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The infection process of Exserohilum turcicum: A microscopy investigation

Quenton Kritzinger: Department of Plant and Soil Sciences, University of Pretoria

<div>Northern corn leaf blight (NCLB) is a foliar disease of maize caused by the fungal pathogen, <em>Exserohilum turcicum</em>. The aim of this study was to determine the infection strategy of <em>E. turcicum</em> race 23N in maize leaves by using various microscopy techniques to better understand the role of the fungus in NCLB development. Leaf samples were collected from inoculated B73 maize plants at 1, 4, 11, 14 and 18 days post inoculation (dpi). Samples were prepared according to standard microscopy procedures and analysed using light and electron microscopy. At 1 dpi the fungus penetrated the leaf epidermis, and at 4 dpi hyphal growth was seen in the epidermis. At 11 dpi, lesions formed on the leaves. Scanning electron microscopy revealed hyphal growth in the xylem at 11 dpi and almost completely blocked by 14 dpi. Hyphae spread into the adjacent bundle sheath cells causing cellular damage, characterised by plasmolysis at 18 dpi. Conidiophores formed through the stomata at 18 dpi. Additionally, the lesions enlarged and started to coalesce and wilting of the leaves was observed. Extensive plugging of the xylem could inhibit water flow through the parasitized tissue, which could account for the formation of lesions. Increased colonization of the fungus in the xylem corresponded to an increase in size of the lesions, which leads to reduction in photosynthetic area. Understanding the infection strategy of the fungus could aid in development of better control and prevention strategies.</div>