Ultrastructural changes associated with induced systemic resistance of cucumber to disease: host response and development of Colletotrichum lagenarium in systemically protected leaves

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Abstract
Systemic resistance to anthracnose caused by Colletoricum lagenarium was induced in cucumber by inoculation with fungal conidia. Control and immunized leaves were examined by both light and electron microscopy before and after subsequent challenge with the fungus. Penetration through the epidermal wall was evident 36 h after challenge and was followed by hyphal proliferation and host-cell destruction in control tissues. Penetration was rarely seen inimmunized tissues before 72 h, but at 25 h, invaginations of the appressorial wall were observed, as well as highly electron opaque epidermal walls beneath the appressoria. At 72 h, thin penetration pegs were seen within the dense host wall, and at 96 h, the peg was embedded within the underlying papilla and aggregates of callose-like material. Penetration through the epidermis was rare in immunized tissue, but when it was observed, development appeared to be similar to that in control leaves. Evidence is presented to show that ultrastructural changes in immunized tissues as well as in the fungus occur within 24 h after challenge of the host. There appears to be a correlation between the comparatively few lesions observed macroscopically in immunized leaves and the sparsity of successful penetrations into host mesophyll cells.