Anredera cordifolia (Basellaceae), Madeira vine, is a perennial, semi- succulent climber native from Paraguay to southern Brazil and northern Argentina. It has a history of weediness and difficulty of control once established. In South Africa Madeira vine has a wide range and distribution with altitudes ranging from 10-1800m above sea level. Described as a transformer species, its sheer weight is capable of breaking branches off trees, causing the potential collapse of forest canopies. Chemical and mechanical control methods are expensive, labour intensive and may provide only temporary relief. A biological control programme was therefore initiated in 2003. Cf Phenrica sp. 2 (Coleoptera: Chrysomelidae: Alticinae), was field collected from A. cordifolia in Brazil, SSW of Cascavel in the Paraná Province during a survey in November 2003. Eggs are laid in groups of 16 with the average fertility rate being 89%. After going though three larval instars, the larvae pupate in the soil with the adults eclosing after a period of 17 days. The total developmental time for a generation from egg to egg ranges between 7-8 weeks. Biological traits that favour the flea beetle as a possible biological control agent include long-lived adults (up to 5 months) and multiple generations during the summer period. Both adults and larvae feed extensively on leaves and stems and although developmental rates will slow down during the winter period, no indication of a definite diapause was found under the prevailing laboratory conditions. After completing the larval no-choice trials with twenty-six plant species from 14 plant families Phenrica sp. 2 proved to be adequately host specific, as larval development was only supported by 3 Basellaceae species (including the control A. cordifolia) and one Portulacaceae species. All of these are introduced species in South Africa. The only indigenous Basella species could not be tested as it has a very marginal distribution, and because it’s inconspicuous nature, it is seldom seen or collected. Adult multi-choice trials were restricted to species that could sustain larval development to give some indication of the acceptability of these species for adult feeding and oviposition. Although adult feeding was initially concentrated on B. alba, the oviposition preference was clear-cut as females only oviposited on A. cordifolia. In order to quantify the impact of Phenrica sp. 2 on plant biomass and to assess the incidence and intensity of foliar damage, a pair of adults was confined to the host plant, for 2 generations, with different levels of larval densities. The results indicated that the host plant, due to both larval and adult feeding, suffered leaf losses of up to 55%. Anredera cordifolia was however still capable of enlarging the root mass despite suffering huge leaf losses. This would imply that A. cordifolia has an effective re-growth capacity and it will only be vulnerable to attack of the storage organs that enable re-growth, or to repeated attack of other plant parts through which reserves are exhausted. Unfortunately the period of exposure (24 days) was too short to prove that Phenrica sp. 2 impacts on the below ground dry mass, but should the plant be completely defoliated, as was observed in the field, the host plant would be forced to deplete stored resources. Phenrica sp.2 has shown to be very host specific and although A.cordifoia loses its leaves during the winter period in most provinces in South Africa, the adults are long-lived and should be able to survive the leafless periods. Further more the relatively short life cycle, high fecundity and 3 generations per year should theoretically insure a strong population build-up that would improve the chances of establishment in the field. All indications are that Phenrica sp. 2 is an agent well worth considering for the biological control of A. cordifolia.

Varieties of Lantana camara (lantana) have been introduced into many countries of the world as ornamental plants and have become invasive weeds in many countries including South Africa. In South Africa, it mostly invades the sub-tropical eastern and northern range. Mechanical and chemical control options are expensive and ineffective. A biocontrol programme was initiated in South Africa in 1961. To date, 22 insect species, and a fungus have been introduced, of these 10, and the fungus have established. Three indigenous lepidopteran species and an exotic generalist pest mealybug are also associated with the weed. The variable success of some of the agents released on L. camara worldwide has been ascribed to a few factors. One important aspect is the large range of varieties encountered in the field. It is therefore essential to be able to predict the possible establishment and impact of agents on many varieties. Laboratory trials on five of the established agents showed clear varietal preferences. In the field, most of the biocontrol agents had limited geographic ranges, linked to altitudinal conditions, as higher populations were recorded at low lying warm summer rainfall areas. A pink and orange flower corolla lobe and throat colour combination and plants with few to medium leaf hairs were most abundant in South Africa. Most of the agent species had individual preferences towards different flower colour combinations, as the agents built up different population levels on varieties in the field, within the suitable geographic region for the insect species. Eight agents preferred smooth leaved varieties, while three preferred hairy leaves, and three had no specific preference to leaf hairiness. Varietal preferences thus did play a significant role in agent populations and accompanied impact achieved in the field. New candidate agents need to be proven specific under quarantine conditions and the results extrapolated to predict specificity in the field, while avoiding potential non-target effects. Many authors have questioned the validity of laboratory host specificity trials. The conventional wisdom is that insects portray a far wider host range in the laboratory than what they would do in the field. In other words, laboratory studies measure the physiological host range of an agent and are conservative and usually don’t reflect the ecological host range of agents in the field. To avoid unnecessary rejections of biocontrol agents, this study has made a retrospective study of the host specificity of agents established in the field. Their laboratory and field host ranges were compared and it was found that virtually all the agents reflect similar or less non-target effects in the field than predicted during multiple choice trials. Of the 14 agents, only one introduced species, Teleonemia scrupulosa, and the indigenous species, Hypena laceratalis and Aristea onychote were able to sustain populations on non-target species in the field in the absence of L. camara. Insect populations on non-target species were much reduced compared to that on L. camara. Furthermore non-target effects were only recorded on plant species closely related to the target weed. The multiple choice trials therefore predict field non-target effects accurately. Predictions of non-target effects of candidate agents can therefore be accurately predicted by laboratory studies, in terms of species likely to be affected and to what extent. One field that need further study though is the impact of non-target effects, especially on Lippia species by L. camara biocontrol agents.

The two-spotted spider mite (TSM), Tetranychus urticae Koch, is an important pest of many greenhouse and field crops worldwide. The development of resistance in TSM populations to chemical acaricides, allied with public health concerns about pesticide residues, has motivated the search for alternative control measures to suppress the pest. Hyphomycetous fungi are promising agents for mite control and the fungus Beauveria bassiana (Bb) (Balsamo) Vuillemin was investigated in this study as a biocontrol agent. The principal objectives of this study comprised: a) screening Bb strains for their pathogenicity against T. urticae; b) testing the effect of adjuvants on the efficacy of Bb; c) studying the effect of plant type on persistence of Bb and the efficacy of control of Bb against T. urticae; d) evaluating the field efficacy of Bb applications against T. urticae; e) testing the compatibility of Bb with selected fungicides; and f) assessing the synergy between Bb and soluble silicon for T. urticae control. Screening bioassays of sixty-two strains of Bb identified the two most effective strains, PPRI 7315 (R289) and PPRI 7861 (R444), that caused mortality levels of more than 80% of adult mites at 9 d post-inoculation with 2 × 108 conidia ml-1. These strains performed significantly better than the Bb commercial strain PPRI 5339, in laboratory bioassays. The two strains also attacked mite eggs, causing 53.4% and 55.5% reduction in egg hatchability at 2 × 108 conidia ml-1 respectively. However, PPRI 7861 showed relatively higher production of conidia in culture and was, therefore, selected for further trials under greenhouse and field conditions. Greenhouse evaluations of the effects of two adjuvants (Break-thru® and a paraffin oil-based emulsion) on efficacy of Bb demonstrated a higher efficacy of the biocontrol agent (BCA) when it was applied with Break-thru® or the oil solution than with water alone. Moreover, Bb conidia applied in Break-thru® solution resulted in greater control of TSM than conidia applied in the mineral oil. There was also a dose-response effect and the control of TSM by Bb increased when the concentration of conidia was increased. The control of TSM by Bb in beans (Phaseolus vulgaris L), cucumber (Cucumis sativus L.), eggplant (Solanum melongena L.), maize (Zea mays L.) and tomato (Solanum lycopersicum L.) was tested in greenhouse trials. On these crops, the persistence of conidia declined over time. The rate of decline was significantly higher on maize. However, TSM mortality was positively correlated with the amount of conidia deposited on leaves immediately after spraying, rather than their persistence over time. Higher levels of mortality of TSM due to Bb application were observed on beans, cucumber and eggplants, suggesting that the type of crop must be taken into consideration when Bb is applied as a BCA. Field efficacy of Bb against mites was evaluated in two trials on eggplants. Based on assessment of population densities of mites and leaf damage assessments; both trials showed that the strain PPRI 7861 controlled TSM in the field. Two commonly used fungicides, azoxystrobin and flutriafol, were investigated in vitro tests on culture medium and laboratory bioassays on detached bean leaves (Phaseolus vulgaris L.) for their effects on Bb. Azoxystrobin (a strobilurin) was less harmful to Bb while flutriafol was found to be inhibitory. Another important finding of this study was the substantial enhancement of Bb efficacy by soluble silicon. When Bb was combined with soluble Si, the control of TSM was better than when either of the two products was applied alone. Moreover, application of soluble Si as a plant fertilizer in hydroponic water nutrient increased accumulation of peroxidase, polyphenoloxidase and phenylalanine ammonia-lyase enzymes in leaves of plants infested with TSM. Increased activity of these defense enzymes in leaves deters feeding behaviour of mites. We suggested that feeding stress renders them susceptible to Bb infection, which would explain the synergy observed between the two agents. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2009.

Thesis (M. Tech) --Central University of Technology, Free State, 2008 / Aphids (Hemiptera: Aphididae) are amongst the most destructive insects in agricultural crop production systems. This reputation stems from their complex life cycles which are mostly linked to a parthenogenetic mode of reproduction, allowing them to reach immense population sizes within a short period of time. They are also notorious as important and efficient vectors of several plant viral diseases. Their short fecund life cycles allow them to be pests on crops with a short growth period, e.g. lettuce (Lactuca sativa L.). It is common practice to provide this crop with some degree of protection from environmental extremes on the South African Highveld. Shadehouses are popular in this regard, but aphids are small enough to find their way into these structures, and their presence on lettuce is discouraged due to phytosanitary issues. In addition, the excessive use of insecticides is criticized due to the negative influence on human health, and because aphids can rapidly develop resistance. This necessitates the use of alternative control options in order to suppress aphid numbers. Biological control is popular in this regard and the use of predatory ladybirds (Coleoptera: Coccinellidae) is a popular choice. This study investigated the aphid and coccinellid species complex encountered under varying shadehouse conditions on cultivated head lettuce in the central Free State Province (South Africa). Their seasonality was also examined, along with variations in their population size throughout a one-year period. Finally, the impact of varying aphid populations on some physical characteristics of head lettuce was examined, and recommendations for aphid control (using naturally occurring coccinellid predators) were made. Two shadehouse structures were evaluated during this study. One was fully covered with shade netting and designed to exclude the pugnacious ant, Anoplolepis custodiens (Hymenoptera: Formicidae), while the other was partially covered with shade netting (on the roof area) allowing access to the ants. Six cycles of head lettuce were planted and sampled four times during each cycle. These were scheduled to monitor the seedling, vegetative and heading stage of lettuce. Four important aphid species were recorded on the lettuce, namely Acyrthosiphon lactucae, Nasonovia ribisnigri, Myzus persicae and Macrosiphum euphorbiae. Both structures harboured similar aphid and coccinellid species, but their population dynamics differed. A. lactucae dominated in the absence of A. custodiens in the fully covered structure (whole study), while N. ribisnigri dominated in the partially covered structure in the presence of these ants during the warmer months (December – January). M. euphorbiae replaced this species as the dominant species in the absence of A. custodiens (April – September). M. persicae occured during the winter (May – August) in the fully covered structure. Promising coccinellid predators were Hippodamia variegata and Scymnus sp. 1, and to a lesser extent, Exochomus flavipes and Cheilomenes lunata. However, the fully covered structure hampered the entrance of the larger adult coccinellid species, resulting in their lower occurrence. Aphid and coccinellid activity peaked during the summer months (October – January), and the fully covered structure attained the highest aphid infestation levels and coccinellid larval numbers during this time. On the other hand, aphid numbers were higher in the partially covered structure during the cooler months of the year (April – July) and this structure also harboured more adult coccinellids. In most cases, aphid infestation levels did not affect the amount of leaves formed. However, symptomatic damage in terms of head weight reduction did occur under severe infestation levels. Specific environmental conditions within a shadehouse structure concurrently contributed to this reduction, with less favourable conditions accelerating this condition. Results from this study have shown that even though the type of shadehouse structure does not influence the insect species complex found on lettuce, it does have an influence on detrimental and beneficial insect population dynamics. Aphid species infesting lettuce have been identified, along with coccinellid predators that could potentially be used in their control. Both types of structures had advantages and disadvantages, and therefore, decisions concerning shadehouses should not be focused on which type of structure to use, but rather which type of structure to use during different seasons of the year.

Interactions between insects have been shown to be important regulators of population abundances and dynamics as well as drivers of spatial segregation and distribution. These are important aspects of the ecology of insects used in biological control and may have implications for the overall success of a particular programme. In the history of biological control there has been a tendency to release a suite of agents against a weed, which in some cases has increased the level of success, while in others little change has been observed. In most of these cases the implications of increasing the level of complexity of the system is not taken into account and there is little research on the effect of releasing another agent into the system. A brief meta-analysis was done on all the biological control programmes initiated in South Africa. Emphasis was placed on multi-species releases and the effects that overlapping niches were having on the number of agents responsible for the success of a programme. Where overlapping niches were present among agents released the number of agents responsible for success was lower than the number established. Water hyacinth, Eichhornia crassipes (Martius) Solms-Laubach in South Africa has more arthropod agents released against it than anywhere else in the world, yet control has been variable. If the biology and host utilisation of all the agents against water hyacinth is considered, a definite overlap of niches is apparent in at least one life stage of all the agents. Therefore the probability of these insects interacting is high, especially if they are established at the same site in the field. Three of the insects released in South Africa have been selected to investigate possible interactions. They are Neochetina eichhorniae Warner, Neochetina bruchi Hustache and Eccritotarsus catarinensis (Carvalho). Y-tube olfactometer bioassays were used to measure responses of these insects to water hyacinth with prior feeding damage by either conspecifics or heterospecifics. This was done to determine whether olfactory cues played a role in host acceptability and avoidance of conspecifics or heterospecifics. The insects were given a choice between damaged and undamaged plants in various combinations. There was a significant preference for the undamaged plants when given a choice between undamaged and damaged plants. However when the insects were given a choice between two damaged plants there was no discrimination between heterospecific or conspecific damaged plants. This may indicate that there is little or no ecological cost for the insect to share a plant with other insects utilising a similar resource. Insect – insect interactions were investigated in a common garden plot experiment to measure the impact that pairwise combinations of the insect may have on their performance. There was a significant interaction between the mirid E. catarinensis and the weevil N. eichhorniae, with the weevil not performing as well when in combination with the mirid than when alone. Interestingly there was a negative interaction between the two weevil species when in combination, however it was impossible to determine which species was being affected if not both. None of the insects performed significantly better when in combination with another insect. A field study on Wriggleswade Dam in the Eastern Cape, South Africa was initiated to determine whether the relationship between the mirid E. catarinensis and the weevil N. eichhorniae could be determined in the field. The performance of the insects at the different sites in the field suggests that there was an interaction between the agents. This interaction did not limit the establishment of either insect at a site, but it did result in one insect dominating at a site over another. Interactions between the three species of insect tested in this thesis suggest that there are both negative and neutral relationships between them. A basic comparison between the insect performances from 15 sites around the country was done to determine if the spatial segregation observed in the field could be extrapolated to the natural South African situation. The interaction observed between N. eichhorniae and E. catarinensis does seem to extrapolate to the general South African situation where there is definite spatial segregation on a landscape level. The co–occurrence of the two Neochetina weevils at these sites suggests that the negative relationship observed between them in the common garden experiment does not extrapolate to the field. The results from this thesis suggest that the interactions between the agents tested would not limit establishment or have significant ramifications on performance. However, there may be spatial and temporal segregation of these species in the introduced range.

Rhizoctonia solani and Pythium spp. are aggressive soil-borne fungal pathogens responsible for seed rot and seedling damping-off of many crops. With increased environmental and public concern over the use of chemicals, biological control of these diseases has been attracting more attention. However, success with this strategy depends on the development of effective antagonists, which requires repeated in vitro and in vivo tests. Bacillus spp. were isolated from a soil sample obtained from a field where sorghum and tef had been grown for at least two years. Potential Bacillus isolates were screened for their ability to inhibit in vitro growth of R. solani and Pythium sp. Among 80 isolates tested, endospore forming Bacillus spp. H44 and H51 gave highest antifungal activity against the two test-pathogens in three consecutive tests. Results demonstrated that both H44 and H51 have potential as biocontrol agents against diseases caused by these two pathogenic fungi. The interaction between three isolates of Trichoderma (T. harzianum Eco-T, Trichoderma spp. SY3 and SY4) and Pythium sp. were investigated using in vitro bioassays together with environmental scanning electron microscopy (ESEM). Visual observation on the dual culture tests revealed that hyphal growth of Pythium was inhibited by these antagonists soon after contact between the two organisms within 3-4 days of incubation. The ESEM investigations showed that all three isolates of Trichoderma grew toward the pathogen, attached firmly, coiled around and penetrated the hyphae of the pathogen, leading to the collapse and disintegration of the host's cell wall. Degradation of the host cell wall was postulated as being due to the production of lytic enzymes. Based on these observations, antibiosis (only by Eco-T) and mycoparasitism (by all three isolates) were the mechanisms of action by which in vitro growth of Pythium sp. was suppressed by these Trichoderma isolates. The reduction of seedling diseases caused by R. solani and a pythium sp. were evaluated by applying the antagonists as seed coating and drenching antagonistic Bacillus spp. (B81, H44 and H51) and Trichoderma (T. harzianum Eco-T and Trichoderma spp. SY3 and SY4). On both crops, R. solani and Pythium sp. affected stand and growth of seedlings severely. With the exceptions of H51, applications all of isoltes to seeds reduced damping-off caused by R. solani in both crops. Application of Eco-T, H44 and SY3 to sorghum controlled R. solani and Pythium sp. effectively by yielding similar results to that of Previcur®. On tef, biological treatments with Eco-T and SY4 reduced seedling damping-off caused by R. solani and Pythium sp., respectively, by providing seedling results similar to the standard fungicides, Benlate® and Previcur®. Most other treatments gave substantial control of the two pathogens on tef. Overall, Bacillus sp. H44 and T harzianum Eco-T were the best biocontrol agents from their respective groups in reducing damping-off by the two pathogens. In all instances, effects of application method on performance of biocontrol agents and adhesive on emergence and growth of seedlings were not significant. A field trial was conducted at Ukulinga Research Farm at the University of Natal, Pietermaritzburg, South Africa, to determine efficacy of biological and chemical treatments on growth promotion and reduction of damping-off incited by R. solani and Pythium sp., and to evaluate the effects of a seed coating material, carboxymethyl cellulose (CMC), on seedling emergence and disease incidence. Seeds of sorghum and tef were treated with suspensions of antagonistic Bacillus H44 or T harzianum Eco-T, or sprayed with fungicides, Benlate® or Previcur®. Application of Benlate® and Previcur® during planting significantly increased the final stand and growth of sorghum seedlings. Seed treatments with both H44 and Eco-T substantially controlled damping-off caused by Pythium, resulting in greater dry weights of seedlings than the standard fungicide. However, they had negative effects when they were tested for their growth stimulation and control of R. solani. The CMC had no significant effect on germination and disease levels. These results showed that these antagonists can be used as biocontrol agents against Pythium sp. However, repeated trials and better understanding of the interactions among the antagonists, the pathogens, the crop and their environment are needed to enhance control efficiency and growth promotion of these antagonists. Some of these biocontrol agents used in this study have the potential to diseases caused by R. solani and Pythium sp. However, a thorough understanding of the host, pathogen, the antagonist and the environment and the interactions among each other is needed for successful disease control using these antagonists. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.

The public concern over synthetic pesticides in foods and the environment has created an interest to find effective and safe non-fungicide means of controlling postharvest pathogens. The overall objective of this thesis was to evaluate the effect of potassium silicate, yeast antagonists and hot water dip treatment to control postharvest grey mould and blue mould of pome fruits, caused by Botrytis cinerea and Penicillium expansum, respectively. Botrytis cinerea and Penicillium expansum were isolated from infected strawberry and pear fruits, respectively. These isolates were found to be non-resistant to YieldPlus® (Anchor yeast, Cape Town, South Africa), a biofungicide containing a yeast Cryptococcus albidus. A total of 100 epiphytic yeast isolates were obtained from the fruit surface of “Golden Delicious” apples and “Packham’s Triumph” pears, and screened against B. cinerea and P. expansum. Fifteen yeast isolates reduced grey mould incidence by > 50%, when applied four hours before inoculation with B. cinerea. Similarly, seven yeast isolates reduced blue mould incidence by > 50%, when applied four hours before inoculation with P. expansum. YieldPlus® and yeast Isolate YP25 provided the best control of B. cinerea, while Isolate YP60 and YieldPlus® provided the best control of P. expansum on “Golden Delicious” apples. A mixture of YP25 and YP60 provided complete control of both B. cinerea and P. expansum, when applied to “Golden Delicious” apples before inoculation with either B. cinerea or P. expansum. Electron microscopy studies showed that yeast Isolates YP25 and YP60 inhibited the mycelial growth of B. cinerea and P. expansum, respectively. Preventative and curative application of potassium silicate resulted in reduced incidence of B. cinerea or P. expansum of “Golden Delicious” apples. Electron microscopy studies indicated that potassium silicate inhibited the growth of B. cinerea and P. expansum. Furthermore, treatment of “Golden Delicious” apples with either potassium chloride or potassium hydroxide resulted in reduced incidence of both B. cinerea and P. expansum. In vivo tests showed that the disease incidence of P. expansum and B. cinerea on “Golden Delicious” apples was reduced by hot water dip treatments at 58-60°C for 60 to 120 seconds, compared with the control fruit treated with sterile distilled water, without causing skin damage. The use of potassium silicate, yeasts (Isolates YP25 and YP60), YieldPlus® and the antagonists mixture (YP25+YP60) in combination, resulted in the control of B. cinerea and P. expansum of “Golden Delicious” apples compared with Imazalil® treated fruit. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.

In the genus Fusarium are many important plant pathogens. The diversity of hosts the genus attacks, the number of pathogenic taxa and the range of habitats in which they cause disease are the greatest in plant pathology. One important species complex within the genus Fusarium is Fusarium oxysporum Schlecht. This species complex consists more than 80 pathogenic forma specialis and is particularly difficult to control. The fungi can survive in soil for decades as specialized spores, known as chlamydospores. Interestingly, however, this species complex also contains beneficial non-pathogenic forms that can be exploited to manage Fusarium wilt diseases. In this study, the ability of non-pathogenic F. oxysporum strains, Trichoderma harzianum Rifai Eco-T®, soluble silicon, and their combination was evaluated on two important crops, banana (Musa sp. L.) and beans (Phaseolus vulgaris L.), for their potential to suppress pathogenic strains of F. oxysporum. The ability of these crops to take up and accumulate silicon in their organs, and its effect on low temperature stress was also investigated. Several endophytic fungi, mainly Fusarium spp. and bacteria, were isolated from healthy mature banana plants. After preliminary and secondary in vivo screening tests against F. oxysporum f.sp. phaseoli on beans (Phaseolus vulgaris L.) cv. Outeniqua, two non-pathogenic F. oxysporum strains were selected for further testing. These two non-pathogenic F. oxysporum strains were found to colonize banana (Musa sp.) cv. Cavendish Williams and bean plants, and to suppress Fusarium wilt of these crops. In order to improve the efficacy of these biocontrol fungi, soluble silicon was introduced. The relationship between plant mineral nutrition and plant diseases have been reported by several authors. Plants take up silicon equivalent to some macronutrients, although it is not widely recognized as an essential element. In this study, we established that roots, the target plant organ for soilborne plant pathogens, accumulated the greatest quantity of silicon of any plant organs when fertilized with high concentrations of silicon. On the other hand, the corm and stem accumulated the least silicon. Such observations contradict the concept of passive uptake of silicon via the transpiration stream in these plants as the only uptake mechanism. The prophylactic properties of silicon have been documented for many crops against a variety of diseases. In vitro bioassay tests of silicon against these wilt pathogens showed that silicon can be toxic to Fusarium wilt fungi at high concentrations (>7840 mg .-1), resulting in complete inhibition of hyphal growth, spore germination and sporulation. However, low concentrations of silicon (<490 mg .-1) encouraged hyphal growth. Silicon fertilization of banana and beans significantly reduced disease severity of these crops by reducing the impact of the Fusarium wilt pathogens on these crops. However, it could not prevent infection of plants from the wilt pathogens on its own. Synergistic responses were obtained from combined applications of silicon and non-pathogenic F. oxysporum strains against Fusarium wilt of banana. Combinations of silicon with the non-pathogenic F. oxysporum strains significantly suppressed disease severity of Fusarium wilt of banana, caused by F. oxysporum f.sp. cubense (E.F. Smith) Snyder & Hansen, better than applications of either control measure on their own. Banana production in the subtropical regions frequently suffer from chilling injury, and from extreme variations between night and day temperatures. Such stress predisposes banana plants to Fusarium wilt disease. Silicon, on the other hand, is emerging as important mineral in the physiology of many plants, ameliorating a variety of biotic and abiotic stress factors. We established that silicon fertilization of banana plants significantly reduced chilling injury of banana plants. Membrane permeability, lipid peroxidation (MDA level) and proline levels were higher in silicon-untreated plants than the treated ones, all of which demonstrated the stress alleviating effect of silicon. Low temperatures damage the cell membrane of susceptible plants and cause desiccation or dehydration of these cells. Levels of sucrose and raffinose, recognized as cryoprotectants, were significantly higher in silicon-amended banana plants than unamended plants. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.

Fourteen spore-forming bacterial strains were isolated and screened for entomopathogenic activity. Five displayed toxicity towards the common mealworm, Tenebrio molitor L., (Coleoptera: Tenebrionidae). The majority of the isolates were obtained from insect larvae and insect rich environments. The three bacterial species identified were Bacillus thuringiensis Berliner, Brevibacillus laterosporus Laubach and Bacillus cereus Frankland and Frankland. Bioassays were conducted using T. molitor larvae. The one isolate of B. cereus required the highest concentration of bacterial cells to achieve its LC50, whereas one of the isolates of B. laterosporus required the lowest cell concentration to achieve its LC50. Dose response curves were generated for the five best isolates, which showed that the isolate of B. laterosporus (NDR2) was substantially more toxic than the other isolates. / Thesis (Ph.D.) - University of KwaZulu-Natal, Pietermaritzburg, 2009.

Powdery mildew (PM) is an important foliar disease of many crops, occurring under both greenhouse and field conditions. The application of biological control and soluble silicon (Si) against PM has received increasing acceptance as a result of increased environmental and public concern over the use of fungicides for disease management, and because many key fungicides are no longer effective because of resistance problems. However, success with these control options depends on the development of effective antagonists and understanding how best to use Si in agriculture. Potential antagonists of PM were isolated from naturally infected leaves of different plants. A total of 2000 isolates were tested in a preliminary screening on detached leaves of zucchini. The best 30 isolates showing consistent results were further tested under greenhouse conditions for their efficacy against PM of zucchini. In a greenhouse trial, 23 isolates provided disease control to levels of 30 to 77%. Application of 29 isolates resulted in significant reductions in values of area under disease progress curve (AUDPC). The best five isolates were identified as Clonostachys rosea (Link) Schroers, Samuels, Seifert & Gams (syn. Gliocladium roseum) (Isolate EH), Trichothecium roseum (Pers.) Link (syn. Cephalothecium roseum) (Isolate H20) and Serratia marcescens (Bizio) (Isolates B15, Y15 and Y41). Three adjuvants (Break-ThruR (BK), PartnerR (PR) and Tween-80R (T-80)) were compared for their ability to improve efficacy of spray application of silicon (Si) and biocontrol agents (BCAs) against PM. Both BK and PR improved the efficacy of Si significantly (P < 0.05). Microscopic studies showed that BK affected PM fungi directly and enhanced the deposition of BCAs on the pathogen. Break-ThruR was only toxic to the pathogen mycelia when used at > 0.25 m. .-1, but phytotoxic to zucchini plants when used at > 0.45m. .-1. However, it did not affect the c.f.u. of bacterial BCAs. Use of BK at 0.2-0.4 m. .-1 can be recommended to assist spray application of Si (at 750 mg .-1) or BCAs for improved control of PM. The effect of concentration, frequency of application and runoff of Si sprays applied to the foliage was evaluated for control of PM of zucchini. Silicon (250-1000 mg .-1) + BK (0.25 m. .-1), was sprayed onto zucchini plants at frequencies of 1-3 wk-1. Spraying Si reduced the severity of PM significantly (P < 0.05). Regardless of the concentration of Si, the best results were obtained when the frequency of the treatment was increased, and when spray drift or spray runoff were allowed to reach the rhizosphere of the plants. When Si was applied onto leaves, direct contact between the spray and the pathogen resulted in mycelial death. Part of the spray (i.e., drift and runoff) was absorbed by plant roots, and subsequently played an important role in the health of the plants. If affordable, soluble Si should be included in nutrient solutions of hydroponics or supplied with overhead irrigation schemes when PM susceptible crops are grown. Under greenhouse conditions, application of BCAs, with or without Si, reduced the severity and development of PM significantly (P < 0.001). Application of Si significantly reduced the severity and AUDPC values of PM (P < 0.05 for both parameters). Silicon alone reduced the final disease level and AUDPC values of PM by 23-32%, and improved the efficacy of most BCAs. In the course of the investigation, antagonistic fungi consistently provided superior performances to bacterial isolates, providing disease control levels of up to 90%. Higher overall disease levels reduced the efficacy of Si against PM, but did not affect the efficacy of BCAs. Under field conditions, Si alone reduced disease by 32-70%, Isolate B15 reduced disease by 30-53% and Isolate B15 + Si reduced disease by 33-65%. Other BCAs applied alone or together with Si reduced the disease level by 9-68%. Most BCAs reduced AUDPC values of PM significantly. For most antagonists, better efficacy was obtained when Si was drenched into the rhizosphere of the plant. However, efficacy of some of the BCAs and Si were affected by environmental conditions in the field. Repeated trials and better understanding of how to use Si and the BCAs, in terms of their concentration and application frequency, and their interactions with the plant and the environment, are needed before they can be used for the commercial control of PM. Elemental analysis was conducted to determine the impact of differing application levels of silicon (Si) in a form of potassium silicate (KSi) in solution in terms of Si accumulation and selected elements in different tissues of zucchini and zinnia and growth of these plants, and to study the effect of PM on the levels of selected elements in these two plant species. Plants were grown in re-circulating nutrient solutions supplied with Si at different concentrations and elemental composition in different parts were analysed using EDX and ICP-OES. Increased levels of Si in the solution increased the levels of Si in leaves and roots of both plants without affecting its distribution to other plant parts. In zucchini, the roots accumulated the highest levels of Si, substantially more than in the shoots. In contrast with zinnia, accumulation of Si was highest in the leaves. Accumulation of potassium (K) in shoots of both plants increased with increased levels of KSi in the nutrient solution. However, K levels in flower of zinnia, fruits of zucchini and roots of both plants remained unaffected. Increased level of Si reduced accumulation of calcium (Ca) in both plants. Adding Si into the nutrient solution at 50 mg .-1 resulted in increased growth of zucchini and increased uptake of P, Ca, and Mg by both plant species. However, application of higher levels of Si did not result in any further biomass increase in zucchini. Levels of Si in the nutrient solution had no effects on elemental composition and characteristics of the fruits of zucchini. In both plant species, the presence of PM on the leaves of plants resulted in these leaves accumulating higher levels of Si and Ca, but less P, than leaves of uninfected plants exposed to the same levels of soluble Si. The highest concentrations of Si were observed in leaf areas infected with PM, and around the bases of trichomes. For optimum disease control and maximum accumulation of different elements in these two plants, hydroponic applications of Si at 50-150 mg .-1 is recommended. Five selected biocontrol agents and potassium silicate, used as source of soluble Si, were tested under hydroponic conditions at various concentrations against PM of zinnia (Glovinomyces cichoracearum (DC) Gelyuta, V.P.). Application of BCAs resulted in reductions in final disease level and AUDPC values of PM by 38-68% and 30-65%, respectively. Both severity and AUDPC values of PM were reduced by 87-95% when plants were supplied with Si (50-200 mg .-1). It is proposed that the provision of a continuous supply of Si and the ability of this plant species to accumulate high levels of Si in its leaves were the major reasons for the good response of zinnia to Si treatments against PM. Silicon played a protective role before infection and suppressed development of PM after infection. The combination of the best selected BCAs and Si can be used as an effective control option against PM of zinnia when grown in hydroponic system. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.