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.