a) Inhibitors released by the plant in its environment:
- Plants exude a variety of leaf and root exudates which contain amino acids, sugars, glycosides, organic acids, etc.
- The inhibitory substances directly affect micro-organisms or encourage certain groups to dominate the environment which may act as antagonists to pathogen.
- Eg : Tomato leaves secrete exudates which are inhibitory to Botrytis cinerea.
- Red scales of red onion contain the phenolic compounds, protocatechuic acid and catechol, which diffuse out to the surface and inhibits the conidial germination of onion smudge fungus, Colletotrichum circinans.
- Resistant varieties of apple secrete waxes on the leaf surface which prevents the germination of Podosphaera leucotricha (powdery mildew of apples).
- Chlorogenic acid present in sweet potato, potato and carrot inhibits Ceratocystis fimbriata.
b) Inhibitors present in plant cells before infection:
- Several phenolic compounds, in cells of young fruits, leaves or seeds are responsible for the resistance of young tissues to Botrytis.
- Saponins have antifungal membranolytic activity which excludes fungal pathogens that lack saponinases. Ex: Tomatine in tomato and Avenacin in oats.
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