Penetration and Infection of Leaves of Black Walnut byMarssonina juglandisand Resulting Lesion Development

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Abstract
Penetration and infection of black walnut (J. nigra) leaves by the walnut anthracnose fungus and the resulting lesion development were observed. A conidial suspension was atomized onto mature leaves. Conidia germinated terminally or subterminally within 48 h and formed short germ tubes and/or appressoria. Penetration through epidermal cells was initiated from appressoria and was either direct or followed subcuticular growth. Papilla formation was associated with resistance to penetration. Intercellular hyphae were observed after 72 h, while intracellular spread through epidermal cells was prominent at 144 h. By 168 h, the mycelium had entered the mesophyll, and host cell necrosis could be seen microscopically. Macroscopic lesions were observed at 240 h and acervulus formation occurred after 240 h.