhouse
trials with the recirculating horizontal mini trough system, yield losses resulting from
Pythium inoculation were greatest at pH 6.0 and 7.0, with no significant reduction in
yield at pH 4.0. Biocontrol activity showed an inverse response with greatest biocontrol
at pH 5.0. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2003.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/5507 |
| Date | January 2003 |
| Creators | Neumann, Brendon John. |
| Contributors | Laing, Mark D., Caldwell, Patricia May. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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