How primary infection and secondary infection occurs ?

Primary infection occurs when the pathogen comes into contact with a susceptible host under favourable environmental conditions. Pathogens penetrate the surface of a plant directly or enter through wounds or natural openings. Growth and development of a pathogen usually occurs on or within infected plant tissue. Secondary infection results from spores or cells produced following primary infection or from other secondary infections. The secondary infection cycle can be repeated many times during the growing season.

The number of cycles is dependent on the biology of the pathogen and its host and the duration of environmental conditions needed for infection.