Conidial germination of Alternaria porri, formation of prepenetration structures, penetration of the onion leaf surface, and the postpenetration processes were studied using light, scanning electron, and transmission electron microscopy. Ninety-six percent of conidia germinated at 25 °C within 24 h of inoculation. Each conidium formed several germ tubes that grew in any direction across the leaf surface. Each germ tube usually terminated in a bulbous appressorium formed directly on the epidermal cell (52.4% of appressoria) or on a stoma (48.6% of appressoria). Following direct penetration of the outer epidermal cell wall or the stoma, bulbous primary hyphae developed below the appressoria. Secondary hyphae developed from the primary hyphae within 48 h after inoculation and grew within the intercellular spaces penetrating mesophyll cells. The changes in ultrastructure of cells in close proximity to hyphae and of infected cells are described. Key words: Allium cepa, electron microscopy, infection process, purple blotch.

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The use of sewage sludge as an organic fertiliser under South African conditions is an alternative disposal route to sacrificial land disposal. However, the lack of research done under South African conditions and the conservative nature of the heavy metal guidelines, when interpreted as total metal content is limiting the agricultural use of sludge. A glasshouse experiment, which forms part of a greater project, was conducted to characterise soil-plant interactions of the main sludge-borne heavy metals (Pb, Cd, Zn and Cu) in two sludges (low metal and high metal) to different soil types (clayey, loamy and sandy) on maize seedlings. Growth differences, heavy metal accumulation in plant parts and soil-metal concentrations (total and potentially available) were determined. The low metal sludge treatment showed the highest yield for maize seedlings when compared to controls (soil unamended and inorganic fertiliser added). The amendment of sludge to the soil did indicate higher heavy metal content, although the increase was not as predicted, owing to the difficulty of obtaining a representative sample in the soil. Except for Cd, heavy metal values in the soils (at the beginning and end of experiment) exceeded guidelines due to very high background values in the soil. No negative effects of heavy metal contamination in plant parts of the crops could be proven. Results showed that application of sludge to different soils could be useful in order to increase crop growth over a 28 d period in the glasshouse. Soil, plant and water quality monitoring, together with the prevention of metals entering the plant, is a prerequisite in order to prevent potential health hazards of sludge application to agricultural land.

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The new South African Wastewater Sludge Guideline Series will replace the document entitled: "Permissible Utilisation and Disposal of Sewage Sludge, Edition 1, 1997" which deals with application of sewage sludge to land. The new guideline series will be published as 5 volumes: Volume 1: Management Options for the Use or Disposal of Wastewater Sludge, Volume 2: Requirements for the agricultural use of wastewater sludge, Volume 3: Requirements for the on site and off-site disposal of wastewater sludge, Volume 4: Requirements for the use of wastewater sludge in the production of commercial products, Volume 5: Requirements for the incineration of wastewater sludge, We envisage that Volumes 1 and 2 will be published at the time of the conference and that work has begun on Volume 3, 4 and 5. The paper aims to communicate the major differences between the previous sludge guidelines and these new sludge guideline series. For example, the paper details the new wastewater sludge classification system and the requirements for the characterisation of the sludge for classification purposes. The paper will also explain the approach followed in the development of Volume 1 and 2 and will elaborate on the proposed approach for Volumes 3, 4 and 5.