An introduction to nematode systematics is provided which deals broadly with the history of the classification of nematodes, the controversial usage of the Phylum names Nemata Cobb, 1919 and Nematoda (Rudolphi, 1808) Lankester, 1877 and the reason why the name Nematoda was used in the present study. The classification, diagnosis and bionomics of the genus Aphelenchoides Fischer, 1894, the genus to which A. arachidis Bos, 1977 belongs is discussed. The section on bionomics is included to capture the astounding ability of this group of organisms to adapt to different trophic levels, a concept that is used to attempt an explanation for the ability of a supposedly African nematode, A. arachidis, to infest an alien crop species (groundnut). The ability of Aphelenchoides spp. to adapt to different host plant species is discussed, as well as the ability of the groundnut plant to mature its pods underground, a characteristic that predisposes these plants to a host of pathogens. The damage caused by two of the most important endoparasitic nematode species on groundnut, A. arachidis and Ditylenchus africanus Wendt, Swart, Vrain & Webster, 1995 were compared with each other. The South African population of A. arachidis was found predominantly in the shells of groundnut, whereas they were found in the shells, roots, hypocotyls and testas of the groundnut plants in Nigeria. The present study showed that A. arachidis and D. africanus occur together in groundnut in South Africa with D. africanus usually being the dominant species. In only one instance, at Bullhill (Vaalharts Irrigation Scheme, Northern Cape), the groundnut shells and testas were infested by A. arachidis alone. The importance of plant quarantine in South Africa is dealt with and the aims and principles of quarantine, as well as the different guidelines that have to be adhered to when deciding on the quarantine status of an organism are explained. Descriptions are provided of the methods used to prepare specimens for viewing with the light microscope (LM) and the scanning electron microscope (SEM), as well as the procedures of the molecular study. A morphological and morphometrical description of A. arachidis specimens from South Africa, as well as a comparison with specimens from Nigeria was done. Differences between the South African and Nigerian populations included, respectively, a lower b-value (7 - 11 vs 10 - 18), more lateral lines (2 - 4 vs 2), a slightly shorter stylet (8-10 m vs 10 - 12 m) and a longer length of the post-uterine sac as a percentage of the distance from vulva to anus (41 - 96 % vs ± 50 %). Scanning electron micrographs of this species are presented for the first time and shows the morphology of the lip region and lateral lines. Since both A. arachidis and A. blastophthorus were detected in the pods, a study was done to evaluate a PCR-based diagnostic method for the identification of these species and to compare the results with those reported in literature. Restriction fragment length polymorphisms (RFLPs) in the rDNA fragment were used to compare and differentiate between nematode species. The differences encountered within the South African population (morphological, morphometrical and molecular) warrant a study of more specimens from more localities. Through this it could be ascertained whether the South African population is a subspecies of A. arachidis or if this species just differs widely between localities. Future research should focus on a survey of the groundnut producing areas in South Africa to determine the distribution and economic impact of A. arachidis. The incidence of A. arachidis on other agricultural crops, especially those used in rotation with groundnut, also needs to be determined. The next issue to address is what enables a supposedly endemic species to Africa, A. arachidis, to parasitize an alien plant species (groundnut) from South America. Screening of the endemic bean family (Fabaceae) in Africa for the presence of A. arachidis, could hold the answer to this question. / Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2008.

Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2008.

An introduction to nematode systematics is provided which deals broadly with the history of the classification of nematodes, the controversial usage of the Phylum names Nemata Cobb, 1919 and Nematoda (Rudolphi, 1808) Lankester, 1877 and the reason why the name Nematoda was used in the present study. The classification, diagnosis and bionomics of the genus Aphelenchoides Fischer, 1894, the genus to which A. arachidis Bos, 1977 belongs is discussed. The section on bionomics is included to capture the astounding ability of this group of organisms to adapt to different trophic levels, a concept that is used to attempt an explanation for the ability of a supposedly African nematode, A. arachidis, to infest an alien crop species (groundnut). The ability of Aphelenchoides spp. to adapt to different host plant species is discussed, as well as the ability of the groundnut plant to mature its pods underground, a characteristic that predisposes these plants to a host of pathogens. The damage caused by two of the most important endoparasitic nematode species on groundnut, A. arachidis and Ditylenchus africanus Wendt, Swart, Vrain & Webster, 1995 were compared with each other. The South African population of A. arachidis was found predominantly in the shells of groundnut, whereas they were found in the shells, roots, hypocotyls and testas of the groundnut plants in Nigeria. The present study showed that A. arachidis and D. africanus occur together in groundnut in South Africa with D. africanus usually being the dominant species. In only one instance, at Bullhill (Vaalharts Irrigation Scheme, Northern Cape), the groundnut shells and testas were infested by A. arachidis alone. The importance of plant quarantine in South Africa is dealt with and the aims and principles of quarantine, as well as the different guidelines that have to be adhered to when deciding on the quarantine status of an organism are explained. Descriptions are provided of the methods used to prepare specimens for viewing with the light microscope (LM) and the scanning electron microscope (SEM), as well as the procedures of the molecular study. A morphological and morphometrical description of A. arachidis specimens from South Africa, as well as a comparison with specimens from Nigeria was done. Differences between the South African and Nigerian populations included, respectively, a lower b-value (7 - 11 vs 10 - 18), more lateral lines (2 - 4 vs 2), a slightly shorter stylet (8-10 m vs 10 - 12 m) and a longer length of the post-uterine sac as a percentage of the distance from vulva to anus (41 - 96 % vs ± 50 %). Scanning electron micrographs of this species are presented for the first time and shows the morphology of the lip region and lateral lines. Since both A. arachidis and A. blastophthorus were detected in the pods, a study was done to evaluate a PCR-based diagnostic method for the identification of these species and to compare the results with those reported in literature. Restriction fragment length polymorphisms (RFLPs) in the rDNA fragment were used to compare and differentiate between nematode species. The differences encountered within the South African population (morphological, morphometrical and molecular) warrant a study of more specimens from more localities. Through this it could be ascertained whether the South African population is a subspecies of A. arachidis or if this species just differs widely between localities. Future research should focus on a survey of the groundnut producing areas in South Africa to determine the distribution and economic impact of A. arachidis. The incidence of A. arachidis on other agricultural crops, especially those used in rotation with groundnut, also needs to be determined. The next issue to address is what enables a supposedly endemic species to Africa, A. arachidis, to parasitize an alien plant species (groundnut) from South America. Screening of the endemic bean family (Fabaceae) in Africa for the presence of A. arachidis, could hold the answer to this question. / Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2008.

Allelopathy occurs when one organism releases a compound into the environment that affects the functioning of another organism. Scientists have long suspected that alleopathic plant compounds could offer novel, softer chemistries to the ongoing battle of controlling pests in agricultural fields. Strawberry growers rely on toxic fumigants to kill soilborne fungal pests, weeds, nematodes, and insects. Increased regulations have reduced the use of fumigants (including methyl bromide), and strawberry growers need new sustainable pest control solutions. We selected four putative allelochemicals with known fungicidal and herbicidal activity (ferulic acid, gallic acid, juglone, and p-Coumaric acid). We assessed the pesticidal activity of these plant compounds both in agar and in soil on two emerging soilborne fungal pathogens (Macrophomina phaseolina and Fusarium oxysporum f.sp. fragariae), and four annual weeds commonly found in strawberry production fields (Malva parviflora, Melilotus officinalis, Poa annua, and Senecio vulgaris). We also assayed lettuce (Lactuca sativa ‘Inferno’), which served as a positive control plant species due to its sensitivity to phytotoxic compounds. Fitted sigmoidal dose-response curves predicted EC50 and EC75 values for each combination of plant compound and pest. All plant compounds inhibited the in vitro radial mycelial growth of the two soilborne fungal pathogens in a dose-dependent manner. Fusarium oxysporum f.sp. fragariae was more sensitive to the plant compounds than Macrophomina phaseolina. Average EC50 values for the radial mycelial growth of two F. oxysporum f.sp. fragariae isolates were 75.1 parts per million by weight (ppmw) juglone, 469 ppmw p-Coumaric acid, and 687 ppmw ferulic acid. Average EC50 values for the radial mycelial growth of two M. phaseolina isolates were 196 ppmw juglone, 2869 ppmw p-Coumaric acid, and 5716 ppmw ferulic acid. The three compounds we assayed in vitro also reduced M. phaseolina colony forming unit counts in soil and the EC50 values were 476 ppmw ferulic acid, 612 ppmw juglone, and 827 ppmw p-Coumaric acid. Metconazole, the conventional fungicide control, did not inhibit M. phaseolina colony forming unit counts in soil at its label high rate. The plant compounds required similar or lower rates to inhibit colony forming units that grew from M. phaseolina overwintering structures (microsclerotia) in soil as to inhibit radial mycelial growth in vitro. Based on the EC50 value in soil assays, ferulic acid was the least expensive plant compound to apply on a per acre basis to inhibit M. phaseolina ($74,226). In F.oxysporum f.sp. fragariae soil assays, the compounds induced hormesis at lower rates and may be germination stimulant candidates. Metconazole and the high rates of every compound effectively or completely inhibited F. oxysporum f.sp. fragariae colony forming units in soil. The plant compounds were more herbicidal than fungicidal in vitro. When combining the in vitro seedling length results for L. sativa, M. parviflora, P. annua, and S. vulgaris the EC50 values differed significantly (p < .0001) and were: 47 ppmw juglone, 120 ppmw p-Coumaric acid, 189 ppmw ferulic acid, and 297 ppmw gallic acid. At least one rate of ferulic acid, juglone, and p-Coumaric acid inhibited the germination of all plant species, while gallic acid only inhibited the germination of P. annua at 1000 ppmw (p < .05). In soil, visible microbial contamination in individual wells of 24-well plates and seed dormancy made it difficult to fit curves to weed seedling length data. The soil assay L. sativa seedling length EC50 values 11 days after initial treatment were slightly higher than in vitro, although plant compounds were in the same order of phytotoxicity: 129 ppmw juglone, 616 ppmw p-Coumaric acid, 644 ppmw ferulic acid, and 1584 ppmw gallic acid. Based on the EC50 value in soil assays, the least expensive compound to inhibit L. sativa seedling length on a per acre basis was gallic acid ($21,676). Germination 26 days after initial soil treatment generally declined in a dose-dependent manner for each compound. There was a direct relationship between plant compound rate and seedling damage in soil with the higher rates of all compounds, except p-Coumaric acid, inducing damage comparable to a conventional herbicide (pendimethalin or oxyfluorfen). Contaminated treatments appeared to be due to an interaction between plant compounds and microorganisms because herbicide and water controls had almost no microbial growth 11 days after initial treatment. Further, there was a significant positive linear relationship between level of contamination in phenolic acid-treated wells (ferulic acid, gallic acid, and p-Coumaric acid, p < .0001) and the in soil rate. This relationship was slightly negative in juglone soil treatments (p = .0167), which may be due to its greater antimicrobial activity than the phenolic acids. We propose that herbicidal effects in soil were due to the joint effect of the plant compounds themselves, and the microbial growth in wells. Microbial growth was either antagonistic or additive to the inhibitory action of the plant compounds. The plant compounds we assayed were inhibitory of emerging fungal pathogens in strawberry production and common annual strawberry field weeds. Evidence presented in this thesis correlates well with past research that not only found plant compounds to be herbicidal and fungicidal, but also described their modes-of-action (such as the production of reactive oxygen species that causes necrotic lesions on roots, and inhibition of glycolytic enzyme activity that prevents germination), and implicate plant compounds as carbon sources for a variety of microorganisms. Compound prices are currently exorbitant, but may decline as demand increases. Whether or not they provide effective pest control may depend on soil texture, organic matter, microbial diversity, and other edaphic factors.

Diss. (sammanfattning) Uppsala : Sveriges lantbruksuniv. / Härtill 4 uppsatser.