In general plants defend themselves against pathogens by two ways:

I)Structural defense mechanisms:

II) Biochemical defense mechanisms:

1. Structural defense mechanisms:

These may be pre-existing, which exist in the plant even before the pathogen comes in contact with the plant or induced.

A) Pre-existing structural defense structures :

These include the amount and quality of wax and cuticle that cover the epidermal cells and the size, location and shapes of natural openings (stomata and lenticels) and presence of thick walled cells in the tissues of the plant that hinder the advance of the pathogen.

i) Waxes: Waxes on leaf and fruit surfaces form a hydrophobic or water repellent surface preventing the germination of fungi and multiplication of bacteria.

ii) Cuticle and epidermal cells:

• A thick cuticle and tough outer wall of epidermal cells may increase resistance to infection in diseases in which the pathogen enters its host only through direct penetration.
• Eg : In linseed, cuticle acts as a barrier against Melampsora lini.
• The silicification and lignifications of epidermal cells offers protection against Pyricularia oryzae and Streptomyces scabies in paddy and potato, respectively.

iii) Sclerenchyma cells:

• The sclerenchyma cells in stems and leaf veins effectively blocks the spread of some fungal and bacterial pathogens that cause angular leaf spots.

iv) Structure of natural openings:

a) Stomata:

• Some pathogens like stem rust of wheat can enter its host only when the stomata are open.
• The structure of stomata provides resistance to penetration by certain plant pathogenic bacteria.
• Ex: The citrus variety, szinkum, is resistant to citrus canker because it posses a broad cuticular ridge projecting over the stomata and a narrow slit leading to the stomatal cavity thus preventing the entry of bacterial and fungal spores into the interior of the leaf.

b) Lenticels:

• Small and suberised lenticels will offer resistance to potato scab pathogen, Streptomyces scabies.

B) Post-infectional structural defense mechanisms/Induced structural barriers:

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 : Corklayer

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