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.