i) Histological defense structures :

a) Cork layer:

• Infection by fungi, bacteria, some viruses and nematodes induce plants to form several layers of cork cells.
• It then blocks the spread of toxin substances secreted by the pathogen.
• Furthermore, cork layers stop the flow of nutrients and water from the healthy to the infected area and deprive the pathogen of nourishment.
• Ex: Potato tubers infected by Rhizoctonia; Prunus domestica leaves attacked by Coccomyces pruniphorae.

Fig : Cork layer

b) Abscission layers :

• An abscission layer consists of a gap formed between infected and healthy cells of a leaf surrounding the locus of infection due to the disintegration of the middle lamella of parenchymatous tissue.
• Gradually, infected area shrivels, dies, and sloughs off, carrying with it the pathogen.
• Ex: Xanthomonas pruni, and Closterosporium carpophylum on peach leaves

Fig : Abscission layer.

c) Tyloses :

• Tyloses are the overgrowths of the protoplast of adjacent living parenchymatous cells, which protrude into xylem vessels through pits.
• In susceptible varieties, few or no tyloses are formed ahead of pathogen invasion.
• Ex: Tyloses form in xylem vessels of most plants under invasion by most of the vascular wilt pathogens.

Fig : Tyloses

ii) Cellular defense structures:

a) Hyphal sheathing:

• The hyphae penetrating the cell wall and growing into the cell lumen are enveloped by a cellulosic sheath (callose) formed by extension of cell wall, which become infused with phenolic substances and prevents further spread of the pathogen.
• Ex: Hyphal sheathing is observed in flax infected with Fusarium oxysporum f.sp. Lini.

Fig : Hyphal sheathing