Thesis (Ph.D. (Plant Protection)) --University of Limpopo, 2012 / Global withdrawal of synthetic fumigant nematicides like methyl bromide due to their eco-unfriendliness resulted in serious consequences in production of crops which do not have genotypes that are resistant to plant-parasitic nematodes. Watermelon (Citrullus lanatus) is one such crop, where infection by highly aggressive root-knot nematodes (Meloidogyne species) invariably results into as high as 50% yield loss, with occasional total crop failures. Initial screening for nematode resistance in Cucumis species indigenous to South Africa suggested the possibility of the existence of nematode resistance, with the probability of these species being compatible with Citrullus species in inter-generic grafting technology. Uses of indigenous genera in Cucurbitaceae family as nematode-resistant seedling rootstocks in watermelon production could promote the South African watermelon industry as outlined in ISO 9001 certification guidelines to have competitive advantage in lucrative watermelon export markets. The objectives of this study were to determine the: (1) host-status and host-sensitivity of C. africanus and C. myriocarpus seedlings using a series of inoculation levels of M. incognita race 2 under various conditions, (2) host-status and host-sensitivity of C. africanus and C. myriocarpus seedlings using a series of inoculation levels of M. incognita race 4 and M. javanica, including the resistance form in these plant species, at least, under selected environmental conditions, (3) host-status and host-sensitivity of C. africanus and C. myriocarpus seedlings using a series of inoculation levels of M. incognita race 2 with multi-nematode xxviii infestations in order to establish whether the observed nematode resistance was sustainable when the plant was attacked by various pests at the root system level, (4) compatibility of inter-generic grafting of Citrullus and Cucumis seedlings in order to establish the potential uses of Cucumis species in olericulture, and (5) influence of the greenhouse whitefly (Trialeurode vaporariorum) infection on resistance of C. africanus to Meloidogyne species in order to establish whether the observed nematode resistance was sustainable when the plant was attacked by pests on complimentary organs. Reliability of measured variables was ensured by using statistical levels of significance (P ≤ 0.05) and coefficient of determination (R2), with validity being ensured by conducting experiments at the same location over two seasons or conducting one experiment during one season at two different locations, viz. the University of Limpopo and the Agricultural Research Council – Institute for Industrial Crops, and/or by setting up factorial treatments. Results consistently demonstrated that C. africanus and C. myriocarpus were non-hosts to M. incognita races 2 and 4 and M. javanica, without the test nematodes inflicting any damage to plants, which in plant-parasitic nematodes is described as nematode resistance. Quadratic relationships between RF values and log10(Pi + 1) transformations, in addition to confirming the density-dependent growth patterns of plant-parasitic nematodes, also suggested that chemical compounds responsible for suppression of nematodes in the two Cucumis species were different. The two Cucumis species were resistant to M. incognita races 2 and 4 and M. javanica, regardless of the environment under which the experiments were conducted. In field studies, the xxix two Cucumis species supported the ring nematodes (Criconema mutabile) and the spiral nematodes (Helicotylenchus dihystera), without these exo-parasitic nematodes inflicting any damage to plants, which in plant-parasitic nematodes is described as tolerance. Interactions among Meloidogyne species, C. mutabile and H. dihystera were either stimulatory or inhibitory, depending on whether Meloidogyne species were in the soil or inside the roots. Mechanisms of nematode resistance in the two Cucumis species were different, with C. africanus and C. myriocarpus depicting pre-infectional and post-infectional forms of resistance, respectively, without any sign of hypersensitivity in roots. When, seeds of Citrullus species were primed in water to hasten germination. Using the developed technology, survival of grafts improved from 36% to 100%, translating to relative improvement of 178%, with nematode-resistant rootstocks retaining their nematode resistant capabilities, while watermelon scions flowered earlier, with relatively higher fruit yield, without any deleterious effect on accumulation abilities of essential nutrient elements in leaves. Resistance of C. africanus to M. javanica was invariably broken by the greenhouse whitefly infection at high population levels, possibly through loss of non-structural carbohydrates, which are essential in synthetic pathways of secondary metabolites. Cucumis africanus and C. myriocarpus contain cucurbitacin B (C32H48O8) and cucurbitacin A [cucumin (C27H40O9), leptodermin (C27H38O8)], respectively, which have high demand for carbon and energy. Consequently, the efficacy of indigenous Cucumis species as nematode-resistant rootstocks in suppression of Meloidogyne species would be dependent upon the management of the xxx greenhouse whitefly population densities. In conclusion, C. africanus and C. myriocarpus have the potential for use as nematode-resistant rootstocks in the production of watermelon cultivars ‘Congo’ and ‘Charleston Gray’ in South Africa, where nematode population densities of M. incognita races 2 and 4 and M. javanica are widely distributed and are highly injurious to watermelons. Although nematode resistance in the two Cucumis species had attributes of sustainability, populations of the greenhouse whitefly broke the resistance. Proposed future research areas included influence of cucurbitacins in fruit quality of watermelons and protocols for mass culturing the nematode-resistant Cucumis rootstocks using tissue culture technology. / the National Research Foundation,the Agricultural Research Council (ARC) and the Landbank Chair of Agriculture-University of Limpopo

Thesis (M.Sc. (Plant Production)) -- University of Limpopo, 2013 / Two phytonematicides were researched and developed from fermented crude extracts of wild watermelon (Cucumis africanus) and wild cucumber (Cucumis myriocarpus) fruits for use as alternatives to methyl bromide in managing root-knot (Meloidogyne species) nematodes in tomato (Solanum lycopersicum) production. Fruits of C. africanus contain cucurbitacin B (C32H48O8), while those of C. myriocarpus contain cucurbitacin A, which comprises cucumin (C27H40O9) and leptodermin (C27H38O8). Phytonematicides from C. africanus and C. myriocarpus fruits are referred to as nemafric-B and nemarioc-A, respectively. The two phytonematicides, due to their origin from plant species with allelochemicals, have high potential of being phytotoxic to crops. The use of the Curve-fitting Allelochemical Response Dosage (CARD) computer-based model assisted in the establishment of concentrations which were stimulatory to growth of tomato (Solanum lycopersicum) plants, while exhibiting nematoxic properties to Meloidogyne species. The two phytonematicides were developed from crude extracts of fruits dried at 52˚C in air-forced ovens and ground in a Wiley mill through 1-mm-opening sieves. However, equipment for drying and grinding fruits would not be accessible to smallholder farmers who wished to prepare their own products on-farm. The objective of this study therefore, was to determine whether nemafric-BL and nemarioc-AL produced from fresh fruit of the two Cucumis species would be suitable for use (i.e. non phytotoxic) in tomato production for managing population densities of M. incognita race 2. In order to distinguish the products of fresh (F) fruits from those of dried (D) fruits, they were code-named nemafricF-BL or nemariocF-BL and nemafricD-BL or nemariocD AL, respectively, where G and L denoted granular and liquid formulations, respectively. Tomato cv. ‘Floradade’ seedlings were infested with 3 000 eggs and second-stage xv juveniles of M. incognita race 2. An equivalent of 40 g and 80 g dried fruit mass of nemafric-B and nemarioc-A, namely, 284 g and 411 g fresh fruit mass for nemafric-B and nemarioc-A, respectively, were separately fermented using EMROSA effective micro-organisms mixed with 16 L chlorine-free tapwater in 20 L container for 14 days at ± 25˚C, allowing pH to gradually decline to ± 3.7. Separate experiments for each product run concurrently. Treatments, namely, 0, 2, 4, 8, 16, 32 and 64% concentrations, where for instance, 2% = 20 ml/1000 ml x 100, were arranged in a randomised complete block design, with 10 replications. Blocking in the greenhouse was done for wind direction which was regularly erected by fans for cooling down the greenhouse. At 56 days after weekly application of each treatment, flower number, fruit number, dry shoot mass, dry root mass, dry fruit mass, plant height, stem diameter and nematode numbers were each subjected to analysis of variance. Nematode data were, prior to analysis, transformed using log10(x + 1), but untransformed data were reported. Using the sum of squares, nemafric-BL and nemarioc-AL treatments affected dry root mass, dry shoot mass, flowers number, fruit number, plant height and stem diameter. Nemafric-BL contributed 67%, 78%, 58%, 43%, 60% and 26%, while nemarioc-AL contributed 71%, 61%, 19%, 35%, 34% and 24% to total treatment variation of the six respective variables. Plant variables with significant (P ≤ 0.05) treatment effects were further subjected to the CARD model to generate seven biological indices, with three distinct phases, namely, stimulation, neutral and inhibition phases. Using the quantified stimulation phase, the mean concentration stimulation range (MCSR) was computed for each variable using two biological indices, namely, threshold stimulation point (Dm) and saturation point (Rh). The CARD model explained 98%, 99%, 98% and 98% of the quadratic models of dry root mass, dry shoot mass, plant height and stem diameter, xvi respectively, against increasing concentrations of nemarioc-AL. Similarly, the CARD model explained 99%, 96%, 84% and 93% of total treatment variation in the respective plant variables. The integrated MCSR [MSCR = Dm + (Rh/2)] for nemafric-BL on tomato plants was 7%, while that for nemarioc-AL was 4%. In the CARD model, the overall sensitivities (∑k) of tomato plants exposed to nemafric-BL and nemarioc-AL were 3 units and 5 units, respectively. Tomato plants were therefore, less sensitive to nemarioc-AL since it had higher ∑k value than nemafric-BL. At 4% nemarioc-AL and at 7% nemafric-BL, the two phytonematicides were each highly suppressive to population densities of M. incognita race 2. In conclusion, on the basis of non-phytotoxicity of the computed MCSR values and their suppressive effects on population densities of M. incognita race 2, the smallholder farmers could produce nemafric-BL and nemarioc-AL phytonematicides on-farm. However, the production of the two products from fresh fruits would not be sustainable since fruits of the two Cucumis species are highly seasonal due to the high incidence of post-harvest decays. / The Land Bank Chair of Agriculture – University of Limpopo, Limpopo Agro-processing Technology Station,and the Flemish Interuniversity Council of Belgium