a) Breakdown of physical barriers.:
- Fungi penetrate host plants directly through a fine hypha produced directly by the spore or mycelium or through a penetration peg produced by an appressorium.
- These structures exert pressure on the surface which results in stretching of the epidermis which becomes thin. Then the infection peg punctures it and effects its entry.
b) Breakdown of chemical barriers:
The host is provided with defense mechanisms against invasion which include
i) presence of cuticular layer on the epidermis,
ii) lack of suitable nutrients for the pathogen in the host cells,
iii) presence of inhibitory or toxic substances in the host cells,
iv) exudation of substances toxic to pathogen or stimulatory to antagonists of the pathogen.
Eg : I) The glands in leaf hairs of begal gram contain maleic acid which is antifungal and provide resistance to infection by the rust fungus (Uromycesciceris arietini).
ii) Protocatecheuic acid and catechol in the red scales of onion provide resistance to onion smudge pathogen, Colletotrichum circinans.
To overcome these physical and chemical barriers, the fungi produce various enzymes, toxins organic acids and growth regulators.